The convergent evolution It is the emergence of phenotypic similarities in two or more lineages, independently. Generally, this pattern is observed when the groups involved are subjected to similar environments, microenvironments, or ways of life that translate into equivalent selective pressures..
Thus, the physiological or morphological traits in question increase the biological adequacy (fitness) and competitive ability under those conditions. When convergence occurs in a particular environment, it can be intuited that said trait is of the type adaptive. However, further studies are needed to verify the functionality of the trait, through evidence that supports that, indeed, it increases the fitness of the population.
Among the most notable examples of convergent evolution we can mention the flight in vertebrates, the eye in vertebrates and invertebrates, the spindle forms in fish and aquatic mammals, among others..
Let's imagine that we meet two people who, physically, are quite alike. They both have the same height, eye color, and hair color. Their features are also similar. We will probably assume that the two people are siblings, cousins, or perhaps distant relatives.
Despite this, it would not be a surprise to learn that there is no close family relationship between the people in our example. The same is true, on a large scale, in evolution: sometimes similar forms do not share a more recent common ancestor.
That is, throughout evolution, traits that are similar in two or more groups can be acquired in a different way. Independent.
Biologists use two general definitions for evolutionary convergence or convergence. Both definitions require that two or more lineages evolve characters similar to each other. The definition usually integrates the term "evolutionary independence", although it is implicit.
However, the definitions differ in the specific evolutionary process or mechanism required to obtain the pattern..
Some definitions of convergence that lack mechanism are the following: "independent evolution of similar characteristics from an ancestral trait", or "evolution of similar characteristics in independent evolutionary lineages".
In contrast, other authors prefer to integrate a mechanism into the concept of coevolution, in order to explain the pattern.
For example, "the independent evolution of similar traits in distantly related organisms due to the emergence of adaptations to similar environments or life forms".
Both definitions are widely used in scientific articles and in the literature. The crucial idea behind evolutionary convergence is to understand that the common ancestor of the lineages involved possessed an initial state different.
Following the definition of convergence that includes a mechanism (mentioned in the previous section), this explains the similarity of the phenotypes thanks to the similarity of the selective pressures that the taxa are experiencing..
In the light of evolution, this is interpreted in terms of adaptations. That is, the traits obtained thanks to convergence are adaptations for said environment, since it would increase, in some way, its fitness.
However, there are cases where evolutionary convergence occurs and the trait is not adaptive. That is, the lineages involved are not under the same selective pressures.
In the literature it is usual to find a distinction between convergence and parallelism. Some authors use the evolutionary distance between the groups to be compared to separate the two concepts.
The repeated evolution of a trait in two or more groups of organisms is considered a parallelism if similar phenotypes evolve in related lineages, whereas convergence involves the evolution of similar traits in separate or relatively distant lineages..
Another definition of convergence and parallelism seeks to separate them in terms of the development paths involved in the structure. In this context, convergent evolution produces similar characteristics through different development paths, while parallel evolution does so through similar pathways..
However, the distinction between parallel and convergent evolution can be controversial and becomes even more complicated when we go down to the identification of the molecular basis of the trait in question. Despite these difficulties, the evolutionary implications related to both concepts are substantial..
Although selection favors similar phenotypes in similar environments, it is not a phenomenon that can be applied in all cases..
Similarities, from the point of view of shape and morphology, can lead organisms to compete with each other. As a consequence of this, selection favors divergence between species that coexist locally, creating a tension between the degrees of convergence and divergence expected for a particular habitat..
Individuals who are close and have significant niche overlap are the most powerful competitors - based on their phenotypic resemblance, which leads them to exploit resources in a similar way.
In these cases, divergent selection can lead to a phenomenon known as adaptive radiation, where one lineage gives rise to different species with a great diversity of ecological roles in a short time. The conditions that promote adaptive radiation include environmental heterogeneity, the absence of predators, among others..
Adaptive radiation and convergent evolution are considered as two sides of the same "evolutionary coin".
In understanding the difference between evolutionary convergence and parallels, a very interesting question arises: when natural selection favors the evolution of similar traits, does it occur under the same genes, or can it involve different genes and mutations that result in similar phenotypes??
Based on the evidence generated so far, the answer to both questions appears to be yes. There are studies that support both arguments.
Although until now there is no concrete answer as to why some genes are “reused” in evolutionary evolution, there is empirical evidence that seeks to elucidate the matter..
For example, the repeated evolution of flowering times in plants, insecticide resistance in insects, and pigmentation in vertebrates and invertebrates has been shown to occur through changes involving the same genes..
However, for certain traits, only a small number of genes can alter the trait. Take the case of sight: changes in color vision must necessarily occur in changes related to opsin genes.
In contrast, in other characteristics the genes that control them are more numerous. About 80 genes are involved in the flowering times of plants, but changes have only been evidenced throughout evolution in a few.
In 1997, Moore and Willmer wondered how common the phenomenon of convergence is.
For these authors, this question remains unanswered. They argue that, based on the examples described so far, there are relatively high levels of convergence. However, they argue that there is still a significant underestimation of evolutionary convergence in organic beings.
In evolution books we find a dozen classic examples of convergence. If the reader wishes to expand his knowledge on the subject, he can consult the book by McGhee (2011), where he will find numerous examples in different groups of the tree of life.
In organic beings, one of the most astonishing examples of evolutionary convergence is the appearance of flight in three lineages of vertebrates: birds, bats, and the now-extinct pterodactyls..
In fact, convergence in current flying vertebrate groups goes beyond having forelimbs modified into structures that allow flight..
A series of physiological and anatomical adaptations are shared between both groups, such as the characteristic of having shorter intestines that, it is presumed, reduce the mass of the individual during the flight, making it less expensive and more affective..
Even more surprising, different researchers have found evolutionary convergences within bat and bird groups at the family level..
For example, bats in the family Molossidae are similar to members of the family Hirundinidae (swallows and allies) in birds. Both groups are characterized by rapid flight, at high altitudes, exhibiting similar wings.
Similarly, members of the Nycteridae family converge in various respects with passerine birds (Passeriformes). Both fly at low speeds, and have the ability to maneuver through vegetation.
An outstanding example of evolutionary convergence is found when analyzing two groups of mammals: the aye-yesterday and the squirrels..
Today, the aye-aye (Daubentonia madagascariensis) is classified as a lemuriform primate endemic to Madagascar. Their unusual diet is basically made up of insects.
Thus, the aye-aye has adaptations that have been related to its trophic habits, such as acute hearing, elongation in the middle finger, and teeth with growing incisors..
In terms of the denture, it resembles that of a rodent in several respects. Not only in the appearance of the incisors, they also share an extraordinarily similar dental formula.
The appearance between the two taxa is so striking that the first taxonomists classified the aye-aye, along with the other squirrels, in the genus Sciurus.