1. Topic 10.3 Gene pools & speciation

2. 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

3. 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)

4. 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

5. 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

6. 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

7. 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

8. 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

10. 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)

11. Skill: Comparison of allele frequencies of geographically isolated pops.
Activity p

12. 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?

13. 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)

15. 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

16. 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.

17. What are some advantages of polyploid plants? Animals?

18. 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