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Convergent Evolution and Front-loading

May 22, 2012

There are many examples of convergent evolution among different life forms. Convergent evolution is simply the independent acquisition of some biological feature among different lineages. For example, both birds and bats have wings (this is an elementary example, of course), but this similarity is not due to common ancestry but instead it is the result of convergent evolution.

So how does convergent evolution relate to front-loading? One of the criticisms of the front-loading hypothesis is that you can’t design a genome in a unicellular organism to evolve specific organs, tissues, biochemical systems, etc., several billion years in the future. But convergent evolution neatly answers this criticism.

A classic example of convergent evolution is the eye in the octopus and the mammalian eye. They are extremely similar, structurally speaking (see the figure, below).

Figure. Source: Ogura et al., 2004.

 

The human eye and octopus eye both have the following tissues:

1. Eyelids.

2. Cornea.

3. Pupil.

4. Iris.

5. Ciliary muscle.

6. Lens.

7. Retina.

8. Optic nerve.

Furthermore, the arrangement of these parts are practically the same. Cool! And these two systems have arisen independently, through convergent evolution (i.e., they are not related through common descent; see Ogura et al., 2004). This means that these two organs have evolved as a result of the initial state of the last common ancestor of mammals and octopuses. In short, the convergent evolution of these two organs demonstrates that a genome can be programmed to evolve a given objective. If we ran the “clock of life” backwards (to borrow from Stephen J. Gould), human-like eyes would probably appear on the scene once again. In other words, the same system keeps popping up again and again. And this is evidence that a given objective can be front-loaded, starting with a specified initial state. The eye is a beautiful example of convergent evolution, wherein 8 separate “parts” independently came together in the same arrangement to produce the function of vision.

Are there examples of convergent evolution in biochemical systems? If so, this would provide evidence that not only can organs be front-loaded, but so too can biochemical systems. More on this later.

References

Atsushi Ogura, Kazuho Ikeo, Takashi Gojobori. Comparative Analysis of Gene Expression for Convergent Evolution of Camera Eye Between Octopus and Human. Genome Research, 14: 1555-1561 (2004).

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