The original coronavirus strain has almost disappeared in the US. One chart shows how variants took over.

• Coronavirus variants have largely replaced the original strain, rendering it essentially obsolete.
• The Alpha variant took over as the US's dominant strain in April. Delta could replace it soon.
• Scientists aren't sure whether more contagious variants will evolve from the dominant ones.

Viruses will do what it takes to survive - even if it means killing off an older, weaker self and replacing it with a more transmissible strain.

For the first several months of the pandemic, however, the coronavirus had no need to become more dangerous: The virus was doing a good job of spreading, with each new infected person passing it to an average of two to three others. At the time, scientists hoped that the original strain of the virus, known as the "wild type," was already contagious enough that it wouldn't evolve further.

But as the pandemic swelled and more people got infected, the coronavirus had more opportunities to replicate, and therefore mutate, incurring small, random changes in its genetic sequence. Most mutations are harmless, but every so often a distinct set yields new properties - a variant.

Scientists now estimate that variants have almost completely replaced the original strain in the US, rendering it essentially obsolete.

"Pretty much all the virus that's circulating right now has one of these variants that make it differ from the original strain that first took off across the world," Tyler Starr, a postdoctoral research fellow at the Fred Hutchinson Cancer Research Center, told Insider.

The chart below shows how a few variants have dominated the US since February. More than 200 less prevalent strains, including the original version of the virus, are listed as "other."

The Alpha variant, first identified in the UK in September, became prevalent in the US from February to April, going from 27% to 70% of all circulating strains. It's about 50% more transmissible than the original strain, according to the Centers for Disease Control and Prevention. Meanwhile, the share of other coronavirus strains (including the original) fell from 20% to 4%.

By May, Alpha had a strong competitor: Delta. An analysis from Public Health England found that the Delta variant was associated with a 60% increased risk of household transmission compared to Alpha, though more recent estimates suggest that difference is closer to 40%.

From May to June, Delta grew from less than 3% of all circulating strains in the US to more than 20%. It's poised to become the US's dominant strain within weeks.

"Basically everywhere, once Delta gets there, it does overtake something like the Alpha variant," Starr said. "That is evidence that, to some degree, it is more transmissible."

Could an even more contagious variant replace Delta?

So far, Starr said, coronavirus variants - even Delta - aren't fundamentally different from the wild type.

"These mutations might be slightly modifying things like transmissibility," he said, but "that trait was there in the original virus and it's just being altered slightly."

In fact, some scientists wonder if the virus is nearing "peak fitness," the point after which it no longer mutates to become more infectious.

Delta is by far the "fittest" variant to date, according to the World Health Organization. In addition to being more transmissible than other strains, it may also be deadlier: Researchers in Scotland found that getting infected with Delta doubled the risk of hospital admission relative to Alpha. (Previous studies have suggested that the Alpha variant may be 30 to 70% deadlier than the original strain.) Vaccines, of course, significantly lower that risk for both variants.

"Delta is absolutely going up the fitness peak - whether it's at the top, I think that's very hard to say until we just don't see any further change," Andrew Read, who studies the evolution of infectious diseases at Pennsylvania State University, recently told Insider.

"If Delta takes over the world and nothing changes," he added, "then we'll know in a while - a year or two - that it is the most fit."

But Starr thinks the virus probably won't ever stop mutating.

"As people continue to get immunity, the virus will continue to evolve to be able to transmit and infect people," he said. "But at the same time, we'll have that low-level immune reaction that makes it a much less severe thing over time."

It's still possible that an entirely new lineage might replace Delta as the dominant variant, or that two variants - Delta and Alpha, for instance - could combine mutations to produce an even more infectious strain. In the worst-case scenario, the virus could evolve into a "variant of high consequence" - one that's far more distinct than the variants currently circulating and highly resistant to vaccines. That hasn't been observed yet.