Monday, March 18, 2013

The "If"s of Natural Selection

We write a lot about genetic determinism here on MT because unfortunately it's everywhere, but we probably don't turn enough of our attention to its corollary, the assumed certainty that traits are here because of adaptive natural selection.  Everyone knows about 'survival of the fittest' and that therefore traits are here because they served a purpose, often if not usually treated as if a specific purpose, in our evolutionary past.  Even those who clearly recognize that selection, when it occurs, is usually highly probabilistic, still talk the talk of determinism as if the adaptationist assumption is latent in their thinking.

Bushy eyebrows (Darwin's)
It's easy to make up a story about how and why a trait evolved.  That's because if you assume everything has an adaptive explanation in order to be here, what is here must have such an explanation;  as scientists, of course, it's up to us to say what that is.  Thus, we've got bushy eyebrows to shield our eyes from the sun; East Africans are fast runners because they were cattle thieves and had to run fast in order to survive, and the story that appeared just last week in the Proceedings of the Royal Society B, that Neandertals had larger eye sockets to be able to see better in the long, dark northern nights, and so were out-competed by our more successful ancestors who, with smaller eyes, could devote more of their cortex to higher thought processes, specifically involving those required for social organization.

But it's fair to say that these kinds of explanations are usually Just-So stories.  Made up since there is no direct evidence for the distant past, and perhaps even plausible, but untestable.  We've said enough times to get into trouble that really, the most robust selection story we have is probably that of malaria and sickle cell (and other anemias that are protective), that these traits evolved as protection against malaria around 10,000 years ago.  Other stories running close behind are lactose tolerance and skin color -- early humans were all lactose intolerant until various groups domesticated dairy animals and adults began to subsist on dairy products, and skin color lightened as humans moved north because of the need for vitamin D.

But even those stories leak a bit.  Remember that for a trait to evolve by natural selection, those with the trait had to have more children than those without.  For many many generations.  But it's at least a bit forced to argue that lactose intolerance systematically lowers the number of children its carriers have.  It may cause occasional or even frequent discomfort, but it's rarely lethal. People report becoming accustomed to it.  It is argued that in times of food shortage, adults can gain nutrition by drinking milk.  But in times of drought what are the cows drinking?  And if there's inadequate food for agricultural humans who held cattle, why would there be cattle food?  Wouldn't the grass have died too?  Whether they or other arguments are true, the issues are not given very close consideration.  So even if there is a lot of good-looking circumstantial evidence, should we really conclude definitively that milk drinking was a strong selective force in the not so distant past?

Skin color lightened because of the need to make vitamin D in northern climes, when sunlight isn't as strong for much of the year?  But, we're able to store vitamin D for months at a time, so probably don't need to make it all year round.  Plus, estimates of required vitamin D levels differ wildly.  Further, darker skinned people tend to have lower vitamin D levels than lighter skinned, on average, yet they also have fewer bone breaks, a marker of bone density and a serious consequence of inadequate vitamin D.  Could the link between vitamin D and bone mineralization be more complex than we realize, or could there be an additional mineralization pathway, as yet unidentified?

Even if we were to grant that these issues can be resolved and the adaptive stories are correct, it is important to note that of all our traits, and our many thousands of functional genomic elements, there are precious few stories of such genetic adaptation that have persuasive documentation.  This is consistent with a much less deterministic view of adaptation, especially if one looks at the gene level.

Normally, estimates of fitness -- very, very difficult to identify directly even in the present, unless involving human activity like antibiotic or herbicide resistance -- are that the difference between the 'fitness' conferred by the better allele at a gene even under rather strong selective pressure is only about 1%.  If continuous, and deterministically systematic, a 1% advantage would indeed lead the 'good' allele to replace the 'bad' one.  But the advantage is that if I carry the good one, I have 100 children while my bad-allele-carrying neighbor has a mere 99!  This is not even testable in most natural human populations, which were in demes too small.

In our book, The Mermaid's Tale, we described natural selection this way:
Natural selection means the systematic differential reproductive success of competing organisms. The idea is simple: if a species over-reproduces so that not all individuals in the next generation can go on to successfully reproduce, and if there is variation in form among that species, and if some forms of an organism do better in a particular environment than other forms, and if the reason for this is included in their heritable genome, and if the environment remains stable long enough over time for this form to be favored persistently, and if the favorable forms are also lucky enough to produce offspring who go on to reproduce, and if they produce more offspring than their competition, then those forms can become ever more common over time at the expense of their competition. If all these contingencies do occur, indeed co-occur, then the more prolific life form will become more suited—better adapted—to the environment in question. If the forms are sequestered from each other by some mating barrier, then they would diverge over time, and this was the explanation Darwin and Wallace proposed for the origin as well as specialization of species.

This reasoning is beyond doubt, and is essentially what Darwin and Wallace were suggesting.  But it hinges on the many ifs. Clearly, natural selection is always possible, and often important, sometimes over-ridingly so. At the same time, it has been too easy to assume the ifs. But when the selective differences are small, or highly variable over time, selection is not as much like a systematic force of nature as its usual image. A force is forever, and it has both strength and direction. Instead, and aside from the importance of chance, it is more accurate and realistic to view natural selection as more nuanced, and as only one of many contributing ways in which life’s success is determined.
A force is infinitesimally divisible (and this, a kind of Newtonian-force model, was explicitly Darwin's idea), but there is far too much chance that affects survival and fertility for  selection to be that kind of force in nature, at least as a rule.

We are all too enamored of simple explanations.  We are happy when we learn that this gene is 'for' that trait, and that trait evolved 'for' this purpose. But that is sloppy thinking that is fundamentally inaccurate, and it is not good science, despite its appeal to the media looking for dramatic stories and simple dog-eat-dog explanations, and despite it being a widespread image of life in many health and life sciences.


Rob Gargett said...

Hi, Anne
Beautifully written, and a perspective that badly needs to be promulgated widely. I investigate the way we became human, which demands equal expertise in biological anthropology and archaeology. In fact, I've read the paper that proposed the brow ridge explanation. We laugh now. But you're quite right that the reasoning it employed is still very much part of the zeitgeist of evolutionary biology, and we must work to replace it with a more nuanced account. My purpose in leaving this comment will now be revealed.
For a long time I've thought that calling it "natural selection" contributes to a fundamental misunderstanding, even amongst scholars of evolution. Calling it "natural selection" is a classic example of what the anthropologists Sapir and Whorf hypothesized in the first half of the twentieth century: that the structure of a language affects the perceptions of reality of its speakers and thus influences their thought patterns and worldviews. I think that by calling it "selection" everyone from Darwin to Anne Buchanan may be inadvertently promoting a view that's the opposite of what's actually happening during evolution. It's really not "selection" at all; it's "rejection." The owners of the genotypes making up a population at any given point in time are those individuals and the descendants of others who persisted after those not so lucky fell by the wayside. To use one of your examples: Thog the Homo ergaster didn't survive 'cause his brow ridges were bigger. Rather, his fellow species members without the big brow ridges lost their eyesight due to sun-blindness before they reached reproductive age.
I'll admit it's a subtle distinction, and, of course it makes no difference in what actually happens during evolution. However, I really do believe that if Darwin had nicknamed it "Natural Rejection" there would be fewer misunderstandings.

Anne Buchanan said...

Thanks, Robert. Yes, and your point goes along with the idea that "survival of the fittest" is exactly the opposite of what actually happens, which is more accurately described as "failure of the frail."

Ken Weiss said...

We used this term in our book ('failure of the frail'). Even Darwin was wary of 'natural selection' for a somewhat related reason of the impression it left that there was a selector.

Purifying selection, which is what you describe, is certainly more pervasive than positive adaptive selection. But the story is more complex even than that. There are other ways for a trait to be here today: genetic drift, pleiotropy involving some other trait that was selected 'for', selection for something else (say, reduced nose or reorienting of the cranium) that left eyebrow ridges prominent simply because there was no selective pressure to reduce them, etc.

One can make up all sorts of stories, as you say.

Survival of the fittest is vague in various ways. Does 'fittest' mean the single most-fit individual, or all who are tied for that state, or just those who are 'fitter' than others?

The irony of all of this is that we should be having this kind of conversation--and the sore need for it!--when absolutely nothing about the points is new. They can even be found in Darwin; though he believed in a force-like selection and that adaptation was everywhere, I am sure he would have recognized what we're saying, and if he had a proper sense of probability he would have seen that selection cannot be as he thought of it, even if it is an important part of living diversity.

Ken Weiss said...

I neglected to add, in my above comment, that our ancestral populations, back for 3.5 or more billion years, _never_ experienced so strong a selective regime as to be unsupportable. Selection is usually very weak. The poster-boy examples, even if they are true as charged, are the exceptions and these should not be driving the rule.

Rob Gargett said...

It probly goes without other contribution to the future re-synthesis is to remind the hominin/nid/oid fossil wranglers that, by and large, selection is acting on the females, whose physical accoutrements make them the lineage's foot-soldiers. All those shredded, big-toothed boy fossils are the equivalent of customized cars. Shiny, maybe. Fully loaded, yeah. But, essentially useless for running around town doing errands. ;-)

Rob Gargett said...

Sorry. Forgot to say 1st, thanks for including me in. And, I l.o.v.e. "failure of the frail."