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 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.

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!

• 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

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 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.

Columbine flower color changes

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 bottleneck 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