Eli5: How much of a species’ sequenced genome is stable enough to represent a species, as opposed to finer stuff that will keep changing really fast as they breed?

Question

I know this would vary a lot between species, so for example, what’s this like in the human genome?

In: Biology

Answer 1

Honestly, a species isn’t really a single genome, but a gene pool. The idea of a species is fluid and changes over time. You can’t define a species as its pool is transforming either, all you can do is look back into the past and go “Ok well at some point it definitely diverged from its relatives”. Gene pool drift is a continuous process, and we can only define species at all by where a gene pool differs from another gene pool in arbitrarily defined ways. Essentially, the idea of a species is just a colloquial thing we use to easily communicate a concept where it’s obvious what we mean but impossible to entirely define it.

The stable parts of a genome, ie, the ones where all the different alternatives in the gene pool are very similar, tend to be the parts where mutation almost always causes a reproduction disadvantage. This mostly means the fundamental pieces that operate cells, like genes that code for ribosomes. Some genes that are really important to the species fitting its niche can also be “protected” like this. These genes are referred to as highly conserved, which essentially means that mutations to these genes almost always get removed from the pool before they can spread, and only stick around if the niche the species needs to occupy changes (and if that happens enough, the species becomes a new species).

There’s not a great deal that’s truly stable about a genome. Most of it can and will change over long enough time spans. However, each individual member of the species inherits normal genes from the gene pool with at most just a tiny handful of mutations. The low mutation rate means that the “average” gene pool remains pretty stable as a whole even though individuals may be changing quite a lot.

Answer 2

I’m not an expert on this, but just want to point out that a genome doesn’t need to be constant to identify a species. If a gene has several options that are typical for humans then having any of these represents a human, even though not all humans have all of them.

Answer 3

It’s also worth noting that gene pool diversity can vary a lot between species. Those that recently experienced near extinction, like humans, have much less variation between individuals. Self cloning species, like marble crayfish, show essentially none.