BIOL 1020- Blog Post Assignment 22/8/12
Divergent evolution (Adaptive Radiation)- Elizabeth MacLeman
As a species’ population increases, the demand for food, habitat and other essential sources also increases. As a result a population may disperse in orders to exploit new habitat and food sources. Over time the population will adapt to their new environment and physical and genetic differences can be identified across the species. Natural selection will have occurred and selected the traits in the population best suited to the species new surroundings, this was first identified by Charles Darwin in regards to the Galapagos island finches.
These finches had significantly different shaped body and beaks to those on the mainland. Darwin identified that the finches that had desirable traits were able to survive and produce offspring. Over time this lead to an observable change in the Galapagos island finches from the mainland finches. Darwin identified that the Galapagos island finches were on their way to becoming a separate species. Darwin had identified divergent evolution.
Often due to geographical borders like mountains, rivers and seas separating populations of the species or due to inherited behavioral or physical traits interbreeding between the populations of the same species does not occur. This means that the genetic differences between the populations are not shared and will over time lead to the formation of separate species, divergent evolution.
In the article Mitochondrial sequence divergence among Antarctic killer whale ecotypes is consistent with multiple species written by Richard LeDuc et al. the idea that the Antarctic orca whale may be separate species or at least experiencing late stage divergent evolution is explored. Three physically distinct types of orca whale were identified in Antarctic waters. Type A has black and white colouring and lives in ice free waters, hunting cetaceans, Type B has grey, white and black colouring, had a large eye patch pattern and a large dorsal cape. Type B lives in pack ice. Type C has a slightly smaller eye patch pattern to that of Type B and lives in denser areas of pack ice.
They were testing claims that type B and C were in reproductive isolation and therefore on their way to becoming two different species, experiencing divergent evolution and that type A was a different species all together, has already experienced divergent evolution.
They compared complete sequences of mitochondrial control region of 81 orca whales of all three types of orca whale. What they found was three fixed differences between type A and the other two types and one fixed difference between C and the other two types. The three fixed differences between type A and the two other types is consistent with the idea that type A is a different species of Orca whale. This is due to behavioral and geological barriers as type A lives in a very different habitat to the other two types. While the argument for reproductive isolation between B and C is weaker as their mitochondrial control regions are almost identical.
While the evidence for formation of a separate species is not conclusive Richard LeDuc et al. present strong evidence for this conclusion, presenting an interesting and relevant example of this slow form of evolution as first discovered by Charles Darwin. Overtime these predictions may well present themselves to be true as evolution continues down its divergent path separating this species to form another.
Original article: Mitochondrial sequence divergence among Antarctic killer whale ecotypes is consistent with multiple species: 23 August 2008 royal society publishing. Written by Richard LeDuc, Kelly Robertson and Robert Pitman
Campbell Biology 9th edition 2012 Pearson; Reece, Meyers, Urry, Cain, Wasserman, Minorsky, Jackson and Cooke
Oxford dictionary of Biology 2008 Oxford University Press
Picture: Three different ecotypes of Antarctic killer whales: (a) type A, (b) type B and (c) type C. Type designation from Pitman & Ensor (2003); not to scale. Illustration by U. Gorter.