2.6 Stabilising Selection

Marcos Antonio de Lima Filho, PhD.

Sometimes, natural selection acts to maintain the status quo rather than acting as an agent of change (Wallace, 2011, p. 320). This process is known as stabilising selection. Birth weight in humans is a classic example of this force: Babies weighing much less or much more than the average at birth have a lower probability of surviving to 28 days (Futuyma & Kirkpatrick, 2017, p. 141). The birth weights of most human babies lie in the range of 3–4 kg (6.6–8.8 pounds), and babies who are either much smaller or much larger suffer higher rates of mortality (Urry et al., 2017, p. 496). This type of selection is still present in most parts of the world, but it has been weakening in the past few decades. This change is due to better medical care for premature babies and increased rates of Cesarean sections for larger babies (Ridley, 2004).

Under normal conditions, stabilising selection reduces variation and tends to maintain the status quo for a particular phenotypic character (Urry et al., 2017). This mode of selection favours individuals whose trait values are near the population’s mean; This is a common form of selection because the means of many traits are near the values that have the highest fitness (often referred to as the optimum phenotype) (Futuyma & Kirkpatrick, 2017).

Stabilising selection can explain the persistence of “living fossils”, such as the horseshoe crab and coelacanth fish, which have undergone little evolutionary change over millions of years (Figure 2.6.1). This is because stabilising selection favours traits that are well-adapted to the organism’s current environment, which may not have changed significantly over time. Consequently, these organisms have been pressured to maintain their ancestral characteristics, rather than evolving into new forms.

Most of the thousands of traits in each species are under stabilising selection most of the time. If it were otherwise, the population would be generally unfit and would likely be on a fast track to extinction (Zeigler, 2014, p. 17). That is, stabilising selection is the force keeping species at one stable optimal “design”:

Stabilising selection is almost certainly the most common form of selection occurring in nature. It occurs whenever a successful trait occurs in the great majority of the individuals comprising a population, and there is currently no better or more successful variation of that trait; If this is the case, then most of the variations from this successful norm that occur (as they likely will in some of the variant offspring) will be selected against, which will in effect maintain the status quo with most individuals possessing that successful trait (Zeigler, 2014, p. 17).

It is a pertinent biological question to ask why evolutionary change has halted almost completely in these species, as it can provide insights into why some products lack innovation in later stages. There are many particular conjectures about why these groups of species have changed so little, but no general theory (Ridley, 2004, p. 607). Proposed hypotheses include stabilising selection, internal constraints, and lack of competition (Futuyma & Kirkpatrick, 2017). Still, there is no evidence that living fossils have peculiar genetic systems that would prevent evolutionary change (Ridley, 2004).

The lack of competition hypothesis is an intriguing explanation for the stagnation of evolutionary change, as it reflects the phenomenon experienced by consumers when markets become more monopolistic, resulting in a halt in innovation. That was the first explanation articulated by Darwin: “These anomalous forms may almost be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition” (Darwin, 1859/2008, p. 82).

The rate of evolution varies among species due to factors such as environmental changes, genetic mutations, and natural selection. Some species may experience rapid evolution, while others evolve at a slower rate:

The theory of evolution does not predict that species will constantly be evolving, or how fast they’ll change when they do. That depends on the evolutionary pressures they experience. Groups like whales and humans have evolved rapidly, while others, like the coelacanth “living fossil”, look almost identical to ancestors that lived hundreds of millions of years ago (Coyne, 2010, p. 4).

Indeed, the pace of evolution can vary considerably even within the same species, as demonstrated by Westoll’s seminal study on lungfishes. While these organisms underwent rapid evolution around 300 million years ago, they have since exhibited little change (Figure 2.6.2).