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Class Notes
Patterns of Evolution
Questions/Main Idea:
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Mechanisms of Change
Genetic variation
Gene tic drift
Mechanisms of
Mechanisms of
- Reproductive
There are various mechanisms by which evolution can occur:
– Genetic Variation, Gene Flow, Genetic Drift, Natural
Gene pool → all the alleles of all the genes in a population
Gene (allele) frequency → proportion of particular type of alleles in
a population
Variation has two sources:
– mutation → change in DNA that leads to new alleles
(harmful or beneficial)
– sex (gene shuffling) → meiosis (recombination of alleles
during crossing over) and fertilization
Gene flow → new alleles (or frequencies) from one population enter
another due to migration
– e.g., native Americans and “recent” immigrants
Genetic drift → random changes in allele frequencies of a gene pool
Has a great effect when…
– Bottleneck → random event radically decreases population
size (and allele frequencies)
• e.g., Earthquake in Haiti
– Founder effect → a new population started by a small
number of individuals (and their allele frequencies)
• e.g., American Amish
Speciation → splitting of one species into 2 different species
Species → a group of organisms that interbreed under natural
conditions to produce fertile offspring
– Natural conditions: not artificially inseminated or forced into
the same location
To maintain a population as a single species, genes must freely flow
through the entire population
Geographic isolation → members of a species are isolated from one
another due to a geographic barrier (mountains, rivers).
– e.g., a few members of a species leave their original location
and are separated from the rest of the species (Darwin's
– e.g., barrier develops slowly (formation of Grand Canyon split
a population of squirrels into 2 isolated groups that have
evolved into separate species, the Kaibab and Albert squirrels)
Before mating:
– Different breeding times (e.g., trees release pollen at different times)
– Different behaviors (e.g., courtship rituals of peacocks)
– Mechanical problems (e.g., genitalia doesn’t fit together)
After mating:
– Hybrids don’t survive
– Hybrids are sterile