Typically when a person thinks of a species, they think of a single unit. Dogs. Cats. Birds. There are derivations of that species, true – Labradors and Terriers, Siamese and Himalayans, Parrots and Sparrows – but even so, individuals belonging to that group are roughly analogous with one another. Mutations from the norm are rare.
Consequently, it may be difficult to conceive of a ‘species’ that is expected to differentiate between individuals. A species so vastly changeable that, indeed, each generation may be wildly different from the one before. Yet such creatures exist, and in most cases they exist right under our noses – and, often, inside us. These are known as quasispecies, and they’re categorized and explained via the quasispecies model of sequence evolution.
A quasispecies is not the same as a species, where offspring are typically genetic replicates of their parents, bearing the same genome all down the line of offspring. Instead, a quasispecies is a collection of similarly-classed creatures, often viruses, which are expected to mutate from one generation to the next. They’re self-replicating organisms that essentially transform as they create new generations, and do so in a fairly quick and radical manner.
Unlike a species, however, a quasispecies is connected. What matters in genotype replication is not that the dominant genotype survives and thrives in the next generation, but that the whole group of beings within the quasispecies bear roughly the same attributes from one generation to the ext. A pair of examples will help demonstrate the difference:
– In lions, the leader of the pride is typically the strongest lion. The male maintains a strict hold over his genetic line, breeding as often as possible to ensure the continuation of his strong genes. If a new male comes in and defeats the leader, he’ll kill the cubs and mate with the female(s) present to promote his own genetic continuation. In short, survival of the fittest.
– Viruses, on the other hand, seek to spread the same attributes over and over, and to destroy would-be suppressants rather than kill one another. Viruses infect cells, replicate to spread and then mutate in order to prevent medicines from curing the diseased cells. If the virus were to fight against itself, or if its constituent parts were to develop different attributes, the virus’ strength would diminish overall. It’s survival of the fittest against outsiders rather than against other, similar viruses.
The key to a quasispecies’ power is its ability to mutate rapidly. Mutation essentially makes ‘dominant’ or ‘fit’ genotypes meaningless, as those genotypes will probably be weeded out within a generation. What’s important is that the various sequences that make up the quasispecies continue to mutate and change in a manner that’s similar to one another. In doing so they replenish one another and ensure that the quasispecies continues to replicate and grow within its own cloud. Were mutation to cease, the various organisms involved would become their own species.