East African Lake Fosters a Melting Pot for Cichlid Evolution

Paper: Meier, J.I., Stelkens, R.B., Joyce, D.A. et al. The coincidence of ecological opportunity with hybridization explains rapid adaptive radiation in Lake Mweru cichlid fishes. Nat Commun 10, 5391 (2019) doi:10.1038/s41467-019-13278-z

Illustrations of Darwin's Finches
An illustration of some of Darwin’s finches, draw by British Zoologist John Gould. Note their morphological differences. Photo Source: Wikimedia Commons.

A few million years ago, a species of finch from around Central or South America found itself on the relatively isolated Galapagos Islands. Over the years, this species grew and adapted to its new habitat, going from ground-dwelling, seed eating finches to cacti and tree-dwelling, seed and insect-eating finches. In fact, these adaptations were so distinct, that they gave rise to 14 new species – all originating from that first migrating finch colony.

This is the story of Charles Darwin’s famed finches. More importantly, this is a story about adaptive radiation: the process in which animals rapidly adapt to conform to particular niches within their environments (such as food availability or habitat type), so much so that new species arise.

According to Joana Meier and her research team, adaptive radiation has been typically thought of as requiring a target species’ relative isolation from its origin (think Galapagos Islands) and from other, related species (like other types of finches).

To test this theory, called the “isolation hypothesis,” Meier and her team examine cichlid fishes in two East African lakes: Lakes Mweru and Bangweulu.

Two Lakes, Two Histories

Lakes Mweru and Bangweulu have geologically different histories. Early Lake Bangweulu formed around the Pliocene Epoch (around 2.5-5 million years BP) in the Zambezi river catchment (where rainfall flows into a river). On the other hand, Lake Mweru most likely formed late in the Pleistocene Epoch (around 11 thousand- 2.5 million years BP) in the Upper Congo river’s catchment.

About 1 million years ago, Lake Bangweulu’s outflow permeated a section of the Congo river, which directed its waters towards Lake Mweru. Consequently, Lake Bangweulu’s cichlids (among other fishes) colonized Lake Mweru downstream; however the existence of waterfalls and rapids made converse upstream movement from Lake Mweru to Lake Bangweulu incredibly difficult. As a result, Lake Mweru’s cichlids evolved with plenty interspecific competition, whereas Lake Bangweulu’s cichlids evolved in relative isolation.

Evolution through Hybridization

African Cichlids
Cichlids are known for some pretty neat features, including how some cichlid species actually “parent” their young. Now, Meier et al. (2019) find that cichlids in two East African lakes show a unique sign of adaptive radiation. Photo Source: Flickr, OakleyOriginals.

The isolation hypothesis would predict that because of Lake Bangweulu’s isolation from other cichlid species, the habitat would be more conducive to adaptive radiation.

However, Meier and her team find just the opposite.

Through extensive sampling of the two lakes and behavioral, morphological, and genomic analyses of over 1,400 cichlids, Meier et al. (2019) find relatively little evidence of adaptive radiation in Lake Bangweulu but significant evidence of radiations in Lake Mweru. The research team found more than 40 previously unknown, distinct species in Lake Mweru that seemed to come from a hybrid of Congolese and Zambezian origin. Each species adapted to a particular niche in the Lake’s environment. Some (Orthochromis polyacanthus, for example) were found only in rocky shores, feasting on algae and benthic invertebrates. Others, like species from the genus Pseudocrenilabrus, fed on insect larvae and small benthic and planktonic crustaceans in multiple habitats. Some, like Serranochromis radiations, evolved to have teeth suited to feed on large insect larvae, other fish, and fish eggs and fry.

Through gene analyses that targeted cichlids’ mitochondrial haplotypes (clusters of mitochondrial sequences that reveal information about evolutionary lineage), Meier et al. (2019) conclude that the cichlid radiations they discovered happened incredibly rapidly and relatively recently. Additionally, analyses indicated that radiations in Lake Mweru occurred only after Zambezian cichlids from Lake Bangweulu colonized the lake.

Meier’s team’s findings suggest that before Lake Mweru’s cichlids adaptively radiated, like Darwin’s finches, they first hybridized with other species of competing cichlids from Lake Bangweulu, unlike Darwin’s finches. Hybridization is when two animals from different species mate to form a new, distinct organism. Meier et al. (2019) found little evidence of interspecific hybridization in Lake Bangweulu, and almost no adaptive radiation; but found high evidence of hybridization in Lake Mweru and consequent high adaptive radiations.

The team’s findings support the “hybridization hypothesis”: that colonization by competing, closely related species in one environment, in addition to other, optimal environmental conditions, can catalyze interspecific hybridization. This hybridization increases the organism’s potential for evolutionary diversification through adaptive radiation. Hybridization provides the opportunity to increase genetic variability between species, which can influence species’ morphology, hardiness, and behavior – priming them for adapting to niches in their environments. Hybridization’s effects are strongest at intermediate genetic distance. If the animals were too closely related, hybridization would not generate enough genetic variability to promote radiations. If the animals were too distantly related, their genes may be incompatible, rendering offspring infertile or inviable. Therefore, adaptive radiation through hybridization is unlikely to occur in isolated ecoregions.

Map of the two lakes and photos of cichlid radiations.
A description of colonization and radiation process (left) and photos of discovered, cichlid radiations (right). Click for larger photo. Photo source: Meier et al. (2019).

The authors caution that these findings may not be completely generalizable to other animals, even other species of fish, and that more studies need to examine if hybridization can similarly stimulate adaptive radiations in other organisms. However, Meier et al.’s (2019) findings indicate that evolution through adaptive radiation need not require the complete isolation depicted in the tale of Darwin’s finches.

Sometimes a bustling environment can equally promote evolution and biodiversity.

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