10.2 Revisiting Sexual Selection
As discussed in Chapter 9, sexual selection plays a major role in physical traits and differences between the sexes. In species with two sexes, there is often a “choosy” sex that invests more in an offspring through gamete production and/or parental care. This choosy sex will typically attempt to only mate with the “highest quality” members of the opposite sex through intersexual selection. After all, individuals that invest energy and time into offspring have a lot more to lose if they mate with an inferior partner. The members of the sex that compete for the attention of the “choosy” individuals compete with one another in intrasexual selection. Both intersexual and intrasexual selection can lead to sexual dimorphism, where one sex has elaborate traits such as bright colors, large horns, large body size, and strong scent markers.
In many species with unequal investment in offspring, females are the “choosy” sex, because eggs, being the larger gamete, take more energy to produce. When looking at the seahorse example, we see something unexpected. While the females invest more energy into gamete production, the males invest more into parental care. This species exhibits a mating pattern known as polyandry—where a female attempts to mate with as many males as possible, leaving the eggs and eventual offspring with the males, who, in contrast, are choosy about which female to mate with.
How did polyandry within seahorses arise? Why is polyandry uncommon?
Like physical traits, many behavioral traits have a genetic basis. Environmental pressures cause these traits to change in frequency through natural selection. Behaviors associated with different mating and mate choice can therefore evolve through natural and sexual selection. Across animals, there are mating patterns that we categorize into four mating systems. These mating systems can be highly varied and often coexist within a species, but typically a species will fall into one of these four categorized systems: monogamy, polygyny, polyandry, and promiscuity.
