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Outline: The Evolution of Populations (Chapter 23)
January 26-31, 2010
Introduction
Evolution is a population-level phenomenonLinking Darwinian evolution and Mendelian inheritanceThe Modern Synthesis
Population Genetics
A population’s gene pool is defined by its allele frequenciesHardy-Weinberg theoremManipulating the H-W equationAssumptions of H-W
Microevolution
Natural SelectionDrift Bottleneck Founder Effect
Darwin’s arguments that life has evolved were accepted more readily than his contention that natural selection wasthe mechanism. This was partly because it was not known how characteristics were passed from generation to generation.
Theodosius Dobzhansky
Ernst Mayr
G. Ledyard Stebbins
Sewell Wright
George Gaylord Simpson
Botany
Genetics
Paleontology
Systematics
PopulationGenetics
The smallest unit that can evolve is the population ...
So what is a population?
A population is a set of individuals of the same speciesthat live close enough together to interbreed.
Mutation and Recombination during Sexual Reprodution produce the genetic variation that makes evolution possible.
Fig. 23-3
13.17 19 XX10.169.128.11
1 2.4 3.14 5.18 6 7.15
9.10
1 2.19
11.12 13.17 15.18
3.8 4.16 5.14 6.7
XX
Gene pool: the total collection of alleles present in a popu-lation 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, genotypefrequencies are given by:
p2 + 2pq + q2
where p is the frequency of one allele and q is the frequency of the other.
RR = .64 Rr = .32 rr = .04
So, if our Phlox population is in Hardy-Weinberg equilibrium, what are the expected genotype
frequencies?
Q. In a population of 500 plants, how many will have the whiteflowered phenotype?
A. 20 plants (0.04 X 500)
R = 0.8; r = 0.2
(p2) (q2)(2pq)
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.
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!
• 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
Individuals, Populations, and Species are Hierarchically Related
Biston betularia,the peppered moth
melanistic and normal forms
Allele frequencies change in response to natural selection
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
Natural selection acts on the phenotype. As particular variants are selected, favorable genotypes are maintained or increased. The unit of selection is the individual.
In this diagram, the white arrow indicatesnatural selection working against the lighter-colored phenotypes. Under directional selection, the average fur colordarkens in the population in response.
Under stabilizing selection,the average phenotype is favored. More extreme variants decrease in frequency in response.
Under diversifyingselection, both the lighterand the darker phenotypesare favored over the medium ones. Thus bothlighter and darker coatswill increase in frequency.
Odor Corolla Flair
wide narrow
sweet COMMON RARE (petals) UP HIGH
Odor
skunky RARE COMMON (sepals) DOWN LOW
DISTUPTIVE SELECTION AND DIVERGENCE: SKY PILOT VARIANTS and ELEVATIONWork of Candace Galen and students, published in Evolution, 1987
Outline: The Evolution of Populations (Chapter 23)
January 26-31, 2010
Introduction
Evolution is a population-level phenomenonLinking Darwinian evolution and Mendelian inheritanceThe Modern Synthesis
Population Genetics
A population’s gene pool is defined by its allele frequenciesHardy-Weinberg theoremManipulating the H-W equationAssumptions of H-W
Microevolution
Natural SelectionDrift 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 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
Bottlenecks in Endangered Species: the Cheetah
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.
By 1855, 1 out of every 25 residents was deaf (the national average at the time was 1/5700).
Inference: Jonathan Lambert brought anallele 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.
Year Families including
deaf members
Deaf Individuals
1694 Founder 1
1817 2 7
1827 11
1850 6 17
1855 21
1880 8 19
Outline: The Evolution of Populations (Chapter 23)
January 26-31, 2010
Introduction
Evolution is a population-level phenomenonLinking Darwinian evolution and Mendelian inheritanceThe Modern Synthesis
Population Genetics
A population’s gene pool is defined by its allele frequenciesHardy-Weinberg theoremManipulating the H-W equationAssumptions of H-W
Microevolution
Natural SelectionDrift Bottleneck Founder Effect