Transcript Nerve activates contraction

Chapter 23 – Population Genetics
The Evolution of Populations (Outline)
• The smallest unit of evolution
• It is the population, not the individual, that
evolves.
23.6.
Caribou populations
in the Yukon. One
species, two
populations.
These two
populations are not
totally isolated.
Yet, individuals of
population I are more
likely to breed with
members of their
own population than
with members of
population II and are
thus more closely
related to one
another than to
members of the
other population.
Chapter 23 – Population Genetics
The Evolution of Populations (Outline)
• The smallest unit of evolution
• It is the population, not the individual, that evolves.
• Gene pool, allele frequencies, Hardy-Weinberg
equilibrium
• Conditions that must be met for a population to
remain in Hardy-Weinberg equilibrium.
• A closer look at natural selection.
• Directional, disruptive and stabilizing selection
Can new alleles be formed?
• Mutation: A change in the nucleotide sequence of an
organism’s DNA.
• Only changes in cells that produce gametes can be
passed on to offspring.
• Even a change in one base pair (a “point” mutation) can
impact the phenotype of an organism (f.e. sickle-cell
anemia)
• Mutations are rare (about 1 mutation in 100,000
genes per generation) and are commonly lethal to the
organism.
• Mutations and sexual reproduction produce genetic
variation.
How do we measure genetic variation? Computing allele frequencies.
Population with N individuals (thus 2N allelles) of red-flowered
(AA, Aa) and white flowered (aa) plants.
# of AA = X
# of Aa = Y
# of aa = Z
 X + Y + Z = N
Each AA individual has 2 A alleles
Each Aa individual has 1 A allele
 total # of A alleles = 2X + Y
Similarly, the total number of a alleles = 2Z + Y
Since the total number of alleles = 2N,
the frequency of the A allele = p = (2X + Y)/ 2N
the frequency of the a allele = q = (2Z + Y)/ 2N
the frequency of the A allele = p = (2X + Y)/ 2N
the frequency of the a allele = q = (2Z + Y)/ 2N
If the allele frequencies are the same in two populations,
are the genotypic and phenotypic frequencies also the same?
Two populations with each 200 individuals
Pop 1: 90AA, 40Aa, 70aa
Pop 2: 45AA, 130Aa, 25aa
Population 1: p = (2X + Y)/ 2N = (180 + 40)/400 = 0.55
q = (2Z + Y)/ 2N = (140 + 40)/400 = 0.45
Population 2: p = (90 + 130)/400 = 0.55
q = (50 + 130)/400 = 0.45
Allele frequencies are the same (same gene pool), but alleles are
distributed differently among genotypes (different genotypic and
phenotypic structure)
Mendelian inheritance
preserves genetic
variation from one
generation to the next.
(see book page 473 for a
discussion of a similar
example).
No matter how many
generations of alleles are
segregated (by meiosis) and
combined (by fertilization)
the allele frequency in the
gene pool will remain
constant unless acted upon
by “selective” forces.
23.8.
Hardy-Weinberg principle.
The gene pool remains constant
from one generation to the next.
Mendelian processes alone do not
alter allele frequencies
The probability of generating
an CRCR genotype is
p2=0.8 X 0.8 = 0.64
The probability of generating
an CWCW genotype is
q2=0.2 X 0.2 = 0.04
The probability of generating
an CRCW genotype is
2pq= 2 X 0.8 X 0.2 = 0.32
P2 + 2pq + q2 = 1
How does this apply to the human condition?
(see also book page 474)
For example: Calculate % of human population that
carries an allele for a particular inherited disease
(i.e., is heterozygous for this allele)
Phenylketon uria (PKU), inherited disorder that may lead to
mental retardation caused by improper metabolic processing
of phenylanaline (essential aminoacid not metabolized due to
a missing enzyme). One in 10,000 babies are born with PKU
Due to a recessive allele so frequency of individuals
in the U.S. born with PKU = q2.
q2 = 0.0001  q = √0.0001 = 0.01. Therefore p = 0.99
Frequency of carriers (that may pass the allele on to offspring)
= 2pq = 2 x 0.99 x 0.01 = 0.0198 or
approx 2% of the U.S. population.
Hardy-Weinberg Assumptions
• 1.
No mutations (because they alter the gene pool)
• 2. Population is isolated, i.e. no migration of
individuals into or out of the population (no gene
flow)
• 3. Population must be very large and made up of
sexually reproducing diploid individuals (small
populations show genetic drift)
• 4.
Mating is random (gametes mix randomly)
• 5. All individuals must survive and reproduce
equally well (no natural selection)
A population that follows
the Hardy-Weinberg rule is
non-evolving!
In a Hardy-Weinberg population, the
frequency of the a allele is 0.4.
What is the frequency of individuals
with Aa genotype?
A. 0.20
B. 0.48
C. 0.60
D. 0.16
E. approximately 1.0
Hardy-Weinberg Assumptions
• 1. No mutations (because they alter the gene
pool)
• 2. Population is isolated, i.e. no migration
of individuals into or out of the population
(no gene flow)
• 3. Population must be very large and made
up of sexually reproducing diploid individuals
(small populations show genetic drift)
23.9. Genetic drift
23.8. Genetic drift
23.8. Genetic drift
23.10.
The bottleneck effect:
an analogy
Cheetahs, the bottleneck effect
23.11. Bottleneck effect and
reduction of genetic variation.
The Illinois population of
prairie chickens dropped from
millions of birds in the 1800s
to just 50 birds in 1993
(habitat loss due to the conversion of
native tallgrass prairies to cropland).
Consequently, as a result of
genetic drift, both the
number of alleles per locus
(mean across six loci studied)
and the percentage of eggs
that hatched decreased.
Hardy-Weinberg Assumptions
• 1.
No mutations (because they alter the gene pool)
• 2. Population is isolated, i.e. no migration of
individuals into or out of the population (no gene
flow)
• 3. Population must be very large and made up of
sexually reproducing diploid individuals (small
populations show gene drift)
• 4.
Mating is random (gametes mix randomly)
• 5. All individuals must survive and reproduce
equally well (no natural selection)
A nonheritable difference within a population
Spring
Seasonal
differences in
coloration are
due to hormones
and not to
genetic
differences.
Late Summer
European Map Butterflies
Polymorphism: Genetic variation within populations.
(somewhat stable frequencies of two or more discrete
phenotypes in a population, maintained by selection)
Genetic variation within a population may also be maintained
by geographical variation.
Cline: Gradual changes in some trait over a geographical area.
23.5. A cline determined by temperature (individuals with
the Ldh-Bb allele can swim faster in cold water than can
individuals with other alleles.
The Ldh-Bb allele codes for the production of an enzyme that
is an excellent metabolic catalyst in cold waters.
Balanced polymorphism: Genetic variation within a
population may also be maintained when the
heterozygote is at an advantage.
Normal and sickled cells: Natural selection maintains
two or more alleles at the same locus
23.17. Mapping malaria and the sickle-cell allele
23.13. Modes of selection
Stabilizing selection for beak size in a Galápagos population of the
medium ground finch (Grant and Grant)
Disruptive selection in a finch population
Smaller-billed birds
feed more efficiently
on small seeds.
Larger-billed birds can
crack hard seeds.
Chapter 23 Review (p.485-486)
• 23.1. Mutation and sexual reproduction produce
the genetic variation that makes evolution possible.
• 23.2. The Hardy-Weinberg equation can be used
to test whether a population is evolving.
• 23.3. Natural selection, genetic drift, and gene
flow can alter allele frequencies in a population.
• 23.4. Natural selection is the only mechanism
that consistently causes adaptive evolution.
In a West African finch species, birds with large and
small bills survive better than birds with
intermediate-sized bills. The type of natural selection
operating on these bird populations is
A. directional selection.
B. deme selection.
C. stabililizing selection.
D. nonrandom selection.
E. disruptive selection.