Evolution and Population Genetics

Evidence for evolutionary change

Mechanics and

Hardy-Weinberg Calculations

1. Fossil record. •Material imprinted in rock•Law of superposition: older fossils on bottom

2. Anatomical evidence•Homologous structures: similar bone structure but different function. Ex: bat wing, whale flipper, human arm•Vestigial structures: no longer functional but retained in anatomy Ex: appendix, wisdom teeth•Modified structures: adapted for new function Ex: Panda’s “thumb” = wrist bones modified to strip leaves from bamboo•Embryologic similarities

3. Molecular evidence•Shared sequences of bases in DNA

Evidence for Evolution

Evidence for evolution (continued)

4. Adaptive radiation• Ancestral pioneer arrives at new habitat.• New species evolve = speciation• By occupying different niches, they reduce competition.• Example: Darwin’s finches – 13 different but related

species on the Galapagos Islands

Archaeopteryx(oldest knownfossil bird)

Fossils

LAW OF SUPERPOSITION

Why does a whale have a kneecap?

The Evolution of Populations

Remember individual organisms do not evolve. Individuals are selected, but it is populations that evolve.

Because evolution occurs when gene pools change from one generation to the next, understanding evolution require us to understand population genetics.

Some terminology

Population: All the members of one species living in single area.

Gene pool: the collection of genes in a population. It includes all the alleles of all genes in the population.

Some terminology

If all individuals in a population all have the same allele for a particular gene that allele is said to be fixed in the population.

If there are 2 or more alleles for a given gene in the population then individuals may be either homozygous or heterozygous (i.e. have two copies of one allele or have two different alleles)

Detecting evolution in nature

Evolution is defined as changes in the structure of gene pools from one generation to the next.

How can we tell if the gene pool changes from one generation to the next?

We can make use of a simple calculation called the Hardy-Weinberg Equilibrium

Hardy-Weinberg EquilibriumIn meiosis, individuals alleles are sorted into gametes (sperm or egg) which may combine to form a zygote.A population of 100 organisms has a minimum of 200 alleles for each trait, one from each parent.To determine probability of two independent events (sorting of parental alleles), multiply the probabilities of individual events.If 80% of the alleles in a gene pool are “A” and 20% are “a”, what are the probabilities for each genotype (AA, Aa, and aa)?

Hardy-Weinberg Equilibrium

If 80% of the alleles in the gene pool are A and 20% are a, we can predict the genotypes in the next generation.

Basic probability: To determine the probability of two independent events both occurring, you should multiply the probabilities of the individual events together.

Hardy-Weinberg Equilibrium

Probability of an AA individual is 0.8*0.8 = 0.64Probability of an aa individual is 0.2*0.2 = 0.04Probability of an Aa individuals is 0.2*0.8 = 0.16, but there are two ways to produce an Aa individual so 0.16*2= 0.32. Note the probabilities add up to 1 (100%)

General formulae

Allele frequencies:p + q = 1.

Genotypic frequencies:p2 + 2pq + q2 = 1,

where p is frequency of one allele and q is frequency of the other allele. NOTE: p2 and q2 are homozygous and pq is heterozygous

Hardy-Weinberg Equilibrium

Hardy-Weinberg equilibrium can be used to estimate allele frequencies from information about phenotypes and genotypes.

Hardy-Weinberg ConditionsNo gene flow: movement of individuals to/from the populationRandom mating: no significant preference when choosing mate. Increases homozygote genotype.Large population size: reduce effect of small, chance events (genetic drift: bottleneck or founder effect)No natural selection which would otherwise select for or against a particular genotype.No mutations.

Bottleneck Effect

The bottleneck effect occurs when some disaster causes a dramatic reduction in population size.

As a result, by chance certain alleles may be overrepresented in the survivors, while others are underrepresented or eliminated. Genetic drift while the population is small may lead to further loss or fixation of alleles.

Humans have been responsible for many bottlenecks by driving species close to extinction.

Bottleneck Effect

The Northern Elephant seal population for example was reduced to about 20 individuals in the 1890’s. Population now >30,000, but an examination of 24 genes found no variation, i.e. there was only one allele. Southern Elephant Seals in contrast show lots of genetic variation.

23.8

Founder Effect

Populations founded by only a few individuals (ex: island communities)

Gene pool is unlikely to be as diverse as the source pool from which it was derived.

Example: polydactylism (having extra fingers) is common among the Amish

Founder effect coupled with inbreeding explains the high incidences of certain recessive diseases among humans in many isolated island communities.

Hardy-Weinberg Equilibrium

If a population is found to depart significantly from Hardy-Weinberg equilibrium this is strong evidence that evolution is taking place.

Natural Selection the primary mechanism of adaptive evolution

Primary mechanism of adaptive evolution

“survival of the fittest” means individuals reproduce thus contributing genes to the next generation

May depend on differences in ability to gather food, hide from predators, or tolerate extreme temperatures, which all may enhance survival and ultimately reproduction

Natural Selection the primary mechanism of adaptive evolution

Three major forms of natural selection: Directional

Disruptive

Stabilizing

Directional Selection

Favors one extreme in the population

Average value in population moves in that direction

E.g. Selection for darker fur color in an area where the background rocks are dark

Disruptive selection

Intermediate forms are selected against. Extremes are favoredE.g. Pipilo dardanus butterflies. Different forms of the species mimic the coloration of different distasteful butterflies. Crosses between forms are poor mimics and so are selected against by being eaten by birds.

Stabilizing Selection

Most commonExtreme forms are selected againstExample: Human birth weights. Highest survival is at intermediate birth weights. Babies that are too large cannot fit through the birth canal, babies that are born too small are not well developed enough to survive