CH 21.3 The Evolution of Populations - Natural Selection, Genetic Drift, and Gene Flow

The five conditions for Hardy-Weinberg equilibrium (Table 21.1) are rarely met in nature.
Deviations from these conditions lead to evolution.
Mutations can alter allele frequencies, but their impact is usually small from one generation to the next.
Nonrandom mating affects genotype frequencies but doesn't directly alter allele frequencies unless it's tied to traits favored by natural selection.
The primary mechanisms that directly alter allele frequencies are natural selection, genetic drift, and gene flow.

Based on differential success in survival and reproduction.
Individuals with advantageous heritable traits produce more offspring.
Results in alleles being passed on in differing proportions.
Example: Insecticide resistance in Drosophila melanogaster.
- Allele frequency for DDT resistance increased from 0% (pre-1930) to 37% (post-1960).
- DDT acted as a strong selective force, favoring resistant alleles.
Adaptive evolution: Traits enhancing survival or reproduction increase over time.

Chance events cause unpredictable fluctuations in allele frequencies, especially in small populations.
Coin flip analogy: Smaller sample sizes deviate more from expected ratios.
Can lead to allele loss.
Allele frequencies can also be affected by chance events during fertilization.

Figure 21.9 illustrates genetic drift, showing the frequency of an allele (c to the w) changing over generations.

A small group becomes isolated and establishes a new population.
The new population's gene pool differs from the source population.
Example: Storm transports a subset of individuals to a new island.
Can explain high frequencies of certain inherited disorders in isolated human populations.

A sudden environmental change drastically reduces population size.
Certain alleles are overrepresented, underrepresented, or absent among survivors due to random chance.
Reduces genetic variation, even if the population later recovers in size.

Prairies converted to farmland led to a plummet in population size.
By 1993, fewer than 50 birds remained.
Low genetic variation and reduced egg hatching rates.
Researchers analyzed museum specimens to estimate pre-bottleneck genetic variation.
The 1993 population had fewer alleles compared to pre-bottleneck and other populations.
Translocation of birds from neighboring states increased genetic variation and improved egg hatching rates.

Significant in small populations: Chance events disproportionately affect allele frequencies.
Causes random allele frequency changes: Allele frequency changes are unpredictable.
Leads to loss of genetic variation: Alleles can be eliminated, reducing adaptability.
Can fix harmful alleles: Slightly harmful alleles can reach 100% frequency, threatening survival.

Transfer of alleles into or out of a population due to fertile individuals or gametes.
Reduces genetic differences between populations; can merge them into a single gene pool.
Can affect adaptation to local conditions.

Mainland snakes are strongly banded; island snakes are unbanded or intermediate. (Figure 21.12)
Banding is advantageous for camouflage in mainland environments. Lack of banding is advantageous on islands.
Gene flow from the mainland introduces alleles for banded coloration to island populations.
Mainland snakes swim to the islands and transfer alleles for banded coloration, preventing full adaptation to the island environment.

Gene flow has spread insecticide resistance alleles in Culex pipiens.
These alleles arose in one or a few locations and spread due to natural selection and gene flow.

Increased human mobility leads to more mating between previously isolated populations.
Results in allele exchange and reduced genetic differences.

Natural selection is more predictable because it consistently favors alleles that increase survival and reproduction in a given environment, whereas genetic drift is random.
Genetic drift is a change in allele frequencies due to chance events, reducing genetic variation. Gene flow is the transfer of alleles between populations, potentially increasing or decreasing variation.
In a plant population exchange example where individuals of genotype $C^fC^f$ are most common (9000 $C^fC^f$ , 900 $C^fC^a$ , 100 $C^aC^a$ ). In the other population, individuals of genotype $C^aC^a$ are most common.