Aging and Natural Selection

Longevity and Selection Pressure

Longevity is a complex trait influenced by multiple genes and environmental factors. The strength of natural selection varies across different life stages, affecting the accumulation of harmful mutations and the evolution of aging.

• Polygenic Trait: Longevity is determined by many genes, each contributing a small effect.

• Selection on the Young vs. Old: Natural selection acts more strongly on traits that affect the young because most individuals experience early life stages, while fewer reach old age.

• Accumulation of Harmful Mutations: Mutations that affect older individuals accumulate more readily since selection against them is weaker (fewer individuals reach old age).

• Example: Harmful mutations causing late-onset diseases are less efficiently removed from the population.

Additional info: This concept is known as the "mutation accumulation theory" of aging.

Case Study: Opossum Aging

Studies comparing island and mainland opossum populations illustrate how environmental factors influence aging rates.

• Island vs. Mainland Opossums: Opossums living on islands, with fewer predators, tend to live longer than those on the mainland.

• Selection Pressure: Reduced predation on islands leads to weaker selection for early reproduction and slower aging.

• Application: Populations with lower extrinsic mortality (e.g., predation) often evolve slower aging.

Predicting Aging Rates

Populations exposed to higher risks of death before old age (e.g., predation, accidents) will show faster aging, as selection favors early reproduction.

• Quiz Example: "Pushed off a cliff" scenario illustrates how increased mortality accelerates aging evolution.

Genetic Drift

Definition and Mechanisms

Genetic drift refers to random changes in allele frequencies within a population, especially pronounced in small populations.

• Random Survival and Reproduction: Which genotype survives and reproduces is determined by chance, not selection.

• Bottleneck Effect: A sharp reduction in population size leads to loss of genetic diversity due to random sampling.

• Founder Effect: When a new population is established by a small number of individuals, allele frequencies may differ from the original population.

• Application: Genetic drift can lead to fixation or loss of alleles independent of their adaptive value.

Additional info: In small populations, genetic drift can overpower natural selection, leading to random changes in traits.

Sexual Selection

Definition and Types

Sexual selection is a form of natural selection that explains the evolution of traits related to mating success, often resulting in differences between males and females.

• Intra-Sexual Selection: Competition among individuals of the same sex for access to mates (e.g., male-male competition).

• Inter-Sexual Selection: Preference by one sex for certain traits in mates (e.g., female choice).

• Detrimental Traits: Sexual selection can favor traits that appear harmful for survival but increase mating success (e.g., elaborate plumage).

Sexual Dimorphism and Resource Investment

Sexual dimorphism refers to differences in appearance between males and females, often driven by sexual selection. The sex that invests more resources in offspring typically chooses mates, while the other competes for access.

• Resource Investment: Females often invest more in offspring (e.g., gestation, parental care), leading to female choice.

• Predicting Sex Roles: The sex with greater investment will be choosier; the other will compete.

• Human Example: Big-headed, helpless human babies require extensive parental care, influencing sexual selection dynamics.

Additional info: Sexual selection can explain the evolution of traits such as antlers, bright feathers, and courtship behaviors.