NOTE: This article was written by a one-time colleague of mine who used the handle "Bethnewt".
Many people believe that evolution proceeds by changing organisms slightly, so that species can be lined up to form a gradual series that shows complexity slowly increasing. This view was once held by most biologists but has since been discarded. However, it has been dumpster-dived by creationists seeking materials for straw-man representations of evolutionary theory. Evolution actually takes the form of a branching family tree. Stephen Jay Gould calls ladders and bushes "the wrong and right metaphors respectively for the topology of evolution" ("Eight Little Piggies", 284)
Creationist Questions and Assertions
- Microevolution isn't true evolution because there is no increase in complexity as it progresses.
- If evolution necessitates progress, why are we -- the pinnacle of evolution -- not the best at pulling, seeing, smelling, running, and working hard?
- If God used evolution to create us, why did He take so long to get to the point?
- Respected evolutionists have even said that baboons [or substitute your favourite primate] aren't in our lineage after all. So there!
- If we evolved from apes, why are apes still around?
- Evolution necessitates that better organisms replace those less highly adapted. So why can 'advanced' grass be wiped out by 'primitive' moss on a lawn?
Real evolutionists consider phylogeny to manifest itself as a branching family tree. 'High' and 'Low' organisms are thus not present in a sequence capped by man. Natural selection does not proceed on an absolute scale, with humanoid complexity as a goal, but adapts populations to their environments. Humans, rather than being the top rung of a ladder, are one branch of a family tree on which each relative has a separate environment to deal with. No general pattern of progress is required. Organisms can become simpler as well as more complex; they can even lose characteristics that their ancestors earned -- what happens depends upon the organism's niche.
Those unfamiliar with evolution often think of it as proceeding along specific lines. The depiction of human evolution as a sequence of monkey-men standing straighter and losing their body hair is misleading, yet it's ingrained in many minds; a few half-centuries ago, many of the big names in evolution assumed that evolution proceeded along a ladder with humans at the top, and even today some scientifically-inclined laymen share this mistaken view. Understanding the way evolution really works involves, for most people, unlearning this common view of evolution.
Lamarck (who has gotten a bad rap he doesn't quite deserve) "found that in many cases he could arrange his molluscs into a modifying series that seemed gradually to proceed from older fossil species to younger extinct species to living ones. Deducing that such series represented connected lineages composed of ancestors and descendants, Lamarck in turn concluded that species could indeed change, slowly, from one into another" (Tattersall, 11). Lamarck was one of the first to consider evolution in a geologically long timeframe. Modern textbooks fault Lamarck for certain proposals he made, none of which was horribly off for the knowledge and attitudes of his day; his thinking was in the right direction, but the mechanism he proposed was the cause of his aforementioned bad rap.
Lamarck proposed that organisms somehow sensed what anatomical changes would be beneficial in their environment, and then implemented the change in their bodies, where it could (again through unknown means) be communicated to future generations. The problem people had with Lamarck's theory was not that it proposed evolution -- that had been done -- nor that it required an indefinite time scale -- that wasn't new either. The (currently much-maligned) notions of inheritance of acquired characteristics and use and disuse were neither new nor unacceptable (a chapter on Use and Disuse had a place in Darwin's Origin). The major flaw in Lamarck's theory was that he assumed a natural progressive drive in organisms. Lamarck's giraffes were born with progressively longer and longer necks, and acted to affect those necks by striving toward tall tree leaves. Lamarck was ostracized by his colleagues for assuming "a sort of inbuilt tendency for organisms to become more complex, although he recognized that the actual history of life as seen in the fossil record was untidier than this would imply" (Tattersall, 11).
Any creationist who refutes a theory of evolution that involves organisms necessarily becoming more complex (including the claim above that microevolution is invalid because it doesn't involve an increase in complexity, and the one that presupposes that all animals were working their way up to us) is only refuting a straw man that has no relevance to modern evolutionary theory.
But what of the tendency toward complexity? Some people love to point out that life began with single-celled critters and progressed through stages that -- let's not kid ourselves here -- consisted of more and more parts, with more and more types of specialized cells, and larger and more interconnected (and harder-to-understand) nervous, endocrine, and other systems. What of that?
Gould explained this quite elegantly ("Eight Little Piggies", 322):
We talk about the "march from monad to man" (old-style language again) as though evolution followed continuous pathways of progress along unbroken lineages. Nothing could be further from reality. I do not deny that, through time, the most "advanced" organism has tended to increase in complexity. But the sequence from protozoan to jellyfish to trilobite to nautiloid to armoured fish to dinosaur to monkey to human is no lineage at all, but a chronological set of termini on unrelated evolutionary trunks. Moreover, life shows no trend to complexity in the usual sense -- only an asymmetrical expansion of diversity around a starting point constrained to be simple. Let me explain that last cryptic remark: For reasons of organic chemistry and the physics of self-organizing systems, life arose at or very near the lower limit of preservable size and complexity in the fossil record. Since diversity, measured as number of species, has increased through time, extreme values in the distribution of complexity can move in only one direction. No species can become simpler than the starting point, for life arose at the lower limit of preservable complexity. The only open direction is up, but few species take that route. Increasing complexity is not a purposeful trend of an unbroken lineage but only the upper limit of an expanding distribution as overall diversity increases. We focus on this upper tail and call its expansion a trend because we crave some evolutionary rationale for our perception of ourselves as a predictable culmination.
What evolutionary biologists believe today -- Darwin's theory as modified by recent developments and referred to as Neo-Darwinism -- is that (with respect to giraffes) the ancestors of modern giraffes were caused, by random mutations, to be born occasionally with longer-than-normal necks and occasionally with shorter-than-usual necks. For some added depth to the argument, Dawkins even explains how easily a long neck can evolve from random mutations: "the giraffe's neck could have sprung out in a single mutational step (though I bet it didn't) . . . The point is that you may only have to change one thing in the developing embryo in order to quadruple the length of the neck. Say you just have to change the rate at which the vertebral primordia grow, and everything else follows." (the rate can also be made slower, as the other half of this random process) A non-random phenomenon (natural selection) can create some apparently progressive effects when paired with the random mutations. Long-necked giraffes get more food, are healthier and more prolific, and can better raise those numerous offspring (it's also possible that sexual selection played a role). Short-necked giraffes were malnourished -- perhaps only slightly, but it was enough to give the longer necks an edge. Over a few million years, the advantages added up. So far, that's progress, despite the random element that causes neck size to vary. Where we get into lack of progressive drive is when we look at animals other than giraffes. If the environment favours short necks (as with cows, for example), those with long necks are at a disadvantage, sometimes because a neck too long can be awkward and sometimes simply because it takes energy and effort, at least on a cellular level, to create and maintain the cells in those extra inches of neck. Like paying to heat or air-condition a building you never use, it's a waste. They are, in the evolutionary parlance, selected against.
Perhaps I should pause here to explain competition a bit. Short-necked giraffes would never starve if there were enough food to go around. But because each pair of giraffes is capable of producing more than enough baby giraffes to be their replacements (and giraffes love to produce baby giraffes -- ask any zookeeper), the savannah could be inundated with giraffes in a generation or two. More giraffes than it can afford to feed, in fact. So, with a small carrying capacity (the number of giraffes the area can maintain), not all of those giraffes are going to make it. All of the giraffes are in competition with each other; some are winners and some are losers. The first-place finisher has a lot of kids; the last-place finisher dies young. Those that can deal the best with the environment end up having the most kids.
Where all of this leads us is along lines of very slow change. Populations can exist more or less unchanged for a few million years. Sometimes a few individuals from the population become isolated from the rest, and in a different environment. The small number of individuals allows new genes to be passed around quite quickly (compare the way certain genes -- achromatism, for instance -- are common among natives of very small islands but rare in the rest of the world. Your chances of hooking up with another carrier are minuscule in the US, but pretty good, due to limited choices of mates, on a tiny island -- or, for that matter, within a highly inbred group like a royal family or a ( :-) )West Virginia town). So, the random part is accentuated by the speed at which its results can spread, and the change of environment is a strong and different non-random factor. That's Gould and Eldridge's punctuated equilibrium, by the way, and it follows rather logically from Darwinian theory. My point -- yes, this is relevant to what I was saying before -- is that the new population changes a good deal before the original population has a chance to evolve much at all. So you have a large branch with a twig sprouting from it. Sometimes the two species are reunited, and one may compete with the other to drive it out of the area, or kill it off entirely. In this case our diagram looks like a branch and twig, but either the branch or the twig may be broken. Or, both branches may go on to split into more branches in a similar fashion. So at the end, we have not one continuous lineage, as Lamarck believed, but a family tree that looks like a bush with a lot of broken branches (representing extinction’s) but with many branches persisting to the present. In the example above with the moss and the grass, both moss and grass are healthy branches that exist in the present. Each has adapted to its niche.
Usually, when two species come together, they compete. The differences between them are such that one species usually dominates the other, rather than some individuals of each surviving. But the mechanics of interspecies competition are part of ecology, not evolution. The thing is, both grass and moss have evolved and survived, and neither is "better" than the other; each is adapted to a niche, and any particular microenvironment -- such as a lawn -- will be colonized by whichever ground cover has evolved for the niche it provides. As in construction, sometimes one tool is better for the job. Just because I choose a hammer rather than a wet saw for pounding a nail doesn't mean that a hammer is somehow "better" on a grand scale; it just means that a hammer is good for pounding nails, and a wet saw is not. Likewise, the fact that when cutting tile I would use a wet saw is not an insult to the hammer.
To finally correct the image, I opened the discussion with, the primate family tree also follows a branching pattern. Sometimes the order of branching events is questioned -- Australopithecus africanus was once considered to be a human ancestor but actually may not be -- but the fact of branching is well-established. Humans and living apes are all the tips of branches. We did not evolve from chimpanzees (though they are our closest relatives), but we and the chimps had a common ancestor a while back. Just like you aren't a descendant of your cousin, but you and your cousin have the same grandparents.
Here is a self-esteem-bashing quotation from Gould ("Dinosaur in a Haystack", 327):
Humans are not the end result of predictable evolutionary progress, but rather a fortuitous cosmic afterthought, a tiny little twig on the enormously arborescent bush of life, which, if replanted from seed, would almost surely not regrow this twig again, or perhaps any twig with any property that we would care to call consciousness.
So, humans are not, in any sense, the pinnacle of evolution. We do, however, happen to have expanded our niche, sometimes resulting in the extinction of other species, and we have reproduced prolifically. But that doesn't change our lineage. If our ancestors did not have eight legs -- in other words, if eight-leggedness evolved on a branch that we had long ago split from -- that won't affect the number of our legs. Similarly, we don't have the best sense of smell because of one of two situations: either a good sense of smell did not happen to have occurred on our branch, or it did but was eventually reduced for the same reason that long-necked cows are at a disadvantage and that cave fish lose their pigmentation: it's a waste of resources. Why have more chemoreceptors than you need cluttering up your mucous membranes? Evolution allows things to disappear as well as to appear. For another example, caecilians are a type of amphibian. They descended from the same ancestor or group of ancestors as salamanders and frogs did, but caecilians have no legs, no ears, no tail, bad eyes, and only one lung. They have adapted to the same environment as earthworms -- a fossorial (underground, burrowing) lifestyle. They did this because there are worms to be had (yum!) for those who dig in the soil the way worms do. Some might consider the loss of legs and eyes to be a sort of reverse evolution, but evolution worked for the caecilians the same way it works for everything else: those that do well have the most offspring. Evolution, great though its results may be, is no more than a change in allele frequency in a population over time.
- The Fossil Trail, Ian Tattersall, 1995
- Field Guide to Early Man, David Lambert, 1987
- Eight Little Piggies, Stephen Jay Gould, 1993
- Dinosaur in a Haystack, Stephen Jay Gould, 1995
- The Origin of Species, Charles Darwin, 1859
- Climbing Mount Improbable, Richard Dawkins, 1996