Evolution of Populations

• Population genetics is the study of how gene frequencies in a population change over time. – The physical representation of genes (phenotypes)

eventually gave rise to the Modern Synthesis which is a comprehensive theory of how evolution effected the change.• population - a local group capable of interbreeding • gene pool - total aggregate of genes

Hardy-Weinberg• The Hardy-Weinberg theorem (p2+2pq+q2 = 1) describes gene frequencies in a

stable population that are well adapted to the environment. It assumes the following:– Extremely large populations – No gene flow between populations – Random mating – No natural selection

• As you can see the chance for a these conditions leading to a stable population is entirely improbable. Any departure in these conditions leads to evolution.

Why it doesn’t work

Agents of evolution• Mutation

– point - changes in a single base • sickle cell disease - heterozygous individual has an advantage in Malaria prone areas

– duplication • although most duplications are harmful some duplications through transposable

elements have no adverse effects on an organism. • over time the duplications lead to an expanded genome

– mutation rate - organisms with a high mutation rate (viruses and bacteria) evolve faster • Seen in drug resistance

Agents of evolution• Sexual recombination

– this single event is more important to adaptation leading to evolution than any other single process

– makes natural selection possible

Or

FON

Agents of evolution• Genetic Drift - deviations from expected (Hardy-Weinberg)

frequencies– bottle-neck effect

• caused by a disaster suddenly altering the environmental pressures causing the genes of a few survivors to dominate

– founder effect • when a few individuals move and start a new population allowing their genes

to dominate

– gene flow • changes to a gene pool as individuals move out and into a breeding population • tends to reduce differences between populations

Stabilization of a gene pool• Directional selection

– selection toward a phenotype that is best adapted to the environment – most common type of selection

• Disruptive selection – occurs when environmental factors favor individuals on both extremes

of the phenotypic range – often to facilitate different food sources

Stabilization of a gene pool• Stabilizing selection

– selects against the extremes of phenotypes – humans

• Balancing Selection – occurs when nature allows 2 distinctly different polymorphisms to

exist in relatively equal frequencies (balanced polymorphism) – leads to heterozygous advantage and frequency dependent selection

• heterozygous advantage - heterozygous individuals exhibit less genetic disorders

• frequency dependent selection - frequency of the dominant phenotype becomes less fit over time (prey species)