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Sources/Mechanisms of Evolutionary Change 1. Natural Selection 2. Mutations 3. Gene Flow (migration) 4. Genetic Drift (bottleneck & founders effect) All of these mechanisms can cause changes in the frequencies of genes in populations Natural selection and genetic drift cannot operate unless there is genetic variation (mutations, gene flow and sexual reproduction) If the population of beetles were 100% green, selection and drift would not have any effect because their genetic make-up could not change. Imagine that green beetles are easier for birds to spot (and hence, eat). Brown beetles are a little more likely to survive to produce offspring. They pass their genes for brown coloration on to their offspring. So in the next generation, brown beetles are more common than in the previous generation. A mutation could cause parents with genes for bright green coloration to have offspring with a gene for brown coloration. That would make genes for brown coloration more frequent in the population than they were before the mutation. Exchange of genes due to mixing of populations Some individuals from a population of brown beetles might have joined a population of green beetles. That would make genes for brown coloration more frequent in the green beetle population than they were before the brown beetles migrated into it. Random change in allele frequencies Some individuals may, by chance, leave more descendants Next generation “lucky” genes not necessarily better adapted Unlike natural selection b/c it is random & doesn’t work to produce adaptations Occurs when populations size is reduced for at least 1 generation Can reduce populations genetic variation a lot May not be able to adapt to new selection pressures Northern elephant seals have reduced genetic variation Hunting reduced their population size in 1890’s to as few as 20 individuals population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted when a new colony is started by a few members of the original population small population size means that the new colony may have reduced genetic variation from the original population Founding individuals may carry alleles that differ in frequency from original population G. equilibrium- allele frequencies remain same; not evolving (this is hypothetical) Hardy-Weinberg Principle- allele frequencies of population should remain constant unless : 1. non-random mating occurs (sexual selection- choose mate with certain trait) (or) 2. small population size (or) 3. Immigration or emigration (or) 4. Mutations (or) 5. Natural selection (if different genotypes have different fitness) One or more of these usually hold true for any population therefore, evolution will occur!