A nice example here of how the environment drives evolution by imposing limits on what is possible and what is optimal, in this case by the physics of fluid dynamics.
Researchers from Northwestern University in Evanston Illinois studied 22 different species in eight clades from three aquatic phyla of a wide range of shapes and sizes that all swim by 'median/paired fin swimming', in other words, by using a pair of long fins, running along each side of their body.
The species studies included flatworms, cuttlefish, cartilaginous skates and rays and bony fish.
Typically, these creatures swim by generating an undulating wave running from front to rear. Just like light or sound waves, these waves can be described by two measures - the wavelength or distance between peaks, and the mean amplitude or the height of the peaks. The effectiveness of this as a locomotory system depends on factors such as friction and the formation of energy-dissipating eddies and vortices.
The not entirely surprising thing was that, in all cases, the ratio of the wave length to mean amplitude was close to 20. This is not entirely surprising because this appears to be the optimum ratio for efficient swimming; any change either reducing the cost-benefit for energy used or reducing swim speed. Evolution has converged on a single trait even though this was absent in the last common ancestor of some 450 million years ago.
Technically, it's chance versus physics. Chance offers many possibilities as to how a fish can swim, but physics and the animal's environment puts constraints on these possibilities. In this case, the selection pressure is very high, pushing the animal to one particular solution, and necessity triumphs.So, although starting from very different body plans, natural selection, by favouring greater swimming efficiency, has caused the different clades to converge on the same optimal solution.
Neelesh A. Patankar
This gives what is probably as simple an example as it possible to find why natural selection acting on inherited variation can result in the appearance of design for purpose. Given the mindlessness but inevitability of the process it should be clear to see why this convergence was inevitable. All that was required was that this solution was possible and so available for evolution.
Bale R, Neveln ID, Bhalla APS, MacIver MA, Patankar NA (2015);
Convergent Evolution of Mechanically Optimal Locomotion in Aquatic Invertebrates and Vertebrates.
PLoS Biol 13(4): e1002123. doi:10.1371/journal.pbio.1002123
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