Evolutionary anachronism



Evolutionary anachronism, also known as "ecological anachronism", is a term initially referring to attributes of native plant species (primarily fruit, but also thorns) that seemed best explained as having been favorably selected in the past due to their coevolution with plant-eating megafauna that are now extinct. Diminished effectiveness and distance of seed dispersal by fruit-eating mammals inhabiting the same ecosystems today suggest maladaptation. Maladaptation of such fruiting plants will intensify as ongoing climate change shifts the physical and ecological conditions within their current geographic range.

The concept was formulated by Costa Rican-based American ecologist Daniel H. Janzen and carried broadly into scientific awareness when he and his coauthor, paleoecologist Paul S. Martin, published "Neotropical Anachronisms: The Fruits the Gomphotheres Ate" in the journal Science. Among the largest of extinct fruit-eating mammals in the American tropics were the gomphotheres, related to modern elephants, which inspired the title chosen by Janzen and Martin for their 1982 paper. As they explained,

"There are prominent members of the lowland forest flora of Costa Rica whose fruit and seed traits can best be explained by viewing them as anachronisms. These traits were molded by evolutionary interactions with the Pleistocene megafauna (and earlier animals) but have not yet effectively responded to its absence."

The Janzen and Martin paper was preceded by a 1977 publication by American ecologist Stanley Temple. Temple attributed the decline of the Mauritius endemic tree tambalacoque to human overharvesting to extinction of a large, flightless bird that had coevolved on the same tropical island: the dodo. It was Janzen who applied the concept to some 18 fruiting plant species or genera in Costa Rica, while Martin took the lead on proposing a distinct seed dispersal syndrome: the "megafaunal dispersal syndrome" by comparing the maladapted neotropical fruits with similar forms in the tropics of Africa and Asia that were documented as dispersed by elephants still inhabiting those continents.



Two decades after the "neotropical anachronisms" concept was published and named, science writer Connie Barlow aggregated its history and subsequent applications into a popular science book: The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms. In shaping the book's title, Barlow drew upon a 1992 essay by Paul S. Martin titled "The Last Entire Earth". Martin had written:

"In the shadows along the trail I keep an eye out for the ghosts, the beasts of the ice age. What is the purpose of the thorns on the mesquites in my backyard in Tucson? Why do they and honey locusts have sugary pods so attractive to livestock? Whose foot is devil's claw intended to intercept? Such musings add magic to a walk and may help to liberate us from tunnel vision, the hubris of the present, the misleading notion that nature is self-evident."

The honey locust mentioned in Martin's excerpt is a native tree of eastern North America. Because it is favored for planting along urban streets and parking lots, Barlow was very familiar with it while she was working on her book in New York City. Its long, curving pods became a prominent part of her book. Later, other writers also popularized its lost partnership with ice age ghosts.

One animal-with-animal form of evolutionary anachronism also gained popular attention. As reported in the New York Times, "Pronghorn's Speed May Be Legacy of Past Predators", John A. Byers hypothesized that the antelope-like pronghorn of America's grasslands was still running from a Pleistocene ghost that had been much faster than America's native wolves. This ghost was the American cheetah.

Megafaunal dispersal syndrome
Seed dispersal syndromes are complexes of fruit traits that enable plants to disperse seeds by wind, water, or mobile animals. The kind of fruits that birds are attracted to eat are usually small, with only a thin protective skin, and the colors are red or dark shades of blue or purple. Fruits categorized as mammal syndrome are bigger than bird fruits. They may possess a tough rind or husk and emit a strong odor when ripe. Because mammals (other than primates) tend to have poor color vision, these fruits usually retain a dull coloration of brown, burnished yellow, orange, or will remain green when ripe. The megafaunal dispersal syndrome refers to those attributes of fruits that evolved in order to attract megafauna (animals that weigh or weighed more than 44 kilograms) as primary dispersal agents. Following the Late Pleistocene megafaunal extinctions, most species of large herbivores have become extinct outside of Africa (and to a lesser extent Asia), thereby reducing the effectiveness of seed dispersal—except for the fruits that attracted cultivation by humans.

Common megafaunal dispersal traits

 * Large fruit, best suited to be consumed whole by large animals without seed loss.
 * Fruit grows on or close to the trunk, or on stout branches.
 * Indehiscent fruit that retains its seeds upon ripening.
 * Seeds deter or elude being ground up by teeth through having a thick, tough or hard endocarp; or bitter, peppering or nauseating toxins. They are also difficult to separate from the pulp, which is tasty and soft, to deter seed spitting.
 * The seeds benefit from—or even require—physical or chemical abrasion to germinate.
 * If tropical, the fruit drops on or just before ripening, stopping monkeys from eating them. In colder climates, the fruit stays on the branch for a prolonged time, keeping it away from predation by ineffectual seed dispersers like rodents.
 * "Looks, feels, smells, and tastes" like other fruits known to be dispersed by megafauna where megafauna still exists.

Ecological indicators of missing dispersal partners

 * The fruit either rots where it falls or is ineffectually disseminated by current dispersal agents.
 * The plant is more common where livestock (proxy for megafauna) are present.
 * The seeds germinate and grow well in upland habitats where planted, but the species almost exclusively inhabits floodplains (where water flow disperses the seeds) in the wild.
 * The geographic range is inexplicably patchy or restricted.

In popular culture
The phenomenon was referenced (though not by name) in the 1259 issue "Bee Orchid" of the online comic strip xkcd by Randall Munroe, published on September 2, 2013. In the comic, it is claimed that the orchid Ophrys apifera mimics the female of a bee species to attract males and ensure pollination, but that the bee is extinct and the orchid is doomed to follow, only delaying it by resorting to less effective self-pollination. In reality, only the first part is true: the flower mimics a bee, and reproduces exclusively by self-pollination in northern Europe, but it is pollinated successfully by bees of the genus Eucera in the Mediterranean region.