2.7 Purifying Selection

Marcos Antonio de Lima Filho, PhD.

Losing a “feature” to improve fitness is not something unusual for living organisms either. Although often neglected in discussions of evolution, adaptation is often accomplished through the loss of formerly useful traits and features (Zeigler, 2014, p. 25). The list of lost traits and vestigial organs in animals can fill an extensive catalogue. The instances below are just a hint of the omnipresence of this natural pattern:

• Apes (including humans) lost their tails except for a few vestigial caudal vertebrae. We, humans, have “lost” (actually just significantly reduced the size of) most of our body hair (Zeigler, 2014, p. 26).

• Many cave animals also have eyes that are reduced or missing; These include fish, spiders, salamanders, shrimp, and beetles (Coyne, 2010, p. 64).

• Ancestral birds had teeth similar to the dinosaurs they had recently evolved from (Zeigler, 2014). They lost those teeth more than sixty million years ago (Coyne, 2010), yet birds still carry “teeth genes” (Zeigler, 2014).

Most studies in evolutionary biology have been centred exclusively on the generation of novel traits. Although considerably less attention has been focused on the reduction and loss of traits, it can be argued that trait modification or loss is just as crucial as gain in providing a complete understanding of evolution as a developmental process and may be one of the first steps in the cascade of events leading to evolutionary innovations (Jeffery, 2009). Such forms of adaptation have puzzled evolutionary biologists since the times of Darwin, who famously questioned the role of natural selection in eye loss in subterranean species: ‘‘As it is difficult to imagine that eyes, although useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse’’ (Darwin, 1859/2008, p. 105).

Still, despite our growing understanding of population genetics and genome, we know little about the evolutionary forces that drive these microevolutionary developments (Protas et al., 2007). Therefore, it is important to study the evolution of novelties within the context of reduced or lost traits (Jeffery, 2009).

The adaptation of whale’s hind legs is a case in point. During the evolution of flippers in marine mammals, significant changes, including reductions and losses, must have occurred in the limbs of their terrestrial ancestors prior to their conversion to perform a swimming function (Jeffery, 2009). About 54 million years ago, whales and dolphins’ ancestors had four legs and were well-adapted for moving on land, including their hips (Zimmer, 2014). The hippopotamus is believed to be the closest living relative of whales.

Through a gradual process spanning about ten million years, the ancestors of modern whales transitioned into the aquatic environment. As their evolution progressed, their forelegs transformed into flippers while their hind legs dwindled in size and nearly vanished. Evidence from whale DNA, as well as vestigial traits like their rudimentary pelvis and hind legs (Figure 2.7.1), show that their ancestors lived on land (Coyne, 2010).

During whale evolution, their leg bones were gradually lost within a span of a few million years, leaving only small hip bones far from the rest of the skeleton. However, a recent study shows that, far from being abandoned by evolution, whale and dolphin hips are still evolving. These bones used to be essential for walking on land, but they also serve other purposes as they anchor muscles that control the sex organs (Zimmer, 2014). Despite being considered “useless vestiges” of their terrestrial ancestry due to their diminished state, cetacean pelvic bones are still subjected to sexual selection (Dines et al., 2014).

Scientists are not sure why such a reverse migration happened because millions of years earlier their ancestors had moved from water to land. One possible explanation involves the exploration of an opportunity. The evolution of whales is thought to have occurred following the extinction of many kinds of giant predatory marine reptiles, such as ichthyosaurs, plesiosaurs, and mosasaurs, which left several predatory niches unoccupied (Zeigler, 2014). With these reptilian competitors gone, whale ancestors seized the opportunity to occupy an open niche, free from predators and abundant in food (Coyne, 2010). This elimination of competition created an ideal environment for new adaptations and the speciation of surviving groups (Zeigler, 2014).

Another argument in support of the importance of opportunity, explains Zeigler (2014), is that “whenever an opportunity exists for life forms to move in, they seem to do so”. This is illustrated by a diverse range of animals, such as flatworms, arthropods, molluscs, and vertebrates, that have adapted to living in dark caves by undergoing convergent changes, including the loss of eyes and pigment, enhanced tactile sensitivity, reduced metabolic rates, and increased longevity (Culver, 1982).

The absence of light in cave environments is a driving force behind the evolution of animals towards troglomorphism. In the dark, eyes and pigmentation lose their functions and tend to regress or disappear over the generations (Protas et al., 2007). This adaptation presents unique challenges for cave animals, requiring them to develop sophisticated solutions to locate food, identify predators, and find mates without the use of vision (Soares & Niemiller, 2013). Also, cave animals typically cope with the scarcity of food by evolving more sensitive tactile and chemical senses and slower or more efficient metabolisms (Protas et al., 2007).

Cave environments require unique survival mechanisms, which leads species to rely on non-visual senses. Cave-dwelling creatures, including cavefishes, offer intriguing insights into these evolutionary processes. Beyond regressive traits, cavefish have also gained constructive features, like larger jaws, more taste buds, larger cranial neuromasts, fat reserves, and possibly a more sensitive olfactory system than their surface fish counterparts (Jeffery, 2009). Such adaptations are all convergent traits that permit cavefishes to survive in subterranean habitats. Cavefishes can be found on every continent except Europe and Antarctica (Figure 2.7.2).

The Mexican blind tetra (Astyanax mexicanus, Figure 2.7.3) consists of two forms: a surface-dwelling form and a cave-dwelling form. These blind tetras have become so adapted to cave life that when both forms are placed in a lighted tank, the surface fish obtains most of it, but in a dark tank, the cavefish has much better food-finding abilities (Jeffery, 2008).

Modern physiology and molecular biology have suggested that the vertebrate retina is one of the most energetically expensive tissues, with a metabolism surpassing even that of the brain (Wangsa-Wirawan & Linsenmeier, 2003). This finding suggests that the energetic cost of their maintenance is sufficiently high for eyes to be detrimental in cave environments; In the dark, it may be costly enough to create an effective selection for eye regression (Protas et al., 2007).

The understanding of how such phenomena drive species evolution can offer insights into the evolution of technology. Like eyesight in dark habitats, each useless component in a product carries its own “energetic cost”. Furthermore, because no markets are static, consumer preferences may shift, or innovators may disrupt the previous order. As a result, once valuable features may become detrimental. Competition compels manufacturers to keep production costs as low as possible to maximise value. This may lead them to discontinue, replace, or reject features that incur energetic costs but do not deliver perceived benefits for end-users.