One of the strangest aspects of our understanding of evolutionary biology is the tendency to conflate a sprawling protean dynamic into a sliver of a phenomenon. Most prominently, evolution is often reduced to a process driven by natural selection, with an emphasis on the natural. When people think of populations evolving they imagine them being buffeted by inclement weather, meteors, or smooth geological shifts. These are all natural, physical phenomena, and they all apply potential selection pressures. But this is not the same as evolution; it’s just one part. A more subtle aspect of evolution is that much of the selection is due to competition between living organisms, not their relationship to exterior environmental conditions.
The question of what drives evolution is a longstanding one. Stephen Jay Gould famously emphasized of the role of randomness, while Richard Dawkins and others prioritize the shaping power of natural selection. More finely still, there is the distinction between those which emphasize competition across the species versus within species. And then there are the physical, non-biological forces.
Evolution as selection. Evolution as drift. Evolution as selection due to competition between individuals of the same species. Evolution as selection due to competition between individuals of different species. And so forth. There are numerous models, theories, and conjectures about what’s the prime engine of evolution. The evolutionary biologist Richard Lewontin famously observed that in the 20th century population geneticists constructed massively powerful analytic machines, but had very little data which they could throw into those machines. And so it is with theories of evolution. Until now.
Over the past 10 years in the domain of human genetics and evolution there has been a swell of information due to genomics. In many ways humans are now the “trial run” for our understanding of evolutionary process. Using theoretical models and vague inferences from difficult-to-interpret signals, our confidence in the assertions about the importance of any given dynamic have always been shaky at best. But now with genomics, researchers are testing the data against the models.
A recent paper is a case in point of the methodology. Using 500,000 markers, ~50 populations, and ~1,500 people, the authors tested a range of factors against their genomic data. The method is conceptually simple, though the technical details are rather abstruse. The ~1,500 individuals are from all around the globe, so the authors could construct a model where the markers varied as a function of space. As expected, most of the genetic variation across populations was predicted by the variation across space, which correlates with population demographic history; those populations adjacent to each other are likely to have common recent ancestors. But the authors also had some other variables in their system which varied as a function of space in a less gradual fashion: climate, diet, and pathogen loads. The key is to look for those genetic markers and populations where the expectation of differences being driven as a function of geography do not hold. Neighbors should be genetically like, but what if they’re not? Once you find a particular variant you can then see how it varies with the factors listed above.
More specifically, the researchers were interested in genetic markers which are likely to be in regions which hint at natural selection, and therefore adaptation. Whether adaptation in human populations is driven by climate, diet, or pathogen load is a million-dollar question, and this is one attempt to answer it. This is particularly relevant to the initial public confusion we’re faced with: that evolution is a process whereby populations adapt to changes in the natural, physical environment. Is that so? For human populations the answer is a resounding no!
Rather, the authors found that adaptation to pathogens exhibited particularly strong signals of local adaptation—in particular, adaptations to varieties of worms. This aligns with the deduction of some evolutionary biologists that host-parasite interactions drive much of adaptive evolution in complex organisms. Why the local adaptation with worms? The authors posit that worms evolve slower than bacteria, and are also more localized in distribution. Climate and diet? Not so much effect. At least for humans the public perception is close to 100% wrong. Humans adapt to local biological forces, not to the local natural environment.
Should this truly surprise us? Minnesotans are no furrier than Floridians. They make a cultural adaptation to climatic differences. In contrast, West Africans do have biological defenses against malaria which Minnesotans lack. And it is a popular “fun fact” that there are 10 times as many bacterial cells within the human body as human tissue cells. Most of these are “good bacteria,” but obviously not all of them are. The fact is that we live embedded in a world of microorganisms. The prevalence and power of microbes is also one leading possible explanation for sex: Because small asexual microorganisms evolve so fast the only way lumbering complex organisms can compete with them in the arms race of evolution is to continuously shuffle and rearrange their own genetic portfolios through sexual reproduction.
Finally, this should perhaps allow us to reconceptualize adaptation. It’s not due to something out there, but something in there. Biological organisms by and large aren’t reacting to geological forces, but to other biological entities. This is what makes biology such a frustrating science when you’re faced with the beauty and linearity of physics. The planets may move, but they move regularly. In contrast, as organisms trace evolutionary paths they exhibit chaotic creativity, responding to each other’s dodges and jabs. Evolution is not a smooth gradual geological process, but a noisy and scattered perpetual re-oganization of living organisms again and again in kaleidoscopic patterns.
Image credit: Zink Dawg
Razib Khan’s degrees are in biochemistry and biology. He has blogged about genetics since 2002 (see his Discover blog, Gene Expression) and is an Unz Foundation Junior Fellow. He loves habaneros.