system
Given the following post, respond as if you're commenting on Reddit:
human
How does the number of chromosomes change upon species differentiation?

I would like to know how the change in the number of chromosomes as an species evolves can be explained against the idea of "irreducible complexity", and what evolutionary advantage it brings to justify it being incorporated into the genome of the entire species.

Bonus question: do species always evolve to have more chromosomes, or does the number sometimes reduce?
gpt
The key event in speciation is the appearance of a barrier to interbreeding. This can (and usually does) happen without any change in chromosome number, but adding or losing chromosomes can be a very effective way of achieving reproductive isolation, because gametes with different numbers of chromosomes (generally) can't combine during fertilization to form viable offspring.

Duplicating an entire genome gives you two copies of every gene, keeping everything balanced while providing the opportunity for each copy to "subfunctionalize" and specialize in a variation of the original function. This helps explain how very complex mechanisms can arise from simpler ones without "breaking" the original system, challenging creationists' "irreducible complexity" argument that evolution is incapable of solving this problem. Genome duplications have occurred twice in vertebrate evolution (and a third time in fishes), and as a result mammals often have 4 genes corresponding to a single homolog in fruit flies.

That said, DNA is heavy and bulky, so new species that make twice as much of it aren't always at an advantage. Plants don't have to worry about this as much, so variations in chromosome number are far more common in plant speciation—often in a single plant genus you can find species with 1n, 2n, 4n, 8n, hybridizing between 4n and 8n to get 6n, etc.