Why does speciation occur on islands

The theory of island biogeography makes a couple of straightforward predictions based on an island's size and how isolated it is. Islands which are easy to reach will be colonized by many species, while those that are more difficult to get to will find themselves home to fewer guests.

Isolated islands, on the other hand, will only be colonized by a few species, and, as a result, all of the species on those islands will be descendants of the handful of original settlers.

The more isolated an island is, the lower its species richness will be. An island's size also affects its biodiversity, since larger islands will have a wider variety of habitats, so species which arrive on the island will diversify to fill up the available niches. All in all, the theory predicts that an island's size sets a maximum for how many species it can host, while its isolation and local speciation on the island will decide how many species it actually has.

Since the theory is framed in terms of isolation and area, it's also a useful tool for addressing questions in conservation biology, where a species' habitat may be reduced to distant patches, which are, effectively, islands.

To test the theory, a team of researchers took advantage of human-driven changes in the populations of Anolis lizards on the Caribbean islands. Until recently, each island was host to its own endemic lizards, but human activity has changed the lizards' distribution on the islands.

The team wanted to see how well these changes fit with the predictions of island biogeography. For example, are new lizard populations more likely to be established on species-impoverished islands, such as large islands which can support high species diversity but only have a few species on them?

And since each island is hosting more and more species, the diversity limits set by island area are playing a bigger role in determining the distribution of species. A surprising finding, however, was that species richness is increasing even on isolated islands. The theory predicts that biodiversity should be lower on isolated islands and should increase only slowly, since they're difficult to reach. So why are the lizards colonizing isolated Caribbean islands so quickly?

The answer turns on the meaning of 'isolation'. All measurements were taken by a single observer BM. To test the significance of the morphological differences between the island and the continental lineages we compared population means for all morphological traits using one-way ANOVA. We also conducted a discriminant function analysis to assess if the three lineages Guadalupe juncos, mainland dark-eyed juncos, and yellow-eyed juncos clustered into morphologically differentiated groups.

To calculate discriminant functions, variables were transformed using natural logarithms, and mass was linearized using a cubic-root transformation.

All analysis were conducted in SPSS Song recordings were obtained using a Roland Edirol R stereo digital recorder and a Sennheiser ME66 shotgun microphone and stored as uncompressed wave files. Localities for the recordings were the following: J. Laguna We analyzed sonograms from song recordings using Raven Pro 1. Variables measured included maximum and minimum frequencies, peak frequency, song duration, number of different syllables and total number of syllables, from which we obtained a measure of repeated syllables.

Mean values for all these measures were calculated for each lineage, and differences were analyzed with SPSS John McCormack provided valuable comments on a previous version of the manuscript. We are grateful to Jonathan Atwell for donating recordings of J. Conceived and designed the experiments: BM PA. Analyzed the data: PA BM. Wrote the paper: PA BM. Browse Subject Areas? Click through the PLOS taxonomy to find articles in your field.

Abstract The evolutionary divergence of island populations, and in particular the tempo and relative importance of neutral and selective factors, is of central interest to the study of speciation. Background As naturally isolated systems, oceanic islands have long served as unique experimental settings for evolutionary biologists [1] — [5].

Download: PPT. Figure 1. Geographic distributions and sampling localities of the different junco populations. Figure 2. Photographs of individual males of the five junco morphs included in the study. Results Genetic Divergence and Diversity Sequencing of two mtDNA markers in 87 individuals Table 1 produced 9 haplotypes for the control region CR , and 7 haplotypes for the cytochrome c oxidase I COI gene, resulting in 14 haplotypes for the concatenated sequence bp.

Figure 3. Table 1. Mitochondrial DNA haplotype frequencies in junco populations. Table 3. Genetic diversity and demographic history indices of junco lineages. Morphological Divergence The discriminant function analysis of morphological traits shows a clear separation between all three forms insularis, hyemalis and phaeonotus , and reflects the overall small size and large bill of insularis Fig. Figure 4. Discriminant function analysis of morphological variables from the different junco lineages.

Table 4. Standardized coefficients for the first two canonical discriminant functions from morphological measurements.

Figure 5. Differences in morphological trait means among junco lineages. Song Divergence Differences in song structure and note diversity are apparent between the three lineages, with the song of continental hyemalis represented here by the Oregon junco J. Figure 6. Song spectrograms for representative song types from the three junco lineages.

Figure 7. Discriminant function analysis of song variables for the three junco lineages. Table 5. Standardized coefficients for the canonical discriminant functions from song variables. Discussion Genetic Divergence and Phenotypic Convergence on Oceanic Islands Our results reveal marked mtDNA divergence between insularis and continental junco lineages, supporting the hypothesis of a long history of isolation for the Guadalupe junco.

Genetic Diversity on Guadalupe Island The relatively high amount of haplotypic and nucleotide diversity within the insularis lineage, together with the non-significant F s values, reveal a surprising absence of population bottlenecks in the history of the lineage, despite the necessarily low population sizes juncos there must have experienced. Phenotypic Divergence of the insularis Lineage Our morphological analysis confirmed that insularis birds are smaller than mainland individuals, yet have significantly larger beaks.

Conclusion In contrast to prevailing taxonomy, genetic data reveal the Guadalupe junco as an old, well-differentiated lineage, whose similarity to mainland juncos in plumage and eye color is due to evolutionary convergence. Methods Field Sampling Junco populations were sampled using mist nets and song recording playbacks to attract individuals to the net when needed. Morphological Analysis The following morphological measurements were obtained from each bird: unflattened wing length using a ruler to the nearest 0.

Song Analysis Song recordings were obtained using a Roland Edirol R stereo digital recorder and a Sennheiser ME66 shotgun microphone and stored as uncompressed wave files. Acknowledgments John McCormack provided valuable comments on a previous version of the manuscript. References 1. New York: Oxford University Press; p. Grant PR Evolution on islands. Oxford, U. Sunderland, Massachusetts: Sinauer Associates, Inc. Or explore different modes of speciation , including:.

Find additional lessons, activities, videos, and articles that focus on speciation. Defining speciation. Reproductive isolation. Subscribe to our newsletter. Email Facebook Twitter. Help your students understand the sexual and asexual reproduction with these classroom resources. The theory of natural selection was explored by 19th-century naturalist Charles Darwin.

Natural selection explains how genetic traits of a species may change over time. This may lead to speciation, the formation of a distinct new species. Select from these resources to teach your classroom about this subfield of evolutionary biology. But the first formal genetic study was undertaken by a monk named Gregor Mendel in the middle of the 19th Century.

Mendel bred peas and noticed he could cross-pollinate them in certain ways to get green or yellow seeds.

Today, the field of genetics is breaking new ground searching for new ways to treat disease or develop crops more resistant to insects or drought. Empower your students to learn about genetics with this collection of resources. A keystone species helps define an entire ecosystem. Without its keystone species, the ecosystem would be dramatically different or cease to exist altogether.

A species is often defined as a group of organisms that can reproduce naturally with one another and create fertile offspring. However, the classification of a species can be difficult—even riddled with controversy.

Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. There are four major variants of speciation: allopatric, peripatric, parapatric, and sympatric. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.

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Related Resources. View Collection. Natural Selection.