Chapter 23 The Evolution of Populations

Population Genetics u The study of genetic variation in populations. u Represents the reconciliation of Mendelism and Darwinism.

Population Genetics u Use population genetics as the means to track and study evolution. u Looks at the genetic basis of variation and natural selection.

Sources of Variation u Sexual Reproduction u Random Assortment of Chromosomes u Random Fertilization u Crossing Over u Mutation

Mutations u Inherited changes in a gene. u Rates low in most cases due to DNA repair etc.

Population u A localized group of individuals of the same species.

Species u A group of similar organisms. u A group of populations that could interbreed.

Gene Pool u The total collective genes in a population. u If evolution is occurring, then changes must occur in the gene pool of the population over time.

Microevolution u Changes in the relative frequencies of alleles in the gene pool.

Hardy-Weinberg Theorem u A mathematical way to measure evolution. u Population is evolving if the allele frequency is changing (favoring one allele)

Basic Equation u p + q = 1 u p = % dominant allele u q = % recessive allele

Expanded Equation u p + q = 1 u (p + q) 2 = (1) 2 u p 2 + 2pq + q 2 = 1

Genotypes u p 2 = Homozygous Dominants 2pq = Heterozygous q 2 = Homozygous Recessives

Importance of Hardy-Weinberg u Yardstick to measure rates of evolution. u Predicts that gene frequencies should NOT change over time as long as the HW assumptions hold (no evolution should occur). u Way to calculate gene frequencies through time.

Example u What is the frequency of the PKU allele? u PKU is expressed only if the individual is homozygous recessive (aa).

PKU Frequency u PKU is found at the rate of 1/10,000 births. u PKU = aa = q 2 q 2 =.0001 q =.01

Dominant Allele u p + q = 1 p = 1- q p = p =.99

Expanded Equation u p 2 + 2pq + q 2 = 1 (.99) 2 + 2(.99x.01) + (.01) 2 = = 1

Final Results u Normals (AA) = 98.01% u Carriers (Aa) = 1.98% u PKU (aa) =.01%

Hardy-Weinberg Assumptions 1. Large Population 2. Isolation 3. No Net Mutations 4. Random Mating 5. No Natural Selection

If H-W assumptions hold true: u The gene frequencies will not change over time. u Evolution will not occur. u But, how likely will natural populations hold to the H-W assumptions?

Microevolution u Caused by violations of the 5 H-W assumptions.

Causes of Microevolution 1. Genetic Drift 2. Gene Flow 3. Mutations 4. Nonrandom Mating 5. Natural Selection

Genetic Drift u Changes in the gene pool of a small population by chance. u Types: u 1. Bottleneck Effect u 2. Founder's Effect

By Chance

Bottleneck Effect u Loss of most of the population by disasters. u Surviving population may have a different gene pool than the original population.

Result u Some alleles lost. u Other alleles are over- represented. u Genetic variation usually lost.

Importance u Reduction of population size may reduce gene pool for evolution to work with. u Ex: Cheetahs

Founder's Effect u Genetic drift in a new colony that separates from a parent population. u Ex: Old-Order Amish

Result u Genetic variation reduced. u Some alleles increase in frequency while others are lost (as compared to the parent population).

Importance u Very common in islands and other groups that don't interbreed.

Gene Flow u Movement of genes in/out of a population. u Ex: u Immigration u Emigration

Result u Changes in gene frequencies within a population. u Immigration often brings new alleles into populations increasing genetic diversity.

Result - Mutations u May change gene frequencies (small population). u Source of new alleles for selection. u Often lost by genetic drift.

Nonrandom Mating u Failure to choose mates at random from the population.

Causes u Inbreeding within the same “neighborhood”. u Assortative mating (like with like).

Result u Increases the number of homozygous loci. u Does not in itself alter the overall gene frequencies in the population.

Natural Selection u Differential success in survival and reproduction. u Result - Shifts in gene frequencies.

Fitness - Darwinian u The relative contribution an individual makes to the gene pool of the next generation.

u End of part 1

Rate of Selection u Differs between dominant and recessive alleles. u Selection pressure by the environment.

Modes of Natural Selection 1. Stabilizing 2. Directional 3. Diversifying 4. Sexual

Homework u Read – Chapter 23, 24, 26 u Lab – Population Genetics u Discussion Board – Fri. 2/25 u Chapter 23 – Fri. 2/25 u Chapter 26 – Mon. 2/28

Stabilizing u Selection toward the average and against the extremes. u Ex: birth weight in humans

Directional Selection u Selection toward one extreme. u Ex: running speeds in race animals. u Ex. Galapagos Finch beak size and food source.

Diversifying u Selection toward both extremes and against the norm. u Ex: bill size in birds

Comment u Diversifying Selection - can split a species into several new species if it continues for a long enough period of time and the populations don’t interbreed.

Sexual Mate selection u May not be adaptive to the environment, but increases reproduction success of the individual. u This is a VERY important selection type for species.

Result u Sexual dimorphism. u Secondary sexual features for attracting mates.

Comments u Females may drive sexual selection and dimorphism since they often "choose" the mate.

Preserving Genetic Variation 1. Diploidy - preserves recessives as heterozygotes. 2. Balanced Polymorphisms - preservation of diversity by natural selection.

Example u Heterozygote Advantage - When the heterozygote or hybrid survives better than the homozygotes. Also called Hybrid vigor.

Result u Can't bred "true“ and the diversity of the population is maintained. u Ex – Sickle Cell Anemia

Comment u Population geneticists believe that ALL genes that persist in a population must have had a selective advantage at one time. u Ex – Sickle Cell and Malaria, Tay-Sachs and Tuberculosis

Question u Does evolution result in perfect organisms?

Answer - No 1. Historical Constraints 2. Compromises 3. Non-adaptive Evolution (chance) 4. Available variations – most come from using a current gene in a new way.

Summary u Know the difference between a species and a population. u Know that the unit of evolution is the population and not the individual.

Summary u Know the H-W equations and how to use them in calculations. u Know the H-W assumptions and what happens if each is violated.

Summary u Know the various modes of natural selection. u Identify various means to introduce genetic variation into populations.