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Chapter 17: Evolution of Populations. Section 17-2: Evolution as Genetic Change in Populations. How Natural Selection Works. Evolutionary fitness = success in passing on genes - PowerPoint PPT Presentation
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Chapter 17:Evolution of PopulationsSection 17-2: Evolution as Genetic Change in Populations
How Natural Selection Works•Evolutionary fitness = success in passing
on genes
•Evolutionary adaptation = any genetically controlled trait that increases an organism’s ability to pass along its alleles
Natural Selection on Single-Gene Traits•Changes allele frequencies•Ex: Body color in lizards
Natural Selection on Polygenic Traits•Range of phenotypes•Fitness varies throughout the curve•Natural selection can affect phenotype
range, changing shape of bell curve
Directional Selection•Occurs when individuals at one end of the
curve have higher fitness than individuals in the middle or at the other end
Stabilizing Selection•Occurs when individuals near the center
of the curve have higher fitness than individuals at either end
•Keeps center of curve, narrows overall graph
Disruptive Selection•Occurs when individuals at the upper and
lower ends of the curve have higher fitness than individuals near the middle
•Acts against the intermediate phenotype, can create two distinct phenotypes
Genetic Drift•Occurs in small populations•Allele becomes more or less common by
chance•Random
Genetic Bottlenecks•The bottleneck effect is a change in
allele frequency following a dramatic reduction in population size
•Ex: Natural disaster
The Founder Effect•Occurs when allele frequencies change as
a result of the migration of a small subgroup of a population
•Can create new populations very different from original group
The Founder Effect
Evolution vs Genetic Equilibrium•If allele frequencies in a population remain
the same it is in genetic equilibrium•No evolution•The Hardy-Weinberg principle describes
the conditions under which evolution will not occur
•States that allele frequencies will remain constant unless something causes them the change
Hardy-Weinberg Principle•5 conditions can disturb genetic
equilibrium and cause evolution:▫Nonrandom mating – sexual selection▫Small population size▫Immigration or emigration▫Mutations▫Natural selection
Sexual Reproduction and Allele Frequency•Sexual reproduction alone does not
change relative allele frequency