Modes of selection

Stabilizing Selection

Cumulative effects of disruptive selection can be large Chose high and low oil plants to breed

Disruptive selection Selected only flies with high or low number of bristles to reproduce

Selection Does Not Mean Evolution!!! Evolution is a change in the frequencies of alleles in a population Selection can lead to evolution if the difference in reproductive success is tied to genetic variation Speed of change (evolution) depends on Amount of variation Selection differential (S) Heritability (h2)

Evolutionary response to selection Imagine a lake stocked with bluegills Evolution only results if the difference in size is due to additive genetic differences between individuals If difference in size is due to environment (temperature or crowding  competition) h2 = 0, If only the larger ones reproduce Offspring will still have same size distribution as parents No evolution Selection can be weak or strong – still no evolution

Selection differential measures the strength of selection Fish example. Fig 7.9

Evolutionary response to selection If h2 = 0, then selection will result in larger fish because breeding parents were larger etc. Evolution occurs R = h2 x S

High heritability results in larger change Calculating the evolutionary response to selection R = h2 x S

Key Concepts Evolution and selection are not the same Selection can occur without evolution The magnitude of change depends on: Strength of selection (selection differential) Heritability

Quantitative trait locus (QTL)analysis links traits with genes Can use technique to map loci of interest – map genome (if desired) Mate two purebred fish parents Mate F1 x F1 to get many, many F2 fish Identify large number of short segments of DNA Single nucleotide polymorphisms SNPs Simple sequence repeats Transposable elements

QTL Analysis Markers selected so that alleles at each one are different depending on whether they are inherited from male or female chromosome Examine F2 offspring for these genetic markers and test whether the genotype at particular markers is associated with the phenotypic trait of interest. Markers that have a statistically significant association with the expression of a phenotypic trait are called quantitative trait loci QTLs

QTL analysis of coat color in mice

QTL analysis of coat color in mice

Much of variation in coat color explained by differences in two genes Epistatic interaction Corin only explains a small amount of variation

Expression of Agouti during development influences coat color

Expression of Agouti during development influences coat color Agouti is expressed in mouse embryo 12 days after conception Mice are not making pigment this early, Agouti lays down a “pre-pattern” As embryo develops, certain cells in epithelium differentiate into pigment–producing melanocytes and migrate into hairs, Agouti delays maturation of melanocytes. Expression strongest on ventral side of embryonic mouse, weakest on dorsal side. So, slow-maturing melanocytes on belly fail to reach hairs and cannot color them Dorsal melanocytes turn the hairs brown

Genetic manipulation of dark mice makes them lighter Engineered mainland mice to be homozygous for lite color of Agouti. Offspring showed movement of Agouti furthur towards the dorsal side.

Key Concepts QTL analysis identifies regions of the genome associated with phenotypic variation

Environmental influences on quantitative traits VP = VG + VE Total phenotypic variance in population Variance due to genetic differences Variance due to environmental differences

Phenotypic plasticity A single genotype produces different phenotypes de-pending on the environment

Reaction norm

Genotype x environment interaction First two rows – difference due to phenotypic plasticity; no genetic effects Phenotype determined by G and E Genotype x environment interaction

Phenotypic plasticity in Caenorhabditis elegans Shows genetic basis of Phenotypic Plasticity

Plasticity can evolve Mate biggest body size difference Mate smallest body size difference Random mating Result Plasticity evolved Bigger difference Smaller difference S. Scheiner

Rapid change can lead to mismatch between plastic traits and environment

Key Concepts When plasticity is heritable, the response can evolve