Genetic Diversity

Population genetics Both genetic variation and environmental factors are causes of evolution and diversity of organisms

Vocabulary Gene pool Relative frequency Single-gene trait Polygenic trait Genetic drift Genetic equilibrium Founder effect

Evidence for evolution Homologous body structures: What similarities in the limbs suggest they developed from the same basic structure? How is the form of each limb adapted for a specific type of movement?

Vestigial organs: Organs that are vestiges (traces) of organs. Legs, tails or organs such as the appendix.

Summary of Darwin’s theory Individual organisms differ and some variation is inheritable. Organisms produce more offspring than can survive, and many that do survive do not reproduce. Competition for limited resources Survival of the fittest (adaptation) Diverse species evolved from common ancestors.

What is genetic variation? Genetic variation: many genes have two forms, or alleles. An individual can also be heterozygous for a gene. This creates a large amount of variation. The gene for ABO blood type has 3 major alleles; How many ABO genotypes are available?

Gene pools Within a population, individuals breed, they share a common group of genes. The relative frequency of an allele is the number of times that the allele occurs in a gene pool. See Figure 16-2: frequency of alleles in the mouse population. What happens to the relative freq. of the b allele if the B allele decreases?

Sources of genetic variation Mutations Gene shuffling: Independent assortment during meiosis. * The 23 pairs of chromosomes found in humans can produce 8.4 million different combinations of genes.

Reshuffling of genes Sexual reproduction can produce many different phenotypes The relative frequency of alleles does not change. Like a deck of cards: shuffling rearranges the alleles, but the number of alleles remains the same.

Single gene and polygenic traits Single gene trait: either dominant or recessive. -Widow’s peak, tongue roll, attached earlobes. -represented by a bar graph.

Polygenic trait: two or more genes that control a trait. This leads to many possible genotypes and phenotypes. - Human height - normal distribution is a bell shaped curve

Natural selection acts on phenotype Genes are never directly acted upon during natural selection, why? Evolution is any change over time in the relative frequencies of alleles in a population. Populations evolve, not single organisms. See figure 16-5; effect of color mutations on lizard survival.

Review Explain whether fur color in the mice shown in figure 16-2 is a single-gene or polygenic trait. Draw two graphs: one to show the frequency of height in our class and another to show tongue rolling. Explain why they are different.

Three distributions of phenotypes as a result of natural selection Directional selection: occurs when one species has a higher fitness than individuals in the middle or end Stabilizing selection: individuals near the center of the curve have higher fitness. Disruptive selection: individuals at the upper and lower ends of the curve have higher fitness than the middle.

Genetic Drift In a small population an allele can become more or less common simply by chance, rather than because it has a positive or negative effect on fitness. Over time, a series of chance occurrences of this type can lead to an allele becoming more common in a population.

Founder Effect Situation in which allele frequencies change as a result of the migration of small subgroups of a population. Pitcaim Island in the South Pacific- limited genetic variability because it was founded by a small group of pirates in the late 1700’s. Population is geographically isolated, few new genes entered the gene pool creating less genetic variation.

Hardy-Weinberg & genetic equilibrium Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors causes those frequencies to change. Constant allele frequency is genetic equilibrium If the allele frequency does not change, evolution does not occur.

Five conditions are required for genetic equilibrium 1. There must be random mating 2. The population must be very large 3. There can be no movement in or out of the population 4. No mutations 5. No natural selection

Hardy-Weinberg equation Dominant allele is represented as p recessive allele is represented as q The sum of allele frequencies must always equal the total population (100%) p+q=1 P2 + 2pq +q2 =1

Speciation: formation of new species As new species evolve populations become reproductively isolated from each other. When two populations cannot interbreed reproductive isolation has occurred. When two populations are able to mate, but courtship rituals are different, behavioral isolation has occurred.

Isolating mechanisms Geographic isolation occurs when two species are separated by geographic barriers. Temporal isolation occurs when reproduction occurs at different times.