Cichlids – An Evolutionary Enigma

By Jasmine Gunton

Cichlids are an enigma for having evolved so many species sympatrically.

Photograph by Munheer Ahmed on Unsplash (December 2019).

The concept and processes of evolution have always been highly controversial and contested amongst academics. Since the publication of Charles Darwin’s famous scientific literature, ‘On

the Origin of Species’, great advancements have been made in understanding how evolution works. However, biology is still a messy subject, with many species displaying exceptions to commonly accepted evolutionary theories.

A salient example that demonstrates exceptions to the rule is the case of freshwater cichlid fish, native to tropical America, Africa, and southern Asia. Cichlids evolved via a process known as sympatric speciation. During this process, hundreds of species of cichlid evolved from a single ancestor while still all occupying the same geographical region. From DNA analysis we can determine that over 1,650 species of cichlids [1] evolved from one ancestor in just 100,000 years (approx.) [2]. To understand why this phenomenon is exceptional, one must have a basic understanding of the principal mechanisms and causes of evolution.

Formation of Species

Most biologists agree that a species can be defined as populations of individuals that are able to interbreed in the wild to produce viable offspring. Most commonly, new species are created when populations get physically separated. This can result from either a geographic event (e.g. formation of a mountain range, moving of tectonic plates, etc.) or when a subset of the population migrates to a different area. The two groups of organisms are often not representative of each other and are also subjected to different selection pressures provided by the two different habitats. More importantly, the two populations are now reproductively isolated and are therefore not influenced by gene flow (migration of organisms from one population to another) [3]. Over a long period of time, two new species are formed. This type of speciation is known as allopatric speciation, ‘allo’ meaning ‘different’, and ‘patric’ meaning homeland. In the case of cichlids, there was no such geographic barrier preventing different populations of fish from interbreeding. This phenomenon has been called sympatric speciation (‘sym’ meaning ‘same’). Due to the large number of distinct cichlid species, we specifically call this type of evolution ‘adaptive radiation’.

Gradualism

Another once commonly accepted theory of cichlid evolution was the concept of gradualism. This theory has since been contested using examples from a number of evolutionary events. Gradualism suggests that a species evolves over a very long period of time, as beneficial phenotypes rarely arise within a given population. However, a noticeable lack of transitional forms exist between the original cichlid ancestor and its descendants, indicating a rapid rate of speciation. This theory of evolution has since been called the punctuated equilibrium model. Contrary to the idea of gradualism, the punctuated equilibrium model suggests that species experience long periods of stasis, followed by short bursts of evolutionary change.

Photograph by Michael Rodock on Unsplash (January 2019)

An Alternative Hypothesis

When considering this question, one may ask whether the 1,650 species of cichlid fish can actually all just be classified under the same genus. This judgement can be disproven using a number of taxonomical techniques. First, cichlids show an incredibly large range of morphological differences, including different colour, pattern, size, and mating behaviour. They have also been shown to exhibit sexual behaviour which prohibits them from forming hybrid offspring. Finally, molecular analysis of individual species shows that under the speciation continuum, the Cichlidae family can be classed as encompassing a number of truly distinct species [4].

The Current Verdict

With this information, biologists have long pondered why this species suddenly diverged for seemingly no reason. Currently, no other vertebrate species have been found to display this type of adaptive radiation [4]. For this reason, East African cichlids were one of the first species of fish to undergo extensive genome sequencing. New research has uncovered the importance of considering ecology in the process of speciation. By analysing the DNA of several cichlid species it is also thought that speciation can still occur with some levels of gene flow between different populations. Current hypotheses propose that the hundreds of cichlid species may have formed by competition and sexual selection. Essentially, it is thought that to avoid competition, different cichlids would occupy slightly different areas within the same habitat, subsequently forming new ecological niches. Sexual selection of male colour patterns further prevented these populations from interbreeding. This current theory might explain how the 1,650 cichlid species evolved from just one ancestor.

Future Research

Research into cases of adaptive radiation is relatively limited in the biological science field, and the underlying mechanisms of adaptive radiation are yet to be fully understood. Therefore, academics are reluctant to label a species' evolutionary history under this category. Due to a lack of research, further analysis is required to fully understand the strange evolution of the Cichlidae family. Sympatric speciation in fish shows that scientific theories are not rigid in nature, but rather ever-changing and ‘evolving’ (bad pun, I know), with new theories being constantly proposed and integrated into our understanding of the world. Although greatly appreciated as one of the most revolutionary pieces of scientific literature, concepts in ‘On The Origin of Species’ have since been contested by members of the scientific community. The same will inevitably occur with our current conclusions about evolutionary biology. Cichlids, on the other hand, will simply keep on living their lives in rivers and lakes, completely unaware of the mystery that surrounds their history.

References

[1] Fishbase. Accessed May 20, 2021. URL: https://www.fishbase.se/Nomenclature/NominalSpeciesList.php?family=Cichlidae

[2] Brawand, David, Catherine Wagner, Yang I. Li, Milan Malinsky, Irene Keller, Shaohua Fan, Oleg Simakov et al. “The genomic substrate for adaptive radiation in African cichlid fish”. Nature 513 (2014): 375-381. URL: https://www.nature.com/articles/nature13726

[3] Ellstrand, Norman & Loren Rieseberg. “When gene flow really matters: gene flow in applied evolutionary biology”. Evol Appl 9 (2016): 833-836. URL: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4947145/

[4] Salzburger, Walter. “Understanding explosive diversification through cichlid fish genomics”. Nat Rev Genet 19 (2018): 705–717. URL: https://doi.org/10.1038/s41576-018-0043-9