1. SGN 26 The Evolution of Populations
Change that proceeds speciation

2. Natural selection acts on individuals but only
Natural selection acts on individuals but only becomes apparent in populations
Therefore understanding population genetics is essential to understanding evolution

3. Population genetics – on one level evolution can be defined as a shift in the frequency of traits within a population, whether the characteristics are quantitative (vary along a continuum) or discrete (exist in only several variations)
Shift in the frequency of traits – change in the amount of any trait variation within the population
Traits are determined by genes; the entire aggregate collection of genes found within a population is known as the gene pool
A population’s gene pool is defined by the frequency of alleles that make up the pool

4. A population’s gene pool is defined by its allele frequencies

5. Population – localized group of individuals of the same species with potential to breed amongst themselves Populations can be isolated or can have dense population cores but have outer edges that overlap with other populations
So a population over many generations shares the same gene pool

6. Gene pool – total aggregate of alleles in a population
Some genes exist in only one type within a population (fixed allele) so all individuals are homozygous for that gene (no variation, no natural selection)
If two or more alleles exist, each allele can be ascribed a frequency (relative proportion) within the gene pool (variation on which natural selection can act)

7. Evolution can be understood as a change in the frequency of alleles in a gene pool over generations
Evolution = change in genotype and phenotype

8. Understanding a nonevolving population (hypothetical) informs us about an evolving population (real)
The Hardy-Weinberg Theorem describes a nonevolving population
In a nonevolving population the frequencies of alleles and genotypes in a population’s gene pool remains constant over the generations; the frequencies are at equilibrium
Hardy-Weinberg equilibrium – sexual process of meiosis and random fertilization maintains the same allele and genotype frequencies that existed in the previous generations; population’s gene pool is in a state of equilibrium

9. Hardy-Weinberg equations
Allows us to calculate frequencies of alleles in a gene pool if we know frequencies of genotypes, and vice versa
In a H-W population (nonevolving) the allele and genotype frequencies stay the same every generation

10. Very large population No migration No net mutations Random mating
A H-W population is hypothetical because no forces are acting upon allele frequencies to change them, which is always the case in the real world
Assumptions of H-W Population
(not true in real population
WHY?)
For a population to be nonevolving…
Very large population
No migration
No net mutations
Random mating
No natural selection

11. One thing that a H-W population and a real population have in common is that genetic variation persists from generation to generation because in the diploid condition the alleles are recombined in sexual reproduction every generation and not blended
Therefore variation persists in each generation provided there are at least two different alleles in the population
Hardy-Weinberg shows how genetic variation is maintained in a population (vs blending), which is necessary for natural selection and evolution
Sexual reproduction and diploidy are so successful because they produce and maintain variation(?)

12. H-W gives us a nonevolving baseline against which we can compare the allele and genotype frequencies of an evolving population in regard to microevolution Microevolution – the generation to generation change in a population’s frequency of alleles and genotypes

13. Microevolution – for example, the kind of evolution described within the Theory of Natural Selection

14. Forces that change allele frequencies Vs H-W nonevolving population
Genetic drift
Natural selection
Gene flow
Mutation
Non-random mating
H-W assumptions
Very large population
No natural selection
No migration
No net mutations
Random mating

15. Genetic drift – change in a population’s allele frequencies (microevolution) due to chance
The smaller the population size the greater probability chance changes in genome will be passed down and spread, thus playing an important role in population’s evolution
Alleles are eliminated from the gene pool by chance, such as random death events (not natural selection), chance failure to mate (wrong place/wrong time), etc

16. Variations on the genetic drift theme
Bottleneck effect – change in allele frequencies in a population due to drastic reduction of population size; by chance some alleles may survive and be over or underrepresented versus the former generation

17. Variations on the genetic drift theme
Founder effect – limited genetic variability in founders of new colony

18. Natural selection Differential success in reproduction (one genotype has disadvantage compared to others) leads to changes in allele frequencies NS only cause of microevolution that will adapt a population to its environment; all the others are random or chance events

19. Gene flow Genetic change due to the migration of fertile individuals or gametes between populations

20. Mutation Will only affect evolution if involved in genetic drift (for example rare mutation is overrepresented migrant/survivor/founder population) and/or acted upon by natural selection (negative or positive effect on survival and reproduction) Only source of genetic variation (something new) that serves as raw material for NS
Point mutations
Chromosomal
rearrangements
Mutation must alter phenotype (at any level – molecular, biochemical, organismal, etc.) and must originate in germ line cells in order to be acted upon by natural selection (whereas genetic drift may bring about change of allele frequency by chance)

21. Non-random mating
Seemingly random (?) preferences in mating partners can bring about the evolutionary accentuation of sexual characteristics
Dimorphism – genders within a species are substantially different in regard to phenotype, especially in regard to size, secondary sexual characteristics, etc.
Why preference of dimorphic phenotypes, even to seeming absurdity? How might something like preference for dimorphic phenotype originate?
Health? Competition? Need for maternal size to produce more energy-demanding eggs? Things we can’t see?

22. Sexual reproduction does not in itself change allele frequency, but does recombine alleles (independent assortment and crossing over) in ways that produce different phenotypes and perhaps new combinations of phenotypes that are more advantageous (so sexual reproduction contributes to microevolution by creating new assortments – recombination)

23. Tendency for NS to reduce variation is countered by mechanisms that preserve or restore variation
Diploidy, sexual reproduction and balanced polymorphism preserves variation

24. Diploidy, including sexual reproduction/recombination preserve variation See H-W equation Heterozygotes protect recessive alleles from natural selection No blending but maintenance of discrete alleles

25. Balanced polymorphism preserves variation
Balanced polymorphism – ability of natural selection to maintain two or more phenotypic forms in a population
Environmental variation promotes more than one morph
Heterozygote advantage
Frequency-dependent selection
Neutral variation

26. Heterozygote advantage

27. Frequency-dependent selection – survival and reproduction of one morph changes in comparison to success of other morphs
Disease is a density dependent factor
The more concentrated the host the more successful the pathogen
As one morph increases in number, pathogens that target that morph increase as well; therefore the more successful morphs, targeted by disease, will decline in comparison to other morphs

28. Neutral phenotypic variation
Shell pattern
Hair color

29. Natural selection can in various ways promote some degree of diversity
Directional selection – change in environment favors one phenotype, as opposed to previous conditions which were neutral or favored another phenotype
Diversifying selection – extreme phenotypes favored
Stabilizing selection – acts against extreme phenotypes

30. Ultimately natural selection favors sexual reproduction, which in turn promotes variation Sex seems disadvantageous in regard to number of offspring produced and a other things but it leads to variation and variation must have advantages for sex to be selected for by natural selection (persist and spread) – resistance to pathogens/disease?? Intersexual vs intrasexual selection involves natural selection and often what seems like random mating preferences (nonrandom if displays increase fitness Sexual selection leads to sexual dimorphism in many species