Charles Darwin admired the “infinite forms, the fairest and most wonderful” produced by evolution, and indeed, The Earth today teems with estimated at 1 trillion species. But how long did it take for these species to evolve?
The answer varies greatly among life forms, “depending on taxa [type of creature] and environmental conditions,” Thomas Smith, professor of ecology and evolution biology at UCLA told Live Science. It ranges from human-observable time scales to tens of millions of years.
Crucial because evolution occurs through inherited changes, the creature’s reproduction rate, or generation time, limits the rate at which new species can form—known as the rate of speciation—according to University of California, Santa Barbara (opens in new tab) (UCSB). For example, because bacteria reproduce so quickly, they “split[ing] in two every few minutes or hours,” they can evolve into new varieties after years or even days, according to American Museum of Natural History (opens in new tab) in New York.
However, it can be difficult to determine which bacterial varieties are considered new species, Smith said. While scientists distinguish species based on whether they can interbreed, bacteria do not reproduce sexually. Nevertheless, a 2008 study in Proceedings of the National Academy of Sciences (opens in new tab) reports that the lineage of E. coli (opens in new tab) bacteria observed for decades have evolved the ability to use citrate as a food source in an oxygenated environment. Because the inability to do so is “a defining characteristic of E. coli as a species,” the change could represent the beginning of a new species, the researchers said — one that evolved within a few years.
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Plants, in a phenomenon known as polyploidy, can duplicate their entire genome in seeds, resulting in extra copies of each chromosome and a new species in one generation. The resulting reproductive isolation “automatically makes a new species,” Smith said.
And since many plants reproduce by themselves, the new polyploid organism can go on to create more of the new species. “Plants often self-fertilize, so then a whole population can start,” UCSB said.
Even in the animal kingdom, speciation can occur on human-observable time scales, especially among rapidly generating insects. Apple maggots (Ragoletis pomonella), for example, historically fed on hawthorn plants, but some switched to domesticated apples after they arrived in the northeastern United States in the mid-1800s. Since then, the two groups have become reproductively isolated, according to a 2006 study in the journal Annals of the Entomological Society of America (opens in new tab)and are now considered “host races” – the first step in a kind of speciation without physical barriers.
Speciation usually moves more slowly in vertebrates, but it can still happen quickly. A 2017 study in the journal Science (opens in new tab) report that a Galapagos finch immigrated to a new island and interbred with a native bird, producing a new reproductively isolated lineage within three generations. This lineage may represent the very rapid initiation of speciation through species hybridization rather than the slower accumulation of adaptations, study co-author Leif Andersson, a geneticist at Uppsala University in Sweden, told Live Science.
“This is a possible scenario for how a new species could form,” Anderson said. “But then how stable it is over a longer period of time is more uncertain.”
The speed record for complete speciation among vertebrates probably belongs to cichlids in Africa’s Lake Victoria, Smith said. These fish exploded into 300 species “from a single founder less than 12,000 years ago,” he said. Some research, such as a 2000 study in the journal Proceedings of the Royal Society B (opens in new tab)questioned that timeline, but cichlid speciation “is extraordinary,” Smith said.
To find an upper limit on speciation time, look for speciation that occurs due to physical barriers, Smith said. For example, boas, which are found primarily in the Americas, and pythons, which are native to Africa and Asia, differentiated after the separation of South America from Africa. That probably represents tens of millions to 100 million years from the continent’s breakup to full speciation, Smith said. (The last common ancestor of these snakes slithered about 70 million years ago during the the age of the dinosaursAccording to Australian National University (opens in new tab)while Africa and South America split into ca 140 million years ago.)
Naming an average or most common time of speciation is challenging, Anderson said, but scientists can estimate the most recent ancestors, giving a rough idea. “In birds and mammals, what we see is that typically … the separation between well-developed species is a million years,” he said.
A 2015 study in the journal Molecular Biology and Evolution (opens in new tab) give another rating. Based on data from more than 50,000 species (although this includes few bacteria), the researchers found that speciation typically requires the accumulation of mutations over 2 million years. This is true for vertebrates, arthropods (a group that includes insects, arachnids and crustaceans) and plants.
However, such models require many assumptions, cautioned other researchers in a Quanta Magazine (opens in new tab) research history. Scientists are on more solid ground about the factors that slow or speed up speciation in general — namely, environmental pressures and reproductive isolation, Smith said. “In all species … the greater the selection pressure and the less gene flow, the more likely you are to get speciation,” he said.
Originally published on Live Science.