Figure 1.
The effects of community complexity on natural selection and evolution.
This figure presents a hypothetical case where increasing complexity in a predator–prey community alters natural selection and the evolutionary response of a single prey population. Panel (A) depicts the genetic covariance between relative fitness and a specific resistance trait in the prey, where the slope of the line is the strength of selection on the resistance trait. The blue dotted line and circles represent a simple community composed of a single prey and a single predator population. The red dashed line and triangles represent a more complex community composed of the same prey species but in the presence of two predator species. Panel (B) shows how the prey species evolves in response to these selective pressures caused by different predator communities. The black line represents the ancestral trait distribution, whereas the dotted and dashed lines represent trait distributions after selection in the simple and complex communities, respectively. Benkman and colleagues identified a similar situation occurring in Rocky Mountain lodgepole pine, which is fed upon by red crossbills and red squirrels [37]. In populations without squirrels, crossbills selected for and caused evolution of longer cones with thicker distal scales and more seeds per cone. When crossbills and squirrels were both present, squirrels imposed stronger selection on cone morphology, which caused the evolution of shorter cones with fewer seeds that had thinner distal scales but thicker basal scales. The presence versus absence of squirrels also altered selection by trees on crossbill bill morphology. Therefore, the presence of squirrels altered selection and coevolution between crossbills and pine trees.
Figure 2.
Evolutionary outcomes caused by differences in community composition.
Cartoon examples of how community complexity can lead to unexpected ecological and evolutionary outcomes in populations. The illustrations are taken from a subset of idealized simulations that are depicted as animations in Movies 1–6. The panels on the left (A,C,E) represent initial conditions at the beginning of a simulation, and the panels on the right (B,D,F) show populations and communities after evolution has reached an equilibrium. In these examples, different consumer species (purple, green, and yellow) move in the environment consuming renewable resources (green circles and red squares; orange diamonds represent excrement). If they consume enough resources they reproduce, and if they do not they die. In all cases, species start as generalist consumers, represented here as non-specialized mouth parts capable of consuming any resource. Species can subsequently evolve to specialize on a resource by changing mouth shape to correspond to resource shape, which increases resource capture efficiency and reproduction. In reality, these examples apply to any case where a trait influences consumer efficiency, whether it involves morphological (e.g., beak morphology), physiological (e.g., metabolic rate), or behavioral (hunting method) change. (A) and (B) represent the evolution of specialization in a one species community. A single generalist species feeds on a common resource and evolves more efficient resource consumption (Movie 1). (C) and (D) represent the evolution of character displacement in a two-species community whereby two generalist consumer species initially compete for two limited resources. Competition causes each species to specialize on different resources and thus avoid extinction (Movie 3). (E) and (F) represent coexistence of three species that evolve to specialize on one of the two limited resources (green circles and red squares) or on the waste products produced by other species (orange diamonds) (Movie 6), as observed by Lawrence et al. [26].