Selection simply refers to the fact that, of the offspring in a given generation, those better suited to the selective environment (due to their variations) tend to live in better health, and ultimately do have a better chance of finding mates and therefore, passing the DNA that made them on to the next generation. This introduces the concept of fitness, which I refer to as an individual’s likelihood of having offspring, a sort of probability figure that, for reasons we will learn soon, can change from moment to moment.
I introduce the concepts of selective agents and selective environments, and ask students to consider these when they observe any species; what were the selective pressures that a given organism’s ancestors had to endure, resulting in the life form we see today?
I note that selective environments are extremely complex and difficult to characterize; they may change, for example, seasonally, and/or through the course of an individual’s pre-mating life, such that different pressures may “evaluate” individual fitness at different times.
It is important here to indicate that (except in the case of human beings and “artificial selection”) selective pressures are not conscious entities calculating how to shape a species; the temperature regime that shapes a species’ pelage over time, for example, is strongly conditioned by geography and physics, not a designing mind. I find that this is a significant intellectual barrier for many undergraduates to overcome. I strive to help students understand that while selective pressures may be thought of as essentially random, the effects of selection are certainly not random, and that they do generate apparent order by “tailoring” organisms “to” certain selective environments, over time. Depending on the constitution of the class, at this time I may or may not introduce the phrase and concept that evolution may be considered the “nonrandom differential survival of randomly varying replicators.”
In Fig. 1c, I continue to draw on the diagram by showing selection via copying the individuals depicted in the variation panel, but crossing out all but one individual. Those crossed out are individuals “selected against” (they do not pass their genes on to another generation). In this case, I propose that a particular new variation (medium-gray shading) makes some of the offspring slightly different from their parents as well as their siblings, and that (for whatever reason) this makes medium-gray shaded individuals better suited to their environment (more fit), such that they are more successful in finding mates and having offspring.
Lesson
It is an observable fact that not all offspring of a given generation have find mates, or even have the same number of offspring, and that is selection against certain variations and for certain variations; over time, species are shaped by selective pressures and their evolutionary heritage.