Organisms found exclusively in a single defined geographic location, such as an island, are called endemic species. These species have evolved over long periods to adapt to their local conditions. In contrast, specialist species can only thrive within a narrow range of environmental conditions or rely on a limited diet. Islands often host a disproportionately high number of both endemic and specialist species.
The Impact of Geographic Isolation
The physical separation of islands from continents is crucial for the development of unique species. This isolation prevents the movement of individuals and genetic material between island and mainland populations. Without this gene flow, the genetic makeup of the isolated island population diverges from its mainland counterparts. Over time, these genetic differences accumulate, leading to distinct traits.
When a small group of individuals colonizes a new island, they bring only a fraction of the genetic diversity from the original mainland population. This is known as the “founder effect.” The limited genetic pool of these initial colonists can lead to unique evolutionary paths on the island. Complete isolation is significant, as even small amounts of migration can slow or prevent this divergence.
Evolutionary Diversification on Islands
Once established and isolated, island populations often undergo rapid evolutionary changes. Adaptive radiation is one such process, where a single ancestral species diversifies into multiple new species, each adapted to different ecological niches. This diversification is well-documented in examples such as Darwin’s finches on the Galápagos Islands, where an ancestral finch species evolved into numerous forms with distinct beak shapes. Hawaiian honeycreepers also diversified into over 50 species with a wide array of bill shapes and feeding behaviors.
Reduced competition and predation pressures on islands can also lead to unusual evolutionary outcomes. Large mainland species may evolve smaller body sizes, a phenomenon known as “island dwarfism.” Examples include the extinct dwarf elephants and hippos that inhabited Mediterranean islands. Conversely, small mainland species may evolve into larger forms, a process called “island gigantism.” The Komodo dragon, a giant lizard, and the extinct dodo, a large flightless pigeon from Mauritius, are classic examples.
Ecological Factors Shaping Specialization
The distinct ecological conditions on islands drive species towards specialization. Islands often present a limited range of resources compared to continents, such as specific food sources or nesting sites. This scarcity forces species to adapt to particular niches, minimizing competition for finite resources. For example, certain anole lizards in the Caribbean have partitioned resources by occupying different physical locations within their habitat.
The relative absence of certain predators or competitors allows island species to evolve unique traits and behaviors. Without constant predator pressure, some island birds, like the dodo, have lost their ability to fly. These relaxed selective pressures can lead to highly specialized forms, as organisms fill ecological roles that might be occupied by generalist species in more diverse mainland ecosystems.