Inclusive Fitness and Parental Care
Some might express concern that parental investment (parental care) is said to contribute to inclusive fitness. The distinctions between the kind of beneficiaries nurtured (collateral versus descendant relatives) and the kind of fitnesses used (inclusive versus personal) in our parsing of nature are orthogonal concepts. This orthogonality can best be understood in a thought experiment: Consider a model of a population of animals such as crocodiles or tangle web spiders. Some species or populations of these spiders and reptiles exhibit parental care, while closely related species or populations lack it. Assume that in these animals a gene, called a, codes for parental care, and its other allele, called A, codes for an absence thereof. The aa homozygotes care for their young, and AA homozygotes don't, and the heterozygotes behave like aa homozygotes if a is dominant, and like AA homozygotes if A is dominant, or exhibit some kind of intermediate behavior if there is partial dominance. Other kinds of animals could be considered in which all individuals exhibit parental care, but variation among them would be in the quantity and quality thereof.
If we consider a lifecycle as extending from conception to conception, and an animal is an offspring of parents with poor parental care, the higher mortality with poor care could be considered a dimunition of the offspring's expected fitness.
Alternatively, if we consider the lifecycle as extending from weaning to weaning, the same mortality would be considered a dimunition in the parents' fecundity, and therefore a dimunition of the parent's fitness.
In Hamilton's paradigm fitnesses calculated according to in the weaning to weaning perspective are inclusive fitnesses, and fitnesses calculated in the conception to conception perspective are personal fitnesses. This distinction is independent of whether the altruism involved in child rearing is toward descendents or toward collateral relatives, as when aunts and uncle rear their nieces and nephews.
Inclusive fitness theory was developed in order to better understand collateral altruism, but this does not mean that it is limited to collateral altruism. It applies just as well to parental care. Which perspective we choose does not affect the animals but just our understanding.
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