Ancient Hybridization and Lake Victoria’s Stunning Fish

Reported last week at the Origins of Adaptive Radiation meeting here, the work is “a tour de force, with many lines of evidence,” says Marguerite Butler, a functional morphologist at the University of Hawaii in Honolulu. It joins other research suggesting that hybridization is a powerful force in evolution. “What hybridization is doing is allowing the good stuff to be packed together,” Butler says.

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Pretty cool:

WAIMEA, HAWAII— In the shallow waters of Lake Victoria, the world’s largest tropical lake, swim some 500 species of cichlid fish with a dizzying variety of appearances, habitats, and behaviors. Genomic studies have shown they arose from a few ancestral species in just 15,000 years, a pace that has left researchers baffled about how so much genetic variation could have evolved so quickly. Now, extensive sequencing of cichlids from around Lake Victoria suggests much of it was there at the start, in the cichlids’ ancestors. Ancient and more recent dallying between cichlid species from multiple watersheds apparently led to genetically diverse hybrids that could quickly adapt to life in the lake’s many niches.

Reported last week at the Origins of Adaptive Radiation meeting here, the work is “a tour de force, with many lines of evidence,” says Marguerite Butler, a functional morphologist at the University of Hawaii in Honolulu. It joins other research suggesting that hybridization is a powerful force in evolution. “What hybridization is doing is allowing the good stuff to be packed together,” Butler says.

And in case you are wondering, hybridization is consistent with common descent, but produces a structure very different from a tree.

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The cichlids of Lake Victoria are a special case, of course, but I’ve long felt that we’re going to find hugely significant, and sudden, evolutionary events will turn out to be down to hybridization.

This work actually seems to vindicate my friend, the Creationist Arthur Jones, one of the very few people to be awarded a PhD in evolutionary biology for a non-evolutionary thesis. Back then, of course, it was trees with everything, and even to his own surprise the cichlids did not appear to fit a tree structure, but something different. Can’t link to the thesis as it’s not online.

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Are you saying in some way that hybridization is NOT part of or NOT a powerful force in evolution? Please explain.

No, I’m saying it’s a more powerful force than was realised in the past, and (as Joshua says) it is one way in which tree-structures are disrupted. It can change things very greatly and very quickly as well. Darwin’s natura non facit saltum becomes a potentially doubtful dictum.

Another instance: a good number of bird of paradise species turn out to be hybrids. Apart from making mincemeat of Fisher’s theory of sexual selection (always opposed by Wallace, who studied them in the field), the potential for a new species to form from two over one or two generations opens a whole range of possibilities up.

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I think it also makes phenotypic plasticity important…
Otherwise hybridisation wouldn’t help much in speciation.

Why have you felt this? It’s pretty easy to spot hybrid speciation if you assemble a lot of loci. There doesn’t seem to be a lot of it in most animal taxa that have been studied, and more in plants, but only around 5%.

Very interesting. I was unaware of that. Do you have a reference?

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Here is the summary/abstract for that link:

“In 2010 a comparison between a Neandertal genome and genomes from people today turned up evidence of ancient liaisons, a discovery that belied the common idea that animal species can’t hybridize or, if they do, will produce infertile offspring—think mules. Such reproductive isolation is part of the classic definition of a species. This discovery brought credence to other work in plants, Darwin’s finches in the Galápagos Islands, tropical butterflies, mosquitoes, and a few other animals showing that hybridization was not just common, but also important in shaping evolution. The techniques that revealed the Neandertal and Denisovan legacy in our own genome are now making it possible to peer into the genomic histories of many organisms to check for interbreeding. As more examples are discovered, researchers are questioning the definition of species and rethinking whether the tree of life is really a “net” of life.”

Think Tigers and Lions… they are capable of having fertile hybrids … the so-called Ligers and Tigons!

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Historically speaking, Tigers and Lions became ripe for classification as 2 separate species for two reasons:

a) they rarely overlapped in territories; and
b) they had very different phenotypes, different social lives and different methods of hunting (lions frequently exist in larger groups called “prides” and survive in broad open areas of Africa, while tigers existing solitarily and mostly in the jungles of Asia and India).

And yet … simple isolation and distance is not enough to guarantee sexual incompatibility, or, in other words, “speciation”.

In fact, I have read summaries that emphasize how closely related many taxa of cats appear to be … which contributes to cat survival by allowing for quick hybridization in response to ecological shifts.

Frith, Clifford B. & Beehler, Bruce M. (1998). The Birds of Paradise: Paradisaeidae. Oxford University Press. ISBN 0-19-854853-2, the main source for Wikipedia’s entry.

The book dates from 1998, before the minor revolution in bird taxonomy that seems to have resulted from genomics, but lists 24 species as known, or presumed, hybrids - and that’s in a taxon containing only 42 spp. in 15 genera. As I understand what I’ve read, some of those hybrids may have occurred across genera.

No doubt the exact classification would be changed by extensive DNA studies, but it seems to me that such widespread hybridization - or any, really - is a challenge to Fisherian runaway, or any other version of sexual selection. Fisher said (1930):

The grossest blunder in sexual preference, which we can conceive of an animal making, would be to mate with a species different from its own, and with which hybrids are either infertile, or, through the mixture of instincts and other attributes appropriate to different courses of life, at so serious a disadvantage as to leave no descendants. … it is no conjecture that a discriminative mechanism exists, variations in which will be capable of giving rise to a similar discrimination within its own species, should such a discrimination become at any time advantageous.

Fisher developed his runaway hypothesis for the group in 1915, from theoretical pop. gen., in direct contradiction of Alfred Russel Wallace, who was ambivalent about sexual selection and had studied birds of paradise extensively in the field, and found that females did not necessarily mate with the most spectacular males at all. Fisher, in London, knew better!

And here’s an article that suggests sexual selection has not been particularly rapid in the group.

From what I can tell, all those “hybrid species” are mostly just F1 hybrids, not actual populations.