Darwin’s shocking eels
|June 27, 2014||Posted by News under Convergent evolution, Natural selection, News|
Worldwide, there are hundreds of electric fish in six broad lineages. Their taxonomic diversity is so great that Darwin himself cited electric fishes as critical examples of convergent evolution, where unrelated animals independently evolve similar traits to adapt to a particular environment or ecological niche. The new work, which includes the first draft assembly of the complete genome of an electric fish, the South American electric eel, identifies the genetic factors and developmental paths the animals used to create an organ that, in some instances, can deliver a jolt several times more powerful than the current from a standard household electrical outlet.
“A six-foot eel is a top predator in the water and is in essence a frog with a built-in five-and-a-half-foot cattle prod,” says Sussman. “Since all of the visceral organs are near the face, the remaining 90 percent of the fish is almost all electric organ.”
As a means of communication and navigation in the dark, the generation of electric fields by fish works much the same as echolocation does for bats, says Albert. “These fish are nocturnal and the vast majority of them live at the bottom of a very muddy river, the Amazon.”
Here’s the abstract:
Little is known about the genetic basis of convergent traits that originate repeatedly over broad taxonomic scales. The myogenic electric organ has evolved six times in fishes to produce electric fields used in communication, navigation, predation, or defense. We have examined the genomic basis of the convergent anatomical and physiological origins of these organs by assembling the genome of the electric eel (Electrophorus electricus) and sequencing electric organ and skeletal muscle transcriptomes from three lineages that have independently evolved electric organs. Our results indicate that, despite millions of years of evolution and large differences in the morphology of electric organ cells, independent lineages have leveraged similar transcription factors and developmental and cellular pathways in the evolution of electric organs. Paywall
A friend notes that they don’t explain how Darwin tried to slip away from the implications of the convergence, and kindly offers some selections:
Darwin, C. R. 1866. On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray. 4th edition.
p.224-225 Cases of special Difficulty on the Theory of Natural Selection: Although we must be extremely cautious in concluding that any organ could not have been produced by successive transitional gradations, yet undoubtedly serious cases of difficulty occur, some of which will be discussed in my future work.
One of the most serious is that of neuter insects, which are often differently constructed from either the males or fertile females; but this case will be treated of in the next chapter. The electric organs of fishes offer another case of special difficulty; for it is impossible to conceive by what steps these wondrous organs have been produced. As Owen has remarked, there is much analogy between them and ordinary muscles, in their manner of action, in the influence on them of the nervous power and other stimulants such as strychnine, and as some believe in their intimate structure. We do not even in all cases know of what use these organs are; though in the Gymnotus and Torpedo they no doubt serve as powerful means of defence and perhaps for securing prey; yet in the Ray an analogous organ in the tail, even when greatly irritated, manifests,as lately observed by Matteucci, but little electricity; so little that it can hardly be of much use for these ends. Moreover, in the Ray, besides the organ just referred to, there is, as Dr. R. M’Donnell has shown, another organ near the head, not known to be electrical, but which apparently is the real homologue of the electric battery in the torpedo. And lastly, as we know nothing about the lineal progenitors of any of these fishes, it must be admitted that we are too ignorant to be enabled to affirm that no transitions are possible, through which the electric organs might have been developed.
[page] 225 These same organs at first appear to offer another and far more serious difficulty; for they occur in about a dozen kinds of fish, of which several are widely remote in their affinities. Generally when the same organ is found in several members of the same class, especially if in members having very different habits of life, we may attribute its presence to inheritance from a common ancestor; and its absence in some of the members to loss through disuse or natural selection. So that, if the electric organs had been inherited from some one ancient progenitor, we might have expected that all electric fishes would have been specially related to each other; but this is far from the case. Nor does geology at all lead to the belief that most fishes formerly possessed electric organs, which their modified descendants have now lost. But when we look closer to the subject, we find in the several fishes provided with electric organs that these are situated in different parts of the body,–that they differ in construction, as in the arrangement of the plates, and, according to Pacini, in the process or means by which the electricity is excited,–and lastly, in the requisite nervous power (and this is perhaps the most important of all the differences) being supplied through different nerves from widely different sources. Hence in the several remotely allied fishes furnished with electric organs, these cannot be considered as homologous, but only as analogous in function. Consequently there is no reason to suppose that they have been inherited from a common progenitor; for had this been the case they would have closely resembled each other in all respects. Thus the greater difficulty disappears, leaving only the lesser yet still great difficulty; namely, by what graduated steps these organs have arisen and been developed in each separate fish.
Of course, in real life—as opposed to Darwinism—no difficulty has disappeared. The fact that they all descended by different paths is a huge difficulty for his view of evolution (the only one now allowed in school, apparently, in Britain and the U.S.). He wanted to show that natural selection acting on random mutation (variation) did it all, and that is just so very unlikely.
A friend asks where this genetic toolbox came from, noting that it sounds like directed rather than Darwin’s directionless evolution.
Shush, fella. Heresy. Darwin is their Jesus!
And the rest of us are laughing. They cant stop us laughing just by making it illegal.
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