More than survival of the fittest

Natural selection, a process through which those most suited to environmental conditions survive to reproduce, has been considered central to theories of evolution. However, a study conducted by Tim Weaver, UC Davis professor of anthropology, may give more weight to the theory of genetic drift – the idea that random chance can explain genetic and phenotypic changes in a species over time.

Weaver compared cranial measurements from modern human skulls and Neanderthal specimens to conclude that genetic drift is a plausible explanation of why modern humans and Neanderthals diverged 40,000 years ago.

This study was based on a previous study where we … determined genetic drift was a likely explanation, Weaver said. We used the amount of divergence in cranial morphology to estimate when Neanderthals and modern humans diverged in the past.

To determine when Neanderthals and modern humans split, Weaver and his colleagues compared previous DNA evidence to their own research.

We said, ‘well, if the differences are due to genetic drift, then the amount of divergence between Neanderthals and modern humans should be proportional to the amount of times Neanderthals and modern humans diverged’,Weaver said.

Weaver compared and measured 2,524 human and 20 Neanderthal skulls. After putting the measurements into a model, Weaver and his team estimated that humans and Neanderthals diverged 370,000 years ago, according to a press release. This estimation corresponded very closely to the date calculated by the previous genetic study. Because of the agreement between both studies, it is likely that genetic drift can explain some of the divergence of humans from Neanderthals.

These findings open up other avenues for explaining why modern humans replaced Neanderthals. Not all differences in fossils are adaptive and other factors determining skull morphology should be explored, said Weaver.

The fact that the differences in skull shape are probably due to genetic drift also means that the reason why modern humans were able to replace Neanderthals probably doesn’t have anything to do with the fact that they have differently shaped skulls, Weaver said. It probably has to do with something else, and I would say it has something to do with cultural or behavioral differences between the two.

The best way to think about genetic drift is as changes that happen in a population solely by chance, Weaver said.

Sometimes some individuals in a population tend to have more offspring than others.… The individuals that have more offspring contribute more to the next generation.

Compare this to natural selection, Weaver said, in which the individuals who have more offspring are able to do so because they are somehow better adapted to environmental circumstances, not chance.

Weaver also points out that from an anthropological perspective, it is important to realize there is more than just one evolutionary theory out there. Other explanations give a broader picture of why and how species have developed. While Weaver and his colleagues are not the first to propose genetic drift as the explanation for Neanderthal and modern human divergence, he said that most researchers just assume natural selection is the reason and then move on.

There are different evolutionary forces, and we’d like to have evolutionary explanations for these forces, Weaver said. I think for Neanderthals and modern humans it’s particularly important because ever since [the first Neanderthal skulls] were discovered 150 years ago, most people have assumed that … the Neanderthal’s skulls were an adaptation for some particular behavior they were doing.

Even though much research has been done on genetic drift and other evolutionary theories such as morphology, natural selection dominates anthropology and biology classrooms. Senior biochemistry major Julia Labadie said that in all of her classes other evolutionary theories are mentioned, but never explained.

I’ve taken classes where they talk about everything, but usually the teacher just refers back to natural selection, Labadie said. I feel like no one talks about other explanations very much. There’s a whole list of other factors that they give you, but then they skip over them and say ‘let’s talk about natural selection’.

Labadie also said that she hopes that maybe teachers will start to explore other theories more in depth in the future.

I don’t know what half of the theories really mean, including genetic drift, because no one takes the time to explain them, Labadie said.

Being a researcher, Weaver said that his findings definitely make a difference in the way he teaches his classes, but is not sure how much of a difference the findings of his study will make in other classrooms.

I think it might make a difference in the sense that for a long time in the way anthropology was taught in terms of evolutionary theory, natural selection has been the main force that has been discussed, and there are actually all these other evolutionary forces, he said. To have a little bit more of a balance between discussing genetic drift and not explicitly talking about natural selection is something I teach in my own class.

Weaver’s study can be found in the Mar. 17 issue of the Proceedings of the National Academy of Sciences.

JACQUELYN FLATT can be reached at science@californiaaggie.com. XXX