7.1 Selection for Adaptations

Before discussing the types of adaptations, we must define the word “adaptation” itself. In general, adaptation is defined as “a change or the process of change by which an organism or species becomes better suited to their environment.” It is important to note that the process of adaptation is not intentional. Rather, variation naturally occurs within a species. If that variation helps individual organisms survive and reproduce in their current environment, we call it adaptive (think “Adaptive Value” in Tinbergen’s Four Questions). If the variation does not help survival and reproduction, it is considered either benign or maladaptive.

Within the process of adaptation, mutations are constantly occurring. This is, in many ways, how a healthy variation remains within any species. When a trait or adaptation is derived from a mutation and offers a reproductive advantage over other traits, the trait will become more frequent within the species. This is called positive selection. Alternatively, if a detrimental trait is removed from the population through the process of natural adaptation, we call it negative selection. It is through this process of positive and negative selection that adaptation by natural selection truly occurs.

It is also possible that a trait, though no longer beneficial, remains within the population because it is not detrimental, either. Vestigial traits are everywhere, even if we do not notice them. For example, humans have a small tail at the end of our spine – hence, the tail bone. However, it has become so reduced it is no longer visible.

Adaptations can also take on a different purpose, or function, from the one for which they evolved. Called exaptations, these are traits that have become co-opted for a new use. For example, when feathers first evolved in dinosaurs, they were not used for flight. More than likely, the feathers evolved for thermoregulation instead. However, as dinosaurs became smaller, and birds emerged, the feathers became useful for balance and control in flight, and for signaling one’s quality to potential mates.

Lastly, some adaptations result in by-products. Called spandrels, these by-products may remain benign, and never experience selective pressures. However, in many cases, spandrels become the foundation for future adaptations. For example, the human chin is likely a spandrel resulting from the overall reduction in our jaw. However, as hominins began to communicate and cooperate, the chin became crucial in the development of complex vocalizations – and eventually spoken language. We can thank a by-product of evolution for our rich language capabilities, song and theatre, and the ability to debate ethics.