Transcript Document

Chap. 23 – Population Evolution
How do Populations
Some characteristics
evolve?
become:
More/Less common
within the population
Microevolution = a
change in the genetic
makeup of a population
from
one generation
Before Drought
After
Drought
to the
next
Definitions:
Population– group of individuals
of the same species living in a certain
area
Species – a group of populations
whose members can interbreed and
produce fertile offspring
‘Happy Feet’ Penguin!
Galapagos Penguin!
Gene pool = ALL alleles of ALL genes of
ALL the individuals in a population
A gene is FIXED if all members of the
population are homozygous for the same
allele.
ex. all bb  Can this change?
Yes! By mutation, you can change a fixed allele (bb)
Mutations + Natural Selection
+ Sexual recombination = Evolution
So, why and when does a
population evolve?

Backwards approach:
 What will be the hallmarks of a
NONEVOLVING population?
What is the frequency of alleles in a
population?
A = pink a = white
500 plants
AA
Aa
aa
320 plants
160 plants
20 plants
There are 1000 copies of the gene in the population
p
AA
Aa
A = 2(320) + 160 = 800 = 80% frequency =0.8 for A
q
aa
Aa
a = 2(20) + 160 = 200 = 20% frequency =0.2 a for a
AA
Aa
320 plants
aa
160 plants
Frequency of allele A = 0.8
20 plants
Frequency of allele a = 0.2
Genotype frequencies are:
AA (320/500) = .64 = .8 x .8
p2
Aa (160/500) = .32 = 2(.8 x .2) 2pq
aa (20/500) = .04 = .2 x .2
q2
p2 + 2pq + q2 = 1
Will this genotype become extinct very soon?
0.5A
0.25AA
0.5 a
0.5 A
0.25Aa
0.64AA
0.16Aa
0.16Aa
0.04aa
0.8A
0.5A
0.25Aa
0.25aa
0.5a
Aa X Aa
0.5 a
0.2a
Aa
Aa X Aa
A = 50%
A = 80%
a = 50%
a = 20%
Hardy-Weinberg Theorem—
Frequencies of alleles and
genotypes in a population’s gene
pool remain constant over
generations
(unless, of course, there are
outside circumstances)
p = frequency of dominant allele (A)
q = frequency of recessive allele (a)
p+q=1
p2 + 2pq + q2 = 1
Hardy-Weinberg
Equation
p2 = frequency of AA
2pq = frequency of Aa
q2 = frequency of aa
p2 + 2pq + q2 = 1
Given: how many people are
homozygous recessive for a certain gene
Can you can figure out the allele
frequencies in the population?
ex: 1/10,000 babies has PKU (recessive)
q2 = _____
0.0001
p2 = _____
0.9801
q = ___
0.01
2pq = _____
0.0198
(carriers)
p = ___
0.99
Hardy-Weinberg equilibrium only
occurs if a population is NOT evolving.
MICROEVOLUTION is a change in
allele frequency from one generation
to the next in a population.
WHITE HAMSTERS = bb
GREY HAMSTERS = BB or Bb
This will keep populations in Hardy Weinberg
Equilibrium (NO EVOLUTION):
1. Very large population size
2. No Migrations
3. No net mutations
4. Random mating
5. No natural selection (NO
“reproductively fit” allele)
Microevolution is caused by
departures from the 5
conditions needed for
equilibrium:
1. small populations (genetic drift
-important)
2. gene flow (migration)
3. mutation
4. nonrandom mating
5. natural selection (important)
Population Genetics—
Darwin – Natural Selection
Mendel – genetic basis for variation
1. Genetic Drift = change in allele frequency due to CHANCE. Ex: Billy goat determines which
plants survives by randomly chewing off some flowers. So the allele frequency may be not 0.5 R and
0.5r in each generation.
2 types of drift….
The Bottleneck Effect (skewed representation of
alleles after disasters) can lead to genetic drift.
‘Bottle neck’ is the disaster! Alleles left after
disaster may not be 0.5R and 0.5r….Ex: cheetahs and
hunting
The Founder Effect—a small number of
individuals colonize a new, isolated area—
this can lead to genetic drift. Ex: eye disease alleles
have a high frequency in the founders of a colony
<>
1814 - Tristan da Cunha – colonized by 15 people!
2. Gene Flow - Migration
If populations aren’t completely
isolated, individuals can migrate
and introduce alleles into another
population. Ex: wind… pollinators…
This may cause a change in allele
frequency in the next generation.
3. Mutations
Mutations can change the frequency of
alleles in a population, but this is a very
slow effect in humans. Bacteria - this is
fast!
Mutation is one of the sources of genetic
variation that leads to natural selection.
4. Nonrandom Mating
Inbreeding and assortive mating cause
an increase in homozygotes.
Allele frequencies will not change, but
genotype frequencies will.
Nonrandom Mating
Eskimo
M/M M/N N/N M/M M/N N/N
0.835 0.156 0.009 0.150 0.700 0.150
Egyptian
0.278 0.489 0.233
Chinese
0.332 0.486 0.182
Australian
.024
0.304
OBSERVED
0.672
EXPECTED from
Hardy-Weinberg
Theorem
5. Natural Selection
Differential success in reproduction
If ‘aa’ is less ‘reproductively fit’, then
the frequency of the ‘a’ allele will
decrease. Ex: Cow likes red flower!
Allele under case study:
The ‘Asian’ getting a B is = failing gene
-AA : must have an A+
-Aa - will be okay with an ‘A’
-aa - what a shame even an ‘A-’ is just
awlright! (tsk tsk tsk)
Initial frequency of these genes in a populations is:
p = 0.8 (A)
q = 0.2 (a)
What 5 ways can you influence this
allele frequency to change and cause
microevolution? Break Hardy
Weinberg’s equilibrium…
Variation within populations
1. quantitative characteristics are
Polygenic (height)
2. discrete characteristics are
determined by a single gene locus
(freckles)
--2 or more forms represented =
polymorphic
Geographical variation—there are
differences in gene structure between
populations in different areas
Genetic variation is created
by
Mutation and sexual
recombination (Crossing over
during Prophase 1,
Independent assortment
during Anahase 1, and
Randomn fertilization).
This variation leads to
natural selection if there is a
selective advantage in one
allele type.
Diploidy prevents natural
selection from quickly
getting rid of an unfavorable
allele. (Aa; ‘a’ hides)
Aa
(aa = albino
AA/Aa = normal)
How are undesirable
alleles maintained in
a population?
Sickle Cell Anemia
1) Diploidy heterozygote carries
the hidden recessive
gene
2) Heterozygote
(aa – anemia
advantange also
Aa = carrier +
maintains
malaria protection
undesirable alleles.
AA = normal)
Another ex. for how Host Parasite
natural selection
Interaction= Red
maintains variation or Queen
undesirable alleles
Frequency-dependent
selection—
Reproductive success of
one ‘morph’ declines
when it becomes too
common in the
population due to
coevolution of predators.
Ex: Brown/green Fox and
mite..
Balanced polymorphism maintains genetic
diversity in a population via natural selection
Heterozygote advantage
Undesirable
alleles/variation is
preserved by
Frequencydependent
selection
Variation is caused by
Sexual
Heterozygote
Recombination
advantage
FrequencyAnd Mutations
Diploidy
dependent
Variation
is
the
Natural selection
Raw
Material
for
selection
TO SUMMARIZE: Natural Selection
Types of
SELECTION:
1) Directional
2) Diversifying
3) Stabilizing
Types of SELECTION:
4) Sexual Selection - natural selection for traits
that increases mating chance
- Intersexual Selection – female selects
reproductively fit male
- Intrasexual selection – males combat for female
ARE YOU A ‘FIT’ INDIVIDUAL?
Darwinian Fitness:
the relative contribution that an
individual makes to the gene pool
of the next generation
So why aren’t we perfect?
1. organisms are locked into historical constraints
2. adaptations often are compromises
3. not all evolution is adaptive
4. selection only can edit variations that exist