[Europan Ocean (OP)]

I just listened to Richard Dawkins say that there was no first man, like Adam. That evolution is gradual, offspring the same as the parents. But that makes me think of DNA counts, such as two chromosomes in fish, maybe four in reptiles... then 46 in our ancestors, who may have cross bred with red headed Neanderthals. And our common ancestors with Gorillas, and the leap from 46 or 44 Chromosomes to a working group of 48 chromosomes in the new species to be Gorillas. Are these offspring immediately different from their parents? How does it happen and how does it work?

[evan_au]

Chromosomes can merge (reducing the count of chromosomes) or split (increasing the number of chromosomes).
- Recent research has found that use of CRISPR can chop up a chromosome, leaving a "spare" bit floating around in a cell nucleus
- Neither of these processes need be immediately fatal to a cell, since you still have the same number of genes
- The problem comes when the cell divides - does each "daughter" cell still have a full set of genes?
- And in the next generation of organism - do they still have a full set of genes?

The centromere is a structure in the center of each chromosome that plays a vital role in duplicating DNA into each daughter cell.
- A split-off portion of a chromosome may lack a centromere, inhibiting gene replication (or sometimes, a new centromere may appear)
- Two joined chromosomes may have two centromeres, which leads to chromosome breakage
- So most cases of chromosome splitting or joining are bad for the individual organism (often leading to cancer) and the descendents (who will suffer high rates of genetic disease and infertility with the parent species)
- But in rare cases, a descendent will receive two copies of the same mutation, allowing speciation
- Gene sequencing can identify cases where chromosomes have split or joined, since biologists can compare which genes occur on which chromosome.
See: https://en.wikipedia.org/wiki/Centromere

The leap from 46 or 44 Chromosomes to a working group of 48 chromosomes in the new species to be Gorillas

The human genome appears to be the result of the fusion of two chromosomes that remained separate in other primates (ie humans went from 48 to 46, while gorillas & chimpanzees remained at 48).
- So "progress" (new species) do not always come from splitting chromosomes (fission).
- New species can come from fusion, too.
https://www.mun.ca/biology/scarr/Human_Ape_chromosomes.htm