Future Darwinism- Convergent or Contingent Evolution

A new discovery in red algae is challenging aspects of the conventional wisdom about plant evolution, at the same time producing another stunning example of the phenomena of ‘convergent’ evolution.

As land plants evolved from aquatic green algae, they needed a mechanism to seal in water and hold themselves upright under the force of gravity. This was achieved through the evolution of a tough polymer molecule- lignin.

Now there is strong evidence for the prior evolution of lignin in the cell walls of red algae– a common seaweed, which has evolved a flexible structure with lignin supporting its cell walls, enabling it to withstand the pummelling of waves. This occurred over a billion years ago, long before land plants evolved from green algae. It therefore appears the use of lignin evolved independently in these two algae in response to basically the same problem- overcoming environmental stress.

Evolutionary convergence postulates that similar forms and functions of life will evolve and converge over time in response to similar environmental pressures.

The alternative theory to ‘convergence’ has traditionally been 'contingency', championed by the late Stephen Jay Gould. This maintains that every species is the unique product of an unpredictable or chance chain of events. This suggests that if the ‘tape of life’ is rerun, the evolutionary outcomes or species that will emerge each time will be quite different.

In his book “Contingency and Convergence”, Simon Conway Morris challenges Gould's assertion that contingency is central to evolution and argues the case for convergence as the more compelling driver. The reality of convergence suggests that no matter how many times the tape of life is rerun, similar key outcomes will always emerge for organisms adapting to similar environmental pressures.

In fact convergent evolution has been tested a number of times by re-running the tape of life- that is tracing life's capacity to regenerate following past cataclysmic events, whether induced by asteroid impact or abrupt climatic change. Although each period of major stress has reduced the number of species by up to eighty-five percent, life has recovered and continued on its overall trajectory of increasing complexity, with sixty percent of species re-emerging over a period of less than several hundred thousand years.

For example, crinoids or sea lilies readapted and spread following extinction on two separate occasions; achieving a similar diverse range of anatomical forms within the same time frame. Also ungulates such as horses and deer, which diversified 50 million years ago only to be drastically reduced by climate change, again emerged in the same form 25 million years later.

Another striking example is deep sea bioluminescence. Fish have evolved a variety of visual pigments in order to detect the wavelengths emitted by marine life. Around 80 percent of marine organisms emit light at great depth. This ability has such a huge advantage that it has evolved perhaps as many as thirty times.

Also many fish have evolved yellow lenses to increase the contrast between blue-green bioluminescence and the background deep ocean illumination. More than ten different yellow pigments with completely unrelated biochemistries have now been detected, indicating the evolutionary capacity to solve the same problem independently an equivalent number of times.

The eye has also evolved many times and converged independently to two general classes of vision- binocular in mammals and compound lenses in insects. The form of wings has converged to a similar design as has the independent development of winged flight in birds, insects and bats, although separated in evolutionary time by hundreds of millions of years.

Convergence is at work in behavioural evolution as well. For example, both insects and frogs have evolved identical strategies and rules for mating song rituals, although vertebrates and invertebrates are separated by hundreds of millions of years. In more recent times, the development of early civilisation in the form of farming and building practices and the invention of writing and numbers, began spontaneously and independently in each of the major river deltas across the planet, in response to the end of the ice age.

Convergent evolution is therefore a powerful tool, not only for understanding the past, but for predicting the future. It provides compelling evidence that evolution acts as a powerful, independent force capable of overcoming the vagaries of history and chance.

It therefore provides hope that we can shape the world to be a better place for future generations.