How does a drastic change to the genome persist and spread?

Question

I just read the article on songbirds in the November, 2019 Scientific American. The article explains that songbirds have an extra chromosome, called GRC (germ-line restricted chromosome) that other bird species don't have, and there's a hypothesis that this may have been partially or wholly responsible for the diversity of this group of species.

The following line caught my attention:

Once the GRC originated in the last common ancestor of songbirds, members of that ancestral species that carried the GRC could produce fertile offsping only with mates that also had the GRC.

But when the mutation that created the GRC occurred, who did that bird mate with in the first place?

My belief is that most genetic changes are relatively minor, so they don't immediately result in incompatible mating. Speciation occurs as genetic changes accumulate over time. But the above quote implies that an entirely new chromosome is too drastic to allow interbreeding at all.

Wouldn't there need to be a whole bunch of birds who all had this extra chromosome at the same time to allow it to spread through the population? That seems unlikely to occur.

Am I just reading the line too literally? Could early birds with the extra chromosome mate with other birds, until the new chromosome accumulated too many changes?

Answer 1

It isn't necessarily true that individuals with different numbers of chromosomes are infertile. Even though common wisdom has it that horse (64 chromosomes)/donkey (62 chromosomes) hybrids are invariably infertile mules, that's not true; a significant number of fertile horse/donkey offspring have been documented. Here is an example where the chromosomes were checked and confirmed to be 63 in the fertile mule parent.

Even more dramatically, the common shrew (a single species) has many different chromosome numbers as natural variability (at least 50 different chromosomal variants were identified in one paper) and this doesn't affect their interfertility.

So there are three general answers to the question here, though I don't know which is the most likely.

1. The extra chromosome arose and the original variant was very fortunate to have offspring; similar to the rare horse/donkey fertile hybrids.
2. The extra chromosome arose but had little effect on interfertility; similar to the common shrew. (The quote from the article in the question argues against this, but I don't know if it's a presumption made by the researchers, a presumption made by the writer, or a demonstrated fact.)
3. The extra chromosome arose in a single litter, so that several siblings shared it, and then interbred. Although incest avoidance is generally quite strong, animals that colonize islands or distance geographic areas (invasive species, often) commonly do interbreed this way.

Answer 2

It seems like one of the rare cases in which the duplication of the specific chromosome is not lethal, but actually advantageous. This means it can happen from time to time to see this extra chromosome by errors in cell-division.

Now although its unlikely, it eventually happened, that two birds with the same duplicated chromosome meet and mate: They replicate their "new species".

Note that the name of this "germ-line restricted chromosome" might be the cause of confusion! Any additional chromosome would be "germ-line restrictive": Unless all chromosomes are matched, cell-division is prevented by control mechanisms.

And some direct answers:

But when the mutation that created the GRC occurred, who did that bird mate with in the first place?

It needs to find a bird with the same additional chromosome!

My belief is that most genetic changes are relatively minor, so they don't immediately result in incompatible mating. Speciation occurs as genetic changes accumulate over time.

True, it takes considerable mutations for 2 chromosomes being unable to be paired, but here we speak of an additional chromosome: It intrinsically cannot be paired.

But the above quote implies that an entirely new chromosome is too drastic to allow interbreeding at all.

Exactly!

Wouldn't there need to be a whole bunch of birds who all had this extra chromosome at the same time to allow it to spread through the population? That seems unlikely to occur.

It takes 2 birds and coincidence!

Am I just reading the line too literally? Could early birds with the extra chromosome mate with other birds, until the new chromosome accumulated too many changes?

I don't think so!