Jason Lee ’16, THURJ Staff
Trade-offs, a central theme in evolutionary biology, say that for every benefit, there exists a corresponding cost. Understanding the relationship between benefits and costs can help us understand the reasons for the evolution of certain physical features and behavior. For instance: due to finite resources and intraspecific competition, rats can only obtain a limited amount of calcium from their environment. They must thus allocate their calcium resources wisely, as calcium is required for both tooth decay resistance and bone development. Furthermore, a female rat must also decide between investing her calcium for survival or for more calcium-rich milk for her offspring. Every option therefore has its benefits and costs.
Yet trade-offs are not restricted to resources – in the case of the colorful guppy (P. reticulata), the males must choose between impressing potential mates with flashy signals, or hiding from predators in the shared environment. Guppies live in a female-based polygynous society in which males compete to inseminate females, because the cost of mating is much higher for females than for males. Consequently, brown or grey-colored females have the choice in selecting males, who are well known for their extreme polychromatism and diversity of spots and stripes on their bodies. The females’ preference for bright male color patterns has driven sexual selection, allowing the brighter males to successfully pass on their genes to. However, bright patterns not only capture the attention of females, but also the attention of natural predators, such as pike cichlids and piranhas, which limits the brightness of male patterns. Thus arises the question: how bright should male guppies be, and where is the equilibrium between sexual and natural selection?
“Thus arises the dilemma: how bright should male guppies be, and where is the equilibrium between sexual and natural selection?”
First, is the underlying assumption that females prefer bright spots really true? If so, why are bright spots beneficial? In 1990, scientists conducted an experiment where they raised female guppies in either high predation or no predation conditions. All the females were later transferred to a container containing males of various color patterns and intensities. Regardless of how they were raised, the females showed greater interest in male guppies with bright, flashing spots and stripes. Furthermore, when raised in a predator-free environment, males tended to display a higher density of spots over generations.
From a molecular perspective, there is a simple explanation: the spots on male guppies require various carotenoid pigments for their colors. Because these pigments are only synthesized through the consumption of select plants and invertebrates, they represent both the nutritional status and foraging ability of the males. For these reasons, females prefer males with brighter spots and colors.
However, spots are simultaneously very reflective, as they contain high concentrations of iridescent pigments. In streams where predation risk is high, the size and brightness of spots are reduced, decreasing the visibility of males to predators. Experiments have corroborated these natural observations: guppies raised with no predators or weak predators (pike cichlids) have a rising density of bright spots over generations, while those raised with strong predators (piranhas) become less colorful, suggesting an intricate balance between the forces of sexual and natural.
Researcher Haskins proposed a hypothesis of natural equilibrium between sexual and natural selection, driving conspicuousness and crypsis respectively. He suggested that coloration, size, and patterns reflect the natural conditions of the environment, specifically predation risk and background color. In an experiment, he manipulated predation risk (type and number of predators per unit volume) and background color (size and color of gravel). The results proved his hypothesis correct: the male guppies adjusted to their environment to display optimal signal levels conspicuous enough to attract mates, but cryptic enough to survive from predators. Over generations, sexual and natural forces were able to select for an optimal combination of these two factors – certainly, there were trade-offs between conspicuousness and crypsis, but the male guppies eventually found the perfect equilibrium.
Indeed, guppies that are commercially available are fancy and beautiful to look at, but much more interesting are those that live in their natural habitats, struggling to find that perfect balance between mate attraction and survival from predators.
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