3.11: Types of simple selection

While something of an oversimplification, we begin with three basic types of selection: stabilizing (or conservative), directed, and disruptive. We then introduce the complexities associated with the random aspects of reproduction and the linked nature of genes. We start with a population composed of individuals displaying genetic variation in a particular trait. The ongoing processes of mutation continually introduces new genotypes with varying effects on phenotype. The effects of mutations that produce new alleles can range from the lethal, the organism that carries the mutation either dies or produces no offspring, to apparently neutral – an organism that carries the new allele displays no obvious change in phenotype or reproductive success. A complicating factor is that the phenotypic effects of a particular new allele often depend upon the rest of the genome - due to genetic background effects. At the same time, changes in the population and the general environment influence the predominant types of selection that occur over time, and different types of selection may well (and most certainly are) occurring for different traits.

For each type of selection, we will illustrate the effects as if they were acting along a single dimension, for example smaller to larger, stronger to weaker, lighter to darker, or slower to faster. In fact, most traits vary along a number of dimensions. For example, consider the trait of ear, paw, heart, or big toe shape. An appropriate type of graph would be a multi-dimensional surface, but that is harder to draw clearly. It is also often the case that a genotype that influences one trait also influences other, apparently independent, trait(s). For simplicity's sake, we will start with populations whose distribution for a particular trait can be described by a simple and symmetrical curve, that is the mean and the median are the same. New variants, based on new mutations (new alleles and combinations of alleles), generally fall more or less randomly within this distribution. Under these conditions, for selection NOT to occur we would have to make an seriously unrealistic assumption, namely that all organisms (or a pair of organisms a sexually reproducing species) are equally successful at surviving and producing offspring, something that is observably not the case. Selection occurs when genetic variation influences reproductive success, although the strength of selection (the difference in the average number of viable offspring produced) can vary dramatically between traits.

Stabilizing selection: Sometimes a population of organisms appears static, that is, the mean and standard deviation of a trait are not changing over time. Does that mean that selection has stopped? Obviously we can turn this question around: if we assume that there is a population with a certain stable mean and standard deviation of a trait – what would happen over time if selection disappeared?

Let us assume we are dealing with an established population living in a stable environment. In this population organisms are capable of reproducing more organisms than are needed to replace them. Now consider the factors that lead to the original population distribution: why is the mean value of the trait the value that it is? What factors influence the observed standard deviation? Assuming that natural selection is active, it must be that organisms that display a value of the trait far from the mean are (on average) at a reproductive disadvantage compare to those with the mean value of the trait (→); there is “negative” selection against extreme (far from the mean) values of the trait. These individuals tend to produce fewer fertile offspring than those with a value of the trait near the mean. We can generate a crude estimate of the "strength" of negative selection by looking at the shape of the trait's population distribution curve. The narrower the distribution (the smaller the standard deviation), the stronger is the negative selection effect. Similarly, where a trait’s distribution is broad we predict that the impact of the trait on reproductive success will be relatively weak. The variation in the trait will be characterized only by maximum and minimum values, reflecting the limits of what the system can produce and remain viable.^[127] We do not know why this is the case and don’t really care at the moment.

Directed selection: Imagine that the population’s environment changes. It may now be the case that the phenotype of the mean is no longer the optimal phenotype in terms of reproductive success, the only factor that matters, evolutionarily. A different value of the trait may now be more favorable. Under these conditions we would expect that, over time, the mean of the distribution will shift toward the phenotypic value associated with maximum reproductive success (→). Once reached, and assuming the environment stays constant, stabilizing selection again becomes the predominant process. One outcome of directed selection is that, as the selected population’s mean moves, it may well alter the environment of other organisms.

For directed selection to work, the environment must change at a rate and to an extent compatible with the changing mean phenotype of the population. Too big and/or too rapid an environmental change and the reproductive success of all members of the population may be dramatically reduced. The ability of the population to change will depend upon both the genetic variation present within the original population and the rate at which new mutations are produced, generally a relatively slow and constant process.^[128] In some cases, the change in the environment may be so fast or so drastic, and the associated impact on reproduction so severe, that selection will fail to move the population and extinction will occur.

Disruptive selection: A third possibility is that a population of organisms find themselves in an environment in which traits at the extremes of the population’s phenotypic distribution have a reproductive advantage over those around the mean. If we think about the trait distribution as a multidimensional surface, it is possible that in a particular environment (which may correspond to multiple geographic regions), there will be multiple distinct strategies that lead to greater reproductive success compared to others. This leads to what is known as disruptive selection (→). In an asexually reproducing population, various lineages will be subject to selective pressures based on the environments (regions) they come to inhabit, and the likelihood that individuals move from environment to environment.

The effect of disruptive selection in a sexually reproducing population will be opposed by the random mating between members of the population. But is random mating really a good assumption? It could be that the different environments, referred to as ecological niches, are physically distant from one another and that organisms do not travel far to find a mate. In the process of adapting to the two different niches, the population may split into subpopulations. Over time, two species may emerge, since when and with whom one chooses to mate with and the productivity of such matings are selectable traits. Disruptive selection can lead to reproductive isolation, a hallmark of speciation. Over time, millions of species can be expected to appear. This was the observation (many species) that Darwin and Wallace set out to explain.

---