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Biology 1001A
Tom Haffie

Biology lecture 15 microevolution Before: The Agents of Microevolution  Populations allele frequencies change over time and do match the hardy-Weinberg prediction  There are several processes that foster microevolution; mutation, gene flow, genetic drift, natural selection and non-random mating Agent Definition Effect on genetic variation Mutation A heritable change in DNA Introduces new genetic variation into population Gene flow Change in allele frequencies May introduce genetic as individuals join a variation from another population and reproduce population Genetic drift Random changes in allele Reduces genetic variation, frequencies caused by chance especially in small events populations; can eliminate alleles Natural selection Differential survivorship or One allele may increase reproduction of individuals frequency or allelic variation with different genotypes can be preserved Non-random mating Choice of mates based on Does not directly affect allele their phenotypes and frequencies, but usually genotypes prevents genetic equilibrium Mutations create new genetic variations Mutation:  Heritable change in DNA, which may be neural, deleterious or beneficial o Usually rare events o They produce little or no immediate effect on allele frequencies in a population o They DO create significant changes over evolutionary time scale** o Since mutation is a mechanism through which entirely new variations arise it is a major source of heritable variation  Only variation in the germ lines is heritable o Except plants where nonreproductive structures may pass on a mutation that can eventually change the gene pool Neutral Mutations  Neither harmful nor helpful  Example: in the construction of a polypeptide chain an amino acid can be specified by different codons o As a result, some DNA sequence changes do not alter the amino acid sequence  Other neutral mutations may change an organisms phenotype without influencing its survival  Although if an environment changes and neutral mutation may become a liability Deleterious mutations  Alter structure, function or behaviour in a harmful way Biology lecture 15 microevolution  Example: Collagen mutation Lethal mutations:  Cause death to the organism carrying that mutation o If lethal allele is dominant then both heterozygous and homozygous suffer from this effect o If recessive then it only affects homozygous individuals  A lethal mutation that causes death before the organism can reproduce is eliminated from the population because it did not pass on gene to offspring Advantageous mutation  Confers benefits on individual that carries it  However small the advantage, natural selection may preserve the allele and may increase frequency over time Gene flow introduces novel genetic variants into populations Gene Flow  Organisms or their gametes move from one population to another  If the immigrants reproduce they may introduce novel alleles into the population they have joined  Violates hardy-Weinberg Different examples: organisms  Gene flow is common in some animal species such as male baboons o They move from one local population to another after experiencing aggressive behaviour by older males  Marine invertebrates o Larvae may be carried long distances by currents Examples: Gamete movement  Dispersal agents (pollen carrying wind/seed carrying animals) o Responsible for most gene glow in plants  Example: blue Jays responsible for gene flow of oaks o They carry acorns to different areas which could potentially germinate in these new areas adding to the gene pool Evolutionary importance  This depends on the degree of genetic variation between populations and the rate of gene flow between them Genetic drift reduces genetic variability within population Genetic Drift  Chance events that cause allele frequencies in a population to change unpredictably  Has extreme effects on small populations Biology lecture 15 microevolution  Leads to the loss of alleles  Two types of genetic drift; population bottleneck and founders effect Analogy of genetic drift:  Tossing a coin 20-30 times you won’t always get 50 heads: 50 tails  Sometimes heads will be predominant and sometimes tails will be  But if you tossed this coin 500 times (larger population) you will get a ratio that is more equal to 50:50 then the larger population  Thus you can see how a small population is more greatly affected by genetic drift than a larger population 1. Population Bottleneck  Population bottleneck is caused by disease, starvation or drought killing many and causes the loss of alleles  This greatly decreases genetic variation  Example: Northern elephant seals were nearly wiped out by hunters
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