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Mechanisms of Evolution

By Christopher Himmelheber. Honors Biology presentation on micro- and macroevolution. Images from Wikimedia Commons, used under Creative Commons Attribution 3.0 unported license.

Christopher Himmelheber

on 26 February 2018

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Transcript of Mechanisms of Evolution

Mechanisms of Evolution
Gene Flow
Hardy-Weinberg Equilibrium
What is a species?
Natural Selection
Reproductive Isolation
Genetic Drift
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)
Non-Random Mating
Gene Flow
Natural Selection
Movement of alleles into or out of a population
Tends to lessen genetic differences between populations
Changes in allele frequency due to
random chance
Much larger effect in small populations
Population Bottleneck
New population may not have same gene pool as previous generations
Drastic reduction in population size
Founder Effect
Few members of a population leave

Establish a new population
Sexual Selection
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
Non-random mating
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
Reproductive Definition
Individuals able to mate and produce fertile offspring
When might this definition not work?
Other definitions:
Prezygotic Blocks
Postzygotic Blocks
Peacock and peahen
Stag beetles
Male giraffes "necking"
Species can be hard to recognize
Cordilleran flycatcher
Pacific-slope flycatcher
Different species
Same species
European (German) male
African (Masai) woman
Isolating mechanisms may occur before or after fertilization
Geographic isolation
Ecological isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
Gametic incompatability
Hybrid inviability
Hybrid infertility
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
Bishop Pine
Monterey Pine
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
Genetic divergence
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...
Wood Frog
Leopard Frog
* Group of one species in a geographic area
Alcohol Dehydrogenase from
D. melanogaster
May lead to formation of new alleles
New alleles may be adaptive
Full transcript