Chapter 11 – Evolution and Extinction
Evolution and Extinction
Learning Objectives
Welcome to Chapter 11. At the end of this chapter you will be able to:
- Define evolution and create an historical timeline of advances in evolutionary theory
- Define Natural selection and explain how it works.
- Discuss the differences between genotype and phenotype
- Identify and discuss the patterns of evolution and their impact on evolutionary theory.
- Compare and contrast background extinction and mass extinction
- Identify the most common causes of mass extinction.
- Define adaptive radiation and identify its importance as a pattern of evolution
Evolution and the Florissant Fossil Beds
In south-central Colorado, at Florissant Fossil Beds National Monument, you can visit a unique deposit of fossil trees, leaves, insects, birds, fish and more from about 35 million years ago. At that time, a lava flow dammed a stream to form a lake. Then, repeated volcanic eruptions dropped ash into the lake, making it silica-rich and favoring growth of diatoms, which have silica shells. Huge numbers of diatoms quickly grew in very thin layers on things that fell into the lake, protecting them from decay until they were buried by paper-thin layers of ash and mud. Like flowers pressed in a phone book, the flowers of that ancient time can still be seen clearly. So can the dragonflies, and bees, and mosquitoes. Before the National Monument was established, private collectors could pay a small fee and hunt for fossils there. The author found a few fossil bees and ants and seeds, and a friend found a beautiful cicada. (Taking fossils from the National Monument is strictly forbidden; they belong to all of us!)
There are, however, certain differences between some of those creatures and similar ones that live today. Remember, back at Arches, we learned about the law of faunal succession—when rocks are placed in order from oldest to youngest, the types of fossils in the rocks also fall into order, becoming more like things alive today in younger and younger rocks. The Florissant rocks are relatively young, and their fossils are immediately familiar to modern people, but the fossils are not identical to modern species.
The law of faunal succession suggests the possibility of evolution, but does not prove it by any means. One early theory held that many creations and extinctions occurred over geologic history, something like the history of automobiles. One type of automobile does not give birth to another; new ones are created. But you can place the automobiles in order from oldest to youngest, and the younger they are, the more they look like modern automobiles. This idea was called catastrophism in geology. The fundamentalist interpretation of biblical creation is a one-event form of catastrophism.
Evidence of Evolution
Catastrophism eventually lost out scientifically to evolution. Evolution does a much better job of explaining the patterns of fossils and of living things, and of predicting events such as the emergence of antibiotic-resistant microorganisms. The triumph of evolution over catastrophism owes much to biology, and more recently to genetics and molecular biology; the identification of the mechanisms driving evolution was especially important (see below).
As geologists collected more data, they also realized that evolution explained the data and predicted the next discoveries better than did any catastrophist model. The early geologists could see the clear march of types over time, but saw catastrophic elements in the record as well. In many places, geologists would find fossils of one type, and then of a somewhat different type, with no transitions between them. Evolution implies rather gradual change, not big jumps. The early geologists knew, however, that there were big time jumps in the records (remember all of the unconformities—time gaps—in the Grand Canyon sequence). So some of the jumps in the fossil record were related to the incompleteness of the rock record. Other jumps really are related to catastrophic events in the record (again, see below).
Further study has shown that many of the evolutionary changes have been geologically fast (but recall that geological time is so long that this can be biologically slow!) and often localized. Suppose that a few animals of some type colonize a small island. Then, they have babies who have babies who have babies, a generation per year, thousands of generations in a geological eyeblink. If the babies differ by just a tiny bit from the parents, eventually a new type or species may emerge. If that species then succeeds in escaping the island (say, because sea-level fell and the island became connected to the mainland), a new type would appear suddenly on the continent. Sediments from the small island may end up being subducted or otherwise destroyed, but fossils on the larger continent are more likely to be preserved. A small island may support only a few individuals, so there never would be many critters to produce fossils that humans could find. On the continent, the species might flourish and produce millions of individuals that would leave easily collected fossils. Thus, the fossil record would show a sudden jump when the actual process was gradual.
In one famous case (of many), trilobites of the genus Phacops are classified in part by the number of columns of elements in their compound eyes. In marine sediments from the Devonian (the middle of the Paleozoic) of Pennsylvania and Ohio, a species with eighteen columns in its eyes occurs for a while. Then, a time gap or unconformity occurs, from a temporary drying of the sea. When the sea returned and began depositing sediments again, the trilobites that returned with it had seventeen columns in their eyes. Not a huge jump, but an apparently sudden one. But wait—over in a small part of New York, the sea did not dry up. There, you can find the old eighteen-column trilobites, then some with seventeen columns plus a partial column containing a varying number of elements, and finally the seventeen-column trilobites. The generations of trilobites changed gradually, and you can see this where the rock record is complete in a small region of New York. In the bigger areas, the record looks more catastrophic because the seaway was dry and no fossils were produced when the changes were occurring through the generations living in New York.
Some so-called “creation scientists” still argue that no transitional forms are known, and so that catastrophism is accurate. This is nonsense; extremely fine gradations are known in many, many lineages. There is one technical sense in which, in some lineages for which fossils are scarce, there are missing transitional types. Suppose you find young fossil type 1 and old fossil type 0. They differ a good bit—you are missing the transitional form 1/2. Now suppose you find type 1/2. It is your “missing link”. You publish your results in important scientific journals, and wait for the fame and fortune to roll in. (You are likely to wait a looooooong time….) But, while you’re enjoying your discovery, someone argues that you are haven’t really found the missing link, because now there are TWO transitional types missing: 1/4 between 0 and your newly discovered 1/2, and 3/4 between your newly discovered 1/2 and 1. So, you go back to work, and after years of effort, succeed in finding both 1/4 and 3/4. Wow! Now your critics point out that you are missing FOUR transitional forms (1/8, 3/8, 5/8 and 7/8). This can be argued to absurdity; we cannot find remains of every creature that ever lived, because almost all remains of almost all creatures are recycled by the efficient ecosystems of Earth (dead things are food to scavengers, worms, bacteria, fungi, etc.). But in many, many lines now, the gaps are vanishingly small, and the transitional forms very well known.
The gaps in evolutionary lineages are especially well-filled for commonly fossilized types, such as shelly marine creatures from shallow water. Shells are hard and resistant—they’re really rocks already—and so shells are preserved well. Although sediments from deep-water sites tend to be dragged down subduction zones and melted, which messes up old records, the underwater edges of continents are not subducted and often escape obduction for a long while, preserving their records of shelly creatures. And while most of land is eroding, most of the ocean is accumulating sediments.
The fact that most of the land surface is eroding complicates study of the fossils that especially interest people. Think about central Pennsylvania for a moment, where Dr. Alley was sitting when he wrote this. The Commonwealth of Pennsylvania has about 1 million deer, and each buck has two antlers, so Pennsylvania deer drop about 1 million antlers per year. If antlers were “preserved,” then after even a few millennia of this, walking in the state should be really dangerous, and we should hear about all sorts of antler puncture wounds from the billions of antlers scattered over the landscape. Of course, we don’t—mice and porcupines eat the antlers for the minerals in them.
In central Pennsylvania now, the only places you can find sediments being deposited are in the reservoirs (which were mostly built in the 1930s, and so give very, very short records), a very few marshes such as Bear Meadows up the road from Penn State’s University Park campus (this marsh formed during the Ice Age and thus is geologically very young), and in a few caves and along a few streams. But, the caves and stream deposits don’t last long—the caves are lost as the surface is lowered, and the streams sweep across their flood plains and move the sediments on. So central Pennsylvania today is not making much of a fossil record.
Despite difficulties such as this, careful study around the world has filled in many of the details of the fossil record, including many “missing links.” (In 2001, for example, road-builders accidentally discovered Riverbluff Cave in Missouri, with loads of ice-age fossils that are offering a new window on that interesting time.) For some types of creatures, such as hominids, fossils are still scarce enough that a new find often makes headlines, and may cause a small change in the prevailing view of evolutionary history. And there was lots of excitement in 2008 when fossils of transitional flatfish were found—Darwin had worried about the way flounders evolved so the adults have both eyes on the same side of their heads, so the discovery of the transitional forms was another in the long string of successes for the theory he advanced. However, you almost never read about the great changes in thinking caused by the latest snail fossil—the record is so wonderfully complete that new insights are much harder to come by than they used to be.
The Theory of Evolution
The basis of evolution is diversity. (Modern social scientists and politicians are about 4 billion years behind nature on this one.) We know that kids do not look exactly like their parents—offspring are diverse or different. We also know that kids share more characteristics with their parents than with less-closely-related people from the parent’s generation. That is, we look a lot like our parents, but we are not exact copies. This arises because of genetics; the biological instructions or programs that guide development of an individual are passed down from the parents, but there are many mechanisms active that serve to experiment a little with the instructions between generations.
Suppose that one of these small experiments is successful (say, it gives a young giraffe a longer neck than her neighbors, which allows her to reach leaves that are out of reach of other giraffes). The long-necked giraffe will be better-fed than others, and eventually is likely to succeed in surviving to have babies of her own. Some of those babies will grow a little taller than their mother, some a little shorter, and some the same height as their mother, but the offspring will average taller than the kids of other giraffes lacking the initial, successful change.
Those other giraffes lacking this new development will be less successful, and so will leave fewer babies who go on to have babies. Most populations are small enough that, if one individual is even slightly more successful than others, after a few thousand generations, all the survivors will be related to the one with the successful experiment; if one individual is even slightly less successful than others, after a few thousand generations it will have no survivors. (You can demonstrate this easily using mathematical models, or with greater difficulty by breeding living types such as fruit flies, but both reach the same answer.)
Once all of the members of a species contain the successful experiment, the species has been changed a little. But over those thousands of generations, other “experiments” are conducted, some successful and some not. The slow accumulation of the successful experiments is evolution. The mechanism by which the changes accumulate is called natural selection—beneficial experiments allow more survival and reproduction and so are preserved and multiplied. When enough changes have accumulated, we say that a new type or species has emerged. (If a population is split into two or more parts, those parts are called new species when they no longer can interbreed.)
Notice that things that happen to adults, such as having their ears pierced or their behinds tattooed or stretching their necks to reach leaves, are not passed on to children. The changes that are passed on occur during reproduction. Sex helps generate new combinations of genetic instructions. Even species that reproduce asexually by splitting in half have ways (proto-sex?) to exchange genetic material. Sometimes, accidents occur owing to radioactive decay or toxic chemicals damaging the genetic instructions in an egg or sperm or asexually reproducing creature; however, these often are changes that hurt rather than help.
More importantly, the mechanisms of reproduction do experiment a little by moving a few things around in the genetic instructions during reproduction. Some species, and some individuals of species, conduct more experiments than others. Overuse and misuse of antibiotics by humans are producing antibiotic-resistant disease-causing organisms. These antibiotic-resistant types are often those that experiment a lot during reproduction, and so were lucky enough to quickly find an experiment that allows survival despite antibiotics.
The virus that causes AIDS is especially hard to “beat” with a vaccine or antiviral drug because the virus experiments a huge amount. This is costly to the virus—many of the experiments are failures, which means those offspring don’t succeed. But, this high rate of experimentation allows the virus to respond quickly to challenges such as new drugs or vaccines by producing offspring with new ways to defeat those drugs or vaccines. In an AIDS patient, the virus infecting the spleen often differs from the virus infecting the liver—the virus has evolved in the person to succeed in the chemically different environments of the different organs. And, given that the AIDS viruses in just one person are so diverse, it is not surprising that the viruses in different people are different. The remarkable advances in molecular biology allow these changes to be measured now.
Evolution is a well-tested, well-established scientific theory. It makes predictions that are borne out. Partial speciation has been achieved in the laboratory in fast-breeding types such as fruit flies. The geological evidence of gradual changes is strong, and becoming steadily stronger as more and more samples are collected.
Evolution is also being used routinely in science. A search on the ISI “Web of Science” in July of 2012 revealed over 3000 scientific papers with the subject “evolution and antibiotic resistance,” with an ongoing rise in the number of papers on the topic. A quick perusal of the titles and abstracts of many of those papers revealed that, as microbes evolve to defeat our antibiotics, the scientists who are trying to keep us alive are using the tools and language of evolutionary biology. Antibiotics are quickly losing effectiveness against evolving microbes, more and more people are dying of infections picked up in hospitals, and scientists are increasingly focusing on the problem, informed by a full understanding of evolution, its rates and processes.
Computer scientists even use evolution—some “artificial-intelligence” approaches have been patterned after the natural processes of evolution. Techniques such as genetic algorithms or evolutionary computation successfully solve complex problems, in essentially the same way that nature does. (For more on this, see the Unit 11 Enrichment.)
In the U.S., some groups continue to oppose evolution based primarily on religious grounds. This opposition has the good effect of keeping the experts “on their toes”—the experts work harder and do better science. This opposition has the unfortunate effect of convincing many people that something is fundamentally wrong with evolutionary theory: again, perhaps figuring that “where there’s smoke, there’s fire.” Many people believe, for example, that evolution is somehow anti-religious, when the majority of church members in the U.S. belong to denominations that endorse evolution as the best description of how the biological world works. Evolution is consistent with the major religions on Earth, and even with rather strict readings of the Christian Bible. The idea that the Earth appears young, and was created recently with all of the modern types of organisms present, was tested in the 1700s and 1800s and proved wrong.
Extinction and Dinosaur National Monument
Dinosaur National Monument lies in western Colorado and eastern Utah. The key rocks were deposited in swamps and and along rivers during the Jurassic in the middle of the Mesozoic, and are called the Morrison Formation. Before the modern Rockies were raised, sluggish streams flowed across the basins of this region, with numerous low, wet floodplains. Dinosaurs flourished. After some died, their bodies were washed up on sandbars, where their bones were buried before they were consumed by scavengers and gnawers. Over time, minerals carried in groundwater reacted chemically with the bone, depositing silica. (For a little more on petrification, see the Unit 11 Enrichment—no magic is involved!)
After the bone was turned to stone at what would become Dinosaur National Monument, the rocks of the region were raised and tilted during the mountain-building that formed the Rockies. Streams, including the Green River, cut through the rocks. The Canyon of Lodore on the Green is a favorite destination for serious white-water rafting. The first scientist in the region was John Wesley Powell, who went on to run the Grand Canyon. In 1909, workers from the Carnegie Museum of Pittsburgh found Dinosaur Ledge, a sandbar-turned-to-stone on which many dinosaur bones had been deposited and fossilized. Today, some of those petrified bones are on display in the Carnegie and in other great museums, but many of the bones have been left in the ledge to be viewed in the park (see the picture above).
What Killed the Dinosaurs?
The dinosaurs were the dominant large animals on Earth for over 100 million years. Many were quite small, but some were gigantic. They included large plant-eaters and large meat-eaters. Some spent at least part of their time flying or gliding, and others swam.
Mammals co-existed with the dinosaurs for most of the dinosaurs’ existence. However, almost all of the mammals remained small creatures—they generally could not outcompete the dinosaurs for the big-creature jobs.
Extinction is a normal process. A new species may arise and be more successful than an existing type, pushing the old type to extinction. Diseases, accidents, or other events may kill an entire population. And, extinction is forever. As populations vary owing to random factors, sometimes the population drops to zero. But when the population hits zero, the species can never come back up—you can’t just borrow a few creatures from a future generation and bring them back to fill in the gap. So all sorts of random events cause extinctions, and most of the species that have lived on Earth have become extinct. (This characteristic of extinction, that when you hit zero, you’re gone, also applies to gamblers at casinos. To see why you’re likely to lose if you gamble, roll on back to the Unit 11 Enrichment.)
About 65 million years ago, at the end of the Cretaceous Period of the Mesozoic Era and the start of the Tertiary Period of the Cenozoic Era (the K/T boundary, because K is used for Cretaceous and T for Tertiary), all of the living dinosaurs died out suddenly. At the same time, many other types became extinct—more than half of the species known from fossils near the end of the Cretaceous became extinct at the end of the Cretaceous. Because survival of even a few individuals from a species can allow the species to persist, it is likely that almost all of the living things on the planet were killed. It was a catastrophic event, one of the most catastrophic in the history of the Earth.
The solution to this puzzle—how the dinosaurs and others were killed—was not found until fairly recently. At the K/T boundary, sedimentary rocks around much of the world contain a thin clay layer. This layer is rich in iridium, an element rare on Earth but common in meteorites. This clay layer contains bits of rock that were melted and refrozen rapidly to form glass, such as are produced by meteorite impacts. Quartz grains in the layer contain shock features, which are caused by very high pressures applied very rapidly, but by no other known mechanisms such as volcanic eruptions. The layer is rich in soot (black carbon) from fires. The layer is thicker in and near the Americas than elsewhere. Around the Caribbean, the layer includes a deposit of broken-up rock such as would be produced by a huge wave. And on the Yucatan Peninsula is a large crater, the Chicxulub Structure, that is dated to the K/T boundary. The crater is partially buried by younger rocks, but easily detected using geophysical techniques, drilling, etc. The crater is at least 110 miles (180 km) across, and perhaps as much as 180 miles (300 km) across.
This evidence indicates that a large meteorite, perhaps 6 miles (10 km) across, hit the Earth (the hole or crater made by an energetic projectile is usually a whole lot bigger than the projectile). Such a collision would have released more energy than all of the nuclear bombs that were on Earth when the U.S. and Soviet arsenals were at their largest.
Debate continues on exactly how such a meteorite would kill things, but it looks like fire and ice, and maybe acid. The impact would have blasted huge amounts of rock, from the meteorite and the Earth, into the stratosphere or above. As this rock fell back to Earth, friction with the air would have generated heat in the same way that a re-entering space capsule or a “shooting star” is heated. For a little while, the air would have been a toaster-broiler oven, cooking and burning everything beneath.
Following that, cold probably descended. The impact site included sulfur-containing rocks. The heat of the impact would have vaporized those rocks, and that vapor would have cooled later to form clouds in the stratosphere. The small particles of these clouds wouldn’t fall fast enough to heat up much; modern space capsules and meteorites fall fast enough to get hot, but raindrops and dust particles do not. But many, many small particles would block part of the sunlight and cool the Earth. We know that such cooling occurs with modern volcanic eruptions—big ones such as Mt. Pinatubo in 1992 cool the Earth by a degree or two for a year or two. A nuclear war might do much more, creating nuclear winter or at least nuclear fall. Even more of the sunlight would have been blocked after a huge meteorite impact, and the world may have frozen for a few years.
The sulfur particles, when they fell, would have made sulfuric acid, giving much stronger acid rain than the recent human-produced pollution. The incoming meteorite may have heated the surrounding air enough to burn the nitrogen in it, forming nitric acid that also would have produced acid rain.
The meteorite impact was not nearly big enough to roll the Earth over, notably move the orbit, rearrange the continents, or anything similarly cataclysmic for the physical behavior of the planet beyond the few years or decades of the heat, cold and acid, but the event was cataclysmic for life—almost all of the living things on Earth would have died. Who would have survived? Plants with long-lasting seeds, hibernators, things that live in ocean sediment or along spreading ridges, scavengers, probably some others with appropriate characteristics. The general pattern is that the surviving animals were small, and mammals did better than dinosaurs. (Although, yes, birds are a branch of the dinosaurs, and the birds are still with us.)
After the fire and ice were over, the “jobs” (ecological niches) of many of the dinosaurs were left open. There were no big plant eaters, or meat eaters, left. Over tens of millions of years, the mammals, freed of the competition from the dinosaurs, slowly came to take over the jobs of the dinosaurs. Some of the larger offspring of some species were successful, although most mammals remained small (mice, voles, etc.). Lining up the fossils over time, we see an evolutionary shrubbery—lots of branches, many extinctions where those branches were cut off, persistence of small creatures, but appearance of some large creatures, with those leading to modern lions and tigers and bears—and people. Biodiversity was hugely reduced by the meteorite, but over the next millions of years new species appeared a little more often than existing species went extinct, so that diversity increased back to more-or-less what it was before the meteorite, just with different species doing the jobs.
If today, Coke and Pepsi and all other soft-drink companies suddenly magically disappeared, new soft-drink companies would be started fairly soon, not because of a magical affinity for soft-drink companies, not because soft-drink companies must exist, but because we usually figure out how to take advantage of opportunities. In the same way, wiping out dinosaurs opened up a space for mammals.
More on Meteorites and Mass Extinctions.
Meteorite impacts have been happening throughout Earth history, and a Mars-sized body colliding with the Earth and blasting things into space is the best explanation for the formation of the moon. Big impacts were common early on. Evan Pugh Professor Jim Kasting of Penn State helped show that the heat from many of the early, huge impacts would have been enough to evaporate the whole ocean during the first few hundred million years of the planet, and that it was probably only about 3.8 billion years ago when the last impactor hit that was large enough to evaporate the sunlit upper layer of the ocean. Since then, collisions have been much smaller. The dinosaur-killer was larger than any that happened since, and than any for a long time before, but was not nearly big enough to evaporate much of the ocean.
There still are many large rocks out in space that go whizzing by the planet, and large impacts remain possible. One scientific estimate found that your chances of being killed by a meteorite impact are about the same as being killed in the crash of a commercial airliner. Commercial-airliner crashes kill a few hundred people per decade, and a meteorite might wait ten million years and then kill hundreds of millions of people, so the statistics are not exactly comparable, but the number is interesting. (In comparison, car crashes kill waaaaay more people, and far out-do tornadoes and hurricanes and earthquakes and food poisoning and bee stings and airliner crashes as killers of people in the developed world. In the short term, the most dangerous thing you do is probably driving a car. In the long term, smoking, over-eating, under-exercising, and other poor health habits are much more important.)
Scientists are coming up with ways to divert asteroids, and are looking for the asteroids, to help avoid such collisions. If we see an asteroid coming from far enough away, then we need to turn its path only a tiny bit to miss the Earth. One idea is to hit the asteroid with a bag of dust that would spread across one side, changing the reflectivity of the surface; the difference between reflecting and absorbing the sunlight would cause a tiny push that would steer the asteroid. Another idea is to send a spacecraft to sit next to the asteroid for a year; the tiny gravity of the spacecraft, tugging on the asteroid, would turn it a tiny bit. The fate of the dinosaurs has stimulated much of the interest in this, so some day we may be saved by paying attention to the big beasts of the past.
Meteorites are not the only ways to cause extinctions. A meteorite ended the Mesozoic age of dinosaurs and ushered in the Cenozoic age of mammals, but there doesn’t seem to have been a meteorite at the even bigger extinction that ended the Paleozoic age of shellfish and brought in the Mesozoic age of dinosaurs. The leading hypothesis for that extinction, still “hotly” debated, is that immense volcanic outpourings (the biggest known hot-spot head, if you remember far enough back to flood basalts and hot spots) over a million years or so released enough CO2 to make the climate really hot, and also produced easily weathered rocks that broke down and supplied fertilizer to the ocean. The warmer ocean held less oxygen because heating drives gases out of water; plants growing in the ocean released oxygen near the surface to go into the air just like always, but when these abundant plants died and sank into the low-oxygen deep layers of the ocean, decay used up the oxygen there and made “dead zones.” The volcanic outpouring would have supplied sulfur as well, and with the oxygen gone that would have led to an ocean containing hydrogen sulfide, which causes the smell in rotten eggs and which is highly poisonous to most things, and some of that hydrogen sulfide would have escaped to the air. Special molecules from photosynthetic bacteria that use hydrogen sulfide are found from rocks of that age. We will discuss human-caused global warming next week, but don’t panic, we do NOT expect it to cause the ocean to belch out deadly gases that kill most of the life on Earth!
GeoMations
Evolutionary Process
Evolutionary Process
Sometime about 3.8 billion years ago, life appears. And it starts coming up. There’s new kinds appear. There’s evolution going on. There’s splitting of the types of things that are alive. And each of these lines represents a lineage. It’s a living type, something that’s alive in the world.
Now we know that about 570 million years ago, as oxygen is rising, as big critters are appearing, as shells appear, that there’s this great diversification giving rise to lots of different kinds of life, which we call the Cambrian explosion. We find more and more types coming up. Each new living thing can give rise to other ones. And there’s always some extinction going on, and so some of these lines don’t come up to the today.
But there’s quite a variety of life. Land plants appear. Land animals appear. And then we see this hideous mass extinction at the end of the Paleozoic, when most of the things on Earth died, probably because a really hot stagnant ocean is belching out poison gases that are really nasty. There’s still a little debate on that.
We know that just a few things managed to get through that, and that they then are spreading, giving rise to new species in the new world. Because all the big critters were gone, there’s room now for diversification. You get dinosaurs appearing and the other interesting things.
The dinosaurs are actually doing very well. They’re flourishing. They are the big critters. Mammals are around, but they can’t beat the dinosaurs. And then the meteorite does, so you get the great mass extinction at the end of the Mesozoic. And just a few kinds, again, succeed in getting through.
Those few kinds that manage to get through again give rise to diversity. There’s space for splitting, for new types to appear. And so over 10s of millions of years, we see the rise of diversity and the things coming up to today. And so the world goes back to being a very rich, very diverse place, again with some level of natural extinction, but with a lot of splitting going on, a lot of types getting up to today.
Now, a couple of things that you may want to notice. One is that there’s different types, that you come over here and you’d see something that might be mammals, or it be some subset of mammals like bears, and they will seem to be very different from some other type. And that difference arises because the split that gave rise to those types is actually way the heck down here.
And you can follow one type up to here, and you can follow the other type on up over here. And so because they came so far down in time that they split, you’ll find that they look like very different types today. The other thing to notice, there’s a reasonable chance that today, as we see marked right there, today may be the next mass extinction if we don’t change our behavior.
More Insight to Evolution
Many very good sources are available on evolution. The interested reader may wish to start with Teaching About Evolution and the Nature of Science (1998)(link is external), National Academy of Sciences, National Academies Press, Washington, DC.
What follows, in question-and-answer format, is a synopsis of some of the objections that the author, Dr. Alley, has heard or read against evolution, together with brief answers. The author expresses some opinions toward the end on teaching of science, but they are quite in line with the broader scientific view and with materials already discussed in class. The author really believes that science is a tremendously useful way for humans to find out how the world works to help us stay fed and clothed and housed and healthy so that we can address big questions. The author also includes quotes from two noted people (Pope John Paul II and US President James Earl Carter) that tend to promote religion as well as science.
Question: Is evolution anti-religious, or religion anti-evolution?
Answer: They don’t have to be. The author is religious, and is convinced of the overwhelming scientific evidence for evolution. When the author wrote a commentary on the subject for Pennsylvania newspapers, the pastors at the author’s church (a mainline Protestant denomination) approved of the piece. Most of the religious people in the U.S. belong to groups that have accepted evolution. Pope John Paul II added the Catholic Church to those groups accepting evolution (“Truth cannot contradict truth;” Address of Pope John Paul II to the Pontifical Academy of Sciences, October 22, 1996).
Perhaps the most famous Sunday-School teacher ever in the U.S., former president James Earl (“Jimmy”) Carter, said in January, 2004 that “he was embarrassed by the Georgia Department of Education proposal to eliminate the word ‘evolution’ from the state’s curriculum” (CNN story). He went on to say, “The existing and long-standing use of the word ‘evolution’ in our state’s textbooks has not adversely affected Georgians’ belief in the omnipotence of God as creator of the universe. There can be no incompatibility between Christian faith and proven facts concerning geology, biology, and astronomy. There is no need to teach that stars can fall out of the sky and land on a flat Earth in order to defend our religious faith.”
There surely are people who believe in evolution and who dislike or even attack religion, and there are many religious people who dislike or attack evolution. But, evolution is not anti-religious in any way. The author is of the opinion that most leaders of evolutionary research wish to coexist with religion, and that most religious leaders wish to coexist with evolution.
Question: Doesn’t evolution lead to Hitler, or ethnic cleansing, or killing of innocent misfits?
Answer: Very often, “what is” becomes entangled with “what ought to be”—“Letters to the Editor” on the subject frequently include the worry that science displacing religion inevitably leads to a lack of divine authority for moral codes, which leads to lack of morality. As noted above, this just doesn’t make sense—the morality of evolution-accepting Jimmy Carter or of the late Pope John Paul II has not been seriously in question. (But, the author suspects that questions of morality are more important than questions of science to many of the critics of evolution.)
In regard to racial purity or some similar such nonsense, consider for a moment the case of the author. He peers out from behind thick glasses, and his daughters both wear corrective contact lenses. It is likely that he has a genetic predisposition causing near-sightedness in individuals who, while still young, use their eyes for much close-up work such as reading. This genetic predisposition seems to be hereditable—he has passed it on. (There is still medical debate about genetic roots of bad eyesight, but the interpretation here is probably correct.) Leaving the author in the gene pool may “weaken” it a little bit. How should the author be dealt with in a world that recognizes evolution? Should he have been sterilized as a youth, or killed, or forbidden to mate? Or, should he be recognized as suffering from a handicap for which he should receive affirmative action? The government could have subsidized lessons for him during his youth on how to be attractive to a potential mate while peering through thick glasses. (Fortunately, he was successful in marrying a wonderful woman, but maybe there are other downtrodden thick-glasses wearers who should have been helped by outreach efforts.) Or, should we just recognize that glasses (and now, contacts) work just fine, and why worry about it? The reality of evolution in no way dictates one’s morality! Evolution is what is, not what ought to be—we have to decide how to use the scientific information about evolution.
Question: Don’t the gaps in the fossil record prove that evolution did not occur?
Answer: No. We covered this one in the text at some length; the fossil record is beautifully consistent with evolution. The gaps present are the gaps expected based on the nature of speciation and the incompleteness of the fossil record, and the gaps are filled by transitional forms in those groups that are commonly fossilized and for which you would expect to find transitional fossils. Even a little consideration shows that not every creature is fossilized, and that big and relatively rare land creatures will have somewhat sketchy fossil records whereas small and relatively common shelly shallow-sea creatures will have rather complete fossil records. This is observed. One can look at the likelihood of fossilization, and then generate predictions on how complete the fossil record will become as more fossils are collected, and these work.
Question: Aren’t there lots of problems with age dating, showing that the world is really young?
Answer: Again, this was discussed a lot in the text. There are commentators who make arguments against science that might seem sensible to those who don’t know the field, but those arguments can be shown to be completely wrong with just a little care.
Consider one that the author has been shown several times. The author has helped count over 100,000 annual layers in a Greenland ice core, and to do all of the careful testing using fallout of historically dated volcanoes and other time markers to show that the results are reliable. The “counter-argument” from the young-Earth supporters was that a flight of World War II planes that was forced to land on the ice sheet had been buried a couple of hundred feet in only 50 years, so a couple of thousand feet would be 500 years, and the 10,000 feet of ice thickness in central Greenland would be about 2,500 years, so the ice sheet started safely after Noah’s flood. Seems perfectly reasonable, doesn’t it?
But, when we discussed glacier flow back a few chapters, we noted that a glacier is a bit like pancake batter, spreading across a griddle under the influence of gravity. If you put a dollop of pancake batter in the middle of a griddle and watch, the layer thins as it spreads. Put another dollop on top, and both spread and thin. Keep putting dollops on top, and letting the batter drip off the end of the griddle, and eventually you’ll have a whole pile of layers. The one at the bottom will have been spreading and thinning the longest, and will be the thinnest.
An ice sheet is similar, spreading and thinning as more snow is piled on top. Very crudely, an annual layer will become half as thick and twice as long while it is moving halfway from wherever it is toward the bed. Friends of the author have directly measured the motion and spreading of the ice, and it fits this pattern beautifully. (These measurements show a little extra downward motion associated with squeezing snow to ice, but glaciologists usually speak in terms of ice-equivalent thickness—the air already mathematically squeezed. And very deep, the halfway-to-the-bed-thins-by-half breaks down, because the ice sheet had to form sometime so the first layers weren’t thinned while flowing through the ice sheet, and because the flow over bumps in waffle-iron fashion also complicates things a little). The fact that Greenland makes icebergs, hence is spreading, means that deeper layers are thinner, and the simple airplane burial calculation is completely wrong.
At the time the author was writing this, a quick search of the Web found numerous sites that slightly improved the young-Greenland calculation while still getting it completely wrong. These sites noted the thinning of layers with increasing depth, picked a place near the edge of the ice sheet where the ice was relatively thin, picked a thickness of annual layers at the surface, picked an erroneously thick annual value at the bottom, and suggested using the numerical average of the surface and bottom thickness in the calculation to get the age of the ice sheet. Fourteen inches thick at the top, less than two inches thick at the bottom, call it two inches, take the average of 14 and 2 inches and get 8 inches per year, and a few thousand feet of ice in the thin margin of the ice sheet still squeaks in after Noah’s flood—the scientists must be confused. Seems reasonable, right?
Absolutely not. Try this very simple equivalent, that you can do in your head. Suppose that the ice sheet is two feet thick, or 24 inches, that the top annual layer is 12 inches thick, and the bottom has twelve annual layers each 1 inch thick. A scientist would count the annual layers and find 13, giving a 13-year age for the ice sheet. But the technique advocated by the young-Earth websites would average the top and bottom thicknesses (6.5 inches per year), and then calculate that fewer than 4 years were required to build up the two feet of ice, not the actual 13 years. The mathematical error made on the websites is a very simple one, and one that most students will have learned to avoid while still in middle school. (The author doesn’t know whether the mistake on these young-Earth websites represents stupidity or deliberate misrepresentation, but the mistake is so flagrant that it is not easy to think of a third option.) Do the calculation right for Greenland, and you end up with a very old ice sheet. If you count the annual layers, or calculate the age using the flow of the ice sheet, or estimate the age by identifying abrupt climate changes and using the ages from tree-ring or other layer counting of those abrupt changes elsewhere, or use any of the other dating techniques, you’ll get the same Answer: Greenland’s ice is much older than written history.
The scientific community is continually improving age-dating techniques, arguing about them, working on them, and thus far, no serious problems have been found with the old age of the Earth. The arguments presented against the old age, such as the buried planes or the 5,000-year-old living clam (see the Enrichment from the Grand Canyon) prove not to be problems after all.
Question: Doesn’t the second law of thermodynamics, or the “law of conservation of information,” prove that evolution cannot have occurred?
Answer: No. The second law of thermodynamics says that entropy increases in a closed system; the law of conservation of information is something promoted by the intelligent-design supporter William Dembski in his writings arguing against evolution, and appears to be basically a special case of the second law of thermodynamics, although there is no scientifically recognized “law of conservation of information.” In fact, order can emerge out of chaos, and does so all the time—a snowflake does form from randomly oriented water molecules, for example. The second law summarizes a great number of observations showing that, in growing the snowflake, heat must be removed, which causes an increase in disorder of the surroundings. Some creationist websites have suggested that followers not use this second-law argument because it is completely wrong.
An interesting note is that a whole field of Evolutionary Computing now exists (linked to genetic algorithms, artificial intelligence, etc.). In trying to “teach” computers to solve complex and difficult problems, computer scientists have found it useful to mimic evolution and natural selection—have the computer start with a possible answer, see if it works, then tweak the answer and see if that works better, throwing away ones that work worse and keeping ones that work better. If, for example, you’re trying to improve the routing of airplanes to fly the shortest distance while carrying the most passengers, you could start with the current route map, and then randomly “perturb” it a little, to see if that works better. You need to define “better” (how many more miles of flying is it worth to allow you to sell another passenger ticket?), but then the technique is successful. Usually, many small perturbations are used in sequence, but occasionally a slightly larger one may be useful; huge ones usually fail. The approach obtains order—a useful optimization of the flight plan—from the chaos of reality by random adjustments followed by selection of those that work better. Selection doesn’t require intelligence, but simply telling the computer to save the coordinates if the cost is lower. In biology, this selection is achieved by survival—if you survive to have kids, you have been selected. The lessons of biological evolution thus have been used to help computer scientists solve hard problems—science works. And the idea that somehow the second law of thermodynamics prevents evolution is just silliness.
Note that, in the absence of the “selection” step, randomly generating new options is almost guaranteed to fail in finding an optimal answer. Many of the anti-evolution websites and other anti-evolution materials point to how incredibly unlikely it is for random processes to generate something useful. These sites are completely correct, but completely misrepresenting evolutionary theory. The step of randomly generating lots and lots of new “experiments” is followed by the step of picking “successful” ones—in evolutionary computing, the successful ones meet some criterion such as saving the airline money; in evolution by natural selection, the successful ones promote survival to have kids.
Question: Isn’t evolution restricted to the “micro-evolution” that we can see (such as breeding tiny white dogs from larger, grayer dogs), and cannot include “macro-evolution” of new types?
Answer: This one takes a bit of discussion; it seems common-sensical, but turns out not to make sense.
To many biologists, evolution is defined as something like “change in gene frequencies over time,” with genes being the basic inherited instructions for making and running living things. There is no “micro” or “macro” in this; the distinction is simply not meaningful in the modern theory of evolution. “Macro” is just more of “micro”; they are not separate things.
A different way to view this question is that, biologically, there is a division between species (either things interbreed, or they don’t, or they do somewhat, as discussed next). All of the other divisions that we draw between different living types (kingdom, phylum, class, order, family, genus) are human constructs to help us understand the world. If different people with well-developed science had named all of the creatures, those people likely would have picked the same species, but may have picked different ways to group the species. “Macro” in this sense presumably is used by the evolution-skeptics to refer to changes between the larger groupings (there are wolves and coyotes and domestic dogs, but these people claim that there is a fundamental difference between “dogness” and “catness”). But larger groupings are primarily conveniences for us; evolutionary biology addresses whether creatures interbreed and exchange genes, or not.
“No, no, no” a skeptic might say, “We don’t care about what biologists think; we care about the reality, and we know that there is ‘dogness’ and ‘catness,’ these are different types or sorts.” This is harder to answer; such people presumably are postulating some unknown barrier that somehow prevents genetic “experiments” beyond pre-defined boundaries—evolution can bring about new types of dogs, or new types of cats, but only to some boundary and not beyond. But what forms such a boundary? Where is the ‘police officer’—biochemical or otherwise—that would check after two dogs had sex to make sure that the potential offspring did not include a genetic experiment that went one DNA base pair outside of the defined limits of ‘dogness’? An omnipotent deity could of course do such a thing, but there exists no scientific evidence for this—make another base substitution in DNA, and you have another experiment. Let nature choose the “good” experiments (those that lead to lots of surviving kids) and evolution happens. “Macro” evolution really means “evolution that takes long enough to occur that humans in their lifetimes won’t see much change in large animals (although plenty of such changes are happening to disease organisms).”
Dogs and cats really are different, because many successful evolutionary experiments have accumulated over tens of millions of years. Evolution really is gradual, and “hopeful monsters” (a dog gives birth to a cat, for example) do not happen. But, given the observed rates at which variability is produced by reproduction, and the rates at which natural selection is observed to function, mathematical modeling shows that there has been more than enough time in geological history for all of evolution to have occurred. There is no problem, for example, in going from the known damage that happens to cells in the bright sun (sunburn), to cells that are a bit more sensitive to light to help a creature know where the light is and avoid it, to groupings of those cells, and on to an eye. The question is not whether evolution could have happened in that much time, but why evolution ran as slowly as it did—most of the time, not a lot of change has been occurring, probably because creatures had found pretty good evolutionary solutions to problems and tend to stick with those solutions.
An additional note is that the species with us today are really not as separate and distinct as some people might have you believe, but have blurry edges, overlaps, etc., as you would expect from the scientific understanding of evolution. For example, “ring complexes” of animals are observed—species A interbreeds with B, which interbreeds with C, but A and C don’t interbreed. Are A and C the same species? Would they be if B became extinct? (Imagine what would happen if we had nothing but miniature poodles and Great Danes—could they ever “get it on”?) The messy world of biology shows that the world is not populated by “types” but by evolving populations with greater or lesser degrees of gene exchange.
Question: What about “Intelligent Design”?
Answer: “Intelligent design” is a resurrection of a very old idea. Proponents of “intelligent design” argue that there exist “irreducibly complex” parts of plants and animals, such that these parts would not have been useful while they were evolving but only after they evolved, and thus that they could not have evolved. If evolution made a useless something that later became part of an eye, that something wouldn’t help the creature and might hurt it, and so wouldn’t be saved and experimented upon to generate the rest of the eye—the intermediate steps usually should be useful for evolution to work.
In decades gone by, the eye was often cited as an irreducibly complex structure—without all of the parts of your eye, you wouldn’t be reading this. But it is very easy to find successful creatures on Earth with no eyes, and others with rudimentary eye spots, and others with slightly more complex eyes, and so on—gradual improvements on the slight sensitivity of all cells to light can lead to an eye. Because each step in the evolution of an eye has utility, the eye is not “irreducibly complex”. (Ask someone whether they would prefer to be completely blind or to be able to discern light and darkness, or vague shapes, and that person is not likely to opt for blindness—less-than-perfect eyes are still valuable.) As their eye-argument failed, intelligent-design advocates have switched to other arguments, such as the flagellum or the clotting of blood, but scientists working on these topics are not finding these structures to be irreducibly complex, either.
Scientists working in many fields related to evolution have been vocal through their leading organizations, such as the National Academy of Sciences, and the American Association for the Advancement of Science, in noting that “intelligent design” is not science, even if it happens to be stated by people with scientific training. Scientists after all can say all sorts of things that are not science. (And yes, you can find a few scientists saying anti-religion things, but those are not science either.) The leading hypothesis of “intelligent design” seems to be that there are some things that evolutionists cannot explain. This hypothesis does not lead to useful predictions that can be tested and falsified, so it just isn’t science. Notice, by the way, that successful scientific explanation of one so-called irreducibly complex item such as the eye has not falsified “intelligent design” to its supporters, who can always propose that something else is irreducibly complex. In a widely watched court case in Dover, Pennsylvania in 2005, a federal court stated these results very clearly: “intelligent design” is not science. (See the Kitzmiller case (.pdf)(link is external) at uscourts.gov if you’d like 139 very interesting pages on this topic.)
After the author wrote a newspaper column advocating the teaching of science in science classes, he received communications (e-mail, phone, letter) from numerous interested people, including many intelligent-design supporters. Aside from a couple of unpleasant “You’re going to burn in Hell” e-mails from the fringe, the exchanges were respectful, interesting, and informative, from a broad spectrum of beliefs. Notice that these are not the leaders of the “intelligent design movement”, but mostly-Pennsylvanian newspaper readers responding to a column. Two observations are:
1) None of the “intelligent design” supporters seemed to seriously consider the possibility that the “intelligent designer” was a flying spaghetti monster or a space alien; where it could be determined, the correspondent identified the “intelligent designer” as God as worshipped in the Judeo-Christian tradition, and the correspondent came from a fundamentalist, conservative, or traditional Christian background.
2) None of the “intelligent design” supporters seem to have come to their religious beliefs based on the perceived difficulty of evolutionary biochemists in explaining blood clotting, flagellae, or eyes. Awe and reverence for the glory of nature did seem to figure in some religious beliefs, but supposedly irreducibly complex items were not in the forefront.
This leads to some additional, important points. The broad umbrella of “intelligent design” allows an old Earth and allows evolution to have occurred, except at those moments when an unspecified intelligence tinkered with the process, so if you are a true sacred-book literalist, “intelligent design” may give you scant comfort. And many religious people object to “intelligent design” based on the argument that it is lousy theology—if an unspecified “intelligent designer” is introduced to high-school biology students based on a claimed difficulty that scientists are having in explaining intermediate steps in blood clotting, might future success in explaining blood clotting raise questions in the minds of those students about the validity of belief in the “intelligent designer”? The pastors at the author’s church seemed remarkably uninterested in getting high school biology teachers to take over religious instruction based on explanations of flagellae.
Question: But shouldn’t we be fair, teach both sides, and let kids decide?
Answer: This is a hard one, because we so strongly believe in fairness and in hearing a diversity of ideas. But, if you present “both sides” in science class, it isn’t science class any more. Science is the human search for ways to make accurate predictions, and that means setting aside the ideas that don’t work (the Earth is flat, the Earth is the center of the universe) and keeping the ones that do. Teaching the controversy over evolution in science class would be akin to teaching the controversy over whether the Earth or the Sun is more nearly the center of the solar system. Students are not really equipped to answer such a question (it took bright people, telescopes, calculations, and observations over centuries to figure out that the Earth does most of the moving in the Earth-Sun system). Teaching “intelligent design” is not good science and does not stimulate good science. “You biology students will fail in explaining and predicting some things” is not an especially motivational approach to teaching. Imagine if you did the same thing for math homework—“Do problems 1-24, but a few of them are impossible, so if you hit a hard one, just skip it”—what kind of motivation would that give to students?
The scientific controversy over evolution by natural selection is similar to the controversy over gravity. We don’t have a complete understanding of gravity yet. The grand unified theories of physics have not succeeded in explaining the quantum world and gravity through one set of equations, gravitational waves have not been observed, and other research frontiers await. But we have a pretty good idea that if you knock your pencil off your desk, the pencil will fall down. There are limits to predictability—although gravity causes things to fall down, and gravity keeps the atmosphere near the Earth, gravitational attraction is weak enough compared to thermal energy that you can’t easily keep track of the position of an air molecule, for example. But, scientists can hit a tiny meteorite with a spacecraft from across the solar system, and can deliver a small exploding device to a particular window in the palace of a particular former dictator, using gravitationally based calculations. In the same way, there are lots of evolutionary questions, things we don’t know, and fascinating research frontiers, but the basic idea that kids are mostly like parents, differences affect success, and this leads to evolution, is very well established with very little uncertainty.
Discussions about “intelligent design” surely can be included in school—a school that does not recognize the reality of religion in the world is not preparing its students for that world. But “intelligent design” is not science, and the author believes that science classes should teach science.
Petrification – How is the Bone Turned to Stone?
No magic is involved in petrification of bone, or wood or other materials. The chemical environment inside organic materials is very different from the chemical environment outside. All groundwaters carry minerals, and when those mineral-carrying groundwaters encounter a change in chemical conditions, some chemicals usually are picked up while others are put down. Remember that water can poison people by picking up lead and other chemicals from old pipes and that water can clog old pipes by putting down scum and hard-water deposits—picking up some chemicals and putting down others is the usual behavior for water. Water softeners work by pulling calcium out of water and putting sodium back in.
When organic material is buried in mud, if enough adding and subtracting of chemicals occurs before the material is eaten by worms or fungi or bacteria, then the organic material can be “turned to stone”. This petrification is rare—most organics are recycled—but occurs often enough to give us plenty of fossils to study.
For example, silica is very common, and is relatively insoluble in acidic solutions but very soluble in basic solutions. Groundwater in dry environments often is basic and so carries much silica. When this silica-carrying groundwater meets the organic acids in a buried tree or bone or other formerly living thing, the silica may precipitate. Actual chemical processes are typically a little more complicated than explained here, but the principle is the same. Materials scientists are even succeeding now in using wood as a template for “growing” ceramics, replacing the wood by zeolites, silicon carbide, or other materials to make useful things by human-accelerated petrification.
Extinction and Absorbing Boundaries
If you go to a casino to gamble, you are likely to lose. This is partly because the games are stacked against you—the odds on all casino games favor the house. (The odds on state lotteries typically are even more in favor of the house, by the way.)
But, you also lose at a casino because you are poor and the casino is rich. Suppose that you start off with $10 and the casino starts off with $999,990. Together, you have $1,000,000. Suppose further that this is a bizarre casino with a perfectly fair game—you and the casino are equally likely to win. If you win $10, then you have $20 and the casino has $999,980. But, there is slight catch—if you lose $10, you are at $0 and the casino will show you the door. If you stay and gamble for a while, the outcome is almost guaranteed; you will hit $0 before the casino does, the casino will have all of your money, and you will go home broke. The $0 mark is an absorbing boundary—you can’t bounce off of it (the casino won’t give you your money back), nor can you go negative and then return (well, you might go take out a loan, but your ability to get loans to cover gambling losses is much smaller than the casino’s ability to get loans, so you’ll eventually reach the point of no return—your money will have been absorbed by the casino).
Extinction works the same way. Once a species is gone, it is gone. You can’t have a negative number of tigers and then later have a positive number. So random fluctuations eventually kill off species. But, at certain times, such as when the meteorite hit at the end of the Mesozoic, or now as we humans spread across the surface of the planet and squeeze others out (more on this coming soon!), extinctions go a whole lot faster than normal.
“Intelligent Design” Newspaper Editorial by Dr. Alley
Column by R.B. Alley, published in Harrisburg Patriot-News (2004) and republished in
Pittsburgh Post-Gazette and Centre Daily Times early in 2005.
The School Board of the Dover (PA) Area School District recently (November, 2004) mandated the teaching of so-called “intelligent design” alongside Darwinian evolution in science classes, and similar actions are at least being considered elsewhere. As a religious person and a scientist, I hope that school boards will avoid mixing apples and angels in the classroom.
Like many scientists, I am fortunate to teach. We know that our students will soon discover things we missed, often correcting our mistakes in the process. Thus, a scientist would be foolish to claim that science gives absolute knowledge of Truth. If I successfully predict the outcome of an experiment, I’m never sure whether my understanding of the world is True, whether I’m pretty close but not quite right, or whether I’m really confused and was just lucky this time.
But, our society has agreed to act as if science is at least close to being true about some things, and this makes us very successful doing those things. Carefully crafted bits of silicon really are computers, airplanes designed on those computers using principles of physics really do fly, and medicines from biological laboratories really do cure diseases. The military has investigated psychics as well as physicists, but continues to rely on the physicists because they are so much more successful. Science, tightly wedded to engineering and technology, really does work, and is the best way we humans have invented to learn to do many things.
Science asks a high price for the value it gives, however, and that price is real dedication to science. The cartoonist Sidney Harris once drew a panel showing two long strings of blackboard equations connected by “Then a miracle occurs,” with one scientific-looking character saying to the other “I think you should be more explicit here in step two.” For a plane to fly, for a medicine to cure disease, every step must be tested, and everyone else must be able to follow those steps. Science students are welcome to rely on divine inspiration, but they cannot rely on divine intervention in their experiments. Scientists, like athletes, must follow the rules of the game while they’re playing.
What, then, are the rules? First, scientists search for a new idea, by talking to people, or exploring traditional knowledge, or in the library or other places. We look for an idea that explains what we see around us, but that also disagrees with an old idea by predicting different outcomes of experiments or observations. Then we test the new idea against the old one by doing the experiments or making the observations. An idea that repeatedly makes better predictions is kept; an idea that repeatedly does more poorly is set aside. An idea that can’t be tested also is set aside; it isn’t scientific. Even if I really love an idea, or really believe it is True, but I can’t think how to give it a fair test, I have to set it aside for now. Some people find this limiting and avoid science; others find it exhilarating and are drawn to science. Doing this well gives us good things from good science.
Who decides what is or isn’t science? Scientists–with other thinkers watching over our shoulders–do the day-to-day deciding, but ultimately the whole bill-paying, newspaper reading community is checking on us to make sure that we really are producing useful insights.
Does science have limits? Will we run out of new ideas? Will we hit problems that we can’t solve? Perhaps. But when I come out of a classroom of bright young students, I am convinced that we’re nowhere near any limits that might exist, and that there is much to discover yet.
So, what about Intelligent Design, or even Young-Earth Creationism, and teaching them in science class? They’re interesting ideas. But, some parts we don’t know how to test. Even if they are said by scientists, they aren’t science. And the testable parts have been tested and found wanting–they don’t do as well as the “scientific” view in explaining what we see around us, or in predicting what we find as we collect new tree-ring records and ice-core samples, or as we search for oil and valuable minerals, or as we watch dangerous new diseases appear faster than our bodies can respond to them. We spend a few hours discussing the main pieces of evidence in class, and a lifetime isn’t enough to cover all the details, but scientists have been working on these questions for centuries and have a pretty good idea of what works now. Evolution “in the dark backward and abysm of time” is scientific theory, not Truth, but it is very good science.
How does this fit into the bigger picture? Although some people are happy to view science as merely a tool, others do believe that the remarkable success of science means that we are getting closer to truth. But even these people disagree about that truth: a mechanistic universe, a benevolent and omnipotent deity, or something else? Fascinating as they are, such questions are for now outside of science. Many scientists and religious people are thinking about such questions, but no experimenter knows how to guarantee the cooperation of an omnipotent deity.
By all means, students should ask deep questions, think and discuss and probe. Science does not tell us what we ought to do, and students will have to join us in addressing what ought to be as well as what is. But if we want to face the big questions with better medicines, with computers that function and planes that fly, with clean water and buildings that don’t fall down, I believe that we should teach science in science class.
Key Takeaways for Evolution and Extinction
Easy Come, Easy Go–Evolution and Extinction
- Offspring–are not identical to parents
- Differences affect ability to survive to produce offspring (natural selection)
- Offspring are similar to parents–receive a bit more or less of what makes parents biologically successful
- This over time gives evolution–successful experiments accumulate, unsuccessful ones eliminated, changing living things over time
- Changes to living organisms are not passed on–just reproductive experiments are passed on
The Unbroken Chain
- Law of Faunal Succession suggests evolution
- Evolution predicts transitional forms over time, special creation or catastrophism don’t
- Transitions found, strongly supporting evolution:
- Transitions common in commonly fossilized types
- New species often emerge geologically rapidly from small populations
- Transitions found as often as evolution predicts; fossil record incompatible with competing hypotheses.
Taking Care of Business
- Theory of evolution is explanatory, predictive, and useful
- Germs are evolving antibiotic resistance, and the scientists trying to keep us alive are using knowledge of evolution
- Computer scientists mimic evolution to solve complex problems (evolutionary computing).
- In particular, evolution is:
- Consistent with second law of thermodynamics
- Supported by fossil record and age dating
Extinction Can Ruin Your Whole Day
- Slow background extinction
- Occasional mass extinctions:
- end-Paleozoic: heat-caused loss of ocean oxygen?
- end-Mesozoic: meteorite
- Dinosaurs doing just fine until meteorite got them
- Freed ecological jobs (“niches”), allowing evolution to produce large mammals over last 65 million years.
The Dinosaur Killer
- Evidence: extinction at odd bed with much iridium (common in meteorites), soot, high-pressure shocked quartz, melted-rock droplets, Caribbean giant-wave deposit; right-age giant crater Yucatan Peninsula
- Mechanisms: meteorite blasted things up, fire from heat of fast-falling things, then cold from sun-block of slow-falling things, with acid rain
