final unit.docx - EVOLUTION A population is a group of...

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EVOLUTION A population is a group of organisms of the same species living and breeding together in the same geographic area Note that individual organisms do not experience changes in gene or allele frequencies over time - so an individual cannot evolve Definitions Gene pool: the total number of alleles for a specific gene in a population Gene frequency: the percentage of one allele out of all alleles for that particular gene in a population Genetic Diversity and the Gene Pool in Different Populations All the alleles of all the genes in a population (sometimes a species) is termed a gene pool Each allele is present at a certain proportion, or allele frequency , relative to the total number of alleles (1/15 red, 2/15 purple, 12/15 green) When allele frequencies change over time, a population evolves Changes in a gene pool can have good, bad, or neutral consequences on a population Why is genetic diversity important/favorable in nature? Genetic diversity helps organisms cope with current environmental variability Genetic diversity is the primary basis for adaptation to future environmental uncertainty The more genetically diverse a population/species is, the more ways there are to adapt to possible environmental changes that affect reproductive success Mechanisms of Evolutionary Change 1. Mutation a. Heritable change in DNA sequence in a population 2. Gene Flow a. Migration of genes between populations 3. Genetic Drift a. Changes in gene frequencies within populations 4. Random mating a. If individuals preferentially choose mats with certain genotypes, including close relatives (interbreeding), random mixing of gametes does not occur 5. Natural Selection a. Differential survival and reproduction of individuals in response to environmental pressure that leads to changes in genes Genetic inbreeding reduces diversity Inbreeding is reproduction from the mating of parents who are closely related genetically Inbreeding depression is reduced fitness in a population due to the increased appearance of deleterious homozygous recessive traits Inbreeding depression is more common in small, isolated populations due to increased likelihood of mating between close relatives Diversity within populations reduces inbreeding depression The source of genetic variation in populations is mutation - heritable changes in DNA sequence of germ cells (small and large scale) Unrepaired mistakes in DNA replication lead to changes in sequence, or chromosome rearrangements
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Genetic drift REDUCES genetic diversity Genetic drift tends to have more dramatic effects in smaller populations (any single individual that does not reproduce could spell the loss of alleles from the population) Events that lead to genetic drift: Small groups of founders of new populations Bottlenecks that greatly reduce existing populations Founders of new populations demonstrate genetic drift Founder effect: when founders (immigrants) begin to reproduce and populate a new
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