The Evolution of Populations Chapter 21

Microevolution Evolutionary changes within a population  Changes in allele frequencies in a population over generations Population – all members of a species living in the same area, interbreed, produce fertile offspring Example – industrial melanism and the peppered moth

Genetic Variation Differences among individuals in the composition of the genes Single gene influence (Mendel) or polygenic Phenotype – physical traits, can be inherited or influence by environment.

Sources of Genetic Variation Formation of new alleles  Mutation Altering Gene number or position  Chromosomal changes – deletion, translocation, inversion and duplication Rapid reproduction – prokaryotes Sexual Reproduction  Crossing over, independent assortment and random fertilization

Hardy-Weinberg principle p2 +2pq + q2 Used to calculate the genotype and gene frequencies of a population States: equilibrium of allele frequencies in a gene pool will remain in effect in each generation of sexually reproducing populations as long as: 1. No mutations 2. No gene flow 3. Random mating 4. No genetic drift 5. No selection

Hardy-Weinberg principle Tells us what factors cause evolution  The 5 conditions are hardly ever met  Allele frequencies do change from one generation to another  Evolution can be detected by seeing any deviation from a Hardy-Weinberg equilibrium  Practice problems p.406

Causes of microevolution Opposite of HWP Genetic mutations – cause for multiple alleles, can be adaptive and include favorable phenotypes Nonrandom mating – inbreeding or breeding between relatives, decreases the heterozygote

Causes of microevolution that alter allele frequency directly Genetic Drift – change in allele frequencies due to chance  Bottleneck effect – natural disaster, reduce in population prevents the majority of genotypes from participating in the production of the next generation  Founder effect – rare alleles occur at a higher frequency in a population isolated from a general population ex. amish

Microevolution Gene Flow – transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.

Natural selection Not random – adaptive evolution Most traits are polygenic, see bell curve in allele frequency 3 major types of selection -  Directional – extreme phenotype favored Resistance to antibiotics and pesticides, malaria  Stabilizing – intermediate phenotype is favored Birth weight survival, sickle cell trait  Disruptive – 2 or more extreme phenotypes are favored

Sexual selection Natural selection in which individuals with certain inherited characteristics are more likely than others to obtain mates.  Sexual dimorphism – differences in males and females (i.e. size, color, …)

Balancing selection Natural selection maintains two or more forms in a population.  Heterozygote advantage – Malaria and sickle cell anemia  Frequency-dependent selection – scale eating fish. Right and left mouthed

Why doesn’t Natural Selection create perfect organisms? Selection can act only on existing variations Evolution is limited by historical constraints Adaptations are often compromises Chance, natural selection and the environment interact