Species Interactions Can Function as Agents of Natural Selection.
For a number of reasons, the interaction between two species will not influence all individuals within the respective populations equally. First, interactions among species involve a diverse array of physiological processes and behavioral activities that are influenced by phenotypic characteristics (physiological, morphological, and behavioral characteristics of the individuals). Secondly, these phenotypic characteristics vary among individuals within the populations (see Chapter 5). Therefore, the variations among individuals within the populations will result in differences in the nature and degree of interactions that occur. For example, imagine a species of seed-eating bird that feeds on the seeds of a single plant species. Individuals of the plant species exhibit a wide degree of variation in the size of seeds that they produce. Some individuals produce smaller seeds, whereas others produce larger seeds (Figure 12.4a), and seed size is a heritable characteristic (genetically determined). Seed size is important to the birds because the larger the seed, the thicker the seed coat, and the more difficult it is for a bird to crush the seed with its bill. If the seed coat is not broken, the seed passes through the digestive system undigested and provides no food value to the bird. As a result, birds actively select smaller seeds in their diet (Figure 12.4c). In doing so, the birds are decreasing the reproductive success of individual plants that produce small seeds while increasing the relative fitness of those individuals that produce larger seeds. The net effect is a shift in the distribution of phenotypes in the plant population to individuals that produce larger, harder seeds (Figure 12.4d). In this situation, the bird population (and pattern of seed predation) is functioning as an agent of natural selection, increasing the relative fitness of one phenotype over another (see Section 5.6). Over time, the result represents a directional change in the genetic structure of the population (gene frequencies), that is, the process of evolution (Chapter 5).
In this example, the predator functions as an agent of natural selection, decreasing the reproduction for certain phenotypes (small seed-producing individuals) within the plant population and increasing the relative fitness of other phenotypes (large seed-producing individuals). But the shifting distribution of phenotypes within the plant population and the resulting change in the distribution of food resources will in turn have a potential influence on the predator population (Figure 12.4b). The directional selection for increased seed size within the plant population decreases the relative abundance of smaller seeds, effectively decreasing the availability of food resources for birds with smaller bill sizes. If the birds with smaller bills are unable to crack the larger seeds, these individuals will experience a decreased probability of survival and reproduction, which increases the relative fitness of individuals with larger bill size. The shift in the distribution of phenotypes in the plant population, itself a function of selective pressures imposed by the bird population, now functions as an agent of natural selection in the predator (bird) population. The result is a shift in the distribution of phenotypes and associated gene frequencies within the bird population toward larger bill size (Figure 12.4e). This process in which two species undergo reciprocal evolutionary change through natural selection is called coevolution .
Unlike adaptation to the physical environment, adaptation in response to the interaction with another species can produce reciprocal evolutionary responses that either thwart (counter) these adaptive changes, as in the previous example, or in mutually beneficial interactions, magnify (reinforce) their effect. An example of the latter can be found in the relationship between flowering plants and their animal pollinators. Many species of flowering plants require the transfer of pollen from one individual to another for successful fertilization and reproduction (outcrossing; Figure 12.5). In some plant species, this is accomplished through passive transport by the wind, but many plants depend on animals to transport pollen between flowers. By attracting animals, such as insects or birds, to the flower, pollen is spread. When the animal comes into contact with the flower, pollen is deposited on its body, which is then transferred to another individual as the animal travels from flower to flower. This process requires the plant species to possess some mechanism to attract the animal to the flower. A wide variety of characteristics has evolved in flowering plants that function to entice animals through either signal or reward. Signals can involve brightly colored flowers or scents. The most common reward to potential pollinators is nectar, a sugar-rich liquid produced by plants, which serves no purpose for the individual plant other than to attract potential pollinators. Nectar is produced in glands called nectaries, which are most often located at the base of the floral tube (see Figure 12.5).
The relationship between nectar-producing flowers and nectar-feeding birds provides an excellent example of the magnification of reciprocal evolutionary responses—coevolution—resulting from a mutually beneficial interaction. The elongated bill of hummingbirds distinguishes them from other birds and is uniquely adapted to the extraction of nectar (Figure 12.6). Their extremely long tongues are indispensable in gaining nectar from long tubular flowers. Let us assume a species of hummingbird feeds on a variety of flowering plants within a tropical forest but prefers the flowers of one plant species in particular because it produces larger quantities of nectar. Thus, the reward to the hummingbird for visiting this species is greater than that of other plant species in the forest. Now assume that flower size (an inherited characteristic) varies among individuals within the plant population and that an increase in nectar production is associated with elongation of the floral tube (larger flower size). Individual plants with larger flowers and greater nectar production would have an increased visitation rate by hummingbirds. If this increase in visitation rate results in an increase in pollination and reproduction, the net effect is an increase in the relative fitness of individuals that produce larger flowers, shifting the distribution of phenotypes within the plant population. The larger flower size and longer floral tube, however, make it more difficult to gain access to the nectar. Individual hummingbirds with longer bills are more efficient at gaining access, and bill size varies among individuals within the population. With increased access to nectar resources, the relative fitness of longer-billed individuals increases at the expense of individuals with shorter bills. In addition, any gene mutation that results in increasing bill length with be selected for because it will increase the fitness of the individual and its offspring (assuming that they exhibit the phenotype). The genetic changes that are occurring in each population are reinforced and magnified by the mutually beneficial interaction between the two species. The plant characteristic of nectar production is reinforced and magnified by natural selection in the form of improved pollination success by the plant and reproductive success by the hummingbird. In turn, the increased flower size and associated nectar production functions as a further agent of natural selection in the bird population, resulting in an increase in average bill size (length). One consequence of this type of coevolutionary process is specialization, wherein changes in phenotypic characteristics of the species involved function to limit the ability of the species to carry out the same or similar interactions with other species. For example, the increase in bill size in the hummingbird population will function to limit its ability to efficiently forage on plant species that produce smaller flowers, restricting its feeding to the subset of flowering plants within the tropical forest that produces large flowers with long floral tubes (see Figure 12.6). In the extreme case, the interaction can become obligate, where the degree of specialization in phenotypic characteristics results in the two species being dependent on each other for survival and successful reproduction. We will examine the evolution of obligate species interactions in detail later (Chapter 15).
Unlike the case of mutually beneficial interactions in which natural selection functions to magnify the intensity of the interaction, interactions that are mutually negative to the species involved can lead to the divergence in phenotypic characteristics that function to reduce the intensity of interaction. Such is the case when the interaction involves competition for essential resources. Consider the case wherein two species of seed-eating birds co-occur on an island. The two populations differ in average body and bill size, yet the two populations overlap extensively in the range of these phenotypic characteristics (Figure 12.7a) and subsequently in the range of seed sizes on which they forage (Figure 12.7b). The selection of seeds by individual birds is related to body and bill size. Smaller individuals are limited to feeding on the smaller, softer seeds, whereas only larger individuals are capable of cracking the larger, harder seeds. Although larger birds are able to feed on smaller seeds, it is energetically inefficient; therefore, their foraging is restricted to relatively larger seeds (see Section 5.8 for an example).
Seed resources on the island are limited relative to the populations of the two species, hence, competition is often intense for the intermediate-sized seeds for which both species forage. If competition for intermediate-sized seeds functions to reduce the fitness of individual birds that depend on these resources, the result would be reduced survival and reproductive rates for larger individuals of the smaller species and smaller individuals of the larger species (Figure 12.7c). This result represents a divergence in the average body and bill size for the two populations that functions to reduce the potential for competition between the two species (Figure 12.7d).
