Extinction Factors and Examples

• Extinction is the death of the last member of a species.
• Functionally extinct refers to sexually reproducing, biparental organisms with only one individual left.
• Extirpation is the extinction of a species from one area while it still survives elsewhere.

Factors Contributing to Extinction

• Deterministic factors are gradual and continuous, like habitat destruction or hunting.
• Stochastic factors are unpredictable and shorter term.
  • Demographic uncertainty: variation in reproduction, such as skewed sex ratios.
  • Environmental uncertainty: unpredictable changes in environmental conditions.
  • Catastrophic uncertainty: extreme environmental events like hurricanes.
  • Genetic uncertainty: less intuitive than other factors.

Heath Hen Example

• Heath hens were closely related to the greater prairie chicken.
• They lived on the East Coast but were extirpated from the mainland by 1870 due to hunting.
• By 1870, 300 heath hens survived on Martha’s Vineyard.
• By 1900, the population was at 2,000
• In 1916, a forest fire, harsh winter, and new predators (goshawks) decimated the population.
• The surviving population was mostly male.
• By 1927, only 12 birds were left, with only 2 females.
• In 1932, the last heath hen died.

Heath Hen Causes of Extinction

• Gradual/deterministic factors included hunting and introduced predators.
• Stochastic effects included catastrophic habitat destruction (fire), harsh winter, and unequal sex ratio.
• Early conservation attempts included hunting bans, habitat protection, and captive breeding, but stochastic events still led to extinction.

Genetics

• Inheritance pattern: allele interactions creating a phenotype.
• Fitness: selective action favoring beneficial alleles.
• Allele frequency: how common an allele is in the population.
• A combination of any of these are possible

Genetic Variation

• Genetic variation can be beneficial to individuals and allows populations to adapt to changing conditions.
• Selection requires variation to act on.

Good Variation Example: Colias Butterflies

• Heterozygotes at the Gpi enzyme locus can fly at a greater range of temperatures than homozygotes.
• Different alleles of the enzyme have optimal activity at different temperatures.
• Female butterflies can lay eggs over a wider range of weather conditions.

More Good Variation

• Major histocompatibility complex (MHC) proteins are codominant and highly polymorphic, allowing for better immune recognition of pathogens.
• Having two different alleles is better.

Glanville Fritillary Butterflies

• Populations with less genetic variation are more likely to disappear.
• Offspring of more heterozygous females were more likely to hatch, had higher survival rates, and lived longer as adults.

Measuring Levels of Genetic Variation

• Hardy-Weinberg Equilibrium calculates the expected proportion of heterozygotes at a gene locus.
• Given the frequency of a homozygote phenotype for a specific gene, can determine heterozygosity.
• Heterozygosity can be averaged across different genes.

Heterozygosity Determinant

• Population size is the biggest determinant of the overall level of heterozygosity.
• Small populations lose heterozygosity faster due to stochasticity, as HWE assumes an infinite population size.

Genetic Drift

• Allele frequencies in one generation are a statistical sample of those from the prior generation.
• Smaller populations are more susceptible to stochastic fluctuation.
• Heterozygosity decreases, and some alleles are completely lost.
• Natural selection is weaker in smaller populations.

Genetic Drift and Effective Population Size

• Population size is the main determinant of genetic drift.
• Effective population size (Ne) is the number of individuals that actually reproduce.
• Ne is the size of a theoretical population that meets HWE assumptions but has the same level of genetic drift as the real one.
• Ne can only be less than N.

Effective population size Factors

• Sex ratio: Ne = 4Nm x Nf / (Nm + Nf).
• Differences in reproductive success
• Inbreeding
• Non-constant population size over time
• Dusky Seaside Sparrow: In the last generation (1981) there were 6 individuals, but they were all male Nm x Nf = 6 X 0 = 0 Aka, functionally extinct

Humans

• Humans have a true population of 8 billion individuals.
• The effective population size is closer to 10,000.
• Human population growth is abnormal
• Humans have less genetic diversity.

Metapopulations

• Gene flow between one population and others, then the effective population size will be larger than expected for that population alone.