Transcript Chapter 23 The Evolution of Populations Population Genetics Combines Darwinian selection and Mendelian inheritance Population genetics - study of genetic variation within a population. Emphasis.

Chapter 23
The Evolution of Populations
Population Genetics
Combines Darwinian selection and Mendelian inheritance
Population genetics - study of genetic variation within a
population.
Emphasis on quantitative characters.
1940s – comprehensive theory of evolution (the modern
synthesis).
Until then, many did not accept that Darwin’s theory of
natural selection could drive evolution.
“The modern synthesis” combined discoveries from
• paleontology,
• taxonomy,
• biogeography, and
• population genetics.
It emphasizes the importance of populations as units
of evolution,
•
•natural
selection as the most important mechanism of
evolution,
•and
gradualism.
Allele frequencies define gene pools
500 flowering plants
480 red flowers
320 RR
160 Rr
20 white flowers
20 rr
As there are 1000 copies of the genes for color,
the allele frequencies are (in both males and females):
320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R
160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r
Population - a localized group of individuals of the same
species.
Species - a group of populations whose individuals have
the ability to breed and produce fertile offspring.
Individuals near a population center are, on average, more
closely related to one another than to members of other
populations.
A population’s gene pool is the total of all genes in the
population at any one time.
If all members of a population are homozygous for a
particular allele, then the allele is fixed in the gene pool.
The Hardy-Weinberg Theorem
Used to describe a non-evolving population.
Shuffling of alleles by meiosis and random fertilization have
no effect on the overall gene pool.
Natural populations are not expected to actually be in HardyWeinberg equilibrium.
Deviation from H-W equilibrium usually results in evolution.
Understanding a non-evolving population, helps us to
understand how evolution occurs.
Assumptions of the H-W Theorem:
- Large population size: small populations can have chance
fluctuations in allele frequencies (e.g., fire, storm).
- No migration: immigrants can change the frequency of an
allele by bringing in new alleles to a population.
- No net mutations: if alleles change from one to another,
this will change the frequency of those alleles.
- Random mating: if certain traits are more desirable, then
individuals with those traits will be selected and this will not
allow for random mixing of alleles.
- No natural selection: if some individuals survive and
reproduce at a higher rate than others, then their offspring
will carry those genes and the frequency will change for the
next generation.
Hardy-Weinberg Equilibrium
The gene pool of a non-evolving population remains constant
over multiple generations; i.e., the allele frequency does not
change over generations of time.
The Hardy-Weinberg Equation:
1.0 = p2 + 2pq + q2
where p2 = frequency of AA genotype; 2pq = frequency of Aa
plus aA genotype; q2 = frequency of aa genotype
But we know that evolution does occur within populations.
Evolution within a species/population = microevolution.
Microevolution refers to changes in allele frequencies in a
gene pool from generation to generation. Represents a
gradual change in a population.
Causes of microevolution:
1) Genetic drift
2) Natural selection (1 & 2 are most important)
3) Gene flow
4) Mutation
1) Genetic drift
Genetic drift = the alteration of the gene pool of a small
population due to chance.
Two factors may cause genetic drift:
a) Bottleneck effect may lead to reduced genetic variability
following some large disturbance that removes a large
portion of the population. The surviving population often
does not represent the allele frequency in the original
population.
b) Founder effect may lead to reduced variability when a few
individuals from a large population colonize an isolated
habitat.
*Yes, I realize that this is not really a cheetah.
2) Natural selection
As previously stated, differential success in reproduction
based on heritable traits results in selected alleles being
passed to relatively more offspring (Darwinian inheritance).
The only agent that results in adaptation to environment.
3) Gene flow
-is genetic exchange due to the migration of fertile individuals
or gametes between populations.
4) Mutation
Mutation is a change in an organism’s DNA and is
represented by changing alleles.
Mutations can be transmitted in gametes to offspring, and
immediately affect the composition of the gene pool.
The original source of variation.
Genetic Variation, the Substrate for Natural Selection
Genetic (heritable) variation within and between populations:
exists both as what we can see (e.g., eye color) and what we
cannot see (e.g., blood type).
Not all variation is heritable.
Environment also can alter an individual’s phenotype [e.g.,
the hydrangea we saw before, and…
…Map butterflies (color changes are due to seasonal
difference in hormones)].
Variation within populations
Most variations occur as quantitative characters (e.g.,
height); i.e., variation along a continuum, usually indicating
polygenic inheritance.
Few variations are discrete (e.g., red vs. white flower
color).
Polymorphism is the existence of two or more forms of a
character, in high frequencies, within a population. Applies
only to discrete characters.
Variation between populations
Geographic variations are differences between gene pools
due to differences in environmental factors.
Natural selection may contribute to geographic variation.
It often occurs when populations are located in different
areas, but may also occur in populations with isolated
individuals.
Geographic variation
between isolated
populations of house
mice.
Normally house mice are
2n = 40. However,
chromosomes fused in the
mice in the example, so
that the diploid number
has gone down.
Cline, a type of geographic variation, is a graded variation in
individuals that correspond to gradual changes in the
environment.
Example: Body size of North American birds tends to
increase with increasing latitude. Can you think of a reason
for the birds to evolve differently?
Example: Height variation in yarrow along an altitudinal
gradient. Can you think of a reason for the plants to evolve
differently?
Mutation and sexual recombination generate genetic variation
a. New alleles originate only by mutations (heritable only in
gametes; many kinds of mutations; mutations in functional gene
products most important).
- In stable environments, mutations often result in little or no
benefit to an organism, or are often harmful.
- Mutations are more beneficial (rare) in changing
environments. (Example: HIV resistance to antiviral drugs.)
b. Sexual recombination is the source of most genetic
differences between individuals in a population.
- Vast numbers of recombination possibilities result in varying
genetic make-up.
Diploidy and balanced polymorphism preserve variation
a. Diploidy often hides genetic variation from selection in the
form of recessive alleles.
Dominant alleles “hide” recessive alleles in heterozygotes.
b. Balanced polymorphism is the ability of natural selection to
maintain stable frequencies of at least two phenotypes.
Heterozygote advantage is one example of a balanced
polymorphism, where the heterozygote has greater survival
and reproductive success than either homozygote (Example:
Sickle cell anemia where heterozygotes are resistant to
malaria).
Frequency-dependent selection = survival of one phenotype
declines if that form becomes too common.
(Example: Parasite-Host relationship. Co-evolution occurs,
so that if the host becomes resistant, the parasite changes to
infect the new host. Over the time, the resistant phenotype
declines and a new resistant phenotype emerges.)
Neutral variation is genetic variation that results in no
competitive advantage to any individual.
- Example: human fingerprints.
A Closer Look: Natural Selection as the Mechanism of
Adaptive Evolution
Evolutionary fitness - Not direct competition, but instead the
difference in reproductive success that is due to many
variables.
Natural Selection can be defined in two ways:
a. Darwinian fitness- Contribution of an individual to the gene
pool, relative to the contributions of other individuals.
And,
b. Relative fitness
- Contribution of a genotype to the next generation, compared
to the contributions of alternative genotypes for the same
locus.
- Survival doesn’t necessarily increase relative fitness;
relative fitness is zero (0) for a sterile plant or animal.
Three ways (modes of selection) in which natural selection
can affect the contribution that a genotype makes to the next
generation.
a. Directional selection favors individuals at one end of the
phenotypic range. Most common during times of
environmental change or when moving to new habitats.
Directional selection
Diversifying selection favors extreme over intermediate
phenotypes.
- Occurs when environmental change favors an extreme
phenotype.
Stabilizing selection favors intermediate over extreme
phenotypes.
- Reduces variation and maintains the current average.
- Example = human birth weights.
Diversifying selection
Natural selection maintains sexual reproduction
-Sex generates genetic variation during meiosis and
fertilization.
-Generation-to-generation variation may be of greatest
importance to the continuation of sexual reproduction.
-Disadvantages to using sexual reproduction: Asexual
reproduction produces many more offspring.
-The variation produced during meiosis greatly outweighs this
disadvantage, so sexual reproduction is here to stay.
All asexual individuals are female (blue). With sex,
offspring = half female/half male. Because males don’t
reproduce, the overall output is lower for sexual
reproduction.
Sexual selection leads to differences between sexes
a. Sexual dimorphism is the difference in appearance
between males and females of a species.
-Intrasexual selection is the direct competition between
members of the same sex for mates of the opposite sex.
-This gives rise to males most often having secondary
sexual equipment such as antlers that are used in
competing for females.
-In intersexual selection (mate choice), one sex is choosy
when selecting a mate of the opposite sex.
-This gives rise to often amazingly sophisticated secondary
sexual characteristics; e.g., peacock feathers.
Natural selection does not produce perfect organisms
a. Evolution is limited by historical constraints (e.g., humans
have back problems because our ancestors were 4-legged).
b. Adaptations are compromises. (Humans are athletic due to
flexible limbs, which often dislocate or suffer torn ligaments.)
c. Not all evolution is adaptive. Chance probably plays a huge
role in evolution and not all changes are for the best.
d. Selection edits existing variations. New alleles cannot arise
as needed, but most develop from what already is present.