BIOL100 Lecture Notes - Inclusive Fitness, Natural Selection, Eusociality
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Why Do Animal Societies Evolve?
Social behaviour evolves when, by cooperating, conspecifics individuals can achieve higher rates
of survival and reproduction than they would if they lived alone.
Social systems are dynamic; individuals repeatedly communicate with one another and adjust
their relationship. Their relationships change regularly b/c costs and benefits experienced by
individuals in a social system change with their age, sex, physiological condition, and status.
Group living confers benefits but also imposes costs
It may improve hunting success or expand the range of food that can be captured.
Could also defend their prey and themselves from other carnivores, and they could tell one
another about the locations of food and enemies.
Social behaviour has many costs: interfere with one another’s abilities, may inhibit one
another’s attempts to produce or injure one another’s offspring, and increases exposure to
diseases and parasites.
Parental care can evolve into more complex social systems
Most widespread form of social system in an association of one or two parents with their
immature, dependent offspring.
Many mammalian species have also evolved social systems based on extended families. In
simple mammalian social systems, solitary females or male-female pairs care for their young. As
the period of parental care increases, older offspring may still be present when the next
generation is born. They often help rear their younger siblings.
In most social mammal species, female offspring remain in the group in which they were born,
but males tend to leave or are driven out. Most helpers are females.
Caring for young involves tremendous costs for parents and helper.
Altruism can evolve by contributing to an animal’s inclusive fitness
Natural selection can favour altruistic acts—behaviours that reduce the helper’s reproductive
chances, but increase the fitness of the helped individual. Why?
An animal’s offspring contribute to its individual fitness.
By helping its relatives, an individual can increase the representation of some of its own alleles
in the population. Individual fitness plus the fitness gained by increasing the reproductive
success of nondescendant kin determine the inclusive fitness of an individual. Occasional
altruistic acts may eventually evolve into altruistic behaviour patterns if their benefits exceed
Species whose social groups include sterile individuals are said be eusocial. Most females are
nonreproductive workers that forage for the colony or defend it against predators. Only a few
females, queens, are fertile and produce all the offspring of the colony.
Eusociality may be favoured if it is difficult or dangerous to establish new colonies. Nearly all
eusocial animals construct elaborate nests or burrow systems within which their offspring are
Inbreeding—the mating of individuals who are closely related—increases genetic relatedness
within a group.