Evolution of Populations Chapter 23

Basics of Population Genetics Linked Mendelian genetics with Darwinian evolution. Population -- Group of organisms which belong to the same species and live in the same area. Species -- Groups of interbreeding natural populations. Allele – A version of a particular gene. Gene pool – All genes in a population at any one time; usually two or more alleles for a gene, each having a relative frequency in the gene pool. Gene flow -- Movement of alleles between populations.

Hardy-Weinberg Hardy-Weinberg equilibrium compares how common certain alleles are in natural populations whose gene pools may be changing. For Hardy-Weinberg equilibrium to be maintained, five conditions must be met: 1. Very large population size. (No genetic drift) 2. Isolation from other populations. (No migration) 3. No mutations. (Changes only due to recombination during reproduction) 4. Random mating. (Non-random can promote inbreeding) 5. No natural selection. (Equal survival and reproductive success)

H-W cont. In real populations, several factors can upset Hardy-Weinberg equilibrium and cause microevolutionary change. Microevolution – a shift in a population's allele frequencies; can be caused by genetic drift, gene flow, mutation, nonrandom mating, and natural selection.

Genetic Drift Genetic drift -- Changes in the gene pool of a small population due to chance. A. Bottleneck Effect: size of a population reduced drastically by a natural disaster which kills organisms non-selectively; reduces overall genetic variability in a population. South African cheetahs -- the large population was severely reduced during the last ice age and again by hunting to near extinction.

Genetic Drift cont. B. Founder Effect -- When a few individuals colonize a new habitat, the smaller the founding population, the less likely its gene pool will be representative of the original population's genetic makeup. Tristan da Cunha islands colonized by 15 people in 1814; frequency retinitis pigmentosa is much higher on this island than in the populations from which the colonists came. Amish also have various recessive genetic disorders; including anemia, dwarfism, polydactyly, etc.

The Blue People

“I’m My Own Grandpaw” Many, many years ago when I was twenty-three I was married to a widow who was pretty as could be. This widow had a grown-up daughter who had hair of red. My father fell in love with her and soon they, too, were wed. This made my dad my son-in-law and changed my very life For my daughter was my mother, 'cause she was my father's wife. To complicate the matter, even though it brought me joy I soon became the father of a bouncing baby boy.

My little baby then became a brother-in-law to dad And so became my uncle, though it made me very sad For if he was my uncle, then that also made him brother To the widow's grown-up daughter, who, of course, was my step-mother. My father's wife then had a son who kept them on the run And he became my grand-child, 'cause he was my daughter's son. My wife is now my mother's mother, and it makes me blue Because, although she is my wife, she's my grandmother too.

If my wife is my grandmother, then I am her grandchild And every time I think of it, it nearly drives me wild. This has got to be the strangest thing I ever saw. As husband of my grandmother, I am my own grandpaw. Chorus I'm my own grandpaw I'm my own grandpaw It sounds funny I know but it really is so Oh, I'm my own grandpaw

Using the Hardy-Weinberg Theorem Genotypes: AA, Aa, aa p = frequency of A allele. q = frequency of a allele. p + q = 1 in a population.(q = 1-p; p = 1-q) p2 = frequency of AA genotype. q2 = frequency of aa genotype. 2pq = frequency of Aa genotype. p2 + 2pq + q2 = 1 in a population.

H-W continued Frequency of A is 0.3. What is freq of a? Freq of AA genotype? p2 = 0.3 x 0.3 = 0.09 Freq of aa genotype? q2 = 0.7 x 0.7 = 0.49 Freq of Aa genotype? 2pq = 2 x 0.3 x 0.7 = 0.42 So, 9% of population are AA, 49% are aa, and 42% are Aa.

More H-W In a population,16% of people show a recessive trait. Determine the frequencies of the 3 genotypes. aa = q2 = 0.16; so q = 0.4 p = 1 – 0.4 = 0.6 AA = p2 = 0.6 x 0.6 = 0.36 (or 36%) Aa = 2pq = 2 x 0.6 x 0.4 = 0.48 (or 48%)

Practice makes perfect… 1 of every 10,000 babies in the United States is born with phenylketonuria (PKU). The allele for PKU is recessive, so babies with this disorder are homozygous recessive. What percentage of the U.S. population are carriers for PKU? q2 = 0.0001, so q = 0.01. p = 1 - 0.01 = 0.99 Carriers (heterozygotes) are 2pq. 2pq = 2(0.99)(0.01) = 0.0198 (or about 2%) Thus, about 2% of the U.S. population are carriers for PKU.

Maintenance of Variation Genetic variation results from mutation and sexual reproduction. Although natural selection tends to produce genetic uniformity, variation is preserved by: 1. Diploidy – hides recessive alleles in heterozygotes; this maintains a large pool of alleles which may be beneficial if conditions change. 2. Heterozygote advantage -- maintains two or more alleles if heterozygous individuals have a greater reproductive success. • Example: Carriers of one allele for sickle cell anemia are resistant to malaria; an advantage in tropical areas where malaria is prevalent.

Malaria and Sickle Cell Allele

Maintenance of Variation cont. 3. Outbreeding – plants mechanisms discourage self-pollination; most animals reproduce sexually; hermaphrodite rarely self-fertilize. Hybrid vigor -- Crossbreeding different inbred varieties often produces hybrids which are more vigorous than the parent stocks.

Fitness Darwinian fitness: contribution an individual makes to the gene pool of the next generation (survival and fecundity). Organisms may produce more progeny because they are more efficient feeders, attract more pollinators, avoid predators, mature earlier, or simply live longer. Example, if pink flower plants (AA and Aa) produce 20% more offspring than white flower plants (aa), then AA and Aa genotypes have a higher relative fitness = 1; relative fitness of white flowers would be 0.8.

Types of Selection 1. Stabilizing Selection Favors intermediate traits; selects against extreme phenotypes. Best suited to relatively stable environments. Ex: average human birth weight 6 – 8 lbs; Much smaller and much higher birth weight babies have a greater infant mortality.

Types of Selection cont. 2. Directional Selection Favors one extreme. Most common during environmental change. Ex: fossils of black bears show increased size after periods of glaciation; decrease during warmer periods.

Types of Selection cont. 3. Disruptive Selection Opposite extremes are favored. Variable environmental conditions. Ex: in a prey population where body color varies, middle shades may be poor camouflage.

Types of Selection cont. Sexual Selection Sexual dimorphism – Different secondary sexual characteristics in males and females. Differences in size, plumage, lion's manes, deer antlers, sports cars, etc. In some species, males use their secondary sexual characteristics to compete for female mates. If a male’s reproductive success is increased, he contributes more to the gene pool of the next generation.