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Transcript
Mechanisms of Evolution Population Genetics & Evolution • Darwin didn’t know about genes. • Genetic information used to explain variation within a population. • Population genetics developed – studies the behavior of genes in a population. Populations Evolve – Not Individuals • Genes determine individual’s features. • If feature – phenotype – is poorly adapted, individual won’t survive to reproduce. • Organism can’t evolve new phenotype in response. • Natural selection acts on ranges of phenotypes. • Population = consists of all members of a species that live in an area. Gene Pool • Gene pool = the sum of all the alleles of a population. Populations Evolve – Not Individuals • Evolution occurs as population’s genes and frequencies change over time. • Allelic frequency = percentage of any specific allele in the gene pool. • Genetic equilibrium = a population in which frequency of alleles remains the same over generations. • A population in genetic equilibrium is not evolving. Evolutionary Mechanisms • Allele frequency = how often an allele appears in a population. – Remains relatively stable, particularly if the population is large and well-adapted to its environment. – Many factors can change allele frequency: • 1. Mutation • 2. Migration • 3. Random change • 4. Artificial or Natural Selection • 5. Non-random mating of individuals Factors Affecting Allele Frequency 1.) Mutation : • Results in the introduction of a new allele and causes an immediate, small shift in equilibrium. • Affects populations of organisms that produce a large number of offspring in a short time period (ex. Bacteria) more so than organisms that produce a small number of offspring in a relatively long period of time (ex. Mammals).. Factors Affecting Allele Frequency 2.) Migration: • The movement of organisms into or out of the population and therefore the gene pool. • If organisms entering or leaving the population have a genotype different from that of the rest of the population, a change in equilibrium occurs. • Gene flow = the movement of genes into or out of a population because of migration. Factors Affecting Allele Frequency 3.) Random change in allele frequencies may result in a population with distinct characteristics. • Usually occurs in small populations. • Genetic drift = Changes in the gene pool of a small population due to chance. – Can occur if a storm or attack by predators kills a large proportion of the population and the survivors may have a different allele frequency than that found in the original group. Genetic Drift = Changes in the gene pool of a small population due to chance Factors Affecting Allele Frequency • 4.) Artificial and Natural Selection: • Determines which individuals in a population will reproduce and pass on their genes. • Individuals with characteristics that helped them survive in their environment could produce more offspring than individuals that did not have such characteristics. Factors Affecting Allele Frequency 5.) Non random mating of individuals in a population. • If certain individuals in a population show a preference for mating with individuals that have a particular phenotype, the mating pattern ceases to be random. Sexual Selection Natural Selection Acts on Variations • Some variations increase or decrease an organism’s chance of survival. • Variations can be inherited and controlled by alleles. • Allelic frequencies in a population’s gene pool will change over generations due to natural selection of variations. Natural Selection Acts on Variations • Three types of natural selection that act on variation: • Stabilizing selection • Directional selection • Disruptive selection Selection Animation http://bcs.whfreeman.com/thelifewire/content /chp23/2302001.html Stabilizing Selection • Natural selection that favors average individuals in a population. • Ex. Spiders of average size favored. – Larger spiders easier to see and be caught by predators. – Small spiders find it difficult to find food. – Average are more likely to survive – have a “selected advantage” Directional Selection • Occurs when natural selection favors one of the extreme variations of a trait. • Ex. Woodpeckers with long beaks are able to reach insects that live deep in the tree bark. Long-beaked woodpeckers have selected advantage over woodpeckers with short or average beak size. Disruptive Selection • Individuals with either extreme of a trait’s variation are selected for. • Ex. Clams with shell colors ranging from light (white) to dark brown. On light-colored rocks – white shells blend in. On darkcolored rocks – brown shells blend in. Tan colored shells stick out and are eaten by predators.