Tuesday, November 26, 2013

Natural selection may not lead to evolution?

I recently discovered a new book called The Princeton Guide to Evolution. It looks pretty authoritative so I ordered a copy.

There are excerpts online. The first chapter is "What Is Evolution?" by Jonathan Losos. I'm not very impressed with his answer but I was shocked to read the following passage.

The logic behind natural selection is unassailable. If some trait variant is causally related to greater reproductive success, then more members of the population will have that variant in the next generation; continued over many generations, such selection can greatly change the constitution of a population.

But there is a catch. Natural selection can occur without leading to evolution if differences among individuals are not genetically based. For natural selection to cause evolutionary change, trait variants must be transmitted from parent to offspring; if that is the case, then offspring will resemble their parents and the trait variants possessed by the parents that produce the most offspring will increase in frequency in the next generation.

However, offspring do not always resemble their parents. In some cases, individuals vary phenotypically not because they are different genetically, but because they experienced different environments during growth (this is the “nurture” part of the nature versus nurture debate; see chapters III.10 and VII.1). If, in fact, variation in a population is not genetically based, then selection will have no evolutionary consequence; individuals surviving and producing many offspring will not differ genetically from those that fail to prosper, and as a result, the gene pool of the population will not change. Nonetheless, much of the phenotypic variation within a population is, in fact, genetically based; consequently, natural selection often does lead to evolutionary change.
I never heard to this idea before (that natural selection may not lead to evolution). I thought that natural selection was DEFINED as a change in the frequency of alleles in a population due to selection. Doesn't it have to have a genetic component?

Does this mean that natural selection may not lead to adaptation? Or, does it mean that adaptation isn't necessarily evolution?

The chapters are written by an impressive group of authors (Jonathan Losos is the editor-in-chief). It must represent the current consensus among evolutionary biologists. I'm surprised that I never heard of this definition of natural selection.