November 29, 2004

Evolutionary Fitness

If someone says 'survival of the fittest', what image pops into your mind? I used to connect it to an idea of a strong animal with big teeth and bigger claws fighting it out with other members of its species, ruthlessly bringing down hapless prey, etc.

When I hear people use 'evolutionary' terms in the context of business, it makes me think that such associations are common. The idea of fitness is always tied up in such associations with a big, nasty predator. But if that's fitness, why are there rabbits?

The true concept of evolutionary fitness, also called natural selection, is far broader and very simple: A fit organism is one that survives to reproduce. By extension, a fit species is one that manages to survive in some form, generation after generation.

One of Darwin's key insights was that far more organisms are born than survive to sexual maturity. Within those offspring are all the varying combinations of traits and periodic mutations that appear in the gene pool of their species, and an enormous amount of variety is possible. Some of them will be well adapted to their environment, some less well adapted.

Given a stable, relatively uniform environment, a species will trend towards an equilibrium and more or less stick with it. As long as members of the species can find enough food, and enough of their offspring survive to maturity, the same traits will continue to be useful and will be passed on to more offspring.

Given a change in the environment, outlying traits may become more useful, even ones that would have been selected against before. That is, individuals that once would have had perhaps a marginal chance of surviving to adulthood and finding mates end up leaving more offspring than their middle-of-the-road siblings. Over time (how long a time depends on the severity of the change), the norm re-establishes itself towards the margins of the species' former range of traits. Given a very long time, and continuous environmental change, their descendants would no longer even recognize the ancestral norm as the same species.

Gradual changes in the environment could include a change in climate, isolation in a new habitat, a change in the availability of food or prey, a change in the numbers of natural predators, or a disease. Most of these conditions, if negative, provide a species with time to adjust itself, and for at least some members to survive. Those individuals, better adapted to their new circumstances, can increase their numbers back to whatever the traffic will bear.

However, sudden and catastrophic change is another issue. A rapid influx of new predators, a natural disaster, a plague, the extinction of a former food source, etc, can reduce a population so rapidly that there isn't time to adjust. Non-selective natural disasters can also reduce a population in ways that have little to do with the fitness of the survivors once the crisis is past. In this case, a species runs the risk of being wiped out because it won't have time to adapt.

I mentioned in a previous post on the topic that one of the early false starts in evolutionary theory was Lamarck's idea of individuals being able to pass on traits acquired over a lifetime. In terms of genetic heritability, this is absolutely false. In a physical sense, evolution occurs only at the species level. The point of change has to do with the survival ratio of offspring with different traits. The survivors go ahead, the other traits fall out of the gene pool. Under these constraints, a too rapid change in their environment spells death for most species. But a human could be forgiven for thinking otherwise.

Human beings are blessed in evolutionary terms even though we don't have claws, especially fearsome teeth, very strong muscles, or tough hides. Even with our adaptation to running upright, we aren't really all that fast. What we have is even better: the intelligence to adapt to environmental change within a single generation through altering our behavior, and the innate ability to cooperate with each other.

These adaptations allow us to survive situations that would kill other species, even those who would be considered 'stronger' than us if paired off in a one-on-one fight. Through culture and language, we can preserve the survival skills of past generations, and convey any needed changes to future generations at once. We can adapt without needing to wait for natural selection to alter our gene pool.

So it makes sense that the first theory of evolution assumed that other species also had some means of rapid adaptation according to the experiences of mature individuals. It may even explain why so many people don't seem to understand the kind of devastation that could result from changes we would regard as minor inconveniences. Still, it's the law of nature, and ignorance is no excuse for the law.

Posted by natasha at November 29, 2004 02:59 PM | Science | Technorati links |

I'll have to dig around in the fifteen or fifty or so boxes of books out in the garage to find it and the author, but there's a great mid-sixties socio-science fiction novel out there (somewhere)... Earth Abides. Sums it all up.

Posted by: Thomas Ware at November 29, 2004 08:37 PM