The Basics of Coevolution

Types of Coevolution

A few different categories of coevolution are often discussed by scientists in ecology and evolutionary biology: pairwise coevolution, diffuse coevolution, and gene-for-gene co-evolution. Pairwise coevolution (or 'specific' coevolution) describes tight coevolutionary relationships between two species. Diffuse coevolution (or 'guild' coevolution) refers to reciprocal evolutionary responses between suites of species. This type of coevolution emphasizes that most species experience a complex suite of selective pressures derived from numerous other species, and their evolutionary responses change the selective environment for other species. Gene-for-gene coevolution (or 'matching gene' coevolution) describes the specific case where coevolution involves gene-for-gene correspondence among species, such as when hosts and parasites have complementary genes for resistance and virulence.

Symbiosis and the Nature of Coevolutionary Interactions

Some of the classic, and most obvious, examples of co-evolutionary interactions involve two species that live in continuing, intimate associations, termed symbiosis. With such tight ecological associations, strong and repeated coevolutionary responses are easy to envision. Symbiosis is commonly used to refer to all relationships between different species, and thus an association need not be extremely close to qualify as symbiotic. There are five

Table 1 The five types of relationships between species. Effects of interaction are negative (-), positive (+), or indifferent (0)

Effects of interaction on

Table 1 The five types of relationships between species. Effects of interaction are negative (-), positive (+), or indifferent (0)

Effects of interaction on

Type of interaction

Species 1

Species 2

Antagonism

-

-

Parasitism

-

+

Amensalism

-

0

Commensalism

+

0

Mutualism

+

+

major types of interspecific relationships: antagonism, parasitism, amensalism, commensalism, and mutualism (Table 1). Antagonism describes the scenario where both members of the interaction are harmed, such as interspecific competition. In parasitic coevolution, one member benefits (parasite), while the other is harmed (host). Parasitism is an extremely common form of coe-volution (see Parasites and Abiotic and Biotic Diversity in the Biosphere). Parasitic interactions do not only include associations where one organism lives in or on a second organism, but also encompass some other very common interactions such as herbivory and predation. Sometimes the benefiting member kills the harmed member (e.g., parasitoid-host, predator-prey), whereas other times the harmed member is merely injured (e.g., parasite-host, herbivore-plant). Amensalism is where one member is harmed, while the other member is neither positively nor negatively affected (see Amensalism). A common example of amensalism is the production of a chemical compound by one member as part of its normal metabolism which is detrimental to another organism (e.g., allelopathy in plants, toxic skin secretions in animals). Commensalism describes a symbiotic relationship where one member benefits, while the other member is neither helped nor harmed (see Commensalisms). It is possible for some interactions to be parasitic under some circumstances (e.g., low host nutrition), but commensal during others (e.g., high host nutrition). Mutualism is a coevolu-tionary relationship where both members benefit (see Mutualism). For instance, clownfish gain protection from sea anemones, and anemones gain food from clown-fish. Many relationships may change in nature over time, space, or ecological context, and the distribution of these types of outcomes for a given interaction (e.g., percentage of outcomes that are antagonistic vs. mutualistic) can be important in determining the coevolutionary responses that will be elicited.

The Red Queen Hypothesis

The Red Queen hypothesis was first proposed by Leigh Van Valen in 1973, and is a coevolutionary hypothesis describing how reciprocal evolutionary effects among species can lead to some particularly interesting outcomes. While Van Valen specifically addressed macroevolution-ary extinction probabilities, the hypothesis has since become much more general, providing an evolutionary explanation for numerous characters (e.g., sex, mating systems, pathogen virulence, maintenance of genetic diversity), and coevolutionary arms races in general. The conceptual basis of the Red Queen hypothesis is that species (or populations) must continually evolve new adaptations in response to evolutionary changes in other organisms to avoid extinction. The term is derived from Lewis Carroll's Through the Looking Glass, where the Red Queen informs Alice that ''here, you see, it takes all the running you can do to keep in the same place.'' Thus, with organisms, it may require multitudes of evolutionary adjustments just to keep from going extinct.

The Red Queen hypothesis serves as a primary explanation for the evolution of sexual reproduction. As parasites (or other selective agents) become specialized on common host genotypes, frequency-dependent selection favors sexual reproduction (i.e., recombination) in host populations (which produces novel genotypes, increasing the rate of adaptation). The Red Queen hypothesis also describes how coevolution can produce extinction probabilities that are relatively constant over millions of years, which is consistent with much of the fossil record. Thus, extinction resistance of lineages does not improve over time, but rather remains fairly constant because the probability of evolutionary change in one species leading to extinction in another species should be independent of species age (they are constantly evolving with their changing environments, not constantly improving with respect to a static background environment).

The Geographic Mosaic Theory of Coevolution

Owing to relatively recent developments in ecological, evolutionary, genetic, mathematical, and phylogenetic studies, coevolution is viewed today as an ongoing, highly dynamic process where populations interact across geographical landscapes. This contrasts with the historical view dating back to Darwin where coevolution was largely visualized as a slow, directional molding of species' traits through long periods of evolutionary time. Recent and ongoing research in coevolution is revealing that ecological and evolutionary timescales are often one and the same, where effects of selection can be very rapid (strong intragenerational shifts), and evolutionary responses and counter-responses can be observable in only a few generations. This conceptualization of coevo-lution places a strong emphasis on the geographic context of coevolution and the continual reshaping of species' traits across geographic landscapes.

Universal coevolutionary hot spots

Complex mosaics

Universal coevolutionary hot spots

Complex mosaics

Geographic Mosaic Theory Coevolution

Figure 1 Hypothetical illustration of the geographic mosaic of coevolution. The example depicts interactions between two species within local communities (arrows within circles). Interaction arrows represent different types of selection acting on different species. Arrows between communities reflect the magnitude of gene flow. Thick circles represent coevolutionary hot spots, while thin circles are cold spots. (a) Coevolution occurs in all communities, although the interaction coevolves in different ways among communities. (b) Coevolutionary hot spots occur within a matrix of cold spots. Adapted from The Geographic Mosaic of Coevolution by John Thompson (see Further Reading).

Figure 1 Hypothetical illustration of the geographic mosaic of coevolution. The example depicts interactions between two species within local communities (arrows within circles). Interaction arrows represent different types of selection acting on different species. Arrows between communities reflect the magnitude of gene flow. Thick circles represent coevolutionary hot spots, while thin circles are cold spots. (a) Coevolution occurs in all communities, although the interaction coevolves in different ways among communities. (b) Coevolutionary hot spots occur within a matrix of cold spots. Adapted from The Geographic Mosaic of Coevolution by John Thompson (see Further Reading).

John Thompson has championed this new framework for studying coevolution, the geographic mosaic theory of coevolution (Figure 1). This theory posits that coevolu-tionary interactions have three components driving evolutionary change:

• Geographic selection mosaics. Natural selection arising from interspecific interactions varies among populations.

• Coevolutionary hot spots. Interactions are subject to reciprocal selection only within some local communities (coevolutionary hot spots), embedded within a broader matrix of communities where selection is nonrecipro-cal or where only one of the participants occurs (coevolutionary cold spots).

• Trait remixing. Spatial distributions of potentially co-evolving genes and traits are continually being altered due to new mutations, gene flow, genetic drift, and extinction of local populations.

Thus, with this theory, populations are placed within a context of geographic selection mosaics, providing a more complicated, but more realistic view, than previous perspectives. A couple of examples from nature help illustrate how this framework facilitates the understanding of how coevolution shapes species traits and interactions across landscapes: (1) garter snakes and newts, and (2) conifers and crossbills. These examples demonstrate how coevolutionary hot spots can result in geographic patterns in coevolved traits. First, Taricha granulosa newts and Thamnophis sirtalis garter snakes inhabit western North America, and show strong evidence of coevolving traits across this region. The newts possess a potent neurotoxin, which paralyzes and often kills a predator that has ingested a newt. However, the garter snake has evolved varying amounts of resistance to the neurotoxin (this resistance is physiologically costly). Newt toxicity levels and snake resistance levels are tightly matched across the geographic landscape (i.e., where newts are more toxic, snakes have greater resistance). In addition, snake populations, where newts do not co-occur, exhibit very low levels of resistance. Research has revealed two coevolutionary hot spots, and a number of intermediate and cold spots. Second, both red crossbills (Loxia spp.) and red squirrels (Tamiasciurus hudso-nicus) prey upon pine cones. Where squirrels are the primary seed predator, conifers have evolved heavier pine cones with fewer seeds and thinner scales, which defends against squirrels. Where crossbills are the major seed predator, pine cones are lighter with more seeds and thicker scales, which defends against crossbills. Crossbills in turn have evolved counter adaptations to consume the seeds, exhibiting deeper, less curved bills where pine cones have thick scales compared to areas where pine cones have thin scales. Thus, trees are evolving in response to crossbills and squirrels in different populations, and crossbills are evolving in response to these evolutionary changes in the trees.

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