Islands hold a special place the study of evolution. Every time a new island forms--bubbling up from a volcanic hotspot, slowly accreted by coral polyps, or cut off from a mainland by rising sea levels--it begins a natural experiment in evolutionary change and the formation of new species.

Charles Darwin paid special attention to islands in The Origin of Species, noting that the plants and animals inhabiting islands were usually close relatives to those living on the nearest mainland. The Galapagos island finches Darwin collected as a naturalist aboard the H.M.S. Beagle are a classic example. Arising from a single common ancestor that colonized the Galapagos archipelago two to three million years ago, Darwin's finches include species that crack and eat particular seeds, feed on the flowers and fruit of cacti, use twigs to dig insects out of tree trunks, and even eat the eggs--and sometimes the blood--of larger birds.

Common cactus finch on Islas Plazas, in the Galapagos.

Since the Origin was first published, biologists have come to use the phrase ecological opportunity to describe the processes that can produce a diverse group of species from a single colonizing ancestor. Islands provide colonizing species with new food resources and an escape from predators and competitors. Under these highly favorable conditions, island species can live at much higher population densities than possible on the mainland--a phenomenon called density compensation. This increase in population size is often accompanied by increased variation among individuals, and greater competition from crowding neighbors creates strong benefits for individuals that try new ways to make a living.

Given enough time, one big, variable population will begin to fracture into smaller populations with different lifestyles. Given even more time, those smaller populations will stop interbreeding, and become different enough to call separate species. If that seems like a stretch of the imagination, consider that the processes of ecological opportunity are occurring all around us--as invasive species spread across the landscape, and viruses multiply in a new victim's bloodstream.

Density compensation in your own backyard

Invasive species are invasive because humans have given them an ecological opportunity. Transported to a new environment lacking strong competitors, predators, or diseases adapted to infect them, invasive species overwhelm their new habitats, growing at much higher densities than they do in their native ranges. Kudzu, the vine smothering forests in the southeastern U.S., is the classic example; and so are the cane toads spreading across northeastern Australia. New Englanders are also all too familiar with the sight of purple loosestrife turning wetlands pink, and in the U.S. southwest acres of desert floor are carpeted with red brome. Invasive species often expand their ecological repertoire as they expand into a new range, adapting to food sources or habitats they never encountered back in the old country.

Purple loosestrife's density compensation turns New England wetlands pink.

On a nearly empty island, density compensation and adaptation to new conditions are the origins of new biodiversity. When it happens in established biological communities, though, one species' ecological opportunity often comes at the expense of the native species it chokes out, displaces, or out-competes.

To a virus, everyone is an island

On a much smaller scale, your body provides an ecological opportunity to every disease-causing microbe that manages to get inside. Thanks to intense study over the last two decades, we understand this especially well in the case of HIV, the virus that causes AIDS.

A new HIV infection is typically descended from one lucky virus. Once the virus establishes itself in the lymph nodes, where there are lots of white blood cells for it to infect, its population density explodes, and so does the genetic variation within that population. HIV's high mutation rate maintains this variation, which fuels evolutionary responses to the host's immune system--and to treatment by anti-viral drugs. Tracking and anticipating the viral population's evolution is a critical component of modern treatment planning.

Early in the progression of an HIV infection, viral population density and genetic diversity grow explosively. This corresponds to the period marked "acute HIV syndrome" in the timecourse diagram above.

Ecological opportunity all around us

Clearly, you needn't be on an island to see ecological opportunity in action. The relatively simple set of processes that sets a population on the road to evolutionary diversification can show up in a vacant lot or the next time you catch a cold. Given its ubiquity, the question that ecological opportunity poses to biologists in the second century after The Origin is not, how does evolutionary diversification get started, but, why doesn't it happen more often?

(My collaborator Luke Harmon, who blogs at Dechronization and just joined Twitter, gave some very helpful feedback on the first draft of this post. I got up to speed with research on the evolution of HIV with considerable help from my friend Luke Swenson, a doctoral student at the British Columbia Centre for Excellence in HIV/AIDS who has just launched a blog about HIV research for YouthCo.)


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Photo credits

Cactus finch via Flickr user kookr; purple loosestrife via Flickr user Muffet;

HIV timecourse via WikiMedia Commons; author photo via Jeremy B. Yoder.

About the author: Jeremy Yoder is a doctoral student studying the evolution of species interactions at the University of Idaho, and planning to graduate this spring. He writes about evolution and ecology at his blog Denim and Tweed, and he's also on Twitter.


The views expressed are those of the author and are not necessarily those of Scientific American.