/* */ Rosa Rubicondior: Ashyxiated Whales Show Evolution In Progress

Friday, 27 November 2015

Ashyxiated Whales Show Evolution In Progress

A: external view of sole’s tail hanging out of blowhole. B: blowhole cut open to the right side, revealing the sole in the vestibular sac. The white arrow indicates the left side of the blowhole opening, while the red arrow indicates the fish’s tail in both images.*
PLOS ONE: Fatal Asphyxiation in Two Long-Finned Pilot Whales (Globicephala melas) Caused by Common Soles (Solea solea)

A sad report published in PLOS ONE serves to illustrate the interaction between a species and its environment and poses a neat little question for those who still doubt that Darwinian Evolution by natural selection is the mechanism of adaptive change in a species.

Long-finned pilot whales (Globicephala melas) normally live and hunt in North Atlantic, living on squid and other cephalopods and only rarely venture into the shallower North Sea. However, for reasons which may be related to the death from disease of one of the pod - a female - who may have become disorientated and led her extended family the wrong way (pods of pilot whales are known to stay close to a sick member) - a pod of 30-40 of these whales found their way into the North Sea in November 2014.

Six weeks later a decomposed long-finned pilot whale was found washed up on the Belgian coast. A post mortem found a flatfish, a common sole (Solea solea), stuck in the blowhole of the dead whale. Three weeks later another dead long-finned pilot whale was found with another common sole stuck in its blowhole. The strong probability is that both these whales were asphyxiated by these flatfish.

Soles have the ability to curl up and fit into small crevices or escape through small spaces. They are also known to asphyxiate porpoises in just this way, so the probability is that these were taken up as unfamiliar food by the pilot whales which could not then cope with them to swallow them and, either in an attempt to escape, or by the efforts of the whales to swallow them, they were pushed up into the blowholes where they became stuck. The effect was to choke the whales.

Long-finned pilot whales (Globicephala melas) are rare visitors to the southern North Sea, but recently two individual strandings occurred on the Dutch coast. Both animals shared the same, unusual cause of death: asphyxiation from a common sole (Solea solea) stuck in their nasal cavity. This is a rare cause of death in cetaceans. Whilst asphyxiation has been reported in smaller odontocetes, there are no recent records of this occurring in Globicephala species. Here we report the stranding, necropsy and diet study results as well as discuss the unusual nature of this phenomenon. Flatfish are not a primary prey species for pilot whales and are rarely eaten by other cetaceans, such as harbour porpoises (Phocoena phocoena), in which there are several reports of asphyxiation due to airway obstruction by soles. This risk may be due to the fish’s flexible bodies which can enter small cavities either actively in an attempt to escape or passively due to the whale ‘coughing’ or ‘sneezing’ to rid itself of the blockage of the trachea. It is also possible that the fish enter the airways whilst the whale is re-articulating the larynx after trying to ingest large, oddly shaped prey. It is unlikely that the soles entered the airways after the death of the whales and we believe therefore that they are responsible for the death of these animals.*

IJsseldijk LL, Leopold MF, Bravo Rebolledo EL, Deaville R, Haelters J, IJzer J, et al. (2015)
Fatal Asphyxiation in Two Long-Finned Pilot Whales (Globicephala melas) Caused by Common Soles (Solea solea).
PLoS ONE 10(11): e0141951. doi:10.1371/journal.pone.0141951

*Copyright: © 2015 IJsseldijk et al. Reprinted under Creative Commons Attribution (CC BY) license

So, what has this got to do with evolution?

What we have here is a species finding itself in a new or unfamiliar environment and trying to adapt it's behaviour, in this case, its feeding habit, to that new situation. However, due either to inexperience or to being ill-equipped to cope with this new prey, some individuals suffering the consequences. If this problem was due to a gene-moderated feature (and it's very likely that the ability to swallow a common sole or the ability to clear a blocked blowhole is at least partly genetic) the gene pool would have changed to contain a higher proportion of genes which enable the survivors to cope with the new food. The change occurred not in individuals but in the allele frequency in the local gene pool.

So, the obvious question for those who doubt that evolution occurs by natural selection: why would you not expect the frequency of alleles favourable to coping with soles as a prey species to increase over time relative to those giving a tendency to choke on this food, if a pod of these whales found itself isolated in an environment like the North Sea for some reason?

A subsidiary question is why would you not expect these whales to be able to move into a new environment like the North Sea if a chance mutation enabled them to cope with soles as a food source?

The relationship between environment, fitness for survival, changing allele frequency and evolutionary adaptation should be perfectly plain in this example for anyone not pre-conditioned to avoid seeing it or too intellectually dishonest to admit to seeing it.

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