Convergent evolution… tell me more
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Concerted convergence
It is self evident that organisms are highly integrated in terms of function. Change one characteristic, and almost certainly other characteristic have to change in sympathy. When a group of adaptive traits consistently appear to change independently but concurrently in unrelated organisms, the term ‘concerted convergence’ is used. It expresses the fact that suites of characteristics may have to shift in tandem in order to maintain optimal adaptive functionality of the whole organism, for obvious structural or subtler underlying genetic reasons. The concept of concerted convergence is related to, and partly explained by the molecular term ‘pleiotropy’, which describes how changes in one gene can affect multiple, seemingly unconnected traits. So-called pleiotropic effects are very common in biology, as the protein(s) or other factors encoded by singles genes are often used in various distinct contexts during development.
Importantly, the phenomenon of concerted convergence suggests that the options available for evolutionary adaptation may be more limited than might otherwise be expected, due to the constant need to maintain high fitness and whole-organism functionality. In addition, concerted convergence and pleiotropy not only help to explain seemingly puzzling correlations, but also encourage a more holistic way of thinking about living organisms. This holistic view contrasts with the alternative approach of reductionism, where only one part of an organism’s genetics, form or function is examined at a time, enabling specific questions to be addressed within a larger framework.
As an explicit concept, concerted convergence is still quite new, but the following example (taken from the work of T.J. Givnish and colleagues, 2005) suggests its promise for the future as a tool to help explain hitherto baffling biological observations:
Flowering plants known as monocotyledons (or monocots) typically have elongated leaves with parallel veins, well known for example in the grasses. However, in the group a very strong connection has been found between the evolution of fleshy fruits and broad leaves with a net-like vein pattern (or venation). On 16 occasions during the last 90 million years of monocot history, different groups have independently evolved fleshy fruits and net-like venation in tandem. Why the association, why this convergence? The answer lies in a habitat shift from open sites to shady forests. In the forest understorey sunlight is less strong so the leaves have to expand their area in order to capture more light, and in doing so require better support than can be provided by a parallel venation. In addition, wind-speeds in the forests are much lower, so the plant’s seeds can no longer be readily dispersed by breezes. Seed dispersal remains essential, so they develop a fleshy exterior that will attract animals that can cart them away. Hence leaves with a net-like venation and fleshy fruits are both excellent solutions to life in a shady habitat, and have evolved by concerted convergence many times in the monocots. The predictability of evolving fleshy fruits and net-like veins in shady forests is further highlighted by examples from outside the monocots, such as Vanilla in the orchid family and Gnetum, a broad-leaved tropical vine that is not an angiosperm at all, but a gymnosperm related to conifers.
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