Using Australo-Pacific kingfishers to study species radiations

Islands and species radiations

Islands have long been used to study speciation in wild populations of animals and plants. Speciation is the process by which new species evolve. Species radiations occur when multiple new species evolve in a single group in a relatively short period of time. In evolutionary terms, a short time may be one or a few million years—so, not a short time by the standards of human history!

Islands are great places to study species radiations because each island is a piece of land surrounded by water. Thus, species that live on different islands are physically separated from one another. Furthermore, many islands have well-studied geologic records. Knowing the geologic record of an island helps scientists understand species radiations because, for example, the age of an island may indicate the likely maximum (oldest possible) age of a species that is unique to that island.

The importance of the Australo-Pacific region to understanding species and speciation

The Australo-Pacific region is the cradle of speciation theory, and studies of Australo-Pacific island animals are still helping scientists to understand how biodiversity is created and structured. The study of animals on these islands has influenced ecological and evolutionary theory, contributing to important ideas and observations like the following:

The theory of island biogeography

The theory of island biogeography was first proposed by two ecologists, Robert MacArthur and E.O. Wilson, and explains why some islands have more species than others. Under this theory, the number of species on an island is determined by the island's size and its distance from large landmasses. This is because an island's size (land area) determines the number of species that it can support, and an island's distance from large landmasses influences how many species can migrate to and colonize the island.

The great speciator paradox

The great speciator paradox is the observation that some types of birds that live on groups of islands have many distinct species and subspecies, while other types of birds that live on the same groups of islands do not. The great speciator paradox was originally discussed by Jared Diamond, Michael Gilpin, and Ernst Mayr, who observed that white eyes (birds in the family Zosteropidae) in the Solomon Islands had diversified into different species on different islands, whereas other types of birds on the islands had not.

Taxon cycles

Taxon cycles were first proposed by E.O. Wilson to explain the persistence or extinction of species over time. Wilson specifically based the taxon cycle on his studies of ants in Melanesia. Melanesia is a region of islands near Australia that stretches from the island of New Guinea in the west to Fiji in the east.

A taxon cycle is a cycle in which a species range expands and then contracts. During a taxon cycle, a species expands its range so that it covers a relatively large area. Then, the species begins to diversify into multiple separate species, each with a relatively small geographic range. At least one of these species will have to expand its range again. If not, the group of species may go entirely extinct.

Adaptive radiations on islands

The best known type of species radiation is probably the adaptive radiation. In adaptive radiations, new species have key innovations or obvious adaptive changes. Famous examples of adaptive radiations of species have been documented in the Galapagos Islands and the Hawaiian Islands.

On the Galapagos Islands, one of the most well known examples of an adapative species radiation is the radiation of Darwin's finches. The finches probably evolved from a type of grassquit, a South America bird. Once in the Galapagos, the finches colonized different islands and also evolved into multiple species with different types of beaks that allowed them to eat different types of foods.

On the Hawaiian Islands, a well known radiation has occurred in the silversword alliance, a group of plants in the aster or sunflower family. Silverswords evolved from North America tarweeds that colonized the Hawaiian Islands; the silverswords include multiple species with a variety of growth forms that are adapted to the different type of habitats on the Hawaiian Islands. The ancestor to all the silverswords is thought to have reached the Hawaiian Islands about 5 million years ago.

Geographic radiations on islands

In contrast to an adaptive radiation, a geographic radiation is a rapid diversification of species across islands in which the species do not have clear adaptive changes. The radiation of white eyes that inspired the great speciator paradox is an example of a geographic radiation. Notably, much of the species diversity of white eyes is concentrated in two island archipelagos: the Indonesian Archipelago (24 species) and the Melanesian Archipelago (32 species). 

The geographic radiation of Australo-Pacific kingfishers

Australo-Pacific kingfishers (Todiramphus) are widespread and include many species and subspecies that originated during a geographic radiation. Australo-Pacific kingfishers are distributed from eastern Africa to French Polynesia in the Pacific Ocean, and scientists think that they diversified more rapidly than other groups of birds that have undergone species radiations. Australo-Pacific kingfishers are ideal for studying geographic radiations because they have a wide distribution on both continents and islands, they have diversified rapidly, and they are divided into many species. The species and subspecies of Australo-Pacific kingfishers also vary in their ecologies, feather patterns and colors, and distributions.

In order to study Australo-Pacific kingfishers, scientists collect DNA and build datasets of physical and ecological traits to understand the biological processes that underlie the species radiation in these birds. They can then compare their results to the results of investigations of species radiations in other kinds of birds and even other types of organisms. By studying and comparing species radiations in different organisms that live in different areas of the world, scientists can better understand rapid radiations in nature generally and how genes and physical traits interact during the origination of new species.

In investigating Australo-Pacific kingfishers, scientists are especially interested in determining:

• How changes in the DNA of Australo-Pacific kingfishers have helped to drive their speciation.
• How the physical and ecological traits of Australo-Pacific kingfishers have influenced the process of speciation.
• How the genes and physical traits of Australo-Pacific kingfishers are related to the environments that they inhabit.