Chromosome numbers and evolution

[mynah]

Even as more and more of the mechanisms involved in the evolution of new species are becoming clear, the one that puzzles me is differences in chromosome numbers and their contribution to evolution. Sure, the mechanisms (translocations, inversions, fusions between acrocentric chromosomes, etc.) have been known for a long time, as has the fact that not all human individuals (to mention one species) have the same number of chromosomes, and that differences in chromosome number is a powerful genetic isolating factor.

What I find puzzling is that changes in chromosome number accompanied by speciation is so common. The mechanisms involved are likely to be deleterious, or at best be neutral to the carrier – but even an asymptomatic individual with a translocation or an unusual number of chromosomes has a greatly increased risk of reduced fertility and/or reduced fitness of offspring. As a result, even a rare carrier who benefits personally from a translocation or inversion is likely to have fewer healthy offspring than one who does not have the chromosomal defect.

Of course, if the condition bestows some selective advantage and if there are enough other carriers around for matings to occur frequently between carriers, chromosome numbers may change, and descendants may become genetically isolated from other members of the species. If selection pressure is sufficient, the change will eventually become homozygous in the population.

The question is, however - how is this point reached, and why has it happened so often during the course of evolution?

[Ahmabeliever]

Need or demand will dictate the event. We may not understand the stressor responsible for specific chromosomal differences but cause will create the effect.

The decreased chance for procreation may in fact be another part of the selection process. The females remaining without progeny as they are unsuitable to carry the blueprint for the chromosomal change correctly.

It may be that the carrier of the chromosomal change is selecting for identical alleles.

Bit over my head, but thought provoking which I always appreciate.

The question is, however - how is this point reached, and why has it happened so often during the course of evolution?

How is the point of the trait being homozygous met? Perhaps through the above selection criteria?

Why has it happened so often during nature? I assume it is to do with neccessity. As plants develop better defenses (say tougher seeds) animals develop better beaks/claws/teeth which calls for better predators or they might get pecked to death... Everything affects everything, in some manner, a change promotes more change.

The reappearance of the sabre tooth several times in the fossil record denotes emerging populations of large herbivores around the time of the emergence of the tigers. Supply and demand. Nothing more. Species may differentiate according to the attributes deemed neccesary.

Why would the human form/make-up change now?

The desire to fly. Pollution filtration or even utilisation. Dietary adjustments. Plus plus plus.

Sorry if I'm not in the right ball park, point it out if so.

And the different humans - australian aboriginals?

[Biochemist]

Originally Posted by mynah

What I find puzzling is that changes in chromosome number accompanied by speciation is so common....even a rare carrier who benefits personally from a translocation or inversion is likely to have fewer healthy offspring than one who does not have the chromosomal defect. ...

The question is, however - how is this point reached, and why has it happened so often during the course of evolution?

This is a great question. Do keep in mind that if reproduciton is sexual, the mates (almost always) have to have the same chromosome number to reproduce. This means that 1) the chromosome anomaly has to occur in two adults in the same generation, and 2) they have to find each other.

Calculate the odds on that one.

Bio

[mynah]

It's always puzzled me how chromosome numbers could increase.

This could provide one explanation.

[mynah]

A relatively simple explanation