Transcript Microevolution Evolution within a population

Microevolution
What is the smallest unit that
can evolve?
Do you remember
how evolution
a) Individual
b) can
Speciesbe
c) Genus
measured?
d) Population
Measure the
Finalfrequency
Answer?
allele
in the gene pool
d!
Calculating Allele Frequencies
Phenotypes for flower color (R = red, r = white)
320 RR
160 Rr
20 rr
160 “R” and 160 “r”
Number of alleles:
640 “R”
Allele frequencies:
FR = 800/1000 = .8
40 “r”
Fr = 200/1000 = .2
Total alleles in
gene pool: 1000
To measure evolution, calculate the
allele frequency before and after a
given amount of time.
If the frequency is the same or
constant  no evolution has taken
place
A non-evolving population is said to
be in “Hardy-Weinberg Equilibrium”
Hardy-Weinberg (HW) Equations
Let’s revisit the previous example involving
flower color….
Let p = allele frequency of dominant allele (R)
Let q = allele frequency of recessive allele (r)
Then… p + q = 1
√ .8 + .2 = 1
HW Equation # 1
Hardy-Weinberg (HW) Equations
If p = allele frequency of dominant allele (R), then
p2 = frequency of homozygous genotype (RR)
If q = allele frequency of recessive allele (r), then
q2 = frequency of homozygous genotype (rr)
If you complete the square, then the frequency of
heterozygous genotype Rr must be 2pq
So…
p2 + 2pq + q2 = 1
√ .64 + .32 + .04 = 1
HW Equation # 2
5 Assumptions for HW equilibrium
1)
2)
3)
4)
5)
Large Population Size
No Migration
No Mutation
Random Mating
No Natural Selection
If all 5
conditions
are met, no
evolution will
occur
Any deviation results in
microevolution of the population
What is microevolution?
A generation to
generation change in
a population’s allele
frequency
What causes microevolution?
5 opposing conditions to HW equilibrium
1)
2)
3)
4)
5)
Genetic Drift
Gene Flow
Mutation
Non-random mating
Natural Selection
Genetic Drift
In a small population, some individuals may,
just by chance, leave behind a few more
descendents (and genes, of course!) than
other individuals.
Genetic Drift always reduces diversity
2 Examples of Genetic Drift
1. Population Bottleneck
– A natural disaster or event causes a population’s
size to be greatly reduced
Initial Diverse
Population
Event that causes
the population to
be reduced in size
Final
population (not
diverse)
Over-poaching has bottlenecked the cheetah
population, which is now susceptible to random
changes in gene frequencies
Reduction in diversity and inbreeding puts this
species at risk of extinction
2 Examples of
Genetic Drift
2. Founder Effect
– Changes in gene
frequencies that
usually accompany
starting a new
population from a
small number of
individuals.
Return to Causes of
Microevolution
Gene Flow
•
•
Individuals from one group move into
another group.
Makes certain genes more frequent in
the population.
Return to Causes of
Microevolution
Mutation
•
A change in a DNA sequence
– usually occurring because of errors in
replication or repair.
•
Mutation is the ultimate source of genetic
variation.
Return to Causes of
Microevolution
Non-Random Mating
• Individuals choose mates based on
preferences for particular traits
• 2 examples
– 1) Sexual Selection – mates are chosen based
upon behavior/appearance
– 2) Inbreeding – the choice to mate with related
individuals
Female peacocks choose mates based upon the male’s
plumage display
Return to Causes of
Microevolution
Natural Selection
• Increases or decreases in allele
Strength of
Selection
frequencies
due to environmental impact
• Natural Selection can act upon a
population in a variety of ways…
Diversifying Selection =
Directional Selection =
Stabilizing Selection =
environmental
favors traits that are at
conditions
are
varied
selection that acts against
one extreme of a range of
such that individuals at
extreme
and
both
extremesphenotypes
are
traits
favors
favored intermediates