MOUSE – SPRING 2011
We talked a bit at the very beginning about the genetic variation that underpins the
features that may or may not be adaptive for the organism, the substrate upon which natural
selection operates. You’ve all had genetics, and we don’t need to beat it to death, but I now need
to give you a blitzkrieg course in population genetics. In fact, we teach a whole course on this,
and one could do several such courses, but the central principles will have to suffice for us here.
– Let me remind us of the definitions of some of the terms I’ll be using, as we go
along. Take a look at Futuyma, Chapter 9, or at your genetics book, to refresh your memory.
– any morphological, physiological, anatomical, or measurable feature of an
organism, interesting where there are differences among individuals. Different genotypes
may manifest the same phenotype (as in dominance or codominance).
– underlying genetic constitution of an individual, coded in its DNA; sometimes in
reference to a particular genetic locus, sometimes in reference to the whole genome. A given
genotype may manifest different phenotypes, depending on other influences.
– a site on a chromosome, or more usually, the gene that occupies that site.
– usually, the unit of DNA that codes for a product (a tRNA or a specific polypeptide).
Basically viewed as a “unit of inheritance”, concept a little fuzzy around molecular edges.
– a particular form (or morph) of a gene, either detected by its phenotypic effects or by
DNA methods. Phylogenetically, a character state for a particular gene (character).
– A string of alleles at different loci (positions) along a chromosome, usually
inherited as a unit, barring recombination, but often having myriad phenotypic effects.
– Any novel variant of a locus, due to a change in the DNA sequence. Also, the
process by which such changes occur.
Sources of Variation:
What natural selection “sees” is the phenotype, not the genes underneath.
Now that is important, because if the trait that matters is utterly plastic, environmentally, then
picking the winners is irrelevant to evolution, because the winners (on average) are genetically
the same as the losers. Phenotypic differences have to be heritable for the trait to evolve.
What are the sources?
- When you look at a phenotype, there are actually 3 or 4 different
things it could represent. In practice, particular traits are a combination of two or more of these
factors, and we often have to disentangle them.