The Evolution of Populations: Models of Change Chapter 23

What you need to know! How mutation and sexual reproduction each produce genetic variation. How natural selection, genetic drift, and gene flow contribute to changing allele frequencies.

Changing Allele Freq. Gene Flow: migration of fertile individuals into or out of a population. This tends to reduce variation between separate populations

Natural Selection The only mechanism that consistently causes adaptive evolution Fitness represents the reproductive success of an individual Higher fitness means more babies Natural selection acts directly on the phenotypes of individuals (indirectly on genotypes)

Genetic Variation Genetic variation within a population are attributable to two sources: Mutation: the ONLY source of new genes and alleles. point mutations change one base in a gene Chromosomal mutations are almost certainly harmful *but not always Sexual Reproduction: crossing over, independent assortment (remember the math 2n), fertilization (2n x 2n)

Changing Allele Freq. Natural selection: alleles passed in different proportions based on adaptations and fitness Genetic drift: random, nonadaptive change in allelic frequencies in small populations. Two models include: Founder effect: small piece of a larger population is isolated in a new region (different gene pool) Bottleneck effect: most of the individuals in a population die, leaving a small sample of the original gene pool

Directional Selection A single extreme adaptation is good, shifting the curve in that direction. Example: peppered moths

Directional Selection

Stabilizing Selection Extreme phenotypes are bad adaptations Example: primate birth weight Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness Selection against both extremes keep curve narrow and in same place. Percentage of Population Birth Weight

Stabilizing Selection

Disruptive Selection Selection against the common phenotype Selection for both extremes Example: weeds are either very tall (difficult to uproot) or very short (survive lawn mowers) 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

Sexual Selection Selection for sexual phenotypes: Male competition: fittest male mates most (antlers, horns, body size) Female choice: attractiveness and mating behavior Sexual dimorphism: diifferent appearance of males and females in certain species

Preservation of Variety Diploidy: two alleles for every gene allows for dominant and recessive allele combinations Heterozygote advantage: Certain genes provide advantages for being heterzygous (i.e. sickle-cell anemia heterzygotes are resistant to malaria, mutts)

Imperfection Natural selection does not fashion the perfect organism because: Selection only edits existing variation Evolution is limited by historical constraints Adaptations are often compromises Chance, natural selection, and the environment interact