By

A. Procrastinating Student

The term 'Evolution' refers to (gradual) change: that is all. Many mechanisms have been suggested to explain how and why organisms have evolved (i.e. changed over time). Some of these have been disproved, or just shown to be illogical, for example the Lamarckian view that the new characteristics acquired by an organism during its lifetime are inherited by its offspring (so for example if I became a body builder I would have stronger children). Even today, a number of different mechanisms are widely thought to drive the evolution of organisms, for example: mutation pressure, which arises from the fact that some parts of the genome are more likely to mutate than others; and random drift, i.e. the accumulation of NEUTRAL mutations, that have no effect on the fitness of the organism. These explain some aspects of the evolution of organisms. However, the only mechanism so far suggested that can explain the evolution of ADAPTATION is natural selection.

The idea that the evolution of adaptation is driven by natural selection has its basis in four key observations about living things. The first is that living things have a high fecundity: they have the potential to produce many offspring (for example, the herring is capable of laying up to a million eggs). Hence we would expect all populations to increase in size exponentially, so long as every individual was enabled by unlimited resources to reproduce to its biotic potential. However, what is observed is that wild population sizes are approximately constant. It therefore follows that resources are not unlimited, and this in turn implies that there is competition between organisms for the resources that are present. The third observation is that variation exists between organisms. This means that some organisms must be better equipped to succeed in this competition than others, and these will therefore contribute more offspring to the next generation. In this way, organisms possessed of characters that increase the probability of their survival are naturally selected. Finally, the fourth observation is that offspring resemble their parents. Successfully breeding individuals therefore pass on the characteristics that made them successful to their offspring, and so these characteristics become present in a greater proportion of the population with each successive generation.

This is all very well when the required change can be brought about by a single mutation. But what of more complex adaptations, such as, for example, an eye, or a wing? Such adaptations cannot arise all at once. If they are explainable by evolution by natural selection then they must appear gradually, as a result of many mutations, and each step along the way must increase the fitness of the organism. So what is the use of half an eye? In fact, intermediate stages in the evolution of eyes have been observed in various molluscs, and all are functioning sense organs, to a greater or lesser degree. However, the assumption that intermediate stages in the evolution of the eye should necessarily do this is a false one. To illustrate why this is the case, I will take the example of the wing. We can imagine that the first precursors to insect wings were very slight outgrowths from the insect. It is difficult to see what aerodynamic advantage such outgrowths could confer. However, there is no good reason to suppose that they could not have some other, entirely unrelated function. For instance, it has been suggested that they may initially have been involved in thermoregulation. Their size could then increase, so long as this conferred thermodynamic advantage. In this way they could become developed enough to begin to confer aerodynamic advantage, and once that threshold was reached, the aerodynamic advantage could be improved upon by natural selection in the normal way. This hypothesis has been tested and is supported by experimental evidence. Other examples of functional discontinuity include the production by plants of the polymer lignin, which was produced initially to serve as an antibiotic and a defence against fungi, but was later co-opted to serve in structural and waterproofing roles.