genetics of natural selection, intro to quantitative traits

role of natural selection

fine tunes phenotypes to their environment

population genetics models of natural selection

• help us see how adaptation comes about

• predict the outcome of selection (equilibrium allele frequency at a locus)

• predict rate of evolutionary change (rate of change in allele frequency at a locus)

• examine the interaction of selection with other population processes

peppered moth example of natural selection

• coating of trees with soot due to coal usage in 19th century Britain

• light-coloured moths became easier to spot by predators

• in about a hundred years (a hundred generations), white form was nearly replaced by dark form

absolute fitness

percentage of individuals who survive to have offspring

relative fitness

relative to the phenotype with the highest survival, differences in fitness due to a particular phenotype

genotype frequency weighted by fitness

w_AAf_AA

w_AA = relative fitness of AA individuals

f_AA = starting frequency of AA genotype

average fitness for population

W_avg = f_AAw_AA + f_Aaw_Aa + f_aaw_aa

genotype frequency after selection

w_AAf_AA / W_avg

types of selection

• directional: selection of one favoured allele

• balancing: two or more alleles can be favoured

  • negative frequency dependent selection

  • heterozygote advantage

negative frequency dependent selection

• rarer form is always favoured

• tends to maintain allele frequencies in population

• ex: right- or left-mouthed fish, selection mediated by learned predator-avoidance behaviour

• ex: self-pollination by flowers, mediated by self-incompatibility and rare mate type advantage

heterozygote advantage

• heterozygous individuals have trait advantageous to either homozygous individual

• both alleles are maintained in population

• ex: sickle cell anemia, heterozygote does not have sickle cell anemia (or only mild) but is resistant to malaria

model of relative fitness for heterozygote advantage

w_AA = 1-t

w_Aa = 1

w_aa = 1-s

deleterious mutation-selection interaction

mutations are constantly reintroduced as selection removes them from the gene pool, resulting in a dynamic equilibrium with a non-zero standing quantity of mutation (genetic disease is maintained)

selective sweeps

• SNPs linked with advantageous novel variant are dragged along and also become more common, reducing genetic variance in that region

• can reduce diversity in genomic regions, especially those with low rates of recombination

• leaves behind a trace of past events that have influenced the genome

complex / quantitative traits

population variated is controlled by many genes and sometimes also environmental factors

quantitative genetic variation

• distribution of phenotypic states is often bell-shaped (bell curve)

• important for understanding the evolution of common diseases, crop improvement, and the rate of evolutionary change

continuous complex traits

traits that display a continuous range of variation rather than distinct categories

threshold complex traits

• trait is either displayed or not

• trait is only displayed after surpassing a certain threshold

• showing of trait depends on environmental liability and genetic liability

variance

measurement of how far each number in a data set is from the mean, and thus from every other number in the set

multifactorial model for complex trait variation

phenotypic variance can be decomposed into variance that is attributable to environmental factors and genetic factors