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Unit 4: Ecosystems // Section 8: Evolution and Natural Selection in Ecosystems


As species interact, their relationships with competitors, predators, and prey contribute to natural selection and thus influence their evolution over many generations. To illustrate this concept, consider how evolution has influenced the factors that affect the foraging efficiency of predators. This includes the predator's search time (how long it takes to find prey), its handling time (how hard it has to work to catch and kill it), and its prey profitability (the ratio of energy gained to energy spent handling prey). Characteristics that help predators to find, catch, and kill prey will enhance their chances of surviving and reproducing. Similarly, prey will profit from attributes that help avoid detection and make organisms harder to handle or less biologically profitable to eat.

These common goals drive natural selection for a wide range of traits and behaviors, including:

Automeris moth

Figure 14. Automeris moth
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Source: D.H. Jansen and Winnie Hallwachs, janzen.sas.upenn.edu.

Natural selection based on features that make predators and prey more likely to survive can generate predator-prey "arms races," with improvements in prey defenses triggering counter-improvements in predator attack tools and vice versa over many generations. Many cases of predator-prey arms races have been identified. One widely known case is bats' use of echolocation to find insects. Tiger moths respond by emitting high-frequency clicks to "jam" bats' signals, but some bat species have overcome these measures through new techniques such as flying erratically to confuse moths or sending echolocation chirps at frequencies that moths cannot detect. This type of pattern involving two species that interact in important ways and evolve in a series of reciprocal genetic steps is called coevolution and represents an important factor in adaptation and the evolution of new biological species.

Other types of relationship, such as competition, also affect evolution and the characteristics of individual species. For example, if a species has an opportunity to move into a vacant niche, the shift may facilitate evolutionary changes over succeeding generations because the species plays a different ecological role in the new niche. By the early 20th century, large predators such as wolves and puma had been largely eliminated from the eastern United States. This has allowed coyotes, who compete with wolves where they are found together, to spread throughout urban, suburban, and rural habitats in the eastern states, including surprising locations such as Cape Cod in Massachusetts and Central Park in New York City. Research suggests that northeastern coyotes are slightly larger than their counterparts in western states, although it is not yet clear whether this is because the northeastern animals are hybridizing with wolves and domestic dogs or because they have adapted genetically to preying on larger species such as white-tailed deer (footnote 9).

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