There are many types of social interactions

Most social interactions can be considered as an action by one individual, the donor of the behavior, directed toward another individual, the recipient of the behavior. One individual delivers food, the other receives it; one individual attacks, the other is attacked. When the attacked individual responds—by standing its ground or by fleeing—it becomes the donor of this subsequent behavior. Every interaction between two individuals has the potential to affect the fitness of both individuals, either in a positive or negative way. To understand how an interaction affects both participants, it can be useful to categorize the interactions. In this section, we will explore the four types of social interactions between donors and recipients and then examine the conditions that favor a donor helping or harming a recipient.

The Types of Social Interactions

Social behaviors can be placed into one of four categories, as illustrated in Figure 10.6: cooperation, selfishness, spitefulness, and altruism. When the donor and the recipient both experience increased fitness from the interaction, we call it cooperation. When one lion helps another kill a gazelle, for instance, both individuals experience a fitness benefit. When the donor experiences increased fitness and the recipient experiences decreased fitness, we call it selfishness. Selfishness is a common interaction between two conspecifics that compete for a resource such as food. The winner of the competition receives a fitness benefit while the loser experiences a fitness loss. Spitefulness occurs when a social interaction reduces the fitness of both donor and recipient. The fourth type of interaction, altruism, increases the fitness of the recipient but decreases the fitness of the donor.

Figure 10.6 The four types of social interactions Cooperation occurs when the donor and recipient of a behavior both obtain a positive fitness effect. Selfishness occurs when the donor obtains a positive fitness effect while the recipient obtains a negative fitness effect. Altruism occurs when the donor obtains a negative fitness effect while the recipient obtains a positive fitness effect. Spitefulness occurs when the donor and recipient both obtain a negative fitness effect.

Under what conditions would natural selection favor each type of social interaction? For both cooperation and selfishness, the interactions benefit the donor. We would therefore expect natural selection to favor any donors that engage in either cooperation or selfishness. In contrast, spitefulness cannot be favored by natural selection under any circumstance given that both participants experience lower fitness. Consistent with this prediction, spitefulness is not known to occur in natural populations. Explaining the evolution of altruism presents a unique challenge because it requires natural selection to favor individuals who improve the fitness of others while reducing their own fitness. We will explore this challenge in the next section.

Altruism and Kin Selection

Altruistic behavior presents an evolutionary problem because it does not lead to an increase in direct fitness. Direct fitness is the fitness that an individual gains by passing on copies of its genes to its offspring. We would expect selfish individuals to prevail over altruistic individuals because selfishness increases the fitness of the donor. Despite this expectation, altruism has evolved in many species. For example, some of the most extreme cases of altruism occur in colonial species, such as leaf-cutter ants and honeybees, in which workers forgo personal reproduction to rear the offspring of the dominant female.

We can explain altruistic behavior by looking beyond direct fitness. When an individual has an altruistic interaction with a relative, it increases the fitness of the relative with which it shares genes through a common ancestor. When you help a relative improve its fitness, you are indirectly passing on more copies of your genes, which gives you indirect fitness. An individual’s inclusive fitness is the sum of its direct fitness and indirect fitness. When considering how selection operates, we say that direct fitness is favored by direct selection. Indirect fitness through relatives is favored by indirect selection, also known as kin selection.

As we have noted, indirect or kin selection occurs because an individual and its relatives carry copies of some of the same genes inherited from a recent common ancestor. The probability that copies of a particular gene are shared by relatives is known as the coefficient of relatedness. As depicted in Figure 10.7, its value for diploid organisms depends on the degree of relationship between two individuals. If we focus on the individual in the red box of the family tree, we see that the coefficient between this individual and its offspring is 0.5 because the individual has two sets of genes but gives only one set to its offspring. As a result, they only have half of their genes in common. This also means that the coefficient of relatedness between our focal individual and its parent is also 0.5. If we next consider the focal individual and its siblings, we see that these two individuals have a 0.5 probability of receiving copies of the same gene from a parent. In the case of two cousins, the probability drops to 0.125 (one in eight) of inheriting copies of the same gene from one of their grandparents, which are their closest shared ancestors. Using these coefficients of relatedness, we can calculate the indirect fitness as the benefit given to a recipient relative (B) multiplied by the coefficient of relatedness between the donor and the recipient relative (r):

Figure 10.7 Coefficients of relatedness The coefficient of relatedness is the probability that one individual possesses the same copy of a gene as another individual through a shared relative. In this family tree, we see that the individual in the red box has a 0.5 coefficient of relatedness with its parents, siblings, and offspring. More distant relatives have lower coefficients of relatedness, as indicated by the boxes with lighter shades of red. The coefficients of relatedness are based on the assumption that none of the mates are related to the highlighted individual.

In the case of nonrelatives, there is a zero probability that an individual carries the same genes from a recent common ancestor. In examining these different coefficients of relatedness, we can see that an individual has a higher probability of leaving more copies of its genes in the next generation by promoting the fitness of its closest relatives and gains nothing by promoting the fitness of nonrelatives.

Understanding the role of kin selection and coefficients of relatedness helps resolve the puzzle of how altruistic social interactions can evolve. Whereas selfish interactions provide direct fitness benefits to the donor, altruistic interactions provide indirect fitness benefits to the donor, weighted by the coefficient of relatedness between the donor and the recipient. If the inclusive fitness of altruistic behaviors exceeds the inclusive fitness of selfish behaviors, then altruism will be favored by natural selection.

The evolution of altruistic behavior becomes clear when we examine the costs and benefits in an equation. Genes for altruistic behavior will be favored in a population when the benefit to the recipient (B) times the recipient’s coefficient of relatedness to the donor (r) is greater than the direct fitness cost to the donor (C):

If we rearrange this equation, we can show that for altruism to evolve, the cost-benefit ratio must be less than the coefficient of relatedness between donor and recipient:

Figure 10.8 shows this relationship graphically. Based on this equation and the figure, we can see that altruism is favored when the cost to the donor is low, the benefit to the relative is high, and the donor and its relative are closely related.

Figure 10.8 Conditions that favor the evolution of altruistic behaviors An altruistic behavior will evolve whenever the ratio of donor costs and recipient benefits (C/B) is less than the coefficient of relatedness between the donor and recipient. The region in red indicates the conditions that favor the evolution of altruistic behaviors.

A study of wild turkeys (Meleagris gallopavo) in California has shown how altruistic behavior can be maintained through kin selection. Male turkeys display at leks by puffing up their feathers and strutting back and forth to attract females. Males may either display alone or as part of a coalition with other males (Figure 10.9). When a coalition of males displays together, only the dominant male copulates with the females that they attract. This raises the question of why the subordinate males in a coalition spend their time and energy displaying when they do not produce any offspring. Researchers obtained their first clue by using genetic data. Males in a coalition were more closely related than males drawn at random from the population. Indeed, the average coefficient of relatedness between groups of displaying males was 0.42, which suggests that paired males represent a mixture of full brothers (r = 0.5) and half brothers (r = 0.25).

Figure 10.9 Wild turkey coalition When two or more male turkeys display together to attract a female, only the dominant male sires the offspring. These individuals live in Texas.

Photo by Larry Ditto/age fotostock.

The researchers then determined the average number of offspring sired by the different types of males. Dominant males in a coalition sired an average of 6.1 offspring; subordinate males in a coalition sired 0 offspring; and males that displayed alone sired an average of 0.9 offspring. With these data, we can evaluate the fitness of the subordinate male in the coalition. By being a part of the coalition, the subordinate male forgoes the ability to breed as a solo male, which would have allowed him to sire 0.9 offspring. Therefore, his cost of being altruistic is 0.9 offspring. By helping his brother or half brother to become highly successful at attracting females, he allows his brother to sire 6.1 offspring. As we have learned, the subordinate male’s average coefficient of relatedness to his brother is 0.42. Therefore, the indirect fitness benefit to the subordinate male can be calculated as:

This means that a subordinate male obtains greater inclusive fitness by helping his brother than by going out on his own to attract females.

ANALYZING ECOLOGY

Calculating Inclusive Fitness

In the pied kingfisher (Ceryle rudis), a fish-eating bird from Africa and Asia, adult males often forgo their own reproduction to help their parents raise offspring. Researchers have identified primary helpers and secondary helpers. Primary helpers are sons of the parents and they work hard to protect the nest and to bring food to the chicks. In some cases, one of the parents disappears and is replaced by an unrelated mate, so the son ends up helping a parent and a stepparent. Secondary helpers—which are unmated males from other families—are not related to the parents and do not work as hard to feed and protect the offspring. After helping for one year, both types of helpers set up their own nest the following year. A third group of males, known as delayers, do not help but simply delay reproduction until their second year.

Researchers followed several nests of kingfishers and determined how much each helper improved the fitness of the parents in the first year (B₁), the probability of surviving and finding a mate in the subsequent year (P_sm), and the fitness of the helper when he bred independently in the second year (B₂). They also quantified the coefficients of relatedness between the helpers and the parents being helped in the first year (r₁) and between the helpers and their own offspring produced in the second year (r₂). The coefficient of relatedness for the primary helpers was 0.32 in year 1. This was the result of some nests retaining both parents of the helper (r = 0.5) and other nests having one parent of the helper plus a stepparent (r = 0.25).

Based on these data, the researchers calculated the inclusive fitness of the primary helpers, secondary helpers, and delayers.

As you can see, the primary helper had inclusive fitness that was a bit higher after 2 years. The primary helpers obtain about half of their inclusive fitness by helping their parents raise their siblings in year 1 and the other half by having their own offspring in year 2. In contrast, the secondary helpers did not gain any indirect fitness in year 1, but had a higher probability of surviving and finding a mate in year 2, which improved their direct fitness. The delayers obtained no indirect fitness in year 1 and had a poor ability to attract mates in year 2, leading to a low inclusive fitness.

If the fitness that primary helpers give to their parents declines to 1.0, their indirect fitness declines to 0.32 and their inclusive fitness declines to 0.73. Under this scenario, the secondary helper strategy would be the most favored by natural selection.

The concept of kin selection has given ecologists a better understanding of the evolutionary reasons underlying a wide variety of altruistic and selfish behaviors in animals. In the next section, we will explore the evolution of an extreme form of altruistic behavior in which individuals completely forgo their reproduction to help others.