Adaptations are not perfect
As we have just seen, gene flow can limit the extent to which a population adapts to its local environment. But even when gene flow does not have this effect, natural selection does not result in a perfect match between organisms and their environments.
In part, this occurs because an organism's environment is not static—it is a moving target because the abiotic and biotic components of the environment change continually. In addition, organisms face a number of constraints on adaptive evolution:• Lack of genetic variation. If none of the individuals in a population has a beneficial allele of a particular gene that influences survival and reproduction, adaptive evolution cannot occur at that gene. For example, the mosquito C. pipiens initially lacked alleles that provided resistance to organophosphate insecticides, as described earlier. For decades, this lack of genetic variation prevented adaptive evolution in response to insecticides, allowing humans to destroy mosquito populations at will—at least up until the time when insecticide resistance alleles arose by mutation and spread by gene flow. Note that in this and in all other cases, advantageous alleles arise randomly; they are not produced as needed or “on demand.”
• Evolutionary history. Natural selection does not craft the adaptations of an organism from scratch. Instead, if the necessary genetic variation is present, it works by modifying the traits already present in an organism. Organisms have certain traits and lack others because of their ancestry. It would be advantageous, for example, for an aquatic mammal such as a dolphin to be able to obtain oxygen using gills. Dolphins lack this capacity, however, in part because of constraints imposed by their evolutionary history: they evolved from terrestrial vertebrates that had lungs and breathed air. Natural selection can bring about great changes, as seen in the mode of life and streamlined body form of the dolphin, but it does so by modifying traits that are already present in the organism, not by creating advantageous traits de novo.
• Ecological trade-offs.
To survive and reproduce, organisms must perform many essential functions, such as acquiring food, escaping predators, warding off disease, and finding mates. Energy and resources are required for each of these essential functions. Hence, organisms face trade-offs in which the ability to perform one function reduces the ability to perform another (FIGURE 6.14). Trade-offs occur in all organisms, and as a result adaptations are the product of compromises in the abilities of organisms to perform many different and sometimes conflicting functions.172" class="lazyload" data-src="/files/uch_group80/uch_pgroup312/uch_uch7303/image/image171.jpg">
FIGURE 6.14 Trade-Off between Reproduction and Survival Female red deer
that reproduced had a lower probability of surviving to the next year than did females that did not reproduce, as the energy and resources invested into rearing young made reproducing red deer more susceptible to disease and environmental stress.
Is the additional risk of mortality that results from reproduction the same for females of all ages? Explain.
(After T. H. Clutton-Brock et al. 1983. JAnimEcol 52: 367-383.) View larger image
Despite these constraints, adaptive evolution is a key component of the evolutionary process. What does the importance of adaptive evolution tell us about the link between ecology and evolution? As we saw in the case of soapberry bug populations (see Figure 6.11), natural selection, and the adaptive evolution that results, is driven by the interactions of organisms with one another and with their environment. Any such interaction is an ecological interaction, and hence ecology serves as a basis for understanding natural selection. Next, we'll consider how ecological interactions influence broader evolutionary changes, such as the formation of new species and the great changes that have occurred during the history of life on Earth.