Problem with Single Locus Models
improbable that many traits are controlled by a single genetic locus and fate of phenotype depends only on that locus. Is more complex.
tendency for alleles at different genetic loci to be inherited together. fate of phenotype and genotype at one locus may depend on selection on another locus because it is linked to it
one gene that affects the phenotype of >1 trait
the expression of a gene at one locus depends on which allele is present at a second genetic locus
a single phenotypic trait is determined by the interaction of many genes.
Condition when (alleles at a locus or) genotypes at different loci are distributed independently of one another
Condition when (alleles at a locus or) genotypes at different loci are not independent of (other allele at a locus or) genotypes at different loci.
Sex vs Linkage
Recombination & independent assortment really is SEX. Linkage is kind of the inverse of sex.
The frequency of B on chromosomes carrying allele A ≠ frequency of B on chromosomes carrying allele a •The frequency of a haplotype ≠ product of frequencies of alleles (slide 10)
D = (h)AB(h)ab - (h)Ab(h)aB ≠ 0
ranges from -0.25 to +0.25 (slide 10)
Causes of Linkage Disequilibrium
1) Non random mating. 2) Selection. 3) Mutation and selection in a small pop. 4) Inversions preventing recombination. 5) Migration.
as a favorable mutation increases in frequency, neutral, or even slightly deleterious mutations closely linked to the favored site will increase in frequency. aka Selective Sweep.
costly, complicated, potentially dangerous.
easy, no mate location complications, or reduced genetic contribution.
How Sex Reduces Linkage Disequilibrium
shuffling multilocus genotypes. (slide 26)
Adaptive Benefits of Sex
1) Genetic drift & mutation creates linkage disequilibrium. 2) Selection on multilocus genotypes creates linkage disequilibrium.
Deleterious mutations passed by females to offspring trans-generationally. (slide 28).
Sex in a Changing Environment
Favored due to sexual parent/ offspring ability to adapt. Asexual parent/ offspring do not adapt so if environment becomes less fit- they lose.
Why Sex Genes are Favored Over Asexual Genes.
because greater fitness of SOME of sexual genotypes in changed environment.
physical/ chemical. Needs to be highly temporally variable for sex to prevail (selection of sub-population displaying sex).
The Red Queen Hypothesis: "It takes all the running you can do to keep in the same place."
Co-evolutionary arms race between coexisting species involving reciprocal adaptations to select one over the other.
Variable Selection in Changing Environments Hypothesis'
Abiotic (physical/ chemical) and Biotic (biological interactions between species.)