The changing state of our knowledge about the evolution of these specialised dolphins, which inhabit river systems as widely separated as the Amazon and its tributaries, the Indus and Ganges rivers in northern India, the Yangtze River in China and the Rio de la Plata in South America, is a good illustration of how science modifies its theories in view of new knowledge and how any current explanation is regarded as provisional and open to revision with unsolved questions freely acknowledged.
On a technical note, the De La Plata dolphin (Pontoporia blainvillei), although in the same family as the river dolphins is actually an estuarine and coastal dolphin, not a freshwater dolphin. Sadly, the Baiji from the Yangtze River (Lipotes vexillifer) has not been seen since 2004 and is now considered extinct in the wild.
So, the first question is how did these dolphins, which were once included in the same Platanistidae family because they all shared the same anatomical features, come to be so widely separated if they were adaptive features for living in rivers? The 'solution' was to assume all these shared anatomical features evolved independently by convergent evolution in response to similar environments. The anatomical features are:
Of course, it's possible that some of these features could have convergently evolved but so many seems unlikely and this is further complicated by the fact that the De La Plata dolphin, P. blainvillei, shares these features but lives in the sea and salt-water estuaries.
- A long thin 'beak' or rostrum
- Reduced eyes
- A large number of teeth in both upper and lower jaws
- A flexible neck
But, if these features evolved independently then there is no real basis for lumping them all together in the same family in the first place. The taxonomy and the theory were inconsistent and unsatisfactory.
Cue DNA and molecular analysis.
This has shown the original taxonomic grouping to be to crude. On the basis of molecular evidence, the Ganges and Indus River dolphins are in one group which, under the rules of nomenclature, retains the Platanistidae family name, being the first to be named. The remaining ones are placed in a new Iniidae family which includes the newly-discovered species, Inia araguaiaensis.
But that still leaves the problem of explaining related species in China and South America, and on opposite sides of India with no communication between the river systems. Here fossil evidence gives us a further clue.
Fossil evidence strongly suggests that the original environment of the ancestors of the Inia, Platanista and Lipotes genuses were all marine, not riverine. It seems that the flexible neck, long rostrum and large numbers of teeth could be adaptations not for rivers but for hunting in sea water, as could the reduced eyes as the ancestors came to rely more on sonar than on sight for prey detection in turbid waters. Turbid waters are found in estuaries and inshore waters as well as in rivers. These adaptations also fitted them equally well for rivers, estuaries and coastal plains, opening up the rivers as potential new niches to move into. The only thing that has probably evolved three or four times is freshwater tolerance.
So here we see the molecular and fossil evidence coming together to solve a previous mystery in dolphin evolution and so giving us a much more satisfactory taxonomic structure. We can also see how features evolved for use in one environment can sometimes open up potential new niches, and how when this happens, species can diversify into these new niches. Again the revealed facts fit comfortably within the Darwinian Theory of Evolution by Natural Selection, the unifying principle which makes sense of biology.
The question creationists need to address is why an intelligent designer would put dolphins in the rivers but the fossils of all their ancestors in the sea and then made the molecular evidence agree with the fossil record to make it look like the river dolphins evolved out of a marine one slowly over time, so giving a perfectly natural explanation for both their morphology and geographical distribution without needing to invoke magic and a magic creator. Any takers?
One can almost hear the groans of disappointment of a creationist pseudo-scientist as he tears up the manuscript of another book explaining to his credulous audience how the distribution of river dolphins can't be explained by evolution and so proves evolution doesn't happen, so their favourite magic god must have made them 6000 years ago by magic and put them in the rivers, therefore science is all wrong and the country should be run by self-appointed Christian fundamentalists who know what's best for everyone.
Wild Mammal Blogs, The Evolution of River Dolphins, 15 July 2011
Geisler JH, McGowen MR, Yang G, Gatesy J. A supermatrix analysis of genomic, morphological, and paleontological data from crown Cetacea. BMC Evol Biol. 2011 Apr 25;11:112. doi: 10.1186/1471-2148-11-112.