21 November 2012

Moment of Science: Evolution & Natural Selection

Part 4 of my Why I'm an atheist series involves refuting the claims of the so-called Intelligent Design movement. While writing it I realized that it would be necessary to make sure we're all on the same page regarding the ideas of evolution and natural selection. Summarizing these concepts was taking up a larger and larger portion of the post, so I decided to split the effort off into its own post here. Originally the title of this post included it in the atheism series (as betrayed by its url), but I've since decided that it would probably be less confusing as a standalone post, given that it does not deal directly with my irreligiousness.

Let's get something straight right off the bat: the terms “evolution” and “natural selection” are NOT synonyms. The first is an observed phenomenon, and the second is an explanation for it. Simply put, biological evolution describes changes among generations of livings things; natural selection is a mechanism for how those changes come about.

More specifically, evolution is the idea that the countless species we see around us today arrived at their current states through a gradual progression of forms. This stands opposed to assertions such as the Biblical creation, which state that all modern species were created instantaneously in their current forms. With today's evidence, the fact that life on Earth has evolved through successive forms over billions of years is essentially undeniable if one approaches it with intellectual honesty. The claims of the Old Testament (or those of any other religious text that describes all life having arisen more-or-less at once) just do not line up with the world we perceive around us.

Even before the discovery of extensive fossil and molecular evidence, the patterns of similarity among living things led people to suspect that modern species were the product of some sort of change acting on ancestral organisms. As such, there have been many proposed mechanisms for evolution, but only a few are supported by current science. One of the abandoned models, Lamarckian evolution, postulated that changes occurring during an organism's lifetime are passed on to its young; the classic example is the giraffe stretching its neck to reach food, thereby starting its young off with longer necks than it was born with itself. But we recognize this as wrong because traits acquired within an individual's lifetime are not passed on (your children do not bear your physical scars, for example). And Intelligent Design, though it is often contrasted with “evolution,” is actually another proposed mechanism for evolution—specifically, that purposeful changes are made by an intelligent being, probably a deity. As such, ID's version of evolution would not be limited to smooth, gradual transitions, but would also involve large discontinuities when the intelligent being decided to interfere. ID is also rejected by modern science, for reasons that I discuss in Part 4 of my Why I'm an atheist series.

There are just a few evolutionary mechanisms that are accepted by science, including sexual selection (changes caused by mate choice) and genetic drift (changes due to chance), but the central mechanism is natural selection. Unlike Lamarkian evolution or ID, it doesn't involve traits acquired during a lifetime, nor intelligent intervention. It's a simple and elegant explanation that seems so obvious that one might wonder how it took us so long to figure it out. What it boils down to is basically this: the organisms that are better at surviving live, and those worse at surviving die off, automatically resulting in the next generation being better suited to survive in their environment. From one generation to the next the differences are small, but they accumulate over thousands and millions of years, until descendants appear and act very differently from their ancestors.

Perhaps what delayed our understanding was the lack of a proper field of genetics (though Darwin was still able to describe NS before Mendel's work on inheritance became widely known). Prior to the 20th century it wasn't obvious that the traits we exhibit are tied to specific “units” of heredity, nor were the patterns in which these units are passed down completely understood. These days it's common knowledge that everything from our physical appearance to our brain chemistry is largely (though not completely) determined by discrete units of hereditary code called genes, which exist as long chains of DNA in each of our cells. It's a dramatic oversimplification, but you can think of each gene as corresponding to a certain physical feature of an organism (such as hair color); there also exist multiple versions of the same gene (such as brown hair, blonde hair, red hair) called alleles. All members of a species have the same set of genes, but each individual has a different combination of alleles. For example: most humans have a functioning gene for hair color, but differ in the particular hair color depending on which allele of the hair color gene they have (in real life you see many shades of variation because hair pigmentation is actually influenced by many different genes and their alleles, but I've simplified things a bit for clarity).

So, each species has a particular set of genes, and each individual in that species has a particular set of alleles for those genes. But to really understand how evolution works, we're not looking at either of those levels. Individuals don't evolve (if you're born a wolf, you'll die a wolf—though your offspring might be a smidge closer to being chihuahuas), and while we certainly talk about species “evolving” that's really only the end result, and not where the process is actively happening. For that, we have to examine the level between those of “individual” and “species”: the population. A population is a mostly self-contained portion of a species, such as two packs of the same species of wolf that are separated by a mountain range. In addition to geographical, the barriers could also be temporal, behavioral, or physiological, but for the purposes of explanation I think land features like mountains or bodies of water are the easiest to understand. Regardless of the type of barrier, the key fact is that little or no reproduction occurs between different populations, so their gene pools are essentially separate.

Any given population will have its own environmental challenges to overcome, and not all alleles are equally useful in a given environment—maybe one fur color blends in better with the surroundings, maybe one variation of a stomach enzyme is better at breaking down chemicals in the local plants. Since each individual in a population has a different set of alleles each of them is suited to the environment to a different degree, and thus has a different chance of survival. Those individuals that have a set of alleles more appropriate for their current environment are more likely to survive, and are able to devote more time and energy to reproduction (this is "fitness" as it applies to the oft-stated but somewhat confusing mantra of "survival of the fittest"). They'll have more offspring, and those offspring will be able to similarly out-survive and out-reproduce others of their own generation, and so on with further generations; in this way, the better-suited alleles will proliferate throughout the population. The changes in allele frequency might be small at first, but since they're in response to (relatively) constant pressures from the environment (rather than purely random) they accumulate over time. And so while there will always be a certain degree of randomness to each individual's genetic makeup (due to the details of sexual reproduction), genes for traits particularly relevant to survival will trend toward the more advantageous alleles. Defined in terms of genetics, evolution is the change in the allele frequencies of a population over time. And natural selection brings about this change because some alleles (and by extension the organisms that carry them) are better suited to the environment than others, and so said alleles have an easier time of propagating in a population.

Since different populations of the same species will often be subject to very different environmental pressures their gene pools will often be pushed in different directions. So the wolf population on the barren, snowy side of a mountain range might evolve white coats to blend in with the snow drifts, while the population on the heavily wooded side might evolve a dark fur color to blend in with the shadows. With increased time and distance the changes can be much more dramatic, so that maybe one group of canids retains a pack hunting lifestyle while another finds that they're more successful with a scavenging, solitary lifestyle. If the gene pools are pushed far enough in different directions it may come to the point when the two groups no longer recognize each other as the same type of creature and won't or can't interbreed. At this point we might say that there's been a “speciation,” that is, the branching of one or more new species from a “parent” species (hence the title of Darwin's book, The Origin of Species). 

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This brings us to the first of several misconceptions about evolution and natural selection that I'd like to address: Please note that the emergence of a new species doesn't necessitate the disappearance of the parent species. The whole parent species doesn't suddenly and totally change over from one form to another; it's a gradual process that usually only happens to one population of a species (if that species were limited to a single population, then speciation could result in a total replacement, but that's not usually how it goes). So the typical creationist "challenge" of "If humans evolved from monkeys, why are there still monkeys?" is totally misguided (even aside from the fact that humans didn't evolve from monkeys as we know them today, only a monkey-like ancestor, or that we're more closely related to apes than monkeys). There are still monkeys, and chimps (and lemurs and rats and fish and sea squirts) because only one population of each ancestral species split off in our direction. Evolution isn't a linear path, it's a constantly branching tree, and just because a new twig appears doesn't mean the parent branch ceases to exist—it can go on growing and eventually sprout new twigs in new directions.

And it's very likely to do just that, since while natural selection very much abides by the mantra of "if it ain't broke, don't fix it" and organisms in relatively stable environments may be able to persist without drastic change for long periods of time, for most livings things there is constant evolutionary pressure. The world of most organisms is constantly in flux, and they face pressure to adapt in the form of competition (both within and among species) and in changes to climate and geography. And so just because an ancestral species bears a new evolutionary branch doesn't mean the ancestral species is "done." Monkeys and chimps still came to be even though the populations that would eventually lead to us branched off because they were still under pressure to change and they continue to evolve further today, as do most extant species.

Of course, extant species make up a small portion of natural history as a whole; the vast majority of species that have ever lived are now extinct, never to return. If we filtered the image of our figurative evolutionary tree to only show the species that still exist, we'd find ourselves with many floating branches, twigs, and leaves with no direct connection to others that sprouted before or after them. Those connections existed, we just can't see them today.  We can bring some of the connections back into our image of the tree if we include those that we've found as fossils, but most things that die do not fossilize, meaning there are some connections that we'll never have direct evidence for. To put it another way: monkeys and chimps aren't our grandparents and parents, they're our cousins (of various degrees and removedness) and our grandparents and parents are gone; we may have photographs for some of them, but not all (a bit of a somber analogy, but hopefully a helpful one).

The particular species that split into the lineages that would lead to chimps and humans is no longer alive and given the low chances of fossilization we'll probably never find it. We often use modern organisms or those we do know from fossils to stand in for such missing connections (as I do in my banner art), so we might just fill in the gap with a chimp, but this is misleading given the fact that species continue to evolve after branching. Chimps are about as dissimilar from our last shared ancestor as we are and have spent just as much time evolving to fit their niche as we have to ours. But we can use chimps and humans to imagine what the creature that gave rise to both might have looked like. We can also inform our approximation by looking to known ancestors of the ancestor, and in the case of humans to the many known members of the lineage between our species and our last common ancestor with the chimps. We just have to keep in mind for each species we use as a guide that they are likely not a direct ancestor and are probably the product of another branching along the way; they had their own time to evolve, and so they very likely possess many traits that the direct ancestors did not. Our picture of exact lineages is always going to be imperfect, but imperfect is a long way from wrong. The image formed by the evidence is clearly one akin to a tree, not a linear path and not a sudden deposit of modern species.

Even the tree analogy isn't ideal, however, since in a real tree you have the central trunk leading straight to the top. The evolutionary tree splits immediately at the base and keeps on splitting, with no central or primary path. When using a normal tree in the analogy it's tempting to put humans at the top, but in the more correct tree analogy there is no "top." Humans simply occupy a twig along the outer edge (indicative of how recently we arose); where it's located in terms of its angle from the ground is irrelevant. We humans aren't the "most" or "best" evolved speciesthe first would be nebulous to define and difficult to determine, and the second is hopelessly subjective and virtually meaningless. Biologically, we are are certainly more complex than a lot of other life, but so are a multitude of other species, particularly our fellow mammals. And let's not confuse "more complex" with "better." A logical measure for evolutionary "success" is the number of individuals spawned from a certain genetic makeup, and while our species is certainly numerous, things like bacteria—very simple when compared to us—beat us by orders upon orders of magnitude. In any case, we don't owe our complexity to being "special" or "blessed" in any way, we owe it to being a recent species that has been able to build upon the adaptations of our ancestors, which is certainly not an advantage exclusive to us.* And no matter where they fall in the tree, no matter how simple or complex they are, all extant species are just that: extant. Say what you will about them, but they're not dead, and that seems to give them all a similar level of "success." But then of course the dinosaurs were around for many tens of millions of years, and it seems petty to call them less successful because of the sudden and unpredictable appearance of an asteroid... When it comes to evolution there's not a whole lot of "better," just a lot of "different."

Lastly, and perhaps the most important thing to remember about natural selection is the fact that it is an unconscious process. Neither the organisms themselves nor some overarching force guide evolution by natural selection in any particular direction. How things proceed is determined only by what genetic options are available and how they interact with the environment. The more useful alleles propagate not because something notices they're more useful, but simply because the organisms carrying them are naturally the ones that survive and reproduce. Our shared ancestor with chimps didn't evolve because something knew it would someday lead to humanity; it arose as an automatic reaction to its local environment, just as the first chimps and the first humans were automatic reactions to their own. Natural selection operates only in the present with the options inherited from the past; the future is a non-factor. Otherwise, the dinosaurs probably would have invested in asteroid-proofing.

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Unlike most explanations of evolution and natural selection, I jumped right into the nitty-gritty of the genetics. I imagine most folks avoid this because of all the terms that need to be defined, but I think an understanding of the actual mechanisms will lead to a better grasp of the bigger picture, and hopefully it wasn't too confusing. Another good way to approach it is the same way that Darwin himself did. In his efforts to understand and describe NS, Darwin looked often to artificial selection—that is, the selective breeding of plants and animals by human beings. With hindsight, the fact that the processes are so similar makes it seem almost a wonder that it took until the 1850s for someone to make the connection. Even without an understanding of the underlying genetic mechanisms, humans have been generating new kinds of creatures from older ones for millennia. We've exacerbated the traits we like in crop plants to get high yields and pleasing flavors, and we've created everything from Great Danes to pugs from the genetic stock of wolves. We do this by encouraging the reproduction of individuals exhibiting favorable traits and restricting the reproduction of those with traits we see as useless or detrimental. No one is surprised if the next generation of a breeder's domestic dogs all have shorter tails than the last if that's what the breeder was going for. Nor should we be surprised when the next generation of a wild population is better suited to its environment than the last. Natural selection is almost exactly like artificial selection. The role of breeder is left vacant (or, if you're feeling poetic, is filled by Nature, though remember not to confuse it with a conscious entity), and the mechanism for deciding which individuals are allowed to breed—instead of being informed by human fancy—is simply determined by which are able to survive long enough to reproduce.

Some people may consider it a disappointment that such an unwitting and simple mechanism is the force behind the living world as we see it today, when compared the alternative of a deity shaping life like an artist and planning everything in advance. But lovely as it may be, that idea doesn't line up with the evidence. And for what it's worth, I think there is staggering beauty in the reality of natural selection. That all the living things we see around us burst forth from a single ancestor and have done so many amazing things from a limited initial palettemost of it ultimately guided by a single, simple ruleis awe-inspiring. As Darwin put it:

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

Obviously this wasn't meant to be a comprehensive overview of evolution and natural selection, just a crash course or refresher to bring every one up to speed so I could properly tackle other topics. Hopefully it was informative or clarifying if you came into it not understanding the concepts as well as you would have liked. If you have any questions, please feel free to ask them. I also realize that it didn't deal with the evidence for what we know about evolution, but that will be at least partly dealt with in Part 4 of the atheism series, and possibly further in future posts.

*There are certainly interesting and compelling arguments to be made about the uniqueness of our brainpower, but I consider that outside the realm of biology and evolution—from a genetic, anatomical, and physiological standpoint there's nothing terribly special about our brains, it's just that we've pushed the same material to such an extreme that astounding things have become possible in the areas of cognition and culture. Other creatures remember, solve problems, use tools, feel, communicate, socialize, and pass on cultural quirks—we just do it all and incredibly robustly in each case. I'm not saying it's not amazing, I'm just saying that the biological underpinnings aren't all that special. There's a lot more I could say about this, but I think I'll stop here, as I could (and probably will) do a whole post on this subject.

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