Explaining Large-Scale Diversity within Midas Cichlid Fishes

A new study published in Evolution Letters capitalises on a natural experiment to shed light on when and how species diversify. Luke Turner reports:

Speciation occurs when a new species is formed from a pre-existing one, and can take place due to a variety of ecological factors. This diversification is not equal among all groups of organisms, with some lineages tending to diversify much more rapidly than others. Ecological opportunity, which can result from the appearance of a new niche, extinction of dominant lineages or the evolution of advantageous characteristics, is thought to be a key influencer of diversification rate. While ecological opportunities can explain diversity between different clades, other factors become relevant when examining the differences in diversification rates among closely related lineages. For example, the timing of colonisation of a new habitat affects how successful a species is in radiating, while secondary waves of colonisation also influence the rate of diversification.

The variation in diversification among different organisms is striking, and certain groups account for much more diversity than others. This is evident within the vertebrates, with just under half of around 60,000 species alive today being teleost fishes. Even more amazing is the overrepresentation of cichlid fish species, as 1 in every 25 vertebrate species in the world is a cichlid fish. This astounding diversity makes the group an ideal case study when asking questions about how and why diversification occurs within species.

Brand new research from Kautt et al. (2018), published in Evolution Letters, investigates the factors that contribute to this diversity amongst the cichlid fishes. The research focussed on a group of fish known as Midas cichlids, which live in Nicaraguan volcanic crater lakes and consist of 11 closely related lineages that have evolved relatively rapidly. All of these Midas cichlids originated from the same source population but have transitioned to the crater lakes and diversified into new species in less than 2000 generations.

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One species of Midas cichlid, Amphilophus citrinellus

After colonising the volcanic crater lakes, all of the Midas cichlids exhibited changes in body morphology, with a general trend towards more slender body shapes. This change is thought to give the fish an advantage due to the great depth of crater lakes. A more slender body is better suited to free swimming rather than manoeuvring, which was more important in the vast but shallow lakes from which they originated. The degree of morphological divergence was found to be related to the size and shape of the crater lake itself, as these new environments are smaller and more remote than the great lakes which they transitioned from. The presence of smaller zones closer to the shore in crater lakes makes them more dissimilar to their original habitats, and more dissimilar crater lakes contain Midas cichlids with greater morphological diversity.

When investigating the speed of this change in body shape, it was found that morphological divergence occurs rapidly after the initial colonisation of a crater lake, but then slows down with time. This is thought to be due to the fish ecologically diverging from their source population and rapidly adapting to the available niche in the crater lake. It also fits with the theoretical prediction that adaptive radiations are characterised by short bursts of rapid diversification, instead of constant rates of change over time.

Another interesting discovery was made when the researchers looked at the crater lakes which contained more than one species of Midas cichlid. Two crater lakes are inhabited by multiple species, which are the product of small-scale adaptive radiation within the overall diversification of the Midas cichlids from the source population. While these lakes provide a home for multiple species, others contain only one Midas cichlid species, so this begs the question; why did further diversification occur in some lakes but not others? The answer to this lies with the depth of a particular crater lake, as deeper lakes are able to sustain more fish and therefore allow for a greater amount of morphological diversity. This variation between habitats demonstrates that ecological opportunity can explain the imbalance in diversification rates between different lakes.

 

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Geographic distribution and morphological diversity of Midas cichlid fish populations studied. Taken from Kautt et al. 2018, Figure 1.

Speciation can often be affected by hybridization (individuals breeding with another species) and genetic introgression (genes from one species moving into the gene pool of another), as genetic exchange between closely related individuals can be an important factor in facilitating diversification. One instance of interbreeding was identified between two closely located crater lakes, with gene flow occurring between them and leading to the two populations being very similar in morphology. In addition, the researchers found evidence that there had been secondary contact between the source population and all of the crater lakes, indicating that a secondary wave of colonisation took place from the source to the crater lakes. This resulted from a rise in water levels as recently as 2000 years ago.

This research provides exciting insights into the factors that determine differences in diversification rates between closely related species. It shows that body shape diversity in the Midas cichlids is largely controlled by ecological opportunity, underlining the powerful role that natural selection plays in shaping biodiversity. Although the Midas cichlid fishes originated from the same source population, individuals adapted to their new habitat by developing more slender bodies, allowing them to move more freely. Additionally, diversity within a single crater lake was influenced by its depth, as deeper lakes allowed further speciation and could sustain multiple Midas cichlid species.

 

Luke Turner is a MSc Science Communication student at the University of Sheffield. The original study is freely available to read and download from Evolution Letters here.