Divergence Along Genetic Lines of Least Resistance & Final Conclusions

Last week Kayla and I finished "The Ecology of Adaptive Radiation"- the first novel in our reading list this semester.

The final (non-concluding) chapter focused on Divergence Along Genetic Lines of Least Resistance and was perhaps the only title that I had no clue what would contain. I hadn't really heard of the concept of genetic lines of least resistance and perhaps that's why I had such a hard time ploughing through the forty or so pages in this chapter. So here's a definition as far as I can manage:

Phenotypic evolution by natural selection is primarily determined by genetic variance and covariance that bias evolution away from greater fitness.

I suppose I have a feeble hold on the concepts of quantitative genetics so this definition scared me. This was further complicated by my tenuous grasp of the idea of 'additive genetic variance'... which is evidently the variation we are talking about in this case. I've come to understand additive genetic variance as the variation in offspring that is a direct result NON-interacting genes from the parents. That is, the sum of genetic variance from the two parents, where there is no epistasis or other interactions.

We have another interesting definition here, that I have some trouble wrapping my mind around.

I find that thinking about lines of least resistance in terms of examples is most useful. The common one is that if you had two traits, beak broadness and beak length, these traits would always be tied to each other such that you could never get a really narrow long beak, even if this was the 'fittest' phenotype. Hence, the 'adaptive hillside' on which the population rests would be a combination of these traits that was genetically feasible, but not necessarily the 'fittest' given the landscape.

I suppose that white sands lizards could also have traits like this. Say colour. Say dorsal (cryptic) colour and ventral (signaling) colour vary in such a way that you can never get to be completely matched to your dark soil surroundings, unless you sacrifice some ventral colour. On the other hand, if you lose pigmentation on your dorsal surface, your ventral colour is enhanced (this appears to be the case in white sands, but the genetic correlation between ventral and dorsal colour, if it exists, is unknown). So if we go into dark soils, even though the best situation would be a bright ventral colour for signaling and a dark dorsal colour for camouflage, this combination can never be achieve... and essentially you have the lizards sitting on an adaptive peak that is below that which is optimum, fitness-wise.

Just a disclaimer here- I think this is a bad example because it is quite probable that selection on dorsal and ventral colour is mostly unrelated. Selection on ventral colour likely has more to do with the environment in which signaling takes place (bright white sands versus ancestral dark heterogeneous habitat).

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Well that's about it for now. I wanted to conclude by saying that I tried to read this book in undergrad and couldn't make it through; however, I am very happy that I picked it up again in my second year of my PhD. This was a much more appropriate time to read it and Schluter has definitely enhanced my understanding of many topics as well as filled in large gaps in my understanding of evolutionary ecology and adaptive radiation, specifically.

... on to SPECIATION!