Download 7.5 Population Genetics

Evolutionary Genetics
Evolutionary Genetics
GENETIC EQUILIBRIUM
Population:
– A group of individuals of the same
species living in the same place at the
same time
– The smallest unit that can evolve
– Evolution: change in the
prevalence of certain
heritable characteristics in a
population over a span of
generations
– Species: a group capable of
interbreeding and producing
fertile offspring
• Gene pool: the total
collection of genes in a
population at any one time
• Microevolution: a change in
the relative frequencies of
alleles in a gene pool
• The gene pool of a non-evolving
population remains constant
over the generations
– In Hardy-Weinberg equilibrium,
the frequency of each allele in
the gene pool will remain
constant unless acted upon by
other agents
– A population in equilibrium over
time is NOT evolving…
– For a population to be in HardyWeinberg equilibrium, it must
satisfy five main conditions
1. The population is very large
2. The population is isolated
3. Mutations do not alter the
gene pool
4. Mating is random
5. All individuals are equal in
reproductive success
– Equilibrium conditions are
rarely met in nature
• We can follow alleles in a
population to observe if HardyWeinberg equilibrium exists
• Equilibrium provides a basis for
understanding how populations
evolve
Webbing
No webbing
Phenotypes
Genotypes
WW
Ww
ww
Number of animals
(total = 500)
320
160
20
Genotype frequencies
320
500
160
500
= 0.64
Number of alleles
in gene pool
(total = 1,000)
640 W
Allele frequencies
800
1,000
= 0.32
160 W + 160 w
= 0.8 W
200
1,000
20
500
40 w
= 0.2 w
= 0.04
Recombination
of alleles from
parent generation
Sperm
W sperm
p = 0.8
w sperm
q = 0.2
WW
= 0.64
Ww
pq = 0.16
p2
W egg
p = 0.8
Eggs
w egg
q = 0.2
wW
qp = 0.16
q2
ww
= 0.04
Next generation:
Genotype frequencies
Allele frequencies
0.64 WW
0.8 W
0.32 Ww
0.04 ww
0.2 w
Evolutionary Genetics
POPULATION GENETICS
• Genetic drift: change in the
gene pool of a population
due to chance
•Can alter allele frequencies
in a population
•Happens when populations
are small (esp. >21)
• Two kinds of genetic drift
•Bottleneck effect: an event that
drastically reduces population
size
Original
population
Bottlenecking
Surviving
population
•Founder effect:
colonization of a new
location by a small
number of individuals
• Population is NOT Isolated
– Gene flow: the movement of
individuals or gametes
between populations
•Can alter allele frequencies in
a population
•Tends to reduce differences
between populations
• Mutation and sexual
recombination generate
variation
– Mutations-changes in the nucleotide
sequence of DNA-can create new
alleles
• Only mutations in cells that produce
gametes can affect a population's gene
pool
• A mutation may rarely improve adaptation
to the environment and thus contribute to
evolution
– Sexual recombination generates
variation by shuffling alleles during
Recap of Meiosis: Crossing over and random assortment
A1
Parents
A1

A2
A3
Meiosis
Gametes
1
A
A1
2
A
A2
3
A
A3
dddddddddddddddddddddddddddd
Fertilization
Offspring
A1
A2
A1
A3
• Mating is NOT Random
– Sexual selection may produce
sexual dimorphism
• Sexual selection (Elk)
– Leads to the evolution of secondary
sexual characteristics (like a large
rack) that may give advantage in
mating
• Sexual dimorphism (Peacocks)
– The distinction between males and
females (as between male and female
peacocks)
• Unequal Reproductive
Success
–Natural selection
•Best-adapted individuals
have the most reproductive
success
•Results in accumulation of
traits that adapt a
population to its
environment
• Natural selection can alter
variation in a population in
three ways
– Stabilizing selection: favors
intermediate phenotypes
– Directional selection: acts
against individuals at one of
the phenotypic extremes
– Disruptive selection: favors
individuals at both extremes
of the phenotypic range
Frequency of individuals
Original
population
Phenotypes (fur color)
Original
population
Evolved
population
Stabilizing selection
Directional selection
Disruptive selection
• Diploidy and balancing selection
preserve variation
– Diploidy (two sets of chromosomes)
helps to prevent populations from
becoming genetically uniform
– Balancing selection allows two or
more phenotypic forms in a population
• Balanced polymorphism may result from
– Heterozygote advantage; example: sickle-cell
disease
– Frequency-dependent selection
– Neutral variation provides no apparent
advantage or disadvantage
• The perpetuation of genes
defines evolutionary fitness
– Evolutionary fitness is the
relative contribution an individual
makes to the gene pool of the
next generation
– Survival of genes depends on
production of fertile offspring
– Selection indirectly adapts a
population to its environment by
acting on phenotype
Frequency of individuals
Original
population
Phenotypes (fur color)
Original
population
Evolved
population
Stabilizing selection
Directional selection
Disruptive selection
• Natural selection cannot fashion
perfect organisms
–
There are at least four reasons why
natural selection cannot produce
perfection
1. Organisms are limited by historical
constraints (Edits and rarely creates)
2. Adaptations are often compromises
3. Natural selection is governed by
randomness
4. Natural selection can only use existing
alleles and building blocks (usually)