Download Chapter 20 - Microevolution & HW

Chapter 20
The Evolution of
Populations
Evolutionary Theory Refined
Darwin- idea of natural selection, but
needed the “how”
“How” – Mendel’s gene work
Population Genetics: Study of genetic
changes & variations among
populations
Definitions
Population: one species in one location
Species: group of individuals who can
potentially interbreed & produce fertile
offspring
Gene pool: Total aggregate of genes in a pop
at any one time
All alleles, all gene loci, all individuals
If all are homozygous for an allele, it is
“fixed”
Often there is a frequency of each
Hardy-Weinberg Theorem (1908)
Frequency of alleles & genotypes remains
constant in population’s gene pool unless
acted upon by something other
than meiosis & random fertilization
If true, alleles are in equilibrium
Serves as a null hypothesis
H-W Equation
For a gene locus with 2 alleles:
p = dominant allele
q = recessive allele
p + q = 1
H-W Equation
When gametes combine alleles during
fertilization:
 p2 = homo-dom
2pq = hetero (pq + qp)
q2 = homo-rec
p2 + 2pq + q2 = 1
H-W Assumptions/Conditions
To be in equilibrium:
Very large pop size
No migrations
No net mutations
Random mating
No natural selection
Any deviation results in evolution
(then use equation as comparison)
Let’s Practice…
You have sampled a population in which you
know that the percentage of the homozygous
recessive genotype (aa) is 36%. Using that 36%,
calculate the following:
Frequency of the aa genotype 36%
Frequency of the a allele 60%
Frequency of the A allele 40% 16%
Frequencies of the genotypes AA and Aa48%
Frequencies of the two possible phenotypes
if A is completely dominant over a 64% & 36%
Causes of Microevolution
When Hardy-Weinberg just doesn’t cut it
Evolution: generation-to-generation change
in a population’s frequency of alleles
Small scale, so microevolution
5 Agents of evolutionary change
Mutation
Gene Flow
Non-random mating
Chemical
Changes to DNA
Migration
Sexual Selection
Genetic Drift
Selection
Small population
Natural Selection
Differential Survival
Causes of Microevolution
1) Genetic Drift
Change in allele freq. due to chance
In small populations, any deviation is
amplified
Causes of Microevolution
a)Bottleneck Effect: genetic drift due to
drastic reduction in pop size
Which alleles are present is random
b)Founder Effect:
some individuals
randomly leave
and start
a new colony
Example: colonization
of the New World
Causes of Microevolution
2) Natural Selection
Differential success in reproduction
Accumulates & maintains favorable
genotypes based on environment
Causes of Microevolution
3) Gene Flow
Genetic exchange due to the
migration of fertile individuals or
gametes between populations
4) Mutation
Change in DNA (original source of
genetic variation)
Raw material for natural selection
So Genetic Variation Results From…
Mutation
Sexual Reproduction
Diploidy (keeps recessive alleles in
population through heterozygotes)
Maintenance of Genetic Variation
Balanced Polymorphism
Ability of natural selection to maintain
stable frequencies of two or more
phenotypes
1) Heterozygote advantage
Heterozygotes have some sort of
benefit
Ex: sickle-cell anemia
Sickle Cell Anemia
inherit a mutation in gene coding for one of
the subunits in hemoglobin
oxygen-carrying blood protein
normal allele = Hb
mutant allele = Hs
recessive trait = HsHs
low oxygen levels causes
RBC to sickle
clogging small blood vessels
damage to organs
often lethal
Sickle Cell Anemia
High frequency of heterozygotes
1 in 5 in Central Africans = HbHs
unusual for allele with severe
detrimental effects in homozygotes
1 in 100 = HsHs
usually die
before
reproductive
age
Malaria
Single-celled eukaryote parasite
(Plasmodium) spends part of its
life cycle in red blood cells
Maintenance of Genetic Variation
2) Frequency-dependent selection
A) Negative f-d selection

Rare phenotypes are favored
B) Positive f-d selection

Common forms are favored, thus
eliminating diversity
Natural Selection & Evolution
Three types of selection trends
Directional selection
Shifts the curve away from the
norm
Diversifying selection
Favors extremes of phenotypic
range
Natural Selection & Evolution
Stabilizing selection
Favors intermediate phenotypes &
reduces
variation
giraffe neck
horse size
human birth weight
rock pocket mice
Natural Selection & Evolution
Benefit of Sex
Genetic variation  numerous alleles
Provides varying resistance
Sexual Selection
Sexual dimorphism: distinction
of secondary sex characteristics
of males & females
Natural Selection & Evolution
Product of sexual selection
Intrasexual: competition of one sex to
mate with another (usually males for
females)
Natural Selection & Evolution
Intersexual: one sex is choosy about
mate (usually females)
Smell and Attraction - Humans
Natural Selection & Evolution
FYI…
Evoution limited by historical constraints
Adaptations are often compromises
Not all evolution is adaptive
Selection can only edit existing variations