Both living and nonliving factors can interact to shape local adaptation, according to a recent study in The American Naturalist. The metaanalysis also entailed the use of a method, borrowed from the social and medical sciences, that identified common themes and research gaps in the literature—such as a lack of underlying theory as to how abiotic differences might drive local adaptation. The findings point to potentially novel research directions for the field, including studies of more mobile, long-lived organisms, and the establishment of a theoretical framework for abiotic influence on local adaptation.

Local adaptation is evolution that occurs when factors such as temperature, rainfall, or pests vary spatially, causing different populations of a species to adapt to survive best under local conditions. Local adaptation, then, can happen because of either abiotic or biotic factors. “But no one has compared to see if one of them is more important than the other,” says coauthor and evolutionary ecologist Ryan Briscoe Runquist, at the University of Minnesota Twin Cities in St. Paul. She wondered if species respond more to selection through living factors, nonliving factors, or through a combination of both. The answer, Briscoe Runquist reasoned, could help explain observed patterns of global biological diversity.

The analysis, which looked across 31 studies, drew on various examples to conclude that the strength of local adaptation can depend on the interaction of both biotic and abiotic factors. For example, in a study of 20 locally adapted tree species in the Peruvian Amazon, some trees had evolved to perform best on clay soils, and others on white sand soil. But when the researchers netted off some areas of both clay and sandy soils to exclude herbivores, they found that on protected treatment plots, clay-specialist seedlings outperformed white sand specialists, regardless of soil type. The suggestion of this finding, says University of Minnesota evolutionary ecologist David Moeller, who also coauthored the recent metaanalysis, is that the biotic factor had a strong influence on local adaptation.

Indeed, this and other studies suggest that “the local genotype does much better than the foreign when both have to deal with the stress of the predator,” says Briscoe Runquist. Without a predator, foreign plants don’t struggle quite as much. The implication, she says, is that both biotic and abiotic factors interact to influence the selective pressures leading to local adaptation across multiple species.

The metaanalysis also found that biotic factors drive relatively more local adaptation in low-latitude tropics, while abiotic factors are more influential in high-latitude temperate regions. This has been a longstanding idea, going back as far as Darwin, who noted that tropical environments are less subject to seasonal change. Organisms can thus interact over longer periods and coevolve to shape each other’s evolutionary trajectories.

The work isn’t necessarily definitive. Briscoe Runquist concedes that the 31-study analysis is not enough “to put an exclamation mark to say definitely biotic or abiotic is more important. She hopes future studies will include more papers in their metaanalyses when additional data become available. “This paper fills an important knowledge gap in exploring the presence and relative strength of local adaptation in light of whether selective forces are biotic, abiotic, or a combination,” says Sharon Strauss, an evolutionary ecologist at the University of California, Davis, who was not involved in the work.

The authors say their study’s approach was also novel. The work included a qualitative metasynthesis method, more common in the social and medical sciences. This method identifies larger trends or themes across a field by analyzing studies that may not have been designed to be analyzed as a group, explains George Noblit, a sociologist and ethnographer, recently retired from the University of North Carolina, Chapel Hill, who pioneered some of the earliest qualitative metasyntheses.

By carefully scrutinizing 91 papers and discussing the relevant quotes, the authors distilled a set of common frameworks and gaps in the existing literature. They noticed that most studies focus on annual plants or other sessile short-lived organisms, and that biotic interactions, such as pollinators’ influence on plants, have more associated coevolutionary theory than do abiotic factors such as rainfall’s influence on plants.

The metasynthesis findings could be useful to plan future work, for example, on longer-lived or more mobile organisms. “A lot of people like numbers,” says Briscoe Runquist. But “thinking about our frameworks, themes, and biases are much more important going forward.”