ch 25 Flashcards
genetic rescue
– introduction of new genetic variation into an inbred population
Small populations lose….
genetic variation over time through inbreeding and genetic drift (change in allelic frequency).
Migration introduces…
new genetic variation that counteracts the effects of genetic drift and inbreeding.
Almost all organisms exhibit
phenotypic
variation.
Much of this variation is hereditary
Genetic variation is the basis of
all evolution.
The extent of genetic variation within a population affects its potential to adapt to environmental change
Mendelian population
– a group of interbreeding, sexually reproducing individuals that have a common set of genes – the gene pool.
Genotypic frequency:
Number of individuals possessing the genotype divided by total number of individuals in sample
Genotypic frequency equations
f(AA) = # AA individuals/N
f(Aa) = # Aa individuals/N
f(aa) = # aa individuals/N
N: total # of individuals
f: frequency each genotype.
The sum of all genotypic frequencies always equals 1.
Allelic frequency
Number of copies of a particular allele present in a sample divided by total number of alleles
The gene pool of a population can also be described in terms
of allelic frequencies.
Allelic frequency equation
frequency of an allele =
# copies of the alleles /
# copies of all alleles at
the locus in a population
Calculating allelic frequencies
look at slide 7
For a locus with only two alleles (A and a), the frequencies are
usually represented by p (dominant) and q(recessive).
X-linked loci allelic freq
slide 8
When calculating allelic frequencies for genes at the X-linked loci, we apply the same principles, but females possess 2 X chromosomes (and therefore has two X-linked alleles)
How many alleles are in this population? 6129 individuals
12,258
What is the allele frequency of LM and LN?
LM indiv= 1787
LMN indiv= 3039
LN indiv= 1303
LM = (1787*2) + 3039/12,258 =0.54
LN = (1303*2) + 3039/12,258 =0.46
should equal 1
The Hardy-Weinberg principle
mathematical relationship
between allele frequencies and genotype frequencies
allows prediction of population’s genotype frequencies from its allele frequencies
The Hardy-Weinberg principle equations
p + q = 1
p^2 + 2pq + q^2 = 1
p^2 + 2pq + q^2 = p + q
When are we in HWE and accept hypoth
If allele frequencies are the same
if changed not in HWE and reject null
random mating
The key assumption underlying the Hardy-Weinberg principle
If mating is random and no differential survival or
reproduction exists among members of the
population, the Hardy-Weinberg genotype
frequencies persist generation after generation
Assumptions for Hardy-Weinberg
Populations are usually not in Hardy Weinberg equilibrium (at least, not for all of
the genes in their genome).
Populations tend to evolve: the allele
frequencies of at least some of their genes
change from one generation to the next
Exceptions to the Hardy Weinberg Principle
Nonrandom mating
Unequal survival
Population subdivision
Migration
Finite population size
genetic drift
Small population size can cause a random
change in allele frequencies
due to a
sampling effect.
– Sampling effects are most important when the
allele is present in a small number of copies.
random genetic drift
Uncertainties of genetic transmission can lead to random changes in allele frequencies
The alleles of segregating genes are randomly incorporated into gametes. There is always uncertainty as to which allele a given offspring will receive
large VS small pop sizes in genetic drift
In large populations, the effect of genetic drift is
minimal.
In small populations, genetic drift may be the
primary evolutionary force.
natural selection
selection for survival and
reproduction in the face of competition
The mechanism that changes
the physical and behavioral characteristics of a species
Fitness
symbolized by W, is the relative reproductive success of a genotype
– the reproductive success of one genotype compared to
another genotype in the population!
Relative Fitness
Does not give the absolute reproductive abilities of the
different genotypes in the two habitats.
Does tell us how well each genotype competes with the other genotypes within a particular environment.
If s1 = 1, then aa is effectively a lethal genotype (it’s relative fitness is 0), and we would expect natural selection to reduce the frequency of the a allele in the population.
If s1 is very small, 0.01, natural selection will still reduce the frequency of allele a, but very slowly.
To calculate fitness for each genotype
, the mean number of
offspring produced by each genotype is divided by the mean number of offspring produced by the most prolific genotype.
slide 49
To calculate fitness of each genotype;
W 1,1 = 10/10 = 1
W 1, 2 = 5/10 = 0.5 - use 10 bc most fit NOT total
W 2, 2 = 2/10 = 0.2
calculating selection coefficient
-In each environment, the fitness of the superior genotype(s) is defined as 1
-The fitness of the inferior genotype(s) is expressed as a deviation from 1
-The fitness deviation (s) is the selection coefficient,
which is the relative intensity of selection against a genotype.
To calculate selection coefficient (1-W);
s11 = 0
s12 = 0.5
s22 = 0.8
