What is Evolution?
Evolution is the process of change in an organism’s genetic makeup over time as it adapts to environmental pressures. With this definition it may be puzzling to conclude that evolution can ever go backwards. Evolution has been happening for more than 3.7 billion years, and over that time span, powerful forces like natural selection have worked to constantly shape, modify, and change species.
Can Evolution Go Backwards?
Traditional evolutionary thought holds that the forces of evolution, such as natural selection, only lead to adaptation and forward progress. Therefore, the question of whether or not evolution can go backwards is a tricky one. But, while full reversals of evolution are not likely to occur, microevolution can go backwards or even stall in some cases.
Definition of Microevolution
Microevolution can be defined as a change in the frequency of genetic variants in a population. It’s a process by which small-scale genetic changes can occur within a single species. For example, when a change in the genetic mutation rate causes an increase or decrease in certain traits, it can influence the phenotype of that species and potential adaptations over time by driving changes in the population.
Does Microevolution Lead to Speciation?
Speciation is the process in which a species splits into two or more distinct species. It occurs when two populations become separated for a certain period of time, and during that time genetic differences accumulate in the two segregating populations. When the two populations eventually recombine, they’re so different from one another that they are no longer able to interbreed and make offspring.
While microevolution can lead to speciation, it is not typically responsible for large-scale changes in the phenotype of a species. It can, however, make modest changes in a species’ traits over a period of time.
How Can Genetic Variation Lead to Bigger Changes?
Small changes in genetic variation can ultimately lead to larger changes in the species phenotype if enough individuals with the same variation survive and reproduce. This is due to the process of natural selection, which is responsible for the survival and increased or decreased reproduction of individuals with certain genetic variations.
Over time, with enough individuals carrying the same genetic variation, the trait of that variation will become more common in that species. This affects the overall phenotype of that species, as the trait has been selected for and thus will be more common.
Can Evolution Go Backward in Response to External Pressures?
Evolutionary reversal, also known as reversion, occurs when a species adapts to changing environmental conditions and begins to revert back to its ancestral form. This can happen in two different ways: through the introduction of new traits from outside sources, or through the elimination of previously adapted traits. Reversion is typically not seen on a large scale and is mostly observed in microevolutionary changes.
Examples of Evolution Backpedaling
An example of evolutionary reversal can be seen in the case of the “walking” Indian fish, Labeo rohita. This fish is thought to have adapted its environment to the point where it could “walk” on its fins, but with changing environmental conditions, the fish has reverted back to its ancestral swimming form.
Other examples of evolutionary reversals can be seen in the backpedaling of penguins’ wings, which have become shorter and stiffer due to the lack of flying pressure; in the case of the Komodo dragon, which has lost much of its brute strength and ability to run fast with large prey due to its reduced landscape; and the now-extinct passenger pigeon, which lost much of its migratory instinct with its heavily reduced environment.
It is possible for evolution to go backwards due to external pressures and environmental changes. This process of reversion is mostly observed in microevolutionary processes, as large-scale reversals are unlikely. Examples of evolutionary reversal can be seen in the walking Indian fish, penguins’ wings, Komodo dragon, and the passenger pigeon.
References
Allman, E. (2007). Evolving brains. New York, NY: W.W. Norton & Co.
“Evolutionary Biology.” (n.d) Encyclopædia Britannica. Retrieved November 21, 2020, from https://www.britannica.com/science/evolutionary-biology
Gemmell, N. J., & Hafner, D. (2003). Evolutionary biology: Concepts, models, and applications. New York, NY: Oxford University Press.
Gleason, R. (2020). What is reversion in evolution? ThoughtCo. Retrieved November 21, 2020, from https://www.thoughtco.com/reversion-in-evolution-definition-and-examples-3972348
Tattersall, I. (1997). Becoming human: Evolution and human uniqueness. New York, NY: Harcourt Brace & Co.
