[18.3-4] natural selection, effects of selection on evolution
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describe the core of natural selection (5)
• genetic diversity in a population due to random mutations (gene pool) • ability to reproduce and pass on alleles (genes) • selective pressures in the environment leads to differential survival • those that have a selective advantage will be more likely to survive and reproduce, thus passing on advantageous alleles • over many generations desired alleles will increase in frequency
what are selection pressures?
environmental factors that limit the population of a species
what are some examples of selection pressures? (3)
• predation • disease • competition
what factors does the process of evolution by natural selection depend on?
• organisms produce more offspring than can be supported by the available supply of food, light, space etc. • genetic variety within the populations of all species • variety of phenotypes that selection operates against
what are the 3 types of natural selection?
• stabilising • directional • disruptive
draw the graph! describe stabilising selection
• favours average phenotype (phenotype around the mean) • selection against extreme phenotypes • tends to occur where the environmental conditions are constant over long periods of time
draw the graph! describe directional selection
• changes phenotype of a popuation • favours phenotypes that vary in one direction from the mean in the population ie. extreme phenotype • due to change in environmental condition
draw the graph! describe disruptive selection
• favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population • occurs when an environmental factor takes two or more distinct forms