Download BioA414 Handout IX-2017

BioA414
Population Genetics
Handout IX
Random Mating
• Random union of gametes
• Probability of mating with someone
is unrelated  genotype
• Genotype frequencies  Punnett
square
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Assortative mating
• Individuals do not mate randomly
but prefer one phenotype to another
• Affects genotype frequencies
• Assortative mating may be positive
or negative
Assortative Mating
• Positive Assortative Mating
– Like marries like
– Increased homozygosity
– Fairly common in humans
• Negative Assortative Mating
– Like repels like
– Increased heterozygosity
– Rare in mammals
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Inbreeding
• Preferential mating of close relatives
• Small populations may show this effect
even with no tendency to select close
relatives
• Acts on allele frequencies ≡ genetic drift
• Self-fertilization  most extreme
example
Geographical Isolation
• Islands, mountains, and glaciers isolate
local populations
• The development of transportation
technology  breakup of local genetic
isolates
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Behavioral Isolation
• Territoriality
– Decreases gene flow between
populations
• Cultural factors
– Language, Religion, Social class
regulate gene flow between groups
Genetic consequences of marriage
patterns
• Polygyny
– More than one wife  results in large fitness
differences among males
• Polyandry
– More than one husband  found in
marginal environments
• Monogamy
– One husband and wife
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The breakup of local population
isolates
• Until recently  marriage with someone
from nearby
• The last half of the 20th century was
characterized by the breakups of local
population isolates
• This major change in breeding pattern 
increased genetic heterogeneity
Effects of inbreeding
• Genetic consequences
– Reduction in genetic variability
– ↑ homozygous recessives
– Recessive alleles exposed to natural selection
• Social consequences
– Concentration of wealth and power
– Reduction of social ties with other groups
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Inbreeding Depression
• Decrease of vigor or
reproductive success
• Inbreeding results in higher
frequency of fetal wastage
Cheetah homozygosity
• Bottleneck effect?
– Modern cheetahs  little genetic diversity
– Homozygous at most loci
– Severe genetic bottleneck
• Consequences
– ↓ capacity for adaptation
– ↓ survival of cheetahs and other endangered
species
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Hereditary diseases in religious
isolates
• Recessive diseases  ↑ freq. in small,
endogamous, religious groups
• The Ellis-Van Creveld syndrome, is
common among the Old Order Amish in
Lancaster county Pennsylvania
– Polydactyly, short limbs, and other
deformities
Inbreeding in the Egyptian royal
family
• Social advantages that sometimes
outweigh its genetic costs
• The preferred brother-sister
marriages of ancient Egypt
• Maintain political power of an elite
dynasty
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Summary of effects of evolutionary forces
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Mutation
Occurs at low rate , creates small changes, and increases genetic variation; balanced
with natural selection and drift
Genetic drift
De creases variation due to loss of alleles, produces dive rgence and substantial
changes in small populations
Migration
– Rates and types of migration vary
– Increases e ffe ctive population size
– De creases dive rgence by encouraging gene flow (and re duces drift)
– Cre ates major changes in allele frequencies
Natural sele ction
– Increases or de creases genetic variation de pending on the environment
– Increases or de creases dive rgence
– Continues to act afte r equilibrium has been achie ve d
– Balances with other forces, e.g., mutation and drift
Non-random mating
Inbreeding de creases variation and fitness
Populations and Species
• Micro-evolution  population level changes in
gene frequencies
• Macro-evolution  the production of a new
species
What is a species?
The Biological Species Concept-a group of
interbreeding natural populations that are
reproductively isolated from other such groups
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Outcomes of evolution
• Microevolution
– Smaller evolutionary changes
– Adaptations  within a species or
population
• Macroevolution
– At or above the level of species
– Extinction and speciation
Speciation and Extinction
• A combination of mutation, gene flow,
genetic drift, and natural selection can
create new species out of a population in
breeding isolation
• 2 models of speciation
– Cladogenesis
– Anagenesis
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Anagenesis
• Phyletic transformation
– Transformation of an entire species into a
descendant species
– Accumulation of changes  gradualism
– New species  enough genetic alteration
from original
– No increase in biodiversity
Cladogenesis
• Splitting or branching of existing
species
– Allopatric speciation
• Speciation among populations living in
two different ranges or territories
• Geographic isolation leads to
reproductive isolation
– Sympatric speciation
• Species living in the same or
overlapping ranges adapt to different
ecological niches
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Speciation  Allopatric model
• As populations become subdivided 
allele frequencies change and populations
diverge
• When populations reunite, they may fail
to mate or produce inviable offspring 
allopatric speciation
Allopatric Model
Peripatric Model
vicariance
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Speciation 
Sympatric/Allochronic models
• In some cases, speciation may be driven in the
absence of allopatry or peripheral isolation
• Speciation and reproductive isolation correlate
with ecological preferences (sympatric) or
timing instead of space (allochronic speciation)
• Sympatric speciation is driven primarily by
adaptation as opposed to vicariance associated
with geography
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Types of barriers to gene flow
• Spatial, temporal, and ecological isolation
• Post-zygotic barriers
– Hybrid sterility/inviability
– Haldane’s rule  sterility and inviability occurs
more often in the heterogametic sex
• Pre-zygotic barriers
– Behavioral incompatibility
– Mechanical isolation (genitalia do not fit together)
– Gametic isolation (gametes fail to fuse)
Populations
are the Units
of Evolution
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What drives evolution?
There are 5 forces of change
Only natural selection
makes a populati on
better adapted (more
fit) to its environment
Conservation of biodiversity
• Maintaining large interbreeding populations
requires large non-fragmented habitats
• Earth’s 2-15 million species and most speciesrich habitats are being exterminated by
humanity at rates ~1,000-fold greater than
historical background extinction rates
• Currently experiencing the most massive
extinction episode in 65 million years
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Conservation of biodiversity
• Mutations that give rise to variation
accumulate slowly over many generations
• Habitat loss and extinction occur much
faster
• Extant biodiversity is the result of a
legacy of 4-5 billion years of genetic
information
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