The Failures of Mathematical Anti-Evolutionism
Download 0.99 Mb. Pdf ko'rish
|
The Failures of Mathematical Anti-Evolutionism (Jason Rosenhouse) (z-lib.org)
Evolution Basics
2.1 what evolution says We have a few pages of table-setting ahead of us before we can get down to our main business. For example, we need to discuss what the theory of evolution actually says and why scientists place so much confidence in it. According to evolutionary theory, modern species arose through a process of descent with modification from earlier species, which themselves arose from even earlier species. By tracing this process backward through time, we eventually arrive at one, or perhaps a small number of, original species that started the process. The modification occurs because offspring are never perfect copies of their parents. They typically resemble their parents because they have inherited the parents’ genes, but they are not exact copies because genetic replication is imperfect. Each new generation there- fore exhibits novel genetic sequences and combinations unknown to its forebears. Some of these novel genes arise from various kinds of mutation, meaning that the sequence of genetic letters in the offspring is different from the corresponding sequence in the parent. In other cases the novelties arise through recombinations of genetic material found in the parents. Since the precise nature and origins of the variations will not be relevant for our purposes, we will, for convenience, refer to these novelties collectively as “mutations.” Over long stretches of time, these mutations accumulate to the point where the current generation scarcely resembles its ancient progenitors. If these ideas are correct, then it follows that any two species alive today share a common ancestor in the perhaps distant past. 19 20 2 evolution basics Just as you and your siblings share a recent common ancestor (your parents), and you and your first cousins share a more distant common ancestor (your grandparents), so too do modern species share common ancestors. For example, humans and apes presumably share a rela- tively recent ancestor, humans and horses a more distant ancestor, and humans and lobsters a more distant ancestor still. This notion sometimes causes confusion since we are in danger of picturing these common ancestors like something from a low- budget horror movie. If you hear that humans and lobsters share an ancestor, and then picture a half-human, half-lobster chimera, then you have the wrong idea. Were the human/lobster ancestor alive for us to examine, it would be unrecognizable as either a human or a lobster. It would instead be a relatively simple and ancient creature, probably resembling something like a worm. That creature would be as surprised as anyone to learn that his descendants gave rise to both humans and lobsters. A useful model for the sort of gradual change we envision is to imagine that a newborn baby is photographed once an hour until she is 50 years old. Any two consecutive photographs will be indistinguishable, and even photographs taken days apart will look the same. But they are not actually the same, since microscopic physical changes are constantly accruing. Photographs taken years apart are very distinguishable, and the first and last in the sequence will not be recognizable as the same person. The biological analogue is that parents and offspring are imme- diately recognizable as the same species, as are parents and grand- offspring. But if we had the complete sequence of photographs con- necting the most ancient life forms to their modern descendants, we would find the small mutations present in each new generation even- tually adding up to very dramatic changes and easily distinguishable species. That is the claim, at any rate. For the baby in the previous paragraph we discussed changes taking place over years, whereas for species we often have to wait for much longer periods of time; however, the principle is the same. 2.1 what evolution says 21 Let us now pause this discussion to confront a potential prob- lem. If we believe that ancient, simple life eventually evolved into modern, complex life, then we must conclude that evolution can produce functional biological systems like wings, eyes, and lungs. It is one thing to claim that an accumulation of small mutations can lead offspring to diverge from their ancestral stock, but it seems like quite another to claim they can lead to modern organ systems. The answer to this objection is that our description of evolution to this point is incomplete. You see, some of these mutations rep- resent not just random changes from the ancestral stock but actual improvements. Living things compete with one another for scarce resources, and it sometimes happens that offspring possess mutations that give them an advantage in this competition. Offspring blessed in this way will deposit more copies of their genes in subsequent generations, where they can then serve as platforms for still further mutational improvements. The process through which gene frequen- cies change by the differential survival and reproduction of their bearers is known as “natural selection.” For example, we might imagine that a favorable mutation in a population of ancient, sightless organisms led to one individual possessing a bit of light-sensitive pigment. This individual and its descendants would then be able to orient themselves with respect to a light source, which in most cases would give them a survival advantage over competitors forced to exist in complete darkness. The genes for light sensitivity would quickly spread through the population. In subsequent generations, such “eyespots” might grow, and then the formation of a small pocket could lead to a rudimentary focusing ability. Any sort of clear fluid could then serve as a primitive lens, and by accumulating such small advantageous changes we are well on our way to evolving modern eyes. There is substantial evidence to support the gradual evolution of eyes over time, but those details are not relevant to the present discussion. What matters is not the specific details of eye evolution but rather the sort of explanation we are proposing. It is true that 22 2 evolution basics a purely random process is unlikely to produce a complex, func- tional structure like an eye. But we can readily imagine a random process producing organisms that are very slightly better than their competitors at gathering information from ambient light. Natural selection will then ensure that these improvements are preserved in subsequent generations, where they can serve as a basis for further small improvements. This, then, is the theory of evolution by natural selection, the one that scientists are routinely heckled for studying: All modern life forms arose by a process of descent with modification from ancient forebears. That portion of evolutionary change that causes organisms to become better adapted to their environment is the result of natural selection. In particular, complex, functional structures arise primarily through the accumulation of small advantageous changes, with natural selection preserving the early stages while waiting for the next improvement to arise. This account is sufficient for our purposes because it covers those aspects of evolutionary theory that are challenged by anti- evolutionists. However, for the sake of completeness we should mention that what I have described is only a small subset of the entirety of modern evolutionary theory. There is far more to the subject than just common descent and natural selection. After all, my brief summary of evolution required just a few paragraphs, while introductory textbooks manage to go on for many hundreds of pages. That acknowledged, we have what we need for the discussions ahead, so let us move on. 2.2 the evidence for evolution Scientists are all but unanimous in believing evolutionary theory to be correct, and they point to numerous lines of evidence in support of their view. The evidence for evolution has been presented at length by numerous writers better qualified than me to do so. I will suggest some useful references in Section 2.7. However, in the interests of 2.2 the evidence for evolution 23 making this book as self-contained as possible, I would like to at least highlight some of the main lines of evidence. First, we have something like our sequence of baby photographs in the form of the fossil record. It sometimes happens that the remains of dead organisms are preserved over vast stretches of time, thereby making them available for study in the present. Geologists have a variety of reliable methods for dating fossils, and that allows us to line them up in chronological order. Each fossil is like a snapshot revealing that an animal with particular physical attributes existed at a certain time and place in natural history. In this way, they reveal something of the sequence of life forms that has played out in natural history, and that sequence is in perfect accord with evolutionary expectations. In particular, the sequence shows that the earliest life forms were also the simplest, precisely as evolutionary theory leads us to expect. Complexity arose only very gradually over long stretches of time. The history of animals begins with relatively simple sea crea- tures, which eventually gave way to fish, and only then to amphib- ians, reptiles, birds, and mammals. Whenever sufficient fossils exist to trace a lineage backward through substantial periods of time, we find, without exception, that modern forms arose as the end result of a coherent historical sequence of predecessors. Moreover, there are many instances of transitional forms that bridge the gaps between major groups of organisms. There are early amphibian fossils that look very much like fish and later amphibian fossils that look very much like reptiles. So copious are the fossils connecting reptiles to mammals that it is commonplace to speak of mammal-like reptiles (though we should note that for technical reasons paleontologists prefer not to use this term). Evolutionary theory predicts that modern whales are the descendants of land- dwelling mammals, and sure enough there is a detailed sequence of intermediates to show precisely how that happened. Modern humans are preceded by a long sequence of predecessors that become distinctly more ape-like as we go backward in time. These are just a few examples. There are many others. 24 2 evolution basics Absent evolution, there is no reason to expect any coherent pattern at all to the fossil record. We can imagine hypothetical fossil records in which the major forms of life appeared simultaneously with no subsequent change, or in which humans appeared first ahead of fish and reptiles. We can imagine the most complex life forms appearing right at the start, or a random assemblage of forms showing no discernible patterns whatsoever. The actual fossil record is nothing like this. It exhibits instead a sequence entirely consistent with evolution, and that is surely a major victory for the theory. Moving on, we could also look at the anatomy of modern animals for evidence of evolution. Just as a seemingly healthy adult might bear scars that speak of past infirmities, so too might animals bear vestiges of their evolutionary history. Indeed, such vestiges are ubiquitous. We have mentioned that whales descended from land-dwelling mammals, and, indeed, they have rudimentary pelvic bones that speak of this history. Since pelvic bones attach legs to bodies, why do whales have them? Humans have an appendix whose primary modern function seems to be to get infected, burst, and kill anyone not fortunate enough to have the necessary surgery. Surely some clue to its origin is gleaned from its strong resemblance to a shriveled up version of an organ other mammals use to digest grass and leaves. For that matter, humans have tailbones, so named for the structures they supported in our evolutionary ancestors. You could literally pick any modern organism and find similar vestiges of an evolutionary past. These vestiges extend to the molecular and genetic levels. Humans and apes are unable to synthesize vitamin C, but evolution holds that we are the descendants of animals that possessed this function (just as most modern animals other than humans and apes possess it). Interestingly, our genomes possess a broken version of the necessary gene for making vitamin C. If we are not the descendants of creatures with a functional version of that gene, then what is it doing there? Animals are replete with these “pseudogenes,” meaning bro- ken genes that are very similar in DNA sequence and chromosomal 2.2 the evidence for evolution 25 location to functional genes in related animals. This makes sense under a hypothesis of evolution, but it is hard to explain as the product of intelligent design. To see why, let us use an analogy from human engineering. Automobiles used to employ a carburetor for combining fuel and air in the proper ratio to achieve combustion within an engine. Carburetors were subsequently replaced with a more efficient technology known as a fuel injection system, and for that reason carburetors are no longer found in modern automobiles. It would be silly for an engineer to build a modern car with a useless, nonfunctional carburetor wasting space underneath the hood, but that is basically what pseudogenes are. They are very common in the genomes of organisms. Sticking with genetic evidence for the moment, we can also look at so-called retroviral scars, by which we mean remnants of past infections that are sometimes found in modern genomes. Humans and apes share many such scars, each appearing at precisely the same place in their genomes. It is possible that this is sheer coincidence, but it is more likely that we all inherited this scar from the common ancestor of humans and apes. For an analogy, suppose that each of ten siblings possessed a rare genetic anomaly. Would you assume they all independently got the same random mutation, or would you assume they all inherited it from their parents? Let us shift now to comparative anatomy. As we survey the ani- mal kingdom, it is commonplace to find similar structures performing very different functions. Darwin himself expressed the point like this in On the Origin of Species: What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat should all be constructed on the same pattern, and should include the same bones, in the same relative positions? Download 0.99 Mb. Do'stlaringiz bilan baham: |
Ma'lumotlar bazasi mualliflik huquqi bilan himoyalangan ©fayllar.org 2024
ma'muriyatiga murojaat qiling
ma'muriyatiga murojaat qiling