Survival of the best fit

While we wait for next year’s commemorations of Alfred Wallace – who independently came up with the theory of evolution through natural selection and for many years was credited alongside Charles Darwin – it’s a good time to reflect on how natural selection works, and how ‘survival of the fittest’ may not be an accurate description.

Two sides of the Darwin-Wallace medal of the Linnaean Society, depicting both Charles Darwin and Alfred Wallace (click to embiggen)
The Darwin-Wallace medal of the Linnaean Society, awarded for major advances in evolutionary biology. Charles Darwin and Alfred Wallace separately but simultaneously developed the theory of evolution by natural selection.
That famous term was first used by the economist Herbert Spencer as an interpretation of the basic premise of natural selection, but Darwin himself adopted it in later editions of his book. However, it’s since been misused and misapplied in many ways.

At its simplest, the Darwin-Wallace theory suggests that organisms reproduce in higher numbers according to how suitable they are for their environment. That suitability is really the ‘best fit’ for the environment, but it can also be referred to as ‘fitness’. The trouble is that this causes confusion because fitness has taken on a different meaning in everyday speech, now being more associated with health.

The word ‘survival’ can also be a bit misleading as it should not be assumed to mean survival of any single organism, because evolution occurs across an entire population. We are instead referring to successful continuation of the species, not individual persistence.

In biology, the success of any organism can be expressed as the relative number of offspring, or even more precisely, as the relative number of genes they pass down to the following generations. This counting of genes is a much more accurate definition, because there is no way of predicting an organism’s success based on physical features at any time during its life cycle. The only meaningful measure of success in biology is the ongoing propagation of particular, inherited traits measured by genes in a population of any organism.

When people talk of competition in evolution, many people will automatically think of an example such as lions hunting grazing animals. These are examples of competition in a way, except that the competition which drives natural selection is that between each lion and the other lions in catching their food, and between each antelope in not being eaten.

These examples are easy enough to understand, but in reality, the competition is often not defined, and is therefore not possible to gauge by looking at past performance.

If the climate in which the antelopes grazed warmed a couple of degrees, or the rainfall increased by a few millimetres a year, the range of plants on which they graze may shift. A higher protein or more easily digested grass may become prevalent, and the antelopes may change their grazing habits or get fatter, meaning they are slower in running away from lions. Or their meat could take on a different flavour and become repellent to lions, or they could develop greater muscle strength and outrun the lions, who may in turn have to become smaller and faster to keep feeding on them.

So there is no way to predict which of the grasses, or the antelopes or the lions will be more successful under the new conditions, so only in hindsight can any of their success be determined.

It has been suggested more recently that competition between organisms is less of a driver of evolutionary change than external changes in the environment opening up new niches in which organisms can diversify, and this highlights the unpredictability of advantage even further.

Taking the fittest in one type of competition will give no indication of their success in an unknown competition, like taking the Australian swimming team and sending them to the 200 metre hurdles to race. They may do well, but there is no way to know until they have finished the race.

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