The Evolution of Populations Chapter 21 Microevolution n

The Evolution of Populations Chapter 21

Microevolution n Evolutionary changes within a population q n n 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 n Differences among individuals in the composition of the genes n Single gene influence (Mendel) or polygenic n Phenotype – physical traits, can be inherited or influence by environment.

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

Hardy-Weinberg principle p 2 +2 pq + q 2 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: n n n 1. 2. 3. 4. 5. No mutations No gene flow Random mating No genetic drift No selection

Hardy-Weinberg principle n Tells us what factors cause evolution q q 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 n 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 q Bottleneck effect – natural disaster, reduce in population prevents the majority of genotypes from participating in the production of the next generation q Founder effect – rare alleles occur at a higher frequency in a population isolated from a general population ex. amish

Microevolution n 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 - n n n Directional – extreme phenotype favored q n Stabilizing – intermediate phenotype is favored q n q Resistance to antibiotics and pesticides, malaria Birth weight survival, sickle cell trait Disruptive – 2 or more extreme phenotypes are favored

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

Balancing selection n Natural selection maintains two or more forms in a population. q q 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? n n Selection can act only on existing variations Evolution is limited by historical constraints Adaptations are often compromises Chance, natural selection and the environment interact