Evolutionary Mechanisms and Natural Selection (1C) - Bio & Biochem

1. What is gene flow (migration) and its typical effect on population differences?

   Allele movement between populations; reduces population divergence. Gene flow homogenizes allele frequencies across populations by introducing alleles via migration, counteracting differentiation caused by drift or local selection.

2. What is fitness in evolutionary biology (as used in natural selection questions)?

   Relative reproductive success of a genotype or phenotype. Fitness measures an organism's ability to pass on genes relative to others, driving natural selection by favoring advantageous genotypes or phenotypes.

3. What is directional selection?

   Selection favoring one extreme phenotype; shifts the mean. Directional selection drives evolutionary change by increasing the frequency of advantageous extreme traits, shifting the population's phenotypic mean accordingly.

4. What is the ultimate source of new alleles in a population?

   Mutation. Mutations introduce novel genetic variation by altering DNA sequences, providing raw material for evolution unlike other mechanisms that redistribute existing alleles.

5. What is the bottleneck effect?

   Drift after a drastic population size reduction. Bottlenecks reduce genetic diversity through drastic population declines, amplifying drift as surviving alleles are randomly sampled from the remnant group.

6. Which five conditions must hold for Hardy–Weinberg equilibrium to apply?

   No selection, no mutation, no migration, random mating, large population. These conditions ensure no changes in allele frequencies, enabling mathematical modeling of genotype proportions under equilibrium.

7. What is disruptive selection?

   Selection favoring both extremes; can increase variance/bimodality. Disruptive selection promotes polymorphism by favoring phenotypic extremes over intermediates, potentially leading to speciation through increased variance.

8. State the Hardy–Weinberg genotype frequency equation in terms of $p$ and $q$.

   $p^2+2pq+q^2=1$. This equation derives from binomial expansion, predicting genotype frequencies from allele frequencies under equilibrium assumptions.

9. What is stabilizing selection?

   Selection favoring intermediate phenotypes; reduces variance. Stabilizing selection maintains phenotypic optima by eliminating extremes, preserving the population mean while narrowing trait distribution.

10. Identify the genotype frequencies under Hardy–Weinberg equilibrium in terms of $p$ and $q$.

    $AA=p^2,\ Aa=2pq,\ aa=q^2$. Under random mating, genotype frequencies follow binomial probabilities, with homozygotes as squares and heterozygotes as twice the product.

11. What is the definition of Hardy–Weinberg equilibrium?

    Allele and genotype frequencies remain constant absent evolutionary forces. Hardy–Weinberg equilibrium assumes no evolutionary forces, allowing prediction of stable frequencies from random mating in idealized populations.

12. What is the definition of evolution in a population genetics context?

    Change in allele frequencies in a population over generations. Evolution is defined as shifts in allele frequencies across generations due to mechanisms like selection, drift, mutation, and gene flow acting on genetic variation.

13. What is the founder effect?

    Drift from a new population started by a small number of individuals. The founder effect arises when a small group colonizes a new area, carrying only a subset of alleles that then drift in the isolated population.

14. What is frequency-dependent selection?

    Fitness of a phenotype depends on its frequency in the population. Frequency-dependent selection stabilizes polymorphisms as rare phenotypes gain advantages, preventing fixation and promoting coexistence of multiple strategies.

15. What is sexual selection?

    Selection on traits that increase mating success. Sexual selection evolves traits that enhance mating opportunities, often through competition or choice, even if they reduce overall survival fitness.

16. What is heterozygote advantage (overdominance)?

    Heterozygote has higher fitness than either homozygote. Heterozygote advantage maintains genetic diversity by conferring superior fitness to carriers of both alleles, as seen in sickle-cell anemia resistance to malaria.

17. Find the heterozygote frequency if $p=0.6$ and $q=0.4$ under Hardy–Weinberg.

    $2pq=0.48$. Heterozygote frequency is twice the product of allele frequencies, derived from Hardy–Weinberg principles for random mating.

18. What is the key evolutionary consequence of inbreeding on genotype frequencies?

    Increases homozygosity (decreases heterozygosity) without changing allele frequencies. Inbreeding increases mating between relatives, elevating homozygote frequencies via non-random mating while allele frequencies remain unchanged.

19. What is natural selection, stated in terms of fitness and heritable variation?

    Differential reproductive success due to heritable trait differences. Natural selection occurs when heritable traits confer varying reproductive success, leading to changes in trait frequencies over generations.

20. Identify the type of selection if heterozygotes have the lowest fitness (both homozygotes favored).

    Disruptive selection (underdominance). When heterozygotes are least fit, selection disrupts intermediates, favoring homozygous extremes and potentially leading to bimodal distributions.

21. What is genetic drift?

    Random change in allele frequencies due to chance sampling. Genetic drift causes unpredictable fluctuations in allele frequencies from random sampling errors, especially pronounced in small populations.

22. State the Hardy–Weinberg allele frequency equation using $p$ and $q$.

    $p+q=1$. For a locus with two alleles, their frequencies sum to unity, forming the basis for Hardy–Weinberg genotype calculations.

23. Which mechanism is more influential in small populations: genetic drift or selection?

    Genetic drift. In small populations, random sampling errors dominate over selective pressures, making genetic drift the primary driver of allele frequency changes.

24. Find $q$ under Hardy–Weinberg if the recessive phenotype frequency is $0.04$.

    $q=0.2$. Recessive phenotype frequency equals $q^2$, so $q$ is the square root, assuming Hardy–Weinberg equilibrium holds.

25. Find $p$ under Hardy–Weinberg if $q=0.3$ for a two-allele locus.

    $p=0.7$. Since allele frequencies sum to 1, $p$ is calculated as $1 - q$ for a biallelic locus in equilibrium.

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