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Transcript
Darwin developed his theory of natural selection before knowledge of genetics Populations evolve, not individuals! An organism is born with its phenotype, and it never changes during its lifetime Evolution occurs as a population’s genes and their frequencies change over time Gene Pool: all of the alleles of the population’s genes Allelic Frequency: percentage of any specific allele in the gene pool A population of organisms in which the allelic frequency remains the same over generations is said to have reached genetic equilibrium Mutations Genetic Drift can occur by environmental factors or chance. most are lethal some may be beneficial change in allelic frequencies due to chance events affects smaller, more isolated populations is responsible for the species Darwin found on the Galapagos Islands Movement of individuals into or out of the population Stabilizing Selection Directional Selection favors the average individual in a population reduces variation of the population nature favors one extreme of the population ex: long necks in giraffes Disruptive Selection nature favors individuals at both extremes of the spectrum complete opposite of stabilizing selection Speciation: evolution of a new species when members of similar populations are no longer able to interbreed and produce fertile offspring Can be caused by: Geographic Isolation when a physical barrier divides a population (islands) over time, the isolated gene pool may become so different that a new species would evolve Reproductive Isolation (two types) genetic material becomes so different that fertilization is impossible behavioral changes (one mates in fall, one in spring) Change in Chromosomal Number caused by nondisjuntion polyploid: an individual with a multiple of the normal set of chromosomes results in immediate reproductive isolation 2 Views Gradualism: idea that speciation occurs slowly and steadily over time Punctuated Equilibrium: speciation occurs quickly (~10,000 years), in rapid bursts, with long periods of genetic equilibrium in between Biologists tend to agree that both occur depending on the circumstances Divergent Evolution Pattern of evolution in which species that once were similar to an ancestral species become increasingly distinct Occurs when populations adapt to very different environments Convergent Evolution Pattern of evolution in which distantly related or seemingly unrelated organisms began to resemble each other. Occurs due to organisms adapting to similar environments Ex: dolphins and sharks or euphorbs and cacti