There many ways of dividing up and categorizing Natural Selection. For example, there are the trichotomies of Natural Selection, Sexual Selection and Artificial Selection, and Modes of Selection (Stabilizing, Directional, and Disruptive) trichotomy.
We sense that these are good because they are “threes” and “three” is a magic number. Here, I’m focusing on the Mode Trichotomy, and asking that we consider that there are not three, but four modes of Natural Selection. This will cause tremors throughout the Evolutionary Theory community because Four is not a magic number, but so be it.
In Stabilizing Selection the extremes of a trait are selected against and the mean value of the trait remains the same. Mutations constantly introduced into the population tht produce traits out at the extremes are selected against. In Directional Selection the values of a trait at one end of the distribution are selected against and/or values at the other end are selected for, so that the distribution of values, and it’s mean, move in one direction. In Disruptive Selection the average values are selected against so that the distribution of the trait becomes bimodal.
That was pretty simple, but operationalizing these definitions, displaying them graphically, and thinking about how they work in shaping the overall pattern of evolution reveal important details that are often sidelined or not discussed. And, we have to consider the fourth mode: Null Selection: This is where there is no selection on the trait at any particular value. As mutations (or allelic novelty of any source) are introduced into the population what might have been a nice bell curve representing the trait’s values spreads and flattens.
One might argue that since “Null Selection” is not really selection, that it should not be a mode. I agree, but I still want it on the list of modes of selection. Why? Because without a concept of null selection, the lack of change in trait values is often incorrectly interpreted as “nothing is happening here.” But in fact, something fairly major and impressive is happening. Stabilizing selection is the process of ongoing introduction of variation and ongoing reduction in variation. It balances out because the more introduction of variation there is, the stronger selection becomes. A trait that remains the same for eons is a trait experiencing a dynamic evolutionary processes. Having no concept of Null Selection does not allow this thought to develop, or if it is mentioned, it may not stick as well.
Below I provide graphics depicting the modes. (They are available for non commercial use. For commercial use, that’s $1,000 Euros each. Oh, and click on the graphic to get a larger version.) I’ve made the graphics very simple but they are also meant to be very precise in selected details. as described below in the text.
As stated above, stabilizing selection occurs when the “central” value of a trait is not selected against or favored by selection but extreme values are selected against. The graphic shows “selection against” only, and this is depicted as rather menacing looking arrows pointing down at the upper and lower reaches of this “bell curve” shaped distribution. Note that the “after selection” graph shows that the extreme values from before selection are gone, the total range of variation is lower, and the mean is unchanged.
Here the nasty looking Force of Selection Arrow is only affecting traits near one end of the distribution. The entire distribution squishes to the right. Note that the upper end of the distribution does not move up … in other words, directional selection does not simply move the bell curve along in one direction. The total range of variation reduces and the mean moves, in this case, to the right.
In this case the central or average value is being selected against and/or the extremes selected for. My favorite example of this, and one often given in the textbooks, is the selection for gamete size. Fitness may be enhanced with a gamete with a certain amount of nutrition stored for use in a growing zygote (seed or embryo). Or, fitness may be enhanced by a small lightweight and mobile gamete (a pollen spore or a sperm). You can’t have both, and the compromise is less than ideal. [see this on Anisogamy] This example also forces us to realize that fitness needs to be considered in relation to the morph … the individual as it exists with a certain gender, developmental age, etc. Monty Python and the Catholic Church notwithstanding, a sperm is an individual with it’s own little genome and it’s own little Darwinian problems. So is an egg or a spore. They don’t have a lot of personality but they do have a fitness function.
Those are the usual three forms of selection, and the one I want to add is “Null Selection.” Is this the same as “relaxed selection” you may ask? If you want it to be that’s OK. Neither have definitions that are both formal and accepted. They are probably the same.
In null selection there are no Arrows of Selection happening to the bell curve, but there is still the constant introduction of mutations, so over time the distribution goes wacky and essentially becomes random.
For this to be really clearly conceptualized, we can go back to Stabilizing Selection and redraw the diagram like this:
Here, the mutations are seen constantly bothering the bell curve from below, and selection is working in an uneven way (more against the extremes) in the opposite direction. In cases where people have actually measured a trait over time, one sees this dynamic process. This is the equilibrium in punctuated equilibrium.