Evolution and chromosomes. Please explain.?

> Ok that helps a lot. But wouldn't a large population need to have such fused chromosomes to maintain a population and generate a new species? What am I missing?

I think what you're missing is that the first individual with fused chromosomes could still produce fertile offspring by mating with other individuals in the population that didn't have fused chromosomes. When the offspring produce gametes, the two unfused chromosomes line up next to the fused chromosome in meiosis, and separate to different gametes, so you don't risk getting gametes with missing or redundant DNA. Over many generations the characteristic of fused chromosomes became more and more common in the population until it was the norm. This could have simply been due to chance, or it's possible the fused chromosome provided some advantage in the given environment.

A modern example is the Eagle Lake trout, which has 58 chromosomes. It is a subspecies of (and can interbreed with) rainbow trout, which have 60 chromosomes. http://www.csus.edu/indiv/c/colemanr/Theses/Worth_... p. 12

Two chromosomes from the family tree are fused in human beings. Specifically, this is on human chromosome number 2, the evidence coming from the presence of inactivated teloremes, an inactive centroreme, and the identical banding dye pattern that the fused portion has with the "extra" chimpanzee chromosome.

EDIT:

"Ok that helps a lot. But wouldn't a large population need to have such fused chromosomes to maintain a population and generate a new species? What am I missing?"

No, because 2 organisms with a different number of chromosomes can still interbreed if they're still closely-related. For example, donkeys have 62 chromosomes and horses have 64, but they can interbreed. Although most mules and hinnies are infertile, this isn't universally true:

http://www.ncbi.nlm.nih.gov/pubmed/4056372?dopt=Ab...

http://news.bbc.co.uk/2/hi/science/nature/2290491....

http://www.npr.org/templates/story/story.php?story...

What likely happened was that a single group of our last common ancestor had a genetic anomaly. Still a member of their species, but 2 chromosomes were fused. As long as the individual (most likely a female, as modern hybrid species indicate that these are more likely to be fertile) can still produce offspring, the "wrong" chrosome count floats within the population, not becoming dominant or disappearing completely, but instead accumulating changes. If the mutations that lead to human-like adaptations, such as a larger brain or a body better adapted to bipedal motion, occurred on the fused segments, and such traits became beneficial to survival, then the members who mate most successfully will be these mutated organisms. The proportion with 23 pairs of chromosomes at that point become much more common, eventually displacing the ancestral chrosomal count.

Because about a million years ago, in a human somewhere, two chromosomes fused. Over time, people with 46 chromosomes won out over people with 48.

A close look at human and chimpanzee DNA shows that human chromosome 2 is very similar to chromosomes 12 and 13 of chimpanzees. At some point, chromosomes 12 and 13 fused together in human ancestors to create chromosome 2. It probably happened in a similar way to what happened with the 44 chromosome man.

That's actually an excellent question, for once. It's something that, back when it was first discovered, could have spelled disaster for the idea of common ancestory with the other apes.

Chromosomes have unique markers at the "beginning" and "end" of each one. However, when this odd fact about the number of chromosomes was discovered, scientists began looking at each individual chromosome and it didn't take long before they found it.

One of the chromosomes (think it was actually the second one) had both a beginning and an ending marker INSIDE the chromosome, as well as those at the ends. This means that the chromosome had merged with another, and thus had caused us to have one less than we normally would have had.

It's something that would have been predicted by evolution and was later confirmed to be true. If nothing else, this is actually very strong evidence that common ancestory is true.

Chimps have 48 chromosomes. Humans "normally" have 46. There is a family in China that have 44 and are perfectly normal. How it does that, we do not know, that it does is well proven.

Looking back millions of years into early human history, current research into human evolution tends to confirm that in some cases, interspecies sexual activity may have been a key part of human evolution. Analysis of the species' genes in 2006 provides evidence that after humans had started to diverge from chimps, interspecies mating between "proto-human" and "proto-chimps" nonetheless occurred regularly enough to change certain genes in the new gene pool.

So in short, nasty sex was responsible.

Chromosome fusion:

http://www.youtube.com/watch?v=x-WAHpC0Ah0

http://www.evolutionpages.com/chromosome_2.htm

http://en.wikipedia.org/wiki/Chromosome_2_%28human...

http://etb-darwin.blogspot.co.uk/2012/03/evidence-...

Oh, the point where the two fused is very obvious. See the ends have markers, and you can see both ends plainly in the strand. The Wiki is right on: http://en.wikipedia.org/wiki/Chromosome_2_%28human...

"Generation and annotation of human chromosomes 2 & 4" Hillier et al, Nature.

Similar to the way identical twins are created, when a mitosis process comes, the chromasome is not duplicated into 2 but 3 chromosomes...get it?