The discovery in a tributary of the Amazon of the first new species of river dolphin in a century illustrates one of the basic ideas in evolutionary theory - diversification by population isolation, giving an isolated gene pool. It also illustrates how the morphological, genetic and geological evidence all line up perfectly with the theory of evolution by natural selection.
The discovery was made by Tomas Hrbek of the Federal University of Amazonas in Manaus, Brazil, and colleagues, who analysed the DNA from dophins in the Araguaia and Tocantins rivers and found them to be different to all other river dolphins found in the Amazon river system. These dolphins also have fewer teeth than other dolphins. The new species has been named Inia araguaiaensis,
DNA analysis gives the point of diversification from other members of the Inia genus at about 2.08 million years which coincides with the geological events which produced the Araguaia-Tocantins basin and cut the rivers off from the rest of Amazonia by waterfalls and rapids which form natural barriers for the dolphins so splitting the original population into two isolated gene-pools.
It's exciting evidence for a previously unrecognised species within the ancient lineage of Amazon river dolphins, yet it's already rare, and its habitat is now fragmented by dams.These are exactly the conditions which would be expected to produce speciation over time as the two different populations evolve on their own paths. Since the two populations may already not have a representative sample of the total population's genome they may even be on their way to diversification at the point of isolation - the so-called 'founder effect'. Differences in the local environments will quickly cause the populations to diversify genetically as different mutations and combinations of alleles will be more advantageous in one environment than they might be in the other, and so will be selectively favoured in one population but not in the other, where different mutations may come to predominate.
Scott Baker, Oregon State University, Newport, USA
Similar geological events are thought to have produced similar sets of rapids isolating the Madeira river and coinciding with the evolution of I. boliviensis 2.87 million years ago, and on the Orinoco river, coinciding with the diversification of the subspecies I. geoffrensis humboldtiana.
Unfortunately, this newly-discovered species is already rare with a population of only about 1000 and it is under severe threat from human activity, especially new dams on the Araquaia and Tocantins rivers, so it may well become extinct in a few years, the victim of environmental change so rapid that the slow pace of evolution can't save it.
'via Blog this'