Matching evolutionary adaptation to selection pressures


By Jack Brooks, PhD student, University of New South Wales
Tuesday, 17 January, 2017


Darwin's finches by gould

At first glance, many evolutionary adaptations appear to be unfavourable. For instance, sickle cell anaemia, a disease that effectively reduces the oxygen-carrying capacity of the blood, appears to be detrimental as it results in over 100,000 deaths per year.

However, a closer look at sickle cell disease and other adaptations reveals that in some cases they are advantageous and that this may be the reason the adaptations persist. While there is a strong body of evidence for evolution, evolutionary biologists have conflicting views on how these adaptations occur and if they have purpose.

Sickle cell anaemia

Sickle cell anaemia, initially documented in 1910, is a genetically heritable disease (requiring the abnormal gene from both parents) in which the blood cells are shaped liked sickles rather than discs. As a result of their abnormal shape, the cells are prone to getting stuck in our blood vessels. Blockages of this nature reduce oxygenation of the tissues and have other damaging knock-on effects on the cardiovascular system. In addition, people with this disease are more prone to malaria than others.

In contrast, carriers of the disease — those that have only one copy of the abnormal gene and do not have any symptoms — are protected from malaria. The protective effect of sickle-shaped blood cells was not discovered until 1954. Over 80% of people with the disease live in Africa, which also has the greatest prevalence of mosquitoes carrying malaria. Thus, it appears that this adaptation in its milder form confers protection against malaria in some cases.

The size-weight illusion

Let’s look at another example. When we lift an object and judge its weight, we are usually rather accurate, with vision, muscle and skin receptors, and experience all contributing to the perceived heaviness. But in the size-weight illusion, participants judge the smaller (visually) of two equally heavy objects as lighter. Since its discovery in 1886 by Frenchman Augustin Charpentier, this illusion has continued to perplex psychologists. However, Geoffrey Bingham from the University of Wyoming used a clever series of experiments to put forward a plausible explanation for the illusion that dates back to our days as hunter-gatherers.

In the first experiment in 1989, participants were presented with a series of spherical balls of different sizes and weights. Participants were highly accurate at selecting the balls that they thought they could throw the furthest. Further experiments revealed that the optimal object weights for throwing at different sizes were also perceived to be of equal weight — a result of the size-weight illusion. In 2016 they completed the story when they investigated African archaeological site dating back around 500,000 years. The paper, published in Nature, showed that rocks selected by hunters for throwing were near optimum for throwability and for inflicting damage.

Synaesthesia

Finally, some people carry a gene that can cause a condition known as synaesthesia, where one sense is perceived as another. One of the most common examples of synaesthesia is seeing numbers as colours. There is a higher than normal incidence of synaesthesia in artists than other professions, perhaps a result of the creativity that artists require. It’s not out of the question that this was a random mutation that conferred no advantage but was conserved only to become useful recently.

But while some scientists believe these examples are proof of the advantageous nature of evolutionary adaptations, others believe they might simply be convenient truths. For example, the genetic abnormalities that cause synaesthesia may not themselves confer any cognitive advantages and it remains an open question as to whether any selection pressure led to conservation of the synaesthesia gene. It may prove easier to study these questions in animals first — the first example of synaesthesia in animals was found in 2011.

Conclusion

Scientists have suggested that understanding of the adaptation must be met with understanding the selection pressures that led to that adaptation, and that these hypotheses are formulated before any experiments begin. This interpretation asks whether adaptations are a consequence of survival of the fittest or if fittest is defined after the fact. Darwin, the father of natural selection, found that finches on different islands had beaks optimally shaped to crack open the seeds available on the island they inhabited. In this case, the majority view in the field is that the beak shapes are a result of selection pressures.

Did evolution achieve other feats like this by preserving fitness characteristics in each generation, or are some adaptations detrimental mutations that became useful at a later date? Answering these questions after the fact is difficult as sometimes the selective pressure leading the adaptation may no longer exist, rendering the adaptation vestigial or non-functional. Further research on a case-by-case basis will thus be required to uncover whether some functions of the body are evolutionary adaptations or simply chance happenings.

Image caption: Darwin's finches or Galapagos finches. Image by John Gould [public domain] via Wikimedia Commons

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