How ecology and evolution interact

The previous chapters also illustrated how ecology and evolution affect each other reciprocally and in diverse ways (Table 16.2). Throughout the book a prominent issue has been how ecology shapes anagenetic evolution through natural selection. In Chapters 4 to 7 we considered how the day-to-day phenotypic variation among species evolves through selection pressures exerted by their environment. These include changes in life history variables, such as reproductive lifespan and age at maturity, changes in allocation of reproductive effort to male verses female function and in the way sex is determined, changes in dispersal ability and dormancy, and plastic changes in phenotype exerted during the lifetime of individuals in response to changes in their environment. In each of these cases, a consideration of the fitness consequences of alternative phenotypes has led us to understand the circumstances that would favour the genotypes that code for them.

Table 16.2 How ecology and evolution affect each other

Influencing factor

Influenced factor

Reason

Ecology

Phenotypic change

Natural selection

Ecology

Speciation

Reproductive isolation, ecological divergence

Ecology

Extinction

Reduction in range or population size

Evolution

Ecology of individuals

Natural selection

Evolution

Population ecology

Rapid evolution or adaptive behaviour

Evolution

Community ecology

Selection on species interactions, changes in species richness

Evolution

Ecosystem ecology

Evolutionary novelties affect geochemical cycling, ecospace occupation

Ecology also affects cladogenesis through its impact on speciation. Speciation involves reproductive isolation and ecological divergence, and ecology is involved in both. Reproductive isolation results from the action of selection or other ecological forces, such as drift, that require specific ecological conditions, and are often a consequence of geographic isolation. Speciation can also occur through hybridization, requiring proximity of two close relatives. Ecological divergence occurs as a result of selection, or neutral processes that require specific ecological conditions.

Ecology also affects evolution through its impact on extinction. Extinction occurs when ecological forces combine to make the range or abundance of a species small, after which a range of stochastic and deterministic events can lead to irretrievable loss of fitness. Ecology also drives the diversity of form and species richness among clades through competition and ecological release.

Evolution also drives ecology. The ecology of individuals is affected by plastic responses to changes in their environment. Evolutionary forces have shaped these such that they are adaptive. Evolutionary forces, acting through these plastic changes, can also affect how the size of a population will change in response to changes in the environment. Rapid evolution can also affect such changes. In addition, evolutionary forces can make a species rare or can make it go extinct once rare. Evolution can also affect the properties of communities: it can affect the number and type of species interactions, making some species generalist and others specialist, some relationships antagonistic and others mutualistic. In general, there has been an increase in the complexity of earth's communities and ecosystems over time, affected by what might be termed major transitions in ecology. Evolutionary forces have shaped those changes through stepwise addition of small evolutionary changes, each of which was advantageous in the ecological circumstances of the organisms concerned. Some of those changes involved an increase in species richness, another property of communites. In fact anagenesis and cladogenesis have affected many of the key macroecological patterns that today dominate the living world.

Thus, evolutionary ecology has so far uncovered a rich array of interactions between ecology and evolution, and knowledge of those interactions has been the key to answering some important questions about our planet.

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