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Chapter 15.3 Biologist now know that natural selection is not the only mechanism of evolution Inheritable variation may come from any of the following: 1. Genetic drift 2. Gene flow 3. Mutation 4. Non-random mating 5. Natural selection Genetic Drift: when the same genes are combined in different ways to produce different results Due to chance with independent assortment In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater. Founder Effect Occurs when 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 be common in the new population. Ex. Blue People of the Appalachian Mountains Bottleneck Occurs when a population declines to a very low number and then rebounds The rebound group has traits that are most similar to the smallest group Gene Flow: Increases genetic variation as individuals move in and out of a population Mutations: Random changes in the genetic code Nonrandom Mating Promotes inbreeding and could lead to a change in allelic proportions favoring individuals that are homozygous for particular traits Natural Selection Acts to select the individuals that are best adapted for survival and reproduction Stabilizing selection eliminates extreme expressions of a trait when the average expression leads to higher fitness. Ex. Babies who have too low birth weights are less likely to survive Directional selection when an extreme trait makes organisms more fit it becomes more common. Ex. Body color of the peppered moth found in England Originally, brown most were white, now most are Why? Disruptive selection a process that splits a population into two groups. Tends to remove individuals with average traits but retains the extreme traits to better suit them for their environment Ex. Same snake species found in two areas Speciation Speciation: The development of new species through evolution A population must diverge and then be reproductively isolated Two Types of Speciation: 1. Allopatric speciation 2. Sympatric speciation Allopatric Speciation A physical barrier divides one population into two or more populations. Abert squirrel Kaibab squirrel Sympatric Speciation A species evolves into a new species without a physical barrier. The ancestor species and the new species live side by side during the speciation process but cannot reproduce. Patterns of Evolution Speciation takes a very long time (compared to a human life) but there is evidence that it occurs: Adaptive Radiation Coevolution Adaptive Radiation (divergent evolution) Can occur in a relatively short time when one species gives rise to many different species Follows large-scale extinction events 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. Rate of Speciation Evolution proceeds in small, gradual steps according to a theory called gradualism. Punctuated equilibrium explains rapid spurts of genetic change causing species to diverge quickly.