During May 2021, some opinions were expressed by scientists regarding crosses or “hybrid animals” as well as different plant species where crossings may have taken place (by Ashley Yeager). Hybrid animals in the wild were extremely rare and therefore not catalysts of evolutionary innovation. Some biologists have been studying these phenomena in nature in depth for several years. Not only were they curious to discover the obstacles that prevented them from becoming new species, but sought to identify the new gene combinations created by hybridization and then try to understand how natural species originated and evolved between them.
Focusing on animals in what scientists call hybrid zones – geographical regions in which two species interbreed to produce offspring of mixed ancestry – researchers in the late 1980’s and early 1990’s began to show that, contrary to the prevailing viewpoint, hybridization was a valid mechanism of evolutionary change – one that could radically influence an animal’s ability to adapt to its environment.
As far back as the 1930’s, botanists realized that hybridization plays a role in the evolution of plant species. In 1938, Edgar Anderson and Leslie Hubricht laid out the idea of introgression to describe the hybridization of species of herbaceous perennial wildflowers of the Tradescantia genus. The crosses led to offspring with an even split of parental genetic material, and typically those offspring then repeatedly bred with one of the original parent species, while still retaining genetic material from the other parent species. Alternatively, hybrids bred with other hybrids, and, eventually, entirely new plant species would emerge.
Zoologists knew about these and other examples of hybridization in the plant world, but there was a perception, Taylor says, that cross-species breeding was much less common in animals. That idea stemmed from biologist Ernst Mayr’s description in the 1940’s of the biological characteristics that defined species—essentially, any animal population that could not or did not breed with other, similar populations. For more than two decades, including in his 1963 book Animal Species and Evolution, Mayr argued that “the evolutionary importance of hybridization seems small in the better-known groups of animals.”
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Wetenskaplikes het gedurende Mei 2021 ‘n paar menings uitgespreek rakende kruisings of “basterdiere” (hibriede diere) asook verskillende plantsoorte waar kruisings moontlik plaasgevind het (deur Ashley Yeager). Hibriede diere in die natuur was uiters skaars en daarom nie katalisators van evolusionêre innovasie nie.
Sommige bioloë het vir etlike jare hierdie vreemde verskynsels in die natuur in diepte bestudeer. Nie net was hulle nuuskierig om die struikelblokke te ontdek wat hulle verhinder het om nuwe spesies te word nie, maar gepoog om die nuwe geen-kombinasies te identifiseer wat deur hibridisering of kruising ontstaan het en daarna te probeer verstaan hoe natuurlike seleksie tussen spesies ontstaan en ontwikkel het.
Reeds in die dertigerjare het plantkundiges besef dat verbastering ‘n rol speel in die ontwikkeling van plantsoorte. In 1938 het Edgar Anderson en Leslie Hubricht die idee van introgressie uiteengesit om die verbastering van spesies kruidagtige meerjarige veldblomme van die Tradescantia -genus te beskryf. Die kruisings het daartoe gelei tot ‘n nageslag met ‘n eweredige skeiding van ouerlike genetiese materiaal en tipies het die nageslag daarna herhaaldelik geteel met een van die oorspronklike moedersoorte, terwyl hulle steeds genetiese materiaal van die ander moedersoorte behou het. Alternatiewelik sou basters wat met ander basters geteel is en uiteindelik moontlik met nuwe plantspesies na vore kom.
Dierkundiges het geweet van voorbeelde van verbastering in die plantwêreld, maar daar is ‘n persepsie, het Taylor gemeen, dat teling tussen spesies egter baie minder algemeen by diere voorkom. Hierdie idee spruit voort uit bioloog Ernst Mayr se beskrywing in die veertigerjare van die biologiese eienskappe wat spesies gedefinieer het – in wese enige dierepopulasie wat nie met ander soortgelyke bevolkings kon of nie teel nie. Vir meer as twee dekades, insluitend in sy boek ‘Animal Species and Evolution’ uit 1963, het Mayr aangevoer dat “die evolusionêre belangrikheid van verbastering klein lyk in die meer bekende groepe diere.”
Why Hybrid Animals May Take Over the North
A genetic analysis of a rare skull found at the Natural History Museum of Denmark showed that in the past few decades a male beluga (Delphinapterus leucas) and a female narwhal (Monodon monoceros) mated, creating a hybrid called a narluga that looked a bit like both. The animal’s 18 teeth were small, like a beluga’s, and twisted, like a narwhal’s tusk. Those teeth may have changed the way the narluga fed, not hunting on cod, squid, and shrimp in the water column as both of its parents did, but instead feeding off the bottom. Its teeth may have allowed it and others like it to occupy a different ecological niche than its parents.
The co-study of a 30-year-old whale skull by a professor at Trent University in Peterborough, Ont., has determined it is the offspring of a beluga and a narwhal.
University of Copenhagen researchers found a beluga whale and narwhal were once an unlikely couple, giving birth to a rare hybrid “narluga,” if you will.
When Seehausen began to study the lake’s cichlids roughly 30 years ago, it wasn’t clear how the hundreds of species there had evolved. They weren’t geographically isolated, a common driver of speciation. Rather, the fish were all living in the same lake and could interact, yet there was still incredible cichlid diversity. Something else appeared to be driving their speciation.
Evidence for hybrid-driven adaptation is perhaps nowhere more profound than in the warm, tropical waters of Lake Victoria in Africa.
Something appears to have happened thousands of years ago in Lake Victoria. Genetic analyses of the cichlids have revealed that their vast diversity can be traced back to a hybridization of two divergent lineages around 150,000 years ago. And Lake Victoria wasn’t the only body of water in the region where hybridization appeared to play an important role in speciation. Further investigation revealed that cross-species mating had happened and continued to occur in nearby lakes, where it was driving cichlid diversity. “This was replicated in several different lakes across Africa,” Seehausen says.
There, more than 500 species of bony fishes called cichlids that sport brilliant orange, yellow, and blue hues, roam the lake’s 2,400 cubic kilometers. Some species eat only plants, others eat invertebrates, the bigger species eat other fish, and still more feed on Lake Victoria’s detritus. “There’s incredible diversity of species that live together in the same ecosystem,” evolutionary ecologist Ole Seehausen of the University of Bern tells The Scientist. “This struck me as a beautiful system, the interaction between ecology and evolution . . . to study speciation.”
Lake Victoria – Africa
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.
Some of Lake Victoria’s cichlids nibble plants; others feed on invertebrates; big ones feast on other fish; lake bottom lovers consume detritus. Species vary in length from a few centimeters to about 30 centimeters; come in an array of shapes, colors, and patterns; and dwell in different parts of the lake. Mutations don’t usually happen fast enough to produce such variety so quickly. “It’s been really hard to figure out what’s going on,” says Rosemary Gillespie, an evolutionary biologist at the University of California, Berkeley.
To explore the cichlids’ genetic history in more detail, Seehausen and postdocs Matt McGee, Joana Meier, and David Marques have now sequenced 450 whole cichlid genomes, representing many varieties of 150 species from the area’s lakes, and from the Congo, Upper Nile, and other nearby watersheds. Clues in the genomes suggest multiple episodes of mixing took place. Periods of drying have repeatedly caused Lake Victoria to disappear, and Seehausen and his team propose that fish in the remaining waterways evolved independently until wetter periods reunited them. This “fission-fusion-fission” process restored genetic diversity each time.
The History of Lake Victoria’s Ecological Crisis
NEW AFRICAN CICHLIDS : LAKE MALAWI, TANGANYIKA, VICTORIA