Topic 10.3 Gene pools & speciation

Understanding: A gene pool consists of all the genes and their different alleles, present in an interbreeding population. Species = group of potentially interbreeding populations, with a common gene pool that is reproductively isolated from other species Geographically isolated populations = two populations of same species separated from being able to breed  possible for two different gene pools to exist Individuals that reproduce contribute to gene pool of next generation Genetic equilibrium = when frequency of an allele within a population does not change from one generation to the next No change means no evolution Exists when all members of pop have equal chance of contributing to gene pool

Understanding: Evolution requires that allele frequencies change with time in populations. Evolution = cumulative change in the heritable characteristics of a pop over time Reasons evolution occurs: Mutations – change genes into different alleles Natural selection – removes certain genotypes from the population, thus increases frequency of certain alleles Gene flow = transfer of genes from one pop to another thru immigration Small pop – so that small, random events have a bigger effect on allele frequency Non-random mating - individuals showing strong preferences in selecting mates (reproduction not random)

Hardy-Weinberg Principle According to the Hardy-Weinberg principle, both the ratios of genotypes and the frequency of alleles remain constant from one generation to the next in a sexually reproducing population, provided other conditions are stable. Conditions for stability (no evolution occurring, population exhibiting H-W equilibrium): 1. random mating 2. no gene flow 3. large pop 4. No mutations 5. No selection

Application: Identifying examples of directional, stabilizing and disruptive selection. Fitness of a genotype/phenotype = likelihood that it will be found in the next gen Selection pressures = environmental factors that act selectively on certain phenotypes Result is “natural selection” 3 patterns of natural selection: Stabilizing selection Disruptive selection Directional selection

Stabilizing selection Selection pressures act to remove extreme varieties E.g. avg birth weight favored over low or high E.g. med clutch size (# of eggs laid) favored over low or high (low = less chance for all to survive; high = not enough food to feed them all

Disruptive selection Selection pressures act to remove intermediate varieties, favoring extremes E.g. red crossbill – bill adapted to cross to either side favored over straight bill, making it easier to extract seeds from conifer cones

Directional selection Population changes as one extreme of a range of variation is favored over the other extreme or intermediates advantageous allele increases as a consequence of differences in survival and reproduction among different phenotypes E.g. longer necks in giraffes favored over medium or short

Understanding: Reproductive isolation of populations can be temporal, behavioural or geographic. Speciation = formation of a new species by splitting of an existing pop Can happen when gene pool of one pop is isolated from another pop Allopatric speciation = speciation that occurs when gene pools are separated due to GEOGRAPHIC isolation E.g. cichlids live in 3 East African lakes; water levels fall in different areas, leading to geographic isolation of pops, then each pop is subject to different selection pressures; pops reunited during rainy season when levels rise, but may be reproductively isolated (can’t interbreed)  NEW SPECIES! Sympatric speciation = speciation that occurs when gene pools in same geographic area, but isolated due to BEHAVIORAL or TEMPORAL isolation Behavioral isolation = different courtship behavior Temporal isolation = different timing of mating E.g. 3 tropical orchid species flower for single day after sudden drop in temp, but time lapse between stimulus & flowering is different in all 3 species (8, 9 and 10 days)

Skill: Comparison of allele frequencies of geographically isolated pops. Activity p 459-460

Understanding: Speciation due to divergence of isolated pops can be gradual. Gradualism = species slowly change through series of intermediates Problem = missing links (gaps in fossil record; absence of intermeds) Possible explanations = we haven’t found all the fossils? Or model is wrong?

Understanding: Speciation can occur abruptly. Punctuated equilibrium = long periods of relative stability in a species are punctuated by short periods of rapid evolution Gaps in fossil record may not be gaps because no long sequence of intermeds Causes of rapid evolution: geographic isolation & new niches formed within shared geographic area Rapid change more common in organisms with short gen times (bacteria, insects)

Nature of Science: Looking for patterns, trends & discrepancies: Patterns of chromo number in some genera can be explained by speciation due to polyploidy. Polyploid organism = has more than 2 set of homologous chromosomes Results from: hybridization events between different species Or when chromosomes duplicate in prep for meiosis, but then meiosis doesn’t occur

Polyploidy Diploid gamete + haploid gamete = fertile polyploid Polyploidy can lead to sympatric speciation (due to behavioral or temporal isolation) Polyploidy occurs most often in plants Animal example: red viscacha = highest chromo # of any mammal; may be result of polyploidy; 2n = 102; cells are double normal size Closest living relative = Andean viscacha-rat; 2n = 56 Hypothesis: Andean v-rat produced tetraploid offspring (4n = 112), then eventually shed some of the additional chromosomes, resulting in 102.

What are some advantages of polyploid plants? Animals?

Application: Speciation in the genus Allium by polyploidy. 50 to 70% angiosperms have experienced polyploidy Allium genus = onions, leeks, garlic, chives Determining # of species in Allium genus is challenge due to many polyploidy events Polyploidy results in reproductively isolated but very similar populations