Evolution of Populations Microevolution

Variations in Gene Pools Microevolution: evolution on the smallest scale – generation to generation change in the frequencies of alleles in a population Gene Pool: consists of all the genes that are present in a population Relative Frequency: The number of times an allele occurs in a gene pool compared with the number of times other alleles for the same gene appear

Relative Frequencies of Alleles Sample Population Frequency of Alleles allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown

Sources of Genetic Variation Mutations: a change in the genetic sequence. Some may produce changes in the phenotype that result in better fitness. Gene Shuffling: which results from the shuffling of gametes and sexual reproduction; produces many different combinations of genes.

Single Gene & Polygenic Traits A single gene trait is controlled by 1 gene with 2 alleles. Ex. Widow’s peak. Polygenic Traits: Each gene has two or more alleles which results in many possible genotypes and phenotypes. Frequency of phenotype Widow’s peak No widow’s peak Frequency Phenotype Height

Natural Selection of Single-gene Traits Can lead to change in allele frequencies and thus evolution. Initial Pop. Gen. 10 Gen. 20 Gen. 30 80% Brown 80%Brown 70% Brown 40% brown 10 % red 0% red 10% black 20% black 30% black 60% black

Genetic Drift The random change in allele frequency. In a small population, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become more common.

Genetic Drift Founder’s Effect: Allele frequencies change Sample of Original Population Descendants Founding Population A Founding Population B Founder’s Effect: Allele frequencies change as a result of the migration of a small subgroup.

Evolution Versus Genetic Equilibrium Hardy-Weinberg Principle (1908) states that allele frequency in a population will remain constant unless one or more factors cause those frequencies to change. Genetic Equilibrium: allele frequencies remain the same.

Five Conditions of Hardy-Weinberg Random mating – no sexual selection Large population – no genetic drift No migrations – immigration or emigration No mutations No natural selection STOP

Natural Selection on Polygenic traits Can affect the distribution of phenotypes in three ways: direction, stabilizing, or disruptive.

Graph of Directional Selection Key Directional Selection Low mortality, high fitness High mortality, low fitness Food becomes scarce. The environment favors one extreme

Graph of Stabilizing Selection Key The average Organism is More fit Low mortality, high fitness High mortality, low fitness Selection against both extremes keep curve narrow and in same place. Percentage of Population Birth Weight

Graph of Disruptive Selection Largest and smallest seeds become more common. Key Low mortality, high fitness Population splits into two subgroups specializing in different seeds. Number of Birds in Population Number of Birds in Population High mortality, low fitness Beak Size Beak Size Organisms with either of the two Extremes are better fit.