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Speciation and Selection Mechanisms of Evolution (don’t need to write down) • Darwin’s theory of natural selection explains how organisms adapt to their environments and how variations can give rise to adaptations within species. • Studies from population genetics and molecular biology have led to the development of evolutionary theory. What is Speciation? It is the evolutionary process by which reproductively isolated populations evolve to become distinct species. How does it happen? Populations of the same species become isolated Geographic Reproductive Geographic Isolation Members of a population may become isolated from one another by geographic boundaries like mountains, rivers or even oceans Reproductive Isolation When the two populations can no longer produce viable (live), fertile offspring (can produce own offspring that can reproduce) Reproductive Isolation Allopatric Speciation • Occurs when a physical barrier (mountains, islands, lava flows, rivers) divides one population into two or more populations Sympatric Speciation • Involves species who evolve into a new species without a physical barrier. • The ancestor species and the new species live side by side during the speciation process. Rates of Speciation • Gradualism: evolution proceeds in small gradual steps. • Theory of Punctuated Equilibrium: Rapid spurts of genetic change cause species to change quickly, these periods punctuate much longer periods when the species exhibits little change. Factors that lead to Evolution Genetic Drift Gene Flow (movement of genes) Mutations (in DNA) Sexual Selection Artificial Selection Genetic Drift Genetic drift is any change in the allelic frequencies in a population that is due to chance Causes a loss of genetic diversity. It is most common in small populations. Founder Effect A small sample of a population settles in a location separated from the rest of the population Alleles that were uncommon in the original population might become common the new population Founder Effect Leads to: reduced genetic variation a non-random sample of the genes in the original population Bottleneck It occurs when an event drastically reduces population size and then the population rebounds Reduced genetic diversity, since less diversity in gene pool The bottleneck effect is genetic drift that occurs after a bottleneck event Gene Flow A population in genetic equilibrium experiences no gene flow No individuals leaving or entering Results in low genetic variation Nonrandom Mating (don’t need to write down) Organisms usually random mate with individuals in a population Nonrandom mating promotes inbreeding and could lead to a change in allelic frequencies favoring individuals that are homozygous for particular traits Mutation Though many mutations cause harm or are lethal, occasionally a mutation provides an advantage to an organism That mutation may be passed on to future generations Natural Selection Natural selection selects individuals that are best adapted for survival and reproduction Those organisms pass on their traits to offspring Acts on an organism’s phenotype and changes allelic frequencies #1 Directional selection favors phenotypes at one extreme. Types of Selection #2 Stabilizing selection favors the intermediate phenotype. Types of Selection #3 Disruptive selection favors both extreme phenotypes. Types of Selection Artificial Selection The process in which humans select for or against particular features in organisms Example Sexual Selection Members of one sex choose mates with particular features Only those with certain traits succeed Examples