Zoogeography is the branch of the science of biogeography that is concerned with
the geographic distribution of animal species
and their attributes. That makes Zoogeography the study of patterns of biodiversity regarding time and space.
Overview
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, and it’s the scientific analysis of the patterns of biodiversity
regarding time and space. Zoogeography integrates information on the historical and current ecology, genetics, and physiology of
organisms and their interaction with environmental processes (continental drift, climate) in regulating geographic distributions
of animals. Scientists use descriptive and analytical approaches useful in hypothesis testing in zoogeography and which
illustrates the applied aspects of zoogeography (e.g. refuge design in conservation).
Branches of Zoogeography
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.
Branches of Biology relevant to Zoogeography
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.
Case Study
Green sea turtles (Chelonia mydas) on Ascension Island – dispersal or vicariance?
A Hawaiian Green Sea Turtle
Green turtles live in tropical oceans worldwide. Ascension Island's rookery is located
on the mid-Atlantic ridge between Brazil and Liberia near west Africa. Their feeding grounds are around the coastal areas of
South America, and their females lay eggs on South American beaches.
With a distance of around 2000km (1242mi) from the main body of the range, how did the turtles establish a colony on Ascension
Island that is so isolated?
Dispersal hypothesis: these animals make very long distance migrations of up to 5,000 km (3,106mi) between feeding and nesting
areas and dispersed from South America 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).
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.
See also
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.
External links
Resources for students of Zoogeography
- Biology 413: A
course outline and collection of Web resources by Dr. Taylor, UBC
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