When I first stepped foot on California’s picturesque Santa Cruz Island, I was in awe. The foxes were tame, the jays were supersized, and the wildflowers grew like trees. I knew that islands were renowned for harboring unusual species. But I didn’t know that there was more to the biodiversity of this small island than met the eye—let alone that I would play a role in discovering it.
Islands have played a central role in the quest to uncover how evolution operates. A comparison of species on the Galápagos Islands and neighboring South America seeded Charles Darwin’s insight that similar species share a common ancestor. And later work on Darwin’s finches revealed that evolution isn’t just a slow, steady process spread out over millennia; it can occur rapidly and alter the characteristics of a population from one year to the next.
Islands are the test tubes of nature. Depending on the island (and the species in question), many of them are closed off, rarely playing host to immigrants. This isolation allows species to adapt to the characteristics of their particular island home without the potentially meddlesome influence of “foreign” genes brought in by individuals from faraway lands. That’s why islands are hotbeds for the generation of new species.
But, as I found out on Santa Cruz Island, evolution doesn’t stop there. The process can also generate biodiversity within islands, not just as you go from one island to another. This came to light during my Ph.D. research spent studying a brilliant blue bird called the island scrub-jay, found only on Santa Cruz.
After catching more than 500 jays across the island—which, at 97 square miles, is roughly the size of New York’s Staten Island—it became apparent that island scrub-jays aren’t just different from their sister species on the California mainland. The species also has different characteristics depending on where you look for it on the island, a finding my colleagues and I reported recently in the journal Evolution.
Island scrub-jays that live within pine forests in three different areas of the island have long, shallow beaks, which allow them to obtain food buried within the crevices of pine cones. Meanwhile, their next-door neighbors in oak forests have shorter, stouter beaks, which are better suited for hammering open acorns.
At first glance, this may seem like your prototypical case of evolution in action. But the reality is far from it. The oak and pine forests on Santa Cruz Island stand directly adjacent to one another, and the birds can and do fly between them. These findings contradict the long-held assumption that evolution generates adaptations that are fine-tuned to local characteristics of the landscape only when populations are separated on opposite sides of a barrier, like an ocean or a stretch of inhospitable habitat.
Surprising as these findings may be, a growing number of evolutionary biologists have reported similar stories.
Take the apple maggot fly. When Europeans first colonized the New World, apple maggot flies fed exclusively on the fruits of wild hawthorn. But when apple trees were planted in North America, some members of the species jumped ship and started to live and breed in adjacent apple orchards, becoming a commercial pest. The two groups evolved adaptations specific to their host plants and, over time, diverged to the point where they rarely mate with one another even though they live nearby. Clearly a lack of isolation didn’t prevent them from taking separate evolutionary paths.
Similar stories have been reported for cichlid fish in Nicaragua, spotted salamanders in the Eastern United States, and blue tits on the island of Corsica. These examples contribute to mounting evidence that our assumptions about the spatial scale of evolution—and specifically the importance of isolation—might be misguided, according to a 2014 paper published by a group of evolutionary biologists.
This means that more biodiversity may exist in nature than we have ever bothered to look for. Not necessarily diversity sufficient to declare two populations separate species, but a more subtle form that includes individuals that are “locally adapted” to different environments.
It’s important to figure that out because the amount of diversity within a species is one of the best predictors of its ability to adapt to changes in environmental conditions. In essence, evolution can do more when it has a wider variety of raw ingredients to work with. And, as climate change disrupts the environment, species the world over will need all of the tools at their disposal to keep up.
That could be especially true for the island scrub-jay. With a population of fewer than 3,000 individuals and a limited ability to move elsewhere, the species will have to adapt to any changes that crop up on the island—or it may go extinct. Protecting the full range of biodiversity contained within the species could be critical for its survival. As an added benefit, it could also make a visit to Santa Cruz Island that much more intriguing.