Antitropical distribution and evolution in the Indo-West Pacific Ocean

Area of Science:

• Marine Biology
• Biogeography
• Evolutionary Biology

Background:

• Antitropical distributions, where species inhabit regions north and south of the tropics but not the tropics themselves, are observed in Indo-West Pacific continental shelf species.
• Previous hypotheses for these distributions, including glacial transgression, isothermic submergence, island integration, Neogene warming, and Mesozoic continental drift, are not well-supported by evidence.

Purpose of the Study:

• To evaluate existing hypotheses for antitropical distributions in Indo-West Pacific species.
• To identify the most plausible mechanism explaining these disjunct biogeographic patterns.

Main Methods:

• Review of evidence from systematic studies across diverse animal and plant groups.
• Analysis of characteristics common to antitropical distribution patterns.
• Re-evaluation of the historical "relict theory" proposed by Theel (1885).

Main Results:

• Common antitropical patterns and systematic data suggest that established hypotheses are inadequate.
• The "relict theory" provides a viable mechanism for generating antitropical distributions.
• This theory aligns with the Indo-West Pacific, particularly the East Indies, functioning as a center of evolutionary origin.

Conclusions:

• Antitropical and disjunct distributions arise when an ancestral species with a wide range is gradually replaced by a descendant species that evolved in the East Indies.
• The older species becomes relegated to isolated populations at the edges of its former range, often north and south of the equator.
• This process can also result in relict populations in areas like the western Indian Ocean.