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.

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 species that live close enough together to interbreed.

Mutation and Recombination during Sexual Reprodution produce the genetic variation that makes evolution possible. Fig XX XX

Gene pool: 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. whiteR 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: p 2 + 2pq + q 2 where p is the frequency of one allele and q is the frequency of the other.

RR =.64Rr =.32rr =.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 white flowered phenotype? A. 20 plants (0.04 X 500) R = 0.8; r = 0.2 (p 2 ) (q 2 ) (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 The phenomenon of industrial melanism in Lepidoptera. Ann. Rev. of Entomol. 6:

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

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 ELEVATION Work 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 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

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 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. YearFamilies including deaf members Deaf Individuals 1694Founder

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