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Transcript
15.3 How Does Evolution Occur? What are the Mechanisms of Evolution? Population genetics Hardy-Weinberg principle states that when allelic frequencies remain constant, a population is in genetic equilibrium and evolution does not occur. What happens if changes occur in the allele frequencies of a population? Evolution is always occurring because these conditions can never be met. Genetic Drift A change in the allelic frequencies in a population that is due to chance This is an issue in small populations. If an individual with recessive alleles breeds more than “normal” the frequency of the recessive allele will increase quickly. This does not happen in large populations, there are too many individuals. Bottleneck Occurs when a population declines to a very low number and then rebounds, the problems is a loss of genetic variability. What is Gene Flow? This is when organisms move from one population to another. *Increases genetic variation within a population Nonrandom Mating Since organisms choose their mates, this decreases variation in a population. What is Natural Selection? The process by which organisms with traits well suited to their environment survive and reproduce at a greater rate than less well-adapted organisms in the same environment. Stabilizing selection operates to eliminate extreme expressions of a trait when the average expression leads to higher fitness. Directional selection: selects for one extreme such as tall giraffes. The population average moves to that direction as those individuals reproduce more often. Disruptive selection is a process that splits a population into two groups. This is often the cause of speciation. It selects against the middle, like polar bears and Canadian Brown Bears. What is Sexual Selection? Sexual selection: Organisms choose a mate based on a behavior or appearance but that behavior is often lethal to the organism. Think: peacocks and peahens. Analyze how reproductive isolating mechanisms affect populations. Postzygotic isolation occurs when an organism that has been produced can not reproduce Prezygotic isolation prevents reproduction by making fertilization unlikely. Often mating behavior. Liger Eastern meadowlark and Western meadowlark What is Adaptive Radiation? Can occur in a relatively short time when one species gives rise to many different species in response to the creation of new habitat or some other ecological opportunity Follows large-scale extinction events Geographic Isolation: A population is separated from another population by the landscape (geography). Chromosome number becomes irrelevant in mating since the two populations do not come in contact with each other Coevolution The relationship between two species might be so close that the evolution of one species affects the evolution of the other species. Mutualism Convergent Evolution Unrelated species evolve similar traits even though they live in different parts of the world. Chapter 15 Evolution Large populations with lots of genetic variation! They are least likely to change Small populations where many mutations occur are most likely to evolve.