The Evolution of Populations
Chapter 23
BCOR 012
January 26-31, 2011
Outline: The Evolution of Populations (Chapter 23)
January 26-31, 2010
Introduction
Evolution is a population-level phenomenon
Linking Darwinian evolution and Mendelian inheritance
The Modern Synthesis
Population Genetics
A population’s gene pool is defined by its allele frequencies
Hardy-Weinberg theorem
Manipulating the H-W equation
Assumptions of H-W
Microevolution
Natural Selection
Drift
Bottleneck
Founder Effect
Darwin’s arguments that life has evolved were accepted
more readily than his contention that natural selection was
the mechanism. This was partly because it was not known
how characteristics were passed from generation to generation.
Botany
Genetics
Population
Genetics
G. Ledyard Stebbins
Theodosius Dobzhansky
Paleontology
Systematics
Sewell Wright
Ernst Mayr
George Gaylord Simpson
The smallest unit that can evolve is the population ...
So what is a population?
A population is a set of individuals of the same species
that live close enough together to interbreed.
Mutation and Recombination during Sexual Reprodution
produce the genetic variation that makes evolution possible.
1
2.4
3.14
5.18
6
7.15
8.11
9.12
10.16
13.17
19
XX
1
9.10
Fig. 23-3
2.19
3.8
4.16
11.12 13.17 15.18
5.14
6.7
XX
Gene pool: the total collection of alleles present in a population is that population’s gene pool.
A population’s gene pool is defined by its allele frequencies.
Example:
• Flower color in Phlox is determined by
alternative alleles at the color locus.
• R is dominant to r, and results in red color. The
rr genotype yields white color.
• In one population, the frequency of R has been
determined to be 0.8, whereas the frequency of r
is 0.2. (Note that allele frequencies sum to 1.0)
Hardy-Weinberg Theorem
The Hardy-Weinberg theorem states that, in
a non-evolving population, allele and
genotype frequencies remain constant
through time.
If a population is in Hardy-Weinberg equilibrium, genotype
frequencies are given by:
p2 + 2pq + q2
where p is the frequency of one allele and q is the frequency of the other.
R = 0.8; r = 0.2
So, if our Phlox population is in HardyWeinberg equilibrium, what are the expected genotype
frequencies?
RR = .64
(p2)
Rr = .32
(2pq)
rr = .04
(q2)
Q. In a population of 500 plants, how many will have the white
flowered phenotype?
A. 20 plants (0.04 X 500)
Assumptions of Hardy-Weinberg:
•
•
•
•
No mutations
Large population size
No migration
No natural selection (i.e., all members
survive and reproduce)
• Random mating
These conditions are almost never met in nature. Thus HW is
an ideal case.
Q. So how do populations evolve?
A. Through natural selection and genetic drift.
Genetic drift can alter population allele frequencies.
Columbine flower color changes
Five premises underlying
Darwin’s theory of Evolution by
Natural Selection:
• Variability: Populations of organisms are variable
• Heritability: Some of the variable traits are passed from
generation to generation
• Overproduction: More individuals are produced in a
population than will survive to reproduce
• Competition: Individuals compete for limited resources
Natural Selection
• Differential Survival: Those individuals better
suited to their environment will leave more
descendents than less well suited individuals.
This is natural selection!
Individuals, Populations, and Species are
Hierarchically Related
• Species
A species is a set of populations that are reproductively
isolated from other such population sets.
• Populations
A population is a set of conspecific individuals
living close enough together to interbreed. The
population is the smallest unit of evolution
• Individuals
Selection acts upon the individual
Allele frequencies change in response to natural selection
Biston betularia,
the peppered moth
melanistic and normal forms
Reference: Kettlewell, H. B. D. 1961. The phenomenon of industrial melanism
in Lepidoptera. Ann. Rev. of Entomol. 6: 245 - 262.
The peppered moths satisfy the
conditions for natural selection:
• the population is variable
• color pattern is inherited
• the different forms have different fitnesses
Genetic Variation and Natural Selection
Variation is the raw material of evolution
Natural selection acts on the phenotype. As particular
variants are selected, favorable genotypes are maintained
or increased. The unit of selection is the individual.
Natural selection can be directional, diversifying, or stabilizing.
In this diagram, the white arrow indicates
natural selection working against the
lighter-colored phenotypes. Under
directional selection, the average fur color
darkens in the population in response.
Under stabilizing selection,
the average phenotype is
favored. More extreme variants
decrease in frequency in
response.
Under diversifying
selection, both the lighter
and the darker phenotypes
are favored over the
medium ones. Thus both
lighter and darker coats
will increase in frequency.
Columbine flower color changes
DISTUPTIVE SELECTION AND DIVERGENCE:
SKY PILOT VARIANTS and ELEVATION
Work of Candace Galen and students, published in Evolution, 1987
Odor
Corolla Flair
wide
sweet
(petals)
COMMON
UP HIGH
narrow
RARE
Odor
skunky
(sepals)
RARE
COMMON
DOWN LOW
Outline: The Evolution of Populations (Chapter 23)
January 26-31, 2010
Introduction
Evolution is a population-level phenomenon
Linking Darwinian evolution and Mendelian inheritance
The Modern Synthesis
Population Genetics
A population’s gene pool is defined by its allele frequencies
Hardy-Weinberg theorem
Manipulating the H-W equation
Assumptions of H-W
Microevolution
Natural Selection
Drift
Bottleneck
Founder Effect
Two situations can shrink a population to a
size small enough for genetic drift to operate:
• The bottleneck effect
• The founder effect
The bottleneck effect
Bottlenecks in Endangered Species: the Cheetah
The African cheetah populations experienced two bottlenecks, one at the
beginning of the Holocene (10,000 ybp) and one 100 years ago.
Consequently, cheetah populations are depauperate in genetic variability.
Cheetah Painting © 2007, Jason Morgan, International Wildlife
Founder effect is the establishment of a new population by a few
original founders which carry only a small fraction of the total genetic
variation of the source population.
Founder Effect: Deafness on Martha’s
Vineyard
Martha’s Vineyard census data recorded the early prevalence of deafness.
1694: Jonathan Lambert is the first documented deaf individual on Martha’s
Vineyard. He had two deaf children.
Year
Families
Deaf
including Individuals
By 1855, 1 out of every 25 residents was
deaf
deaf (the national average at the
members
time was 1/5700).
Inference: Jonathan Lambert brought an
allele for deafness from Kent England,
The trait is recessive. The allele frequency
increased to high levels because the islanders
did not interbreed with mainlanders in early
times.
This illustrates a Founder Effect.
1694
Founder
1
1817
2
7
1827
1850
11
6
1855
1880
17
21
8
19
Outline: The Evolution of Populations (Chapter 23)
January 26-31, 2010
Introduction
Evolution is a population-level phenomenon
Linking Darwinian evolution and Mendelian inheritance
The Modern Synthesis
Population Genetics
A population’s gene pool is defined by its allele frequencies
Hardy-Weinberg theorem
Manipulating the H-W equation
Assumptions of H-W
Microevolution
Natural Selection
Drift
Bottleneck
Founder Effect