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Mechanisms of Evolution
Transcript of Mechanisms of Evolution
What is a species?
A population in Hardy-Weinberg equilibrium has constant allele frequencies over generations
Conditions for Hardy-Weinberg
1. Random mating
2. Large population
3. No natural selection
4. No mutations
5. No gene flow
Changes in allele frequency in a population*
Five main processes
Small Population (Genetic Drift)
Movement of alleles into or out of a population
Tends to lessen genetic differences between populations
Changes in allele frequency due to
Much larger effect in small populations
New population may not have same gene pool as previous generations
Drastic reduction in population size
Few members of a population leave
Establish a new population
Birds of Paradise
Selection against both extremes
Favors the average, most common form
Selection against average form
Favors the two extremes
May lead to divergence
Selection against one extreme
Favors the opposite extreme
Competition for mates can lead to evolution of traits that do not directly effect survival
Leads to sexual dimorphism
Evolution of separate gene pools
Changes above the species level
There are multiple definitions of a species
Individuals able to mate and produce fertile offspring
When might this definition not work?
Peacock and peahen
Male giraffes "necking"
Species can be hard to recognize
European (German) male
African (Masai) woman
Isolating mechanisms may occur before or after fertilization
Mechanisms that prevent mating attempts
Species are separated by a geographic barrier and so rarely if ever meet
Species may live in same area, but are in separate, unique ecological niches
Species mate at different times of year
Specific courtship/mating rituals must be followed in order for mating to occur
Sexual organs are not compatible for mating
Mechanisms that prevent cross-breeding after a mating attempt
Sperm from one species may not be able to penetrate eggs of a different species
Hybrid offspring fail to survive to maturity
Hybrid offspring are sterile
Two factors must be present for speciation to occur
Allopatric: Geographical separation of populations
Sympatric: No geographic separation
Apple maggot fly
Rate of Speciation
Example Hardy-Weinberg problem
Within a population of butterflies, the color brown (B) is dominant over the color white (b). If 40% of all butterflies are white, calculate the following:
a.The percentage of butterflies in the population that are heterozygous.
b.The frequency of homozygous dominant individuals.
* Only natural selection leads to adaptive change
These should look familiar...
* Group of one species in a geographic area
Alcohol Dehydrogenase from
May lead to formation of new alleles
New alleles may be adaptive