Population Genetics & Evolution

Population Genetics The study of evolution from a genetic point of view

Causes of Variation Mutations Recombination Random fusion of gametes

Allelic Frequency Determined by dividing the number of a certain allele by the number of alleles of all types in the population. Consider a population of ten individuals and a given locus with two possible alleles, A and a. Suppose that the genotypes of the individuals are as follows:genotypes –AA, Aa, AA, aa, Aa, AA, AA, Aa, Aa, and AA Then the allele frequencies of allele A and allele a are: – so there is a 70% chance of the population getting that allele – and there is a 30% chance of the population getting this allele

Gene Pool The total genetic information available in a population.

Hardy-Weinberg Principle Godfrey Hardy( ) = British mathematician Wilhelm Weinberg(1877 – 1947) = German Physician

The H-W Principle Allele frequencies in a population tend to remain the same from generation to generation unless acted on by outside influences. Genetic Equilibrium Formula p + q = 1 p 2 + 2pq + q 2 = 1

What do the letters mean? p = dominant allele q = recessive allele p 2 =homozygous dominant q 2 = homozygous recessive 2pq = heterozygous p + q = 1 p 2 + 2pq + q 2 = 1

H-W Based on Assumptions No net mutations occur Individuals don’t enter or leave the population The population is large Individuals mate randomly Selection does not occur

Disruptions to Genetic Equilibrium Mutations occur constantly at a slow rate Some mutations will be bad and some will be good for the population

Disruptions to Genetic Equilibrium Individuals and their genes frequently move in and out of populations Sometimes males will leave when they mature to form their own group

Disruptions to Genetic Equilibrium Genetic Drift – The changes of allele frequencies in small populations due to chance Small populations can have genetic issues quickly

Disruptions to Genetic Equilibrium Nonrandom mating Most organisms choose their mates Females often pick “more fit” males to breed with.

Disruptions to Genetic Equilibrium Natural Selection is an ongoing process in nature Traits are being selected for and against all the time.

Species Evolution A species is a population of organisms that can successfully interbreed but cannot breed with other groups

Forming a New Species Geographic Isolation - when the habitat of a species is divided over a long period of time, two new species can arise.

Forming a New Species Reproductive Isolation - Barriers to successful breeding between population groups in the same area

Reproductive Isolation Prezygotic - before fertilization Chromosomal problems Seasonal mating differences

Reproductive Isolation Postzygotic = after fertilization Creation of hybrids that will be born infertile.

Rates of Speciation Gradualism – idea that species originate through a gradual buildup of new adaptations. Punctuated Equilibrium = idea that speciation occurs quickly in rapid bursts, with long periods of stability in between.