Families and Kin Selection

Introduction to Kin Selection

Kin selection is a concept in evolutionary psychology and biology that explains how natural selection can favor behaviors that increase the reproductive success of relatives, even at a cost to the individual. This is closely related to the idea of inclusive fitness, which considers both direct and indirect genetic contributions to future generations.

• Kin Selection: Changes in gene frequency are driven by natural selection, as predicted by the degree of biological relatedness between individuals.

• Inclusive Fitness: The sum of an individual's own reproductive success plus the effects the individual's actions have on the reproductive success of relatives.

Coefficient of Relatedness (R)

The coefficient of relatedness (R) quantifies the probability that two individuals share a given gene inherited from a common ancestor. It ranges from 0 (no relatedness) to 1 (identical twins).

• R = 1: Monozygotic (identical) twins

• R = 0.5: Siblings, parents and children

• R = 0.25: Grandparents and grandchildren, half-siblings

When considering altruistic behavior, the benefit to the recipient is multiplied by the coefficient of relatedness. As relatedness decreases, the benefit required to favor altruism increases.

• Example: If the benefit (B) to the recipient is 6:

  • For a twin (r = 1):

  • For a sister (r = 0.5):

  • For a grandparent (r = 0.25):

Altruistic behavior is more likely when the probability of benefiting a close relative is high enough to offset the cost to the individual.

Reciprocal Altruism

Reciprocal altruism refers to the exchange of beneficial acts between individuals, where the benefit is returned in the future. The classic strategy is known as "tit for tat," where cooperation is met with cooperation and defection with defection.

• Key Point: Reciprocal altruism can evolve even among non-relatives if individuals interact repeatedly and can recognize each other.

Parental Investment

How Much Should Parents Invest?

Parental investment theory explores how much time, energy, and resources parents should allocate to their offspring. The optimal strategy depends on many factors, including environmental conditions and the likelihood of offspring survival.

• r-strategy: Produce many offspring with minimal investment per offspring. High mortality rate, but enough survive to reproduce.

• K-strategy: Produce few offspring with substantial parental care. Most offspring survive to adulthood and reproduce.

Current vs. Future Reproductive Success

• Maximizing Current Reproductive Success: Investing heavily in current offspring, possibly at the expense of future reproduction.

• Maximizing Future Reproductive Success: Conserving resources to increase the chance of future reproductive opportunities.

• Environmental Influence: The optimal balance between current and future investment can depend on environmental stability and resource availability.

Additional info: In humans, cultural and social factors also influence parental investment strategies.

Adoption

Adoption is a form of parental investment in non-biological offspring. It can be explained by kin selection if the adopted child is a relative, or by reciprocal altruism and social factors in other cases.

Recognizing Relatives

Mechanisms for Kin Recognition (Slater, 1994)

Recognizing kin is crucial for directing altruistic behavior and avoiding inbreeding. Slater (1994) identified four main mechanisms:

1. Spatial Cues: Based on location; individuals assume those nearby are kin.

2. Discrimination Based on Familiarity: Learning cues or "signatures" from close relatives during early life.

3. Phenotypic Matching: Comparing the characteristics of new individuals to those of known relatives. This does not require shared experience; people may treat those who look like them more favorably.

4. Genetic Systems: Genes can produce signals that help identify kin, especially in mating contexts. An example is the major histocompatibility complex (MHC).

Major Histocompatibility Complex (MHC)

Role of MHC in Kin Recognition and Mate Choice

The MHC is a group of genes important for immune system function. There are many different types of MHC in humans, and greater diversity in parental MHC leads to better immune function in offspring. MHC is also linked to mate selection and social behaviors.

• Key Point: People tend to prefer mates with dissimilar MHC, which increases offspring immune diversity.

• Example: Wedekind (1990) found that women preferred the scent of T-shirts worn by men with dissimilar MHC genes.

• Additional info: Couples with similar MHC genes have been found to have higher divorce rates.

Table: Kin Recognition Mechanisms