Lect. 9: Microevolution and Natural Selection

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natural selection

a mechanism of evolution when individuals with one version of a heritable trait has greater reproductive success than individuals with a different version of the trait

reproductive success

individual’s production of offspring per breeding attempt or lifetime

natural selection occurs when what 3 conditions are satisifed?

1. variation for a trait

2. heritability

3. differential reproductive success

variation for a trait

different versions of a trait are present within a population

heritability

the different versions of a trait may be passed from parents to offspring

differential reproductive success

individuals with the version of a trait most suited to reproduction in their environment have more offspring than individuals with other versions of the trait

natural selection leads to __ and __ of a population

Natural selection leads to fitness and adaptation of a population

fitness

measure of the amount of reproduction of an individual with a particular phenotype relative to the reproduction of individuals of the same species with an alternative phenotype

traits with higher fitness will increase in…

allele frequency

fitness depends on…

• environment

• reproductive success compared to others in the population

adaptation

the process by which organisms become better matched to their environment. also, specific features that make an organism more fit

organisms will not be perfect because…

• environment quickly changes, natural selection too slow to adapt organisms

• variation is needed as basic material of selection, a mutation creating a new “perfect” version of gene may never rise

• different alleles for a trait may cause individuals to have the same fitness

natural selection leads to fitness and adaptation, not the _____

perfect organism

• example: galapagos finches

3 modes of natural selection

1. directional

2. stabilizing

3. disruptive

directional selection

individuals with one extreme from the range of variation in the population have the highest fitness

stabilizing selection

individuals with intermediate phenotypes have the highest fitness

disruptive selection

individuals in both extreme phenotypes have the highest fitness