Evolutionary Change in Populations: Population Genetics, Selection & Drift

region biosphere landscape ecosystem community interaction population individual The characteristics of populations of individuals change through time

By the end of class today, you should be able to: describe the basic elements required for evolution to occur use the Hardy-Weinberg equation to predict allele and genotype frequencies explain some of the major factors that can change allele frequencies in populations differentiate between directional, stabilizing and disruptive selection distinguish between sexual selection and natural selection

“Biological evolution... is change in the properties of populations of organisms that transcend the lifetime of a single individual.” - Douglas Futuyma, in Evolutionary Biology

Heritability of traits from parents to offspring Naturally occurring variation in traits within species Limited resources (number of offspring exceeds carrying capacity of environment) Differential survival / higher reproductive success of organisms with a particular trait Evolution, Selection

Darwin’s theory of evolution via natural selection faced one major scientific challenge: Mechanism and physical basis of inheritance was unknown Blending theory prevailed at end of 19 th century

Mendelian Genetics

Mendel’s experiments

Parental generation (P) First filial generation (F1) Second filial generation (F2)

Parental generation (P) First filial generation (F1) Second filial generation (F2)

Mendel’s Laws Law of segregation – two hereditary factors (two genes) from parent get separated so that each egg/sperm only gets one copy

Mendel’s Laws Law of independent assortment – heredity factors (genes) for each trait are transmitted independently to the gametes

Darwin + Mendel = Evolutionary Synthesis Mendel published his work in 1866 – largely ignored until its rediscovery in 1900 Application of Mendelian genetics to Darwin’s theory of evolution by natural selection laid the foundation of modern population genetics Evolution = change in allelic frequencies in a population

Hardy-Weinberg Equilibrium Allele frequencies in a population will remain constant from generation to generation unless disturbed by specific factors

Hardy-Weinberg Equilibrium At equilibrium, frequencies of genotypes will follow a predictable ratio: p + q = 1 for diploid genotype: (p + q) 2 = 1 p 2 + 2pq + q 2 = 1 p = frequency of allele 1 q = frequency of allele 2

Hardy-Weinberg Equilibrium is disturbed by any of the following: non-random mating mutation small effective population size immigration selection (unequal fitness of different genotypes)

Hardy-Weinberg Equilibrium The expected ratio of genotype frequencies at equilibrium, p 2 + 2pq + q 2 = 1, makes it possible to estimate genotype frequencies in natural populations - This is significant, as usually only the frequency of the homozygous recessive phenotype is easily observed and documented

Hardy-Weinberg Equilibrium – sample problem Albinism is a rare genetically determined trait that is only expressed in the phenotype of homozygous recessive individuals (aa) Frequency of albinism (aa genotype) in North American population is about 1 / 20,000 ( = ) What are the frequencies of the homozygous dominant (AA) and heterozygous (Aa) genotypes in the population? p 2 + 2pq + q 2 = 1

How does selection change genotype / phenotype frequencies in natural populations? Three main patterns are observed: – Stabilizing selection – Directional selection – Disruptive selection

Stabilizing Selection Selection acts against extreme phenotypes, organisms with average phenotype have the greatest fitness The graph is based on infants born in London from 1935 to From Karn and Penrose (1951).

Directional Selection One extreme phenotype has fitness advantage over more average phenotypes Fishing selects for reduced size in salmon populations

Disruptive Selection Two or more extreme phenotypes have fitness advantages over average phenotype e.g., common goldfields of CA – serpentine specialized races

Phenotype is determined by multiple factors genetic makeup of organism (genotype) environmentally influenced “plastic” development of organism (environment) random developmental quirks (noise)

The effectiveness of selection on a particular phenotype depends on the extent to which the phenotype is controlled by the genotype selection can only operate on the heritable component of the phenotype

Not all evolution occurs through natural selection Genetic Drift Sexual Selection

Genetic Drift Sampling error leads to the fixation / elimination of certain alleles occurs when population size is small due to factors such as: – founder effects (small peripheral population becomes isolated) – genetic bottleneck (population size crashes to very low level, as a consequence genetic diversity is lost)

Sexual Selection also proposed by Darwin, but largely criticized / ignored by Victorian scientists a mechanism in which female preference for mates with particular showy traits leads to higher fitness of males with those traits sexually selected traits are often NOT advantageous to the males in terms of natural selection