29 terms

Genetic Divergence

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Gene Flow
The physical movement of alleles into and out of a population through emigration and immigration
1. alleles are lost from a population via emigration
2. alleles enter a population via immigration
-the physical flow of alleles (gene flow) tends to counter genetic differences
-can slow a population from fully adapting to new environments
- common way to introduce new alleles to a population
Examples of Gene Flow:
human populations and blue jays
for example there is huge diversity of alleles (skin colors, eye colors etc from all different parts of the world, peopel coming from different countries and mating making new alleles)
Genetic Drift
the random change in allele frequencies over the generations, brought about by chance.
1. changes in allele frequences due to genetic drift occur regardless of the fitness of the individuals (BECAUSE ITS BY CHANCE)
2. Gnetic drift either favours the elimination or fixation of an allele (0% or 100%)
Sampling error
the smaller the sample size, the greater the chance of deviation from predicted results (applies for random mating and fertilization in a population)
In natural populations...
deviations from expected results because real populations are finite in size (small population size)
genetic drift tends to...
reduce variation and leads to the homozygous ocndition
genetic drift:
2 situations that increase the impact of genetic drift on a population and rapidly alter allelic frequencies : the bottlebeck effect and the founders effect
the bottleneck effect
a severe reduction in population size brought about by intense selection pressure or natural disaster
ex: contagious diasease, habitat loss, hunting, volcanoes, etc.

1. survivors gene pool may no longer represent the original population
2. by chance certain alleles may be over represented, under represented or eliminated
3. genetic drift may continue to alter the gene pool and reduce variation when the population size is small.
Examples of bottleneck effect
elephant seals (severe hunting resulted in bottleneck and a reduction in variation)
EX: cheetah (monomorphic for many genes)
Founders Effect:
when individuals become isolated from a larger population
1. occurs when a few members migrate to a new habitat (establish a new population in a new habitat)
2. isolated gene pool may no longer represent the original population

In the absence of gene flow, natural selection will alter allele frequencies due to genetic drift

EXAMPLE: ISOLATED ISLANDS
Genetic divergence:
any structural, functional or behavioral difference that favors reproductive isolation is a by product of genetic change
1. genetic changes between populations can be countered by gene flow( homogenizes genetic differences)
2. barriers prevent the exchange of genes between populations and leads to evolutionary divergence of each population
genetic divergence leads to differences between the gene pools of the separate popoulations
(mutation, selection, genetic drift)
REPRODUCTIVE ISOLATION
1. VARIATION
2. NATURAL SELECTION
3. GENETIC DIVERGENCE
4. REPRODUCTIVE ISOLATION
5. SPECIATION
reproductive isolating mechanisms
any heritable feature of body form, function, or behavior that prevents interbreeding breeding between one or more genetically divergent populations

RIMSs result as a by-product of genetic divergence
Maintains positive adaptations for specific environments
What are the two types of reproductive isolating mechanisms?
1. prezygotic isolating mechanisms
2. postzygotic isolating mechanisms
prezygotic isolating mechs
mechs of isolation before or during fertilization
postzygotic isolating mechs
mechs of isolation after fertilization
prezygotic isolation: (1)
1. Ecological isolation:
isolation due to adaptation to different microclimates in the same habitat

ex: california oaks (Santa ynez valley)-3 species
1. valley oak (At the top dont need to much water, small thick leaes, can tolerate wild fires)
2. live oak
3. shrub oak
- separation is based on physiological mechanisms to promote conservation of water
Prezygotic isolation (2)
2. Geographic isolation:
isolated by distance or geographic barriers (do not come in contact)
-distance or barrier relative to the size of the organism

Ex: lice (2 subspecies)
1. head lice
2. body lice

they cannot cross over your collar bone because theres no little hairs there so the chances of them getting caught crossing those areas are extremely high.

GEOGRAPHIC ISOLATION IS THE HAIRLESS REGION OF THE NECK
Prezygotic isolation (3)
Temporal isolation: isolated by reproductive events that occur at different times (mating, flowering days or seasons do not overlap)

Ex: Cicadas - species
-insect mature underground, reproduce every 17 years
-each species has sibling species, reproduces every 13 years
-only once every 221 years do the sibling species release gametes at the same time

Ex: Primates: 2-4 estrus cycles/year
humans: estrus all year long
Prezygotic Isolation (4)
Ethological isolation:
- isolated by behaviors
-copulation proceeded by courtship displays
-reciprocity produces sexual stimulation

Ex: Birds
-females genetically equipped to recognize behavioral signals by males
-females of other species are not genetically equipped to recognize these signals
Prezygotic isolation (5)
Mechanical isolation:
isolated by incompatible reproductive part size or shape
Ex: insects (tsetse flies)
-mechanical isolation is often severe, wrong mating types can result in mortality to both males and females
Prezygotic isolation (6)
gametic isolation (incompatibility): -gametes of different species are incompatible at the molecular level resulting in mortality
Ex: Plant (pollen grains)
Gametic wastage:
-minimum # of sperm required per species to achieve fertilization
Ex: Humans
Postzygotic Isolation:
Isolation takes effect after fertilization
1. Zygotic Wastage
minimum # of zygotes required per species to achieve a viable adult.
Ex: fish and invertbrates
2. Hybrid Sterility
No viable sperm or egg )ex. mule)
3. Hybrid inviability:
egg of one species is fertilized by sperm form another species, but fertilized egg cannote develop past early embryonic stages
4. Hybrid breakdown:
offspring fertile but their offspring are sterile or have genetic abnormalities (hybrids have low fitness)

-Unsuitable interactions among individuals, genes, or gene products leads to death, sterility or low hybrid fitness
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