Population Genetics

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Population Genetics

• 6th Edition, Chapter 19, pps 356-380
• 5th Edition, Chapter 21

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• Remember the four levels of Ecology
• Individual (autoecology, life history)
• Population (one species)
• Community (many species)
• Ecosystem (many species and the environment)

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Population genetics refers to all aspects of the
genetic make-up of populations. Some genetics
terms Genotypes the sum of all hereditary
information in an individual (directs the
development of the individual). Phenotypes is
the external or observable expression of the
genotype. Phenotypic plasticity refers
to the ability of a genotype to give rise to a
range of phenotypic expressions under different
environmental conditions. (Best examples are
among plants, e.g. sun and shade leaves).
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Population Genetics and Ecology

• From a population genetics standpoint, the basic
  conservation views are as follows
• 1) genetic variation is good.
• 2) gene flow of any kind is good.
• 3) decreased genetic variation equals decreased
  fitness and this is bad.
• 4) decreased genetic variation equals a
  decreased ability to adapt to changing
  environments and this is bad.
• The views considered bad are those that lead to
  extinction, whereas all views that are good
  tend to prevent extinction.

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1. Genetic variation is good.

• Higher genetic variation higher biodiversity
  (generally we think of biodiversity as number of
  species, but can also consider it as a measure of
  genetic variation).
• Genetic variation is reduced in small populations
  (E.g. problems with habitat fragmentations and
  leaving small populationsgtgtgtgt from populations to
  sub-populations).
• Founder effect
• Interbreeding (mating between relatives)
• Genetic drift
• Bottlenecks
• ALL REDUCE GENETIC VARIATION

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2. Gene flow of any kind is good...

• Panmictic populations have free interchange
  of genes through individuals in a population.
• Discrete subpopulations- involves two populations
  without gene flow between the two, but free
  interchange within the subpopulations.
• Isolation by distance local interchange of
  genetic materials only.
• Influenced by
• - male/female sex ratios - age of sexual
  maturity
• - dispersal ability - habitat fragmentation
• - genetic effective breeding size
• - genetic isolation

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GENETIC EFFECTIVE BREEDING SIZE If there are
500 individuals in a population, do all of them
find mates and reproduce? Ne 4 ( 1 / Nem )
( 1 / Nef ) 1 Where Nem and Nef are the
effective numbers of males and females. Thus,
mate selection and breeding L/H influences
effective population size.
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3. Decreased genetic variation decreased
fitness and this is bad...

• A strong association exists between mean
  heterozygosity and fitness-related traits.
• Fitness is the probability that a particular
  phenotype will survive and leave offspring.
• Examples of fitness-related traits
• Fecundity (e.g. clutch size)
• Disease resistance (fish studies show
  heterozygous individuals have higher disease
  resistance).
• Bilateral symmetry (Univ. of Mexico movie star
  study)

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4. Decreased genetic variation a decreased
ability to adapt to changing environments and
this is bad...

• Decreased genetic variation inhibits
  polymorphisms and other characters that are
  important to evolutionary process.
• Small populations are more vulnerable to
  extinction in part because of reduced genetic
  variation.as population size the possibility
  of extinction .
• Minimum viable population size population size
  that will ensure its persistence for a specific
  period of time.this is difficult to determine.
  The original benchmark was a 99 chance of
  persistence for 1000 yrs. This is difficult to
  model as many factors determine MVP.

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The Hardy-Weinberg Law

• Is a fundamental concept of population genetics.
• P 2 2pq q 2 1
• Assumptions
• 1. The population is infinitely large, or large
  enough that sampling error is negligible.
• 2. Mating within the population occurs at
  random.
• 3. There is no selective advantage for any
  genotype that is, all genotypes produced by
  random mating are equally viable and fertile.
• 4. There is an absence of other factors such as
  mutation, migration, and random genetic drift.

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P 2 2pq q 2 1 For Example In a
population, a particular gene has two alleles A
and a, where A is dominant. The frequency of A
in the population is represented by p. The
frequency of a in the population is represented
by q. p q 100 of all the alleles for that
gene in the population.
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The probability of getting a homozygous dominant
is given by P 2 2pq q 2 1 .A Punnet
square can be used to show the random combination
of gametes.
Sperm A(p) a(q)
A(p) Eggs a(q)
AA (p2)
Aa (pq)
Aa (pq)
aa (q2)
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For two heterozygous parents if 70 of alleles
w/n a population are A and 30 are a, then
Sperm A (p 0.7) a (q 0.7)
AA (p2) (0.49)
Aa (pq) (0.21)
A (p 0.7) EGGS a (p 0.3)
Aa (pq) (0.21)
aa (q2) (0.09)
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Lets try another example.
If the proportion of the dominant allele A if 56
and for a is 44, then what proportion of the
population will be 1. Dominant A? ____
2. Recessive a? _____ 3. Heterozygous (Aa)?
_____
Sperm A (p 0.56) a (q 0.44)
AA (p2) (0.314)
Aa (pq) (0.246)
A (p 0.56) EGGS a (p 0.44)
Aa (pq) (0.246)
aa (q2) (0.194)
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Some other terms Concepts.

• Genetic drift a random process that results in
  reduced genetic variance. Occurs in small
  populations.
• The amount of heterozygosity is a measure of
  genetic variation, but a gene can become fixed
  for homozygosity no genetic variation for that
  gene in a population (Fixed for A or for a).
• A population in which the frequency of a given
  gene remains constant is in a state of genetic
  equilibrium.

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Some other terms Concepts.

• Natural selection a non-random reproductive
  success.
• N.S. is a two-part process
• 1. Variation exists (genetic and morphological)
  that is heritable.
• 2. Certain morphological traits are selected
  against.
• The individual phenotype is the unit of
  selection. There are arguments for the gene
  (Dawkins) and for groups of individuals
  (Winn-Edwards).
• IMPORTANT Traits are Selected against not
  Selected for.

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FINAL COMMENTS..
Assumptions of the H-W Law are rarely
met. However, if all the assumptions are met,
these gene frequencies will not change from
generation to generation. Hence, evolution will
not occur!