Population
Genetics
Variation is
basis for evolution
Discontinuous:
sexes, blood groups, coloration phases
Continuous:
range of character states: lengths, scales, spines,
Genotype:
genetic content
Phenotype:
expressed genetic traits modified by interaction
Phenotypic
plasticity: expression range of a genotype
Inducible
vs Constitutive genes
Diploid
= 2x copies, locus is gene position of alleles
homozygous
= alleles same; heterozygous, alleles different
Sources of new
genetic variations:
Reassortment
/ recombination of genes and chromosomes:
meiosis,
crossing over, transposition
Mutations:
Inheritable change in genetic material
Macro
mutation: polyploidy, chromosome deletion, large
structural
changes that are often lethal
Micromutation:
change in one or few bases within genes:
neutral,
quasi-neutral mutations
radiation/chemical
damage
Mutations must
become fixed
Passive or
directed evolution?
Induced
Variation in response to selective pressure
Selective
pressures on humans: Eugenetics - Socialism spectrum
Evolution in
genetic terms:
change
in the proportion of an expressed gene.
Hardy Weinburg
Law
the
gene pool: looking at genes rather than individuals
can
be misleading, since individuals are comprised of many traits
For diploid,
sexually reproducing organisms, phenotypes genotypes, and genes will approach
equilibrium at:
1 = p2
+ 2pq + q2
p = frequency of
one allele
q = frequency of
alternate allele
homozygous = pp
or qq
heterozygous =
pq
Gene frequencies
will approach and remain in equilibrium as long as some conditions are met:
assumptions of equilibrium
This
distribution of alleles gives us a starting point for examining factors that
induce changes in gene frequencies.
Equilibrium of
genes will be maintained if these hold:
1) random
mating:
like
to like increases homozygosity: inbreeding depression
outbreeding
increases heterozygosity: Hybrid vigor
2) Mutations do
not occur, or the rate and reverse are equal
3) Population is
closed:
Immigration and
Emigration: selective pressures force particular traits in or out
4) Population
size is infinite
5) No Natural
Selection:
directional
change of gene frequencies: non-random reproduction
Hardy Weinburg
allows us to quantify natural selection as a
null
hyptothesis
Genetic Fitness:
a genotype’s contibution to the next generation:
Absolute or
Crude Fitness as proportional change F1/P:
B1B1=
0.6; B1B2= 1.0; B2B2 =1.5
Relative
genotype fitness: AF/AFmax
B1B1=
0.4; B1B2= .67; B2B2 =1.0
Coefficient of
selection s = 1-(AF)
Can
use s in Hardy Weinburg to get change in gene frequency
Dq = change
in gene frequency; s= 1/2sq(1-q)
Individuals
selected against: do not reproduce: Genetic Death
Proportion
of populations carrying negatively selected
traits
= Genetic Load
Nonrandom Reproduction:
Evolution
Antibiotics, Insecticides
Pyrethrin
Selection: Science 270:1497-1499; 1995
Na channels
locus change in tobacco budworm
Hypothesis:
50 other loci should be homogeneous
from
gene flow TX to GA
lower
proportion of selective pressure in GA, greater fixation in TX
Change in
selective pressures
will enhance
survival of previously outcompeted or lethal mutations/phenotypes
DDT,
Antibiotic resistance, etc., sickle cell
Specialized
vs simplified; convergence vs divergence,
Chilids
and water quality,
Genetic
flexibility/variation positively associated with fitness,
vs
genetically specialized:
toleration of mutation or extinction
loss of genetic
variation a concern: endangered species later
Types of
Selection
Stabilizing:
extremes don’t survive
Disruptive: Ends
favored, may result in split
Directional: one
end favored
Variability
Selection: (Science 273:922; 1996):
adaptive
flexibility correlated with environmental oscillation
complex
specializations as a result of temporal disparities in fitness
Constant vs
Periodic selection: punctuated equilibrium theory
Meteor/asteroid
impact theories: mass extinction events
Guppies,
Bacteria
Group and Kin
Selection (unit of selection)
Raising fitness
of another a cost of the individual’s fitness
Such genes
should be selected against, yet can be found:
Warning
calls, warning coloration
True Altruism
may be rare.
Wynne-Edwards:
reproductive restraint: staying below K
Subpopulations that exceed K will
starve,
can be replaced by those who have group traits
keeping population in check
How could an
altruistic trait survive? Fig. 19.11
Kin Selection:
helping those with related genetic makeup
Table
5-9: shared genes: three brothers or nine cousins
Helping:
non-breeding individuals in a group:
Eusocial
species: reproductive altruism
Tropical
wrens, Florida Scrub Jays, Red Cockaded Woodpeckers:
Jackals
Fig 19.14
Inbreeding
Increase in
Homozygosity with inbreeding Fig. 19.16
Frequency
of Alleles remains the same
Frequency
of homozygosity increases
Genetic
variation within individuals becomes
variation between individuals
Increased
expression of recessive deleterious mutations
May
purge deleterious traits or cause extinction.
Outbreeding
depression: disruption of local population adaptations
Genetic Drift
Loss of alleles
by random sampling in gametes
Larger the
population,
more
likely the F1 generation will have all the same alleles
large
populations as metapopulations: genetically small popuations
Like Inbreeding
(non-random mating)
Genetic Drift (loss of alleles from random mating)
increases homozygosity
Effective
Population Size
Members of a
population contributing to gametes to successful reproduction
Skewed Sex ratios in polygamous species
decreases EP
Bottlenecks and
Founder Effects
Minimum Viable
Population MVP