(i) What is "zoogeography"? Zoogeography is the study of the patterns of the past, present, and future distribution of animals (and their attributes) in nature and the processes that regulate these distributions. It’s the scientific analysis of the spatial and termporal patterns of biodiversity.
It’s part of a more general science known as biogeography. Phytogeographers are concerned with patterns and process in plant distribution. Most of the major questions and kinds of approaches taken to answer such questions are held in common between phyto- and zoogeographers.
Zoogeography is often divided into two main branches: "ecological zoogeography" and "historical zoogeography". The former investigates the role of current day biotic and abiotic interactions in influencing animal distributions; the latter are concerned with historical reconstruction of the origin, dispersal, and extinction of taxa.
(ii) Example: Green sea turtles (Chelonia mydas) on Ascension Island – dispersal or vicariance?
Green turtles are found in tropical oceans throughout the world. A rookery is located on Ascension Island (no. 1 in the map below) on the mid-Atlantic ridge between Brazil and Liberia (west Africa). Most populations are found feeding on near coastal areas of South America and females lay eggs on S.A. beaches.
How did the turtles establish such a colony on Ascension Island that is so isolated from the main body of the range (about 2,000 km from Brazil)?
Dispersal hypothesis: these animals make very long distance migrations (up to 5,000 km) between feeding and nesting areas and "simply" dispersed from S.A. to Ascension Island.
Vicariance hypothesis: aka the "Carr-Coleman" hypothesis after two long term investigators of turtle biology. Hypothesis suggests that ancestors of Ascension Island turtles nested on beaches of islands adjacent to S.A. coast throughout the late Cretaceous (135-65 mya).
Over the last 70 my, these islands have been displaced by "sea-floor spreading" (2 cm/year). This, coupled with the natal homing ability of turtles, resulted in the present colony on Ascension Island.
Q: How can zoogeographic investigation provide a test to distinguish these hypothesis? Dispersal and vicariance hypotheses are part of an age-old divide in zoogeographic inference (more on that later!). What predictions do the two hypotheses make that can be used to distinguish between them by collecting data?
A: One approach was taken by Bowen et al. (1992) who used molecular assays (mitochondrial DNA) to address this problem. They reasoned that the "vicariant hypothesis" implies that the Ascension and S.A. rookeries have been largely isolated over 70 million years and that such long term isolation should result in major genetic differences between the rookeries.
By contrast, the dispersal hypothesis predicts very recent contact between the S.A. and Ascension Island rookeries (perhaps even to the present day) and hence little long term evolutionary isolation and consequently there should be little genetic divergence between the rookeries.
What was the result? In a nutshell, sequence divergence estimates between Ascension Island and S.A. rookeries were VERY low (about 0.2% sequence divergence). Most "haplotypes" were identical (i.e. shared) between the two rookery areas which suggested that the rookeries had only been isolated for only a very short time (less than 1 million years) and that this isolation was incomplete (there was current dispersal between Brazil and Ascension Island rookeries).
Fig.2. Phenogram of genetic divergence estimates among gree turtle mtDNA haplotypes. Note the low level of divergence among haplotypes from Brazil and Acension Island (most are identical, 0% divergene) relative to the large divergence between Atlantic and Pacific haplotypes (0.67% divergence).
The shallow genetic divergence (contrasted with a major split at about 0.7% divergence between Atlantic and Pacific groups of C. mydas) was inconsistent with long term isolation predicted by the vicariance hypothesis. These results, coupled with ecological knowledge of the dispersal capabilities of green turtles strongly suggest that the dispersal hypothesis for the origin of the Ascension Island rookery is correct.
(iii) What’s the bottom line?
Think about how ecological and genetic methodologies were used to test predictions of two hypotheses put forth to explain the current distribution of a group of animals. That’s, in large part, what zoogeography is all about!!
References:
1. Brown, J.H. & Lomolino, M.V. 1998. Biogeography. 2nd edition. Chapter 1.
2. Avise, J.C. 1994. Molecular markers, natural history and evolution. Chapman and Hall. Pp. 224-226.
3. Bowen, B.W. et al. 1992. Global population structure and natural history of the green turtle (Chelonia mydas) in terms of matriarchial phylogeny. Evolution 46: 865-881.