Shaping Evolutionary Theory Chapter 15 Section 15.3 Shaping Evolutionary Theory

Shaping Evolutionary Theory The theory of evolution continues to be refined as scientists learn new information Darwin’s theory of natural selection remains a central theme in evolution Scientists now know that natural selection is not the only mechanism of evolution Population genetics and molecular biology have led to the development of evolutionary theory,

Mechanisms of Evolution ______________as the raw material Population genetics _______________________________: In the absence of forces that cause change, the ____________ frequency remains the same and ______________ doesn’t occur When allelic frequencies remain constant, a population is in genetic equilibrium. Figure 15.14 pg. 431(owls)

Hardy-Weinburg Principle Five conditions The population is very large There is no immigration or emmigration Mating is random Mutations do not occur Natural selection does not occur Table 15.3 pg.432

Genetic Drift Any change in the allelic frequencies in a population that is due to chance In large populations, enough alleles “drift” to ensure that the allelic frequency of the entire population remains relatively constant from one generation to the next In smaller populations, the effects of genetic drift becomes more pronounced and the chance of losing an allele becomes greater

Founder Effect An extreme example of genetic drift Can occur when a small sample of a population settles in a location separated from the rest of the population Evident in the Amish and Mennonite communities in the United States Old Order Amish have a high frequency of six-finger dwarfism

Bottleneck An extreme example of genetic drift Occurs when a population declines to a very low number and then rebounds Reduced diversity Cheetahs in Africa: appear inbred which decreases fertility – may be a factor in the potential extinction of this endangered species

Gene Flow The random movement of individuals between populations, or migration, increases genetic variation within a population and reduces differences between populations

Nonrandom Mating Rarely is mating completely random in a population Usually organisms mate with individuals in close proximity – promotes inbreeding and could cause a change in allele proportions favoring individuals that are homozygous for particular traits

Mutation A random change in genetic material Many mutations cause harm or are lethal Occasionally a mutation provides an advantage to an organism Mutations provide the raw material upon which natural selection works How does mutation violate the Hardy-Weinburg Principle?

Natural Selection Hardy-Weinburg Principle requires that all individuals in a population be equally adapted to their environment and contribute equally to the next generation Natural selection acts to select the individuals that are best adapted for survival and reproduction acts on an organism’s phenotype and changes allelic frequencies

Natural Selection Figure 15.16 page 434 Three main ways natural selection alters phenotypes Stabilizing selection: selection against both extremes Most common – little variation Directional selection: selection against one extreme Extreme variation – peppered moths (industrial melanism) Disruptive selection: selection against the mean Splits population into two groups A fourth type: sexual selection – ability to attract a mate - occurs in populations where males and females differ drastically in appearance (peacocks)