Download The Evolution of Populations

The Evolution of Populations
Chapter 23
BCOR 012
February 03 & 05, 2010
Outline: The Evolution of Populations (Chapter 23)
February 03, 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
Example from health science – Cystic fibrosis
Assumptions of H-W
Microevolution
Causes
Drift
Bottleneck
Founder Effect
Natural Selection - the English Peppered Moth
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.
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)
A Real Example:
Frequency of cystic fibrosis phenotype in the caucasian
population = 1/2500. What proportion of the population
are carriers? (The CF allele is recessive.)
q2 = 1/2500 = 0.0004
q = √.0004 = 0.02
2pq = 0.0392
or about 1 in 25 people
p = 0.98
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In tufted titmice, crest height is controlled by a pair of alleles
at the crest locus. A tall crest is determined by the dominant
allele T. Individuals with the tt genotype have a short crest.
In a population of 100 titmice, 9 birds are short-crested.
What proportion of the tall-crested birds are homozygous at
the crest locus?
A.
B.
C.
D.
E.
91%
49% Clicker
21%
a little over half
42%
Question
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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 genetic drift and natural selection
Genetic drift can alter population allele frequencies.
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
The African cheetah population experienced one (perhaps two) bottlenecks.
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.
The Juan Fernandez Islands, located some 700 km west of Santiago in Chile.
There and nowhere else in the world exists Thyrsopteris elegans, a genus of
tree ferns endemic to the islands.
Q. What permits novel life forms like
Thyrsopteris elegans and Darwin’s finches to
evolve on islands?
A. Genetic change in small populations as a
result of founder effect, and adaptation to the
new environmental conditions.
Outline: The Evolution of Populations (Chapter 23)
February 5
Microevolution
Natural Selection - the English Peppered Moth
Natural Selection as the Mechanism of Adaptive Evolution
Modes of Selection
Directional
Disruptive
Stabilizing
Sexual
Constraints on Natural Selection
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
Five premises underlying
Darwin’s theory:
• 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
Genetic variation occurs both within populations
and between populations
Shell color variation within a
population of marine snail
Plant height varies between yarrow
populations in the Sierra Nevada.
Sources of genetic variation:
I. Mutation, including:
• Point mutations
Sickle cell anemia results from a point mutation. Just one nucleotide
substitution in the hemoglobin gene resulted in an amino acid substitution
that causes the sickle cell disease.
• Chromosomal rearrangements
Humans have 46 chromosomes, whereas chimpanzee, gorilla, and orangutan
have 48. This major karyotypic difference was caused by the fusion of two
ancestral chromosomes to form human chromosome 2.
(Yunis and Prakash 1982)
Sources of genetic variation:
II. Crossing Over, Independent Assortment,
and Sexual Recombination
A closer look at natural selection
as the mechanism of adaptive
evolution
Two measures of fitness:
• Darwinian fitness - the contribution an
individual makes to the next generation
relative to the contribution of other
individuals.
• Relative fitness - the contribution a
particular genotype makes to the next
generation compared to the contributions of
alternative genotypes at the same locus.
Relative fitness, an example:
Consider our population of red- or white-flowered
Phlox. Let’s say the red-flowered plants are
visited preferentially by pollinating insects and so are
more successful at producing offspring. The whiteflowered plants, by contrast, leave only 75% of the
descendents that red-flowered plants do. We arbitrarily
assign a fitness value of 1.0 to the more successful genotypes; thus the fitness of RR and Rr is 1.0 and that of rr
is 0.75.
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 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.
Under stabilizing selection,
the average phenotype is
favored. More extreme variants
decrease in frequency in
response.
Which of the three
selection types would
result in black or white
snuzzles but no gray
ones?
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Clicker Question
A. Directional Selection
B.
C.
D.
E.
Stabilizing Selection
Diversifying Selection
None
All
Sexual selection has led to the evolution of sexual dimorphism in
many animal species.
Constraints on Evolution:
• Natural selection can operate only on existing features; it
can’t fashion new ones de novo
• Adaptations are often compromises - what is an optimal
solution for one challenge is not necessarily optimal for
another
• Not all evolution is adaptive