Thursday, 15 August 2019

Animal Doppelgangers: Convergent Evolution

This is an example of retrograde evolution

In today’s post, I’ve decided to address another evolutionary topic: ‘Convergent Evolution’ (homoplasy). In principle, the concept is relatively simple. Organisms in similar ecological environments (niches) evolve independently to form similar body morphologies and/or analogous body parts, physiology or biochemistry. Or in other words, similar problems yield similar answers. A good example would be a comparison between sharks and dolphins. Sharks and dolphins are not closely related in reference to phylogeny. However, their body habitus is remarkably similar. Thus, both are streamlined and their respective skins have a particularly low coefficient of friction. Also, both species are endowed with a fin configuration that is essentially the same. A key difference relates to the tail fins. The shark has a tail fin with a vertical aspect while the dolphin’s fin is distinctly horizontal. These similarities exist in spite of the fact that dolphins are more closely related to humans than they are to sharks. Of course, a shark’s body shape can vary according to individual species and the various lifestyles pursued. However, this does not detract from the main thrust of the thesis especially when we compare classic pelagic sharks such as the great white (Carcharodon Carcharias), with the dolphin. Both species have evolved as fast-swimming predators. Evolution, over aeons, has honed the shark and dolphin body shape to suit the open sea environment and cement their role as top predators. From a behavioural perspective, there are marked differences between these species. The great white is essentially a solitary hunter while the dolphin hunts within a cooperative pack (pod).

Clearly, the torpedo body shape has evolved under evolutionary pressures and constraints to maximise the efficiency of these two very successful predatory species. Indeed, if we look to the remote evolutionary past we can observe the fossil remains of the long-extinct ichthyosaur (fish-lizard). Again this common and distinctive body shape is resplendent and replicated in the ichthyosaur, although this creature is designated a reptile.

I’ve added images of the great white shark, dolphin and ichthyosaur for comparison: note the similarity in musculature and body form.

As an aside and a point of clarity: the dolphin’s horizontal tail fin is a consequence of anatomical/economical constraints. Evolution often reworks existing structures and moulds them into different anatomical formulations conducive to their new functional role(s). It is to be remembered that dolphins originally evolved from land-dwelling mammals that ‘decided’ to return to the sea. The tail fin evolved from bipedal (is there any other sort?) legs and feet and therefore evolution had to fashion a tail from these existing structures, therefore, resulting in a horizontal fin.

Another good example concerns the convergent evolution of eutherian mammals (classic mammals) and marsupials. Marsupials evolved in wonderful geographical isolation from standard eutherian mammals and these fundamentally distinct 'mammal types' are not closely related. Even so, we observe, once again, that species occupying coinciding ecological niches are spectacularly similar in anatomical and morphological form. Consider the wolf and the recently extinct, marsupial, thylacine (Tasmanian wolf). Both species evolved as pack animals to take on large herbivorous prey and their physical resemblance is marked. As a digression: it is quite difficult to distinguish the skulls of wild dogs and thylacines from each other on gross anatomical inspection.

Convergent evolutionary pressure is yet another example of evolution in wondrous action. I have cited just two main examples and these solely relate to morphology or body form. Other important examples would include the independent evolution of flight in bats, birds and insects. Our vegetative cousins (twice removed) are not exempt from this fascinating aspect of evolutionary force. The independent and repeated evolution of C4 photosynthesis represents an important representation of convergent evolution in plants and algae.   

It is probably a good idea, at this stage in the proceedings, to address and dispel a major misconception concerning the evolutionary process. Detractors (more fool them) often view evolution as a 'purposeful' process. By this I mean that evolution is wrongly envisaged as being part of a master plan or template which is to be followed with intent. Let me be clear: there is no prospective plan to follow. Evolution is a blind force which shapes individual variation through selective environmental forces. From this 'perspective' evolution is not predictable in any grand sense. Environmental selective pressures can change rapidly and in some instances catastrophically. Nothing is certain: chance and circumstance determine all. One of the reasons for the misconception, I think, lies with the normal use of written language and our tendency to invoke metaphor to express complex ideas. Thus our written expression may wax (but not wain) lyrical and provide an erroneous impression/expression of the evolutionary concept.  

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