3.17: A brief reflection on the complexity of phenotypic traits

We can classify traits into three general types: adaptive, non-adaptive, and deleterious. Adaptive traits are those that, when present increase the organism’s reproductive success. These are the traits we normally think of when we think about evolutionary processes. Non-adaptive traits are those generated by stochastic processes, like drift, founder effects, and bottlenecks. These traits become established not because they improve reproductive success but simply because they happened to have become fixed within the population. If an allele is deleterious independent of its environment, it will be expected to rapidly disappear from the population, unless other factors are in play. Rare, strongly deleterious alleles are, most likely, the result of new mutations, or they may lead to a selective advantage in specific situations, which are no longer relevant.

When we consider a deleterious allele we are referring to its effects on reproduction. An allele can "harm" the individual carrying it yet persist in the population because it leads to an increase in the number of viable offspring produced. Similarly, there are traits that can be seen as actively maladaptive, but which occur within the population because they are linked mechanistically to some other positively selected trait. Many genes are involved in a number of distinct processes and their alleles can lead to multiple phenotypic effects. Such alleles are said to be pleiotropic. Not all of the pleiotropic effects of an allele are necessarily of the same type; some can be beneficial, others deleterious. As an example, a trait that dramatically increases the survival of the young but leads to earlier senility and sudden death in older adults could be positively selected for. In this scenario, the maladaptive senility/death trait would persist because of its association with a highly adaptive juvenile survival trait. What is happening is a form of population-level cost-benefit analysis. If the net evolutionary benefits of an allele exceeds its costs, the allele and the trait associated with it can be subject to positive selection. If the costs exceed the benefits, it will be selected against. It is worth noting that a trait that is advantageous in one environment may be disadvantageous in another. Consider the effects of the GULO1 mutation. All of which is to say that when thinking about evolutionary mechanisms, it is best not to assume that a particular trait exists independent of other traits, that it acts independently of the environment, or that the presence of a trait is evidence that it is beneficial.

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