RELATIONSHIP BETWEEN ADAPTATION AND DIFFERENTIAL SURVIVAL OF LIVING BEINGS - BIOLOGY - 2024

In evolutionary biology, a central theme is the study of adaptations. These can be defined in terms of processes or states. If we consider it as a process, it is the portion of evolutionary change that is driven by the mechanism of natural selection. In contrast, in terms of state it is a characteristic whose current condition has been shaped by natural selection.

Natural selection is an evolutionary mechanism and is defined as the differential reproduction of living beings. Thus, some organisms reproduce more than others thanks to the possession of some trait or character that increases their fitness.

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These fundamental ideas were developed by Charles Darwin in "The Origin of Species." Evolution is the only known mechanism that can lead to adaptations.

That is, there is a relationship between adaptation and the differential reproductive success of certain individuals who present traits that increase their fitness. When the latter occurs in populations, it generates adaptations.

Adaptations, natural selection and

In evolution, there are several core concepts such as adaptation, natural selection, and fitness. There are other important terms (such as gene drift), but for the purposes of this article we will focus our attention on these three.

Fitness is the ability of an organism to survive and reproduce, leaving fertile offspring. There are several ways to quantize it, and the parameter varies between 0 and 1.

When an inheritable trait gives some individuals an advantage in terms of fitness (compared to their peers who do not possess it), something inevitable happens: these individuals will reproduce more than others and will increase their frequency in the population. This is known as natural selection.

The term 'selection' is often misleading, as there is no conscious selection by some individuals in the process.

As a process, adaptation is defined as evolution caused by natural selection that results in the accumulation of favorable changes.

As a character, adaptation is a trait that evolved gradually and that fulfills a specific biological role. In terms of fitness, this trait was superior when compared to other states of the trait in the evolutionary history of the species.

What is adaptationism?

A popular view in evolutionary biology is called adaptationism. According to the defenders of this perspective, the great majority of the characteristics present in organic beings can be considered as adaptations and their state is optimal.

There are notable scientists in the branch of evolution who support the adaptationist program, such as John Maynard Smith or William Hamilton, among others. One of his biggest opponents is the renowned paleontologist Stephen Jay Gould and his colleague Richard Lewontin.

One of the consequences of adaptationism is the division of the organism into areas not connected to each other, evaluating the traits in isolation. His opponents argue that the existence of a trait today should not always be understood as an adaptive characteristic.

Are all features adaptations?

When we evaluate the characteristics of an organic being, we cannot conclude without proof that all of its features correspond to adaptations. There are other processes that can explain the presence of some features. Note that one of the consequences of a trait not being adaptive is that it is not the product of natural selection.

It may be that the characteristic we observe is simply a consequence of its chemistry or physics. For example, no one would think that the characteristic bright red color of blood is adaptive. It is simply a consequence of its structure - which is probably adaptive, since it ensures the transport of oxygen.

It may also be a trait that has been fixed by gene drift, a second evolutionary mechanism. In fact, the consequence of drift is non-adaptive evolution, since there is differential reproductive success but not linked to a characteristic that increases the fitness of individuals.

Another possibility is that the characteristic that we observe and think is adaptive is linked to another (for example, genes are close together on the same chromosome, so the probability of recombination is low) than if it is being selected.

How do we check if a trait is adaptive or not?

In case we have suspicions that a trait is an adaptation, we must prove it in the same way that we would prove any other fact in the biological sciences: using the scientific method.

We must consider a series of experiments to help us check if the trait in question is adaptive. For example, we suspect that the white color of polar bears serves as camouflage.

Although it would not be very practical, one of the possible experimental designs would be to paint a bear brown, paint a bear white (this would be the procedural control to ensure that the paint per se has no effect in our experiment) and a bear normal.

Later we would quantify if any facet of the life of the experimental organisms is affected. We must apply this reasoning to any suspicion of adaptations, not assuming that the trait is adaptive.

Exaptation: an alternative view

In 1982, researchers Stephen Jay Gould and Elisabeth Vrba published an article in the journal Paleobiology formalizing a new concept in biology: exaptation.

For the authors, exaptation is a necessary term in evolutionary biology to describe characteristics that have been shaped by natural selection and that currently perform a different function.

Examples of exaptations

We can use our nose as an example. It is very likely that the current characteristics of this cartilaginous prolongation are related to benefits in respiration. However, we use this structure to support our glasses.

That is to say, natural selection did not favor individuals with current noses because it favored the use of glasses.

Extrapolating this example to a more specific biological situation, we have the panda's thumb - Gould's famous example. The diet of pandas is based solely on bamboo, so its correct handling is crucial for the life of the animal. The panda uses a "sixth" thumb for this purpose.

However, the thumb is not a true finger, it is an extension of a small bone originally belonging to the wrist, called the radial sesamoid.

In evolutionary development, it was advantageous for some individuals to have an elongated radial sesamoid, similar to a finger, since it probably improved the handling of their only food item.

References

1. Gould, SJ, & Lewontin, RC (1979). The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist program. Proceedings of the Royal Society of London. Series B. Biological Sciences, 205 (1161), 581-598.
2. Gould, SJ, & Vrba, ES (1982). Exaptation-a missing term in the science of form. Paleobiology, 8 (1), 4-15.
3. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. McGraw – Hill.
4. Kardong, KV (2006). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill.
5. Kliman, RM (2016). Encyclopedia of Evolutionary Biology. Academic Press.
6. Losos, JB (2013). The Princeton guide to evolution. Princeton University Press.
7. Nielsen, R. (2009). Adaptionism-30 years after Gould and Lewontin. Evolution: International Journal of Organic Evolution, 63 (10), 2487-2490.
8. Rice, SA (2009). Encyclopedia of evolution. Infobase Publishing.
9. Starr, C., Evers, C., & Starr, L. (2010). Biology: concepts and applications without physiology. Cengage Learning.