This week’s Evolution 101 blog post is by MSU graduate student Sara Garnett.
As soon as spadefoot toad tadpoles are born, the pressure’s on. Adults lay their eggs in temporary ponds, created from sudden rainstorms in the southwestern desert. Because these ponds don’t last long, anything that helps tadpoles to develop and turn into toads before the pond dries can be a huge advantage. Some tadpoles even develop the ability to be carnivorous, since fairy shrimp – and other tadpoles – can be a good source of nutrition when rapid growth is necessary. Despite this environmental pressure, these cannibal tadpoles have often been observed releasing some of their potential tadpole prey after “tasting” them. With so much riding on getting out of the pond, why would tadpoles expend the energy to catch another tadpole, only to give it up?
Researchers noticed this happening in cases where the prey tadpole was a sibling of the cannibal (individuals from the same clutch of eggs can develop both ways); the cannibal released the other tadpole if it tasted a sibling, but it consumed its prey if the other tadpole were unrelated. Such kin discrimination behavior – behaving differently depending on whether the other individual is related or not – seems at odds with a simplified understanding of evolution as an intense struggle to maximize personal success without regard for other members of one’s own species, but it’s more common than you might think. Many species with cannibalistic tendencies have some way of reducing the risk of harming relatives, even if the individual might benefit from a meal. Nestling birds adjust how intensely they beg for food depending on the frequency of half-siblings relative to full-siblings in the nest. Some plants change how they distribute resources to different structures depending on whether they’re surrounded by strangers or siblings. In addition to these examples of restraint shown toward kin, there are also examples of individuals seeming to actively put themselves in harm’s way; certain types of alarm calls that make an animal more likely to be caught by a predator are more likely to be given in the presence of kin.
To understand why these behaviors exist, we need to go back to what it means to succeed in an evolutionary sense. Survival is pretty important, but what really matters is how successful an individual is at passing on copies of its genes. This success is what is described by the term “fitness.” Although there is some debate within the field about how best to define and measure fitness, one determination of it can be based on the number of offspring an individual produces. While it might seem like this would capture how many copies of one organism’s genes show up in the next generation, it ignores the fact that many of those genes are not unique to one individual. If the gene isn’t a new mutation, at least one parent has a copy, meaning that siblings may also have received a copy. Depending on how far back the gene goes, it may also be found in half-siblings, cousins, grandparents, or other relatives as a result of common descent. What this means is that, if we broaden our concept of fitness, helping relatives to survive and reproduce may also increase the chances of passing on genes, an idea known as kin selection.
Kin selection provides a mechanism for how helping relatives may increase an individual’s fitness, in spite of personal costs. Evolutionary biologist W. D. Hamilton laid out a set of conditions, now known as Hamilton’s rule, under which genes governing such behavior should spread in a population. This rule takes into account the genetic relatedness between an actor and the recipient of the behavior (that is, how likely that a given gene is shared between the two individuals due to common descent), the additional fitness benefit the recipient gets as a result of the behavior, and the fitness cost to the actor. If the fitness benefit to the recipient multiplied by the relatedness is greater than the cost to the actor, it will actually ultimately benefit the actor to perform the behavior. This ultimate benefit comes from thinking about an individual’s inclusive fitness, which takes into account not just the direct fitness gained by producing offspring, but also the adjusted fitness benefits gained by relatives who share genes.
Thinking about Hamilton’s rule illustrates how changing each of these parameters might be expected to alter kin-directed behavior. We might expect to see more instances of behavior that helps relatives as well as the individual when the cost to the actor is not very low, or when the benefit to the relative is very great. Individuals should also be willing to incur greater costs if relatedness to the recipient of the action is greater. The chance that a given gene will be identical due to common descent is greater between siblings than half-siblings (1/2 vs. 1/4 on average), which is greater than the average relatedness between cousins (1/8), affecting the inclusive fitness that results from helping different relatives. This relationship is captured well by a quote attributed to evolutionary biologist J.B.S. Haldane, who claimed he “would lay down [his] life for two brothers or eight cousins.”
Giving up a meal to avoid consuming a sibling makes more sense for a spadefoot toad tadpole when inclusive fitness is taken into account. As long as an individual is not in immediate danger of starvation, it receives a fitness benefit of its own by not taking a relative out of the gene pool. Kin selection and inclusive fitness provide some ideas for understanding behaviors that don’t initially seem to make sense from an evolutionary perspective, even if we still don’t always understand how we tolerate our own siblings.
