Download The evolution of populations Change can be rapid

The evolution of populations
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Change can be rapid-Ecologists Peter and Rosemary Grant observed finches in the Galapagos.
– A drought in 1977 reduced soft seeds finches preferred, but large tough seeds that only the large
beaked finches could eat were still available
– The next year there was a drastic increase in large beaded finches.
Selection
Artificial selection: a breeder selects for desired characteristics such as for dogs
Natural selection: environmental conditions determine which individuals in a population produce the most
offspring
– 3 conditions for natural selection to occur
• Variation must exist among individuals in a population
• Variation among individuals must result in differences in the number of offspring surviving
• Variation must be genetically inherited
Maintenance of Variation
Frequency-dependent selection: depends on how frequently or infrequently a phenotype occurs in a
population
– Negative frequency-dependent selection: rare phenotypes are favored by selection
– Positive frequency-dependent selection: common phenotypes are favored; variation is eliminated from
the population
– Strength of selection changes through time
– Fitness of a phenotype does not depend on its frequency
– Environmental changes lead to oscillation in selection
Fitness and Its Measurement
– Fitness: A phenotype with greater fitness usually increases in frequency
– Most fit is given a value of 1
– Fitness is a combination of:
• Survival: how long does an organism live
• Mating success: how often it mates
• Number of offspring per mating that survive
Directional selection: acts to eliminate one extreme from an array of phenotypes
Stabilizing selection The intermediate phenotype is favored and becomes more common in the population
Disruptive Selection Both extreme phenotypes are favored in a population.
Gene Flow The movement of alleles from one population to another.
Genetic Drift
– A small population can produce changes in the frequency of alleles by chance. (Example picking flowers)
– Bottle neck affect- occurs after a population has been greatly reduced the few members left do not
have the original diversity of the larger population.
– Founder affect- Occurs when a small number of individuals colonize a new area.
– Evolution by drift is aimless, not adaptive. (may be good or bad)
Sexual Selection This is referred to as non-random mating
– This occurs when certain traits increase mating success.
• Intrasexual- males compete to mate. Example bighorn sheep.
• Intersexual- males display traits that attract females. Example birds of paradise
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Speciation Through Isolation
– Speciation- the rise of two or more species from one existing species
– Reproductive isolation- occurs when members of two or more population can no longer mate
successfully. Last step in becoming new species.
– Behavioral isolation- differences in courtship or mating behaviors
– Geographic isolation- physical barriers
– Temporal isolation- reproduction at different times. Example - cicadas
Patterns of Evolution
Natural selection is not random!
Convergent evolution- when two unrelated species develop similar characteristics.
Divergent evolution- When closely related species become increasingly different.
Coevolution- Two or more species evolve in response to one another.
Extinction- The elimination of a species from the Earth.
– Background extinctions- continuous low rate of extinction that are a part of the life cycle
– Mass Extinctions- caused by catastrophic event
Patterns of speciation
– Punctuated equilibrium-bursts of activity followed by long periods of stability
– Adaptive radiation- the change of one species into many different descendents