THE EVOLUTION OF POPULATIONS

Individuals are selected, but populations evolve Species Gene pool Population genetics Modern synthesis

Population distribution Fir populations Not totally isolated, interbreeding may occur Eastern U.S. People mobile, but more likely to choose mate locally

ALLELE FREQUENCIES Gene pool – all alleles in a population Diploid organisms – 2 alleles at each locus Two or more alleles in a population – each has a relative frequency (proportion) Population = 500 organisms = ? alleles 320 homozygous dominant = ? alleles 160 heterozygous = ? alleles 20 homozygous recessive = ? alleles

HARDY - WEINBERG p + q = 1 p2 + 2pq + q2 = 1 p = frequency of dominant allele q = frequency of recessive allele PKU 1/10,000 births; q2 = ?

Hardy-Weinberg Equilibrium Very large population No migration No net mutations Random mating No natural selection No genetic drift

Causes of Microevolution Evolution is a generation-to-generation change in a population’s frequencies of alleles. Genetic Drift – a change in a population’s allele frequencies due to chance bottleneck effect: drastic reduction in population size founder effect: new colony, not representative of the larger population Natural Selection – allele frequencies in offspring generation different than parental due to differential reproductive success

GENETIC DRIFT

BOTTLENECK EFFECT

GENE FLOW: genetic change due to migration, tends to reduce differences between populations. Gene flow & Human Evolution MUTATION: a change in an organism’s DNA

VARIATION WITHIN POPULATIONS Quantitative characters - vary along continuum Discrete characters - either/or Polymorphism - 2+ forms represented - applies to discrete characters Gene diversity - average % heterozygous loci Nucleotide diversity - about 0.01% in humans

GEOGRAPHIC VARIATION Clinal variation

GEOGRAPHIC VARIATION Madeira: 2 mouse populations separated by mountains Squirrel populations & the Grand Canyon

BALANCED POLYMORPHISM DIPLOIDY – recessive alleles cannot be selected for/against in heterozygotes BALANCED POLYMORPHISM Heterozygote advantage: malaria & sickle cell anemia Frequency dependent selection: survival & reproduction of 1 morph declines if that phenotype becomes too common NEUTRAL VARIATION

HETEROZYGOTE ADVANTAGE

FREQUENCY DEPENDENT SELECTION IN HOST/PARASITE RELATIONSHIP

Modes of selection

Directional selection

DIVERSIFYING SELECTION Intermediate selected against – inefficient at cracking both size seeds

Sex: 2 fold disadvantage

Sexual Selection Sexual dimorphism: secondary sex characteristics Intrasexual selection: direct competition among same sex individuals Intersexual selection: mate choice, individuals of one sex (usually female) are choosy in selecting mates