Geographic Mosaic Theory of Coevolution: Key Points and Patterns

Species interactions shape evolution, but these relationships are not uniform across landscapes. The Geographic Mosaic Theory of Coevolution explains how coevolutionary processes vary across environments, leading to diverse evolutionary outcomes. This variation influences how species adapt and evolve in response to each other over time.

Core Principles

The Geographic Mosaic Theory of Coevolution posits that coevolutionary interactions fluctuate based on local environmental conditions, genetic variation, and species distributions. Three major components—hotspots, coldspots, and reciprocal selection—define how these interactions unfold across different regions.

Hotspots

Hotspots are locations where strong reciprocal selection drives rapid coevolutionary change. These regions experience intense selective pressures that shape adaptations in interacting species. For example, studies on rough-skinned newts (Taricha granulosa) and garter snakes (Thamnophis sirtalis) reveal geographic variation in toxin resistance. In certain areas, newts produce high levels of tetrodotoxin (TTX), while local snake populations evolve heightened resistance. Research published in Evolution (2018) found that these coevolutionary dynamics are concentrated in specific areas, fueling localized evolutionary arms races.

Coldspots

Coldspots are regions where coevolutionary interactions are weak or absent due to environmental factors, demographic constraints, or species distributions. If a predator rarely encounters a toxic prey species, selective pressure for resistance remains minimal. Research on plant-pollinator interactions, such as those between Gilia capitata and its pollinators, shows that in some locations, pollinators exert little selective pressure due to alternative resources or lower abundance. A study in The American Naturalist (2020) found that certain Gilia capitata populations exhibited little coevolutionary change because pollination was primarily driven by generalist species rather than specialized mutualists. Coldspots buffer the intensity of coevolution, leading to spatial heterogeneity in evolutionary outcomes.

Reciprocal Selection

Reciprocal selection occurs when two interacting species impose selective pressures on each other, driving evolutionary changes. This process varies in strength depending on ecological and genetic factors. A well-documented example is the relationship between crossbills (Loxia curvirostra) and lodgepole pines (Pinus contorta). In regions where crossbills are the primary seed predators, pinecones develop thicker scales to reduce seed predation, while crossbills evolve stronger beaks to access seeds. A study in Science (2019) highlighted that reciprocal selection is strongest where squirrel populations are low, reinforcing how geographic variation shapes coevolutionary dynamics.

Geographic Structure Of Coevolution

The geographic structure of coevolution reflects how evolutionary interactions vary across regions, creating a patchwork of selective pressures that shape adaptation. These spatial differences arise due to ecological factors, genetic variation, and species distributions, leading to distinct evolutionary trajectories.

Local environmental conditions influence coevolutionary dynamics by altering selection pressures. Climate, habitat structure, and resource availability determine whether a particular interaction fosters strong reciprocal selection or remains weak. For example, in plant-herbivore systems, nutrient-rich environments may support dense plant populations that tolerate herbivory without significant evolutionary change, whereas nutrient-poor regions may drive stronger defensive adaptations. Studies on milkweed (Asclepias spp.) and monarch butterflies (Danaus plexippus) show that plant populations in different regions exhibit varying levels of chemical defenses, with some fostering intense selection for increased cardenolide production while others show minimal response due to lower herbivory pressure.

Genetic differences between populations also influence coevolutionary outcomes. Spatially structured gene flow can reinforce local adaptations or introduce genetic variation that disrupts ongoing coevolution. Research on mycorrhizal fungi and plant mutualisms shows that fungal strains adapted to specific soil conditions enhance plant growth in one region but provide little benefit elsewhere, leading to geographic variation in mutualistic strength. A 2021 study in Ecology Letters found that localized genetic differentiation in fungal populations resulted in varying degrees of plant dependence on these symbiotic relationships.

Historical factors shape coevolution by influencing the evolutionary history of interacting species. Past climate fluctuations, geological events, and species migrations establish interaction patterns that persist over time. In predator-prey systems, historical isolation can lead to the independent evolution of defensive traits, creating regional differences in coevolutionary dynamics. The interaction between rough-skinned newts and garter snakes exemplifies this, as isolated populations develop different levels of toxin production and resistance. A 2022 review in Trends in Ecology & Evolution emphasized that historical contingencies significantly influence present-day coevolutionary patterns.

Role Of Gene Flow

Gene flow, the movement of genetic material between populations, shapes coevolutionary dynamics by either reinforcing or disrupting local adaptations. High gene flow can homogenize genetic variation, weakening coevolutionary interactions by introducing alleles that are not locally advantageous. Conversely, restricted gene flow fosters pronounced genetic differentiation, allowing coevolution to progress independently in different regions.

In species with widespread distributions, gene flow spreads beneficial adaptations across populations. For example, in plant-pollinator interactions, traits that enhance mutualistic efficiency may transfer between populations through pollen or seed movement, leading to a more uniform coevolutionary response. However, when selection pressures vary geographically, gene flow may introduce maladaptive traits, slowing or reversing local adaptations. Studies on butterfly mimicry rings show that gene flow can either reinforce mimicry patterns when selection is strong or disrupt them when foreign alleles dilute locally adapted traits.

Species’ dispersal abilities and ecological barriers shape the extent of gene flow. Highly mobile species, such as migratory birds or wind-dispersed plants, experience greater gene flow, moderating localized coevolutionary pressures. In contrast, species with limited dispersal, such as many amphibians or island-endemic plants, tend to evolve in isolation, leading to pronounced geographic variation. A 2021 study in Molecular Ecology found that predator-prey populations with restricted dispersal exhibited stronger local adaptations, whereas those with frequent genetic exchange showed weaker divergence in coevolutionary traits.

Variation In Local Adaptations

Local adaptations arise when populations evolve traits that enhance survival and reproduction in response to specific environmental pressures. Because evolutionary pressures vary, traits beneficial in one location may be neutral or disadvantageous elsewhere. This variation results in populations of the same species exhibiting distinct characteristics, reflecting diverse selective landscapes.

For example, wild mustard (Brassica) species develop increased glucosinolate concentrations in regions with high herbivore pressure, deterring insect and mammal feeding. However, populations in areas with fewer herbivores allocate fewer resources to chemical defenses, investing instead in growth or reproduction. A similar pattern is seen in Darwin’s finches, where food availability and competition shape beak size and strength differently across islands, leading to distinct feeding strategies.

Patterns In Multiple Species Interactions

Coevolution rarely occurs between just two species. Instead, interactions often involve multiple species, creating complex networks of evolutionary relationships. These multi-species interactions influence the direction and intensity of coevolutionary change by introducing additional selective pressures, competition, or facilitation.

For example, plants, their pollinators, and herbivores interact in ways that shape coevolutionary trajectories. In some ecosystems, plants evolve floral traits to attract specific pollinators, enhancing reproduction. However, herbivores feeding on these plants impose counteracting selective pressures, leading to trade-offs between attracting pollinators and deterring herbivory. In regions with high herbivore pressure, plants may evolve increased chemical defenses that inadvertently deter pollinators, altering evolutionary outcomes. Studies on Passiflora species and their interactions with pollinating bees and herbivorous butterfly larvae illustrate how these conflicting pressures drive different evolutionary pathways.

Predator-prey relationships also highlight how multi-species interactions shape coevolution. In aquatic ecosystems, dragonfly larvae prey on mosquito larvae, but the presence of fish as an additional predator alters evolutionary responses. Mosquito larvae in fish-dominated environments evolve greater escape responses at the cost of slower growth, while those in dragonfly-dominated regions develop more cryptic behaviors to avoid detection. These variations demonstrate how additional species shift selective pressures, leading to geographically distinct adaptations.