Convergent Evolution

Convergent evolution is how different organisms independently evolve certain similar traits. 

Let us take an example to understand this. You might have seen both sharks and dolphins and, in some cases, been confused between the two. This is because of convergent evolution.

Sharks, as well as dolphins, look quite similar, but they are totally unrelated. Sharks are actually egg-laying fishes that have a very sharp and lethal ability to sniff blood in the water. In contrast, dolphins are interactive creatures that navigate by making certain clicking sounds and listening to their echoes. 

Their similarities arise from the common ancestor they share, who swam across seas about 290 million years ago. Evidence suggests that one lineage of the ancestor went out on land and later evolved into mammals, then returned to the water to evolve in whales and dolphins. Whereas the other lineage stayed in the water to become the shark that we know today.

Well, despite the paths that were so different, they did end up with evolutionary niches that were quite similar.

This process is what biologists term convergent evolution, i.e., when two organisms share characteristics that they did not jointly inherit from a common ancestor.

Difference between Convergent and Divergent Evolution

You must have read about the classical theory of evolution proposed by Charles Darwin, which proposed that species can evolve and change and diverge into separate newer forms. 

Let us take an example. Invertebrates like monkeys, goats, birds, and whales have a general set of bones; however, their forelimbs have been modified very differently so that each of them can use them in different ways. 

However, in convergent evolution, it is the opposite where non-related organisms share similar characteristics that they did not jointly inherit from a common ancestor.

The significant differences between convergent and divergent evolution are as follows.

1. Convergent evolution depicts how species that have evolved separately can have analogous (similar) structures. In comparison, divergent evolution shows how species might have certain homologous (common) structures that have been evolved for different usage.
2. Convergent evolution generally happens in organisms that are not closely related, whereas divergent evolution occurs in organisms that are pretty closely related.
3. In convergent evolution, the relationship between the analogous or similar structures in different species that evolved is less distinct in comparison to common or homologous structures that did have a common ancestor.

Convergent Evolution and Parallel Evolution

The fossil record shows that similar patterns existed in different extinction events in terms of legs, shells, and a similar environment. This process has led many evolutionary experts and biologists to understand whether evolution happens randomly or is fixed by its environment.

However, it is not quite easy to distinguish convergent evolution amidst other forms of evolution as convergent evolution shares a lot of similarities with parallel evolution. 

Parallel evolution means when two or more lineages do change in similar ways, so their descendants are pretty similar to their ancestors. For example, scientists have termed the evolution of the marsupial in Australia and placental mammals’ evolution in the world as parallel evolution. Even though both these organisms, i.e., Australian marsupials and placental mammals, evolved independently, they have resemblances due to their similar adaptation of life. 

Hence, convergent and parallel evolution give us a conclusion that natural selection does not have a specific and predetermined path. Species, in general, can converge, diverge and again converge. The primary basis of evolution is that species work and adopt certain survival strategies in a particular environment regardless of how they are adapting to it. 

Recognition of the Phenomenon of Convergence Evolution

Previously, convergent evolution was recognised in a more intuitive way, and it was thought to be a self-evident phenomenon. However, since it has not been recognised with the backing of a strong hypothesis, it led to certain false conclusions. Let us take an example of the forelimbs of moles and the forelimbs of crickets which are so similar, but the lack of comparative data makes it challenging to identify whether convergence evolution did occur.

Today, researchers have developed various measures as well as indices through which convergent evolution can be checked through patterns and process-based concepts. Pattern-based convergence refers to a pattern and facts that may represent convergence, whereas process-based concept refers to evolution due to the action of a given process. 

Among different other methods, one of them is the Wheatsheaf Index by Arbuckle, which measures the strength of a convergent pattern after the convergence is established. This index calculates the ratio of the pairwise distance between all species in a particular dataset to the average pairwise distance among all species that are convergent to another.

A significant value indicates that the convergent family species are more tightly clustered than other families of species.

Apart from this method, there are several other statistical tests and models which try to recognise convergence evolution as either pattern-based or process-based concepts. 

Examples of Convergent Evolution

You have already read about the convergent evolution example of dolphins and sharks. Let us now look at other examples. 

An example of convergent evolution is the wings of insects, birds, and bats. Though their wings might not have the same composition anatomically, the wings serve a purpose that is flight. Though these animals come from different ancestral lines, they gradually evolved wings for travelling through air.

Another example is the eyes of vertebrates, cubozoan jellyfish, arthropods and cephalopods. You can understand just by the name itself that these are very different organisms, but these animals did evolve separately, but they evolved eyes as organs for vision capabilities.

Conclusion

Now, you must have a clear understanding of convergence evolution and its relationship and differences with divergent and parallel evolution. Identifying particular instances of convergent evolution is not easy as species can converge, diverge, and again converge. However, strategies and measurements can be applied to proactively quantify it.