Many biological phenomena result from conflict or cooperation among organisms or among genes. The evolution of most interactions can be explained best by selection at the level of individual organisms or genes.
Altruism benefits other individuals and reduces the fitness of the actor, while cooperative behavior need not reduce the actor’s fitness. Cooperation can evolve because it is directly beneficial to the actor, although the benefit may be delayed. It can also evolve by reci-procity, based on repeated interactions between individuals in which the fitness interests of the associates are aligned. Cooperative interactions can be maintained in part by “policing,” or punishment of cheaters.
Altruism can evolve by kin selection. An allele’s inclusive fitness is the sum of its direct fitness (the average number of copies that a carrier leaves to the next generation) and its indirect fitness (additional copies left by the carrier’s relatives as the result of the carri-er’s behavior). Hamilton’s rule describes the conditions for the increase of an allele for altruistic trait in terms of the benefit to the recipient, the cost to the actor, and the coefficient of relationship between them.
Conflict and kin selection together affect the evolution of many interactions among family members. The genetic benefit of caring for offspring is an increase in the number of current offspring that survive. The genetic cost is the number of additional offspring that the parent is likely to have if she or he abandoned the offspring and reproduced again. Parental care is expected to evolve only if its fitness benefit exceeds its fitness cost. Whether or not one or both parents evolve to provide care can depend on the ratio of fitness costs and benefits for each parent.
Evolutionary conflicts between parents and offspring are widespread. A parent’s fitness may be increased by allocating some re-sources to its own survival and future offspring rather than to its current offspring. Selection acting on the offspring, however, often favors taking more resources from its parents than is optimal for the parents to give. This principle may be one of several reasons why in some species, parents may reduce their brood size by aborting some embryos or killing some offspring.
The most extreme examples of cooperation and altruism are in eusocial species, in which some individuals reproduce little or not at all, and instead help relatives rear their offspring. In eusocial insects, nonreproductive workers rear reproductive queens and males, as well as other workers. Many social interactions in these colonies are governed by kin selection and policing by workers.
Under some conditions, selection acting on groups can cause the evolution of altruism. This form of group selection can be viewed as a type of kin selection. Group selection acting on a pathogen sometimes favors the evolution of decreased virulence when increased host survival increases the number of new hosts that the pathogen infects.
Conflicts may exist among different genes in a species’ genome that are inherited by different pathways. Selection acting on loci that are transmitted through only one sex favors alleles that alter the sex ratio in favor of that sex. The changed sex ratio creates selec-tion at other loci for suppressors that restore the 1:1 sex ratio.
Kin and group selection explain three of the major transitions in the evolution of life on Earth. Eukaryotes evolved by the union of two organisms, in which the fitness of each depends on the other. The union of such a eukaryote with cyanobacteria produced photosyn-thetic eukaryotes: algae and plants. Multicellular organisms could evolve only because their cells are nearly genetically identical, and so cooperate due to kin selection.