environmental change and evolution

I was talking with a senior Bio teacher a few days back & she said it would be good if I could deconstruct some of the questions in 90717 (patterns of evolution), as this was an area where her students seemed to have difficulty. I’m not exactly going to do that here. But one of the themes that does seem to come up again & again is the role of the environment in evolution. Hence this post – because it’s about a paper that looks at the relationship between the origins of biodiversity and environmental change.

In ecology a ‘hotspot’ is an area where there’s very high species diversity (‘diversity’ in this sense meaning number of species). Both ecologists & evolutionary biologists have a strong interest in understanding the origins of these biodiversity hotspots. This review paper by Renema et al. (2008) looks at a series of marine biodiversity hotspots, particularly the one in the Indo-Australian Archipelago (the IAA hotspot).

The authors describe the IAA as both a cradle (an origin) for biodiversity and a museum (with the potential to preserve biodiversity). This underlines the importance of understanding the evolutionary origins of the species found in this area, & in other hotspots round the globe. Renema et al. looked at 3 distinct hotspots that appeared (& in some cases disappeared) over the last 50 million years, using both fossils and molecular biology techniques to examine the histories of a range of organisms. These included foraminifera (or ‘forams’: marine protozoa that form supporting ‘tests’ or shells), mangroves, corals, and molluscs.

Data from foram, mollusc, coral & mangrove pollen fossils showed a series of biodiversity hotspots over that 50-million-year period. The first, 42-39 million years ago (mya), was in the Tethys Sea (shown below).

With the closure of the Tethys & the formation of the Mediterranean, that hotspot disappeared, but 23-16mya 2 new hotspots developed: one in the warm shallow seas between what is now Egypt & Iran/Pakistan (the Arabian hotspot), and another around Australia/PNG./Indonesia. An expansion of that third region produced todays IAA marine biodiversity hotspot. The IAA turns out to be surprisingly old, with the fossil record of the majority of its species dating back at least 20mya – a history supported by molecular studies.

That sequence of events, from the Tethys hotspot of 40mya ago through to the IAA today, offered Renema & his colleagues the chance to see how these were related to global environmental change. They comment that One of the most striking features of these three hotspots is that each in turn marks the location of a major collision between tectonic plates. For example, the Tethys & Arabian hotspots lay within the zone where Africa and Europe were converging. As the two continental plates collided, the Tethys Sea shrank & was replaced by the much smaller Mediterranean.

Similarly, rifting and formation of the Arabian plate also led to the formation of new shallow seas and expansion of the Arabian hotspot, which was then lost as the Arabian plate collided with what is now Iran. And the IAA lies in an area of convergence between Pacific, Indo-Australian, & Eurasian plates, a complex area which has also seen the development of extensive shallow seas and new islands.

The authors concluded that these same geological processes act as drivers of evolution in today’s ecosystems. Plate tectonics affect the amount of shallow-sea habitat availble & also have an effect on ocean currrents (which may in turn either assist or block gene flow between populations. So does the formation of new islands. All of these processes have been identified as key drivers in the origination, maintenance, and extinction of marine taxa (Renema et al. 2008). Environmental changes play a significant role in evolution of populations, species and ecosystems.

W. Renema, D.R. Bellwood, J.C. Braga, K.Bromfield, R. Hall, K.G. Johnson, P. Lunt, C.P. Meyer, L.B. McMonagle, R.J. Morley, A. O’Dea, J.A. Todd, F.P. Wesselingh, M.E.J. Wilson, & J.M. Pandolfi (2008) Hopping hotspots: global shifts in marine biodiversity. Science 321: 654-657  

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