Intaba said:
Human chromosome #2 is the result of the fusion of two ancestral chromosomes.
I know humans now have one less chromosome pair when compared to an ancestral population from a while ago, that the fusion didn't happen at once from certain parents to sons.
Why not?
Mutations that fuse chromosomes are still occurring in humans and other organisms. Usually, the fusion is "unbalanced". When that occurs, there is a large effect on the mutated organism that is usually very bad. However, the fusion is sometimes "balanced". The effect in the case of a "balanced" mutation is very likely to go unnoticed.
A fusion can take place in one step. However, the fusion mutation is not always a saltation. Sometimes, the effect can be very small. When the effect is small, it is more likely to be neutral or slightly positive. If the phenotypical changes are smaller, the chances that the mutation will be carried on to the descendents are improved.
Intaba said:
So what steps happened to arrive from 24 chromosome pairs population to 23 chromosome pairs population?
P.S Sorry if I sound confusing
There isn’t an intermediate state between two fused chromosomes. However, the effect of a chromosome fusion varies with the physical details of the process. A chromosome fusion under some circumstances can have a small or even an insignificant effect on the phenotype of the offspring. If so, it is a small mutation.
Not all chromosome fusions maim or kill the offspring. Scientists have observed chromosome fusions in all sorts of organisms including human beings. Although some fusion are malignant (e.g., cancer forming), some chromosome fusions have no significant effect on the offspring.
When biologists say that evolution is the accumulation of small mutations, they are talking about the physical effect of the mutation. Two chromosome can fuse with no significant affect on the immediate offspring. The effect of the fusion on the offspring of the offspring may be itself be a slight increase in the rate of mutation.
The fusion that you are referring to appears to be in the class of fusions that had little effect on the offspring. For example, the chromosome in question has two centrometers. Thus, it could pair during meiosis with the corresponding unpaired chromosomes. The parents and siblings probably couldn’t see any significant difference in the first individual with a fused chromosome.
The effect of the fusion mutation on fertility and viability of the descendents was also probably small. It may have contributed to a hybridization barrier, in that there was a small selection against hybrids. However, this would have a very small effect.
The chromosome fusion is best seen as an infinitesimal step itself in the evolution of human beings. The mutation probably contributed something to the reproductive isolation. However, the effect was so small that no one without a microscope and staining methods would have noticed it.
Here are some links explaining today’s chromosomal fusions. They emphasize that some fusion mutations have very small effect. There may be bad effects a few generations later. So far no one has seen any significant good effects. Never the less, these references do point out that the physical effect of a chromosomal fusion can be insignificant.
http://en.wikipedia.org/wiki/Chromosomal_translocation
“Robertsonian translocations
his type of rearrangement involves two acrocentric chromosomes that fuse near the centromere region with loss of the short arms. The resulting karyotype in humans leaves only 45 chromosomes since two chromosomes have fused together. This has no direct effect on the phenotype since the only genes on the short arms of acrocentrics are common to all of them and are present in variable copy number (nucleolar organiser genes).“
This article describes Robertsonian translocations. It describes some maladaptive features of this type of mutation. However, notice that these maladaptive changes occur at least two generations after the mutation event if they happen at all. The grandchildren of the first mutation do not have Down's syndrome. They are just slightly more likely.
http://en.wikipedia.org/wiki/Robertsonian_translocation
“In humans, when a Robertsonian translocation joins the long arm of chromosome 21 with the long arm of chromosome 14 (or 15), the heterozygous carrier is phenotypically normal because there are two copies of all major chromosome arms and hence two copies of all essential genes.[2] However, the progeny of this carrier may inherit an unbalanced trisomy 21, causing Down Syndrome.
About one in a thousand newborns has a Robertsonian translocation.[3] The most frequent forms of Robertsonian translocations are between chromosomes 13 and 14, 14 and 21, and 14 and 15, and occur when the long arms of two acrocentric chromosomes fuse at the centromere and the two short arms are lost.[4]
A Robertsonian translocation in balanced form results in no excess or deficit of genetic material and causes no health difficulties.”
This article conjectures that the effects of a chromosomal fusion can be positive. The scientists show that some genes very close to the point of fusion are still functioning, but in a different manner than they did before the fusion.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC187549/
“This study illustrates that the duplication and rearrangement of subtelomeric and pericentromeric regions have functional relevance to human biology; these processes can change gene dosage and/or generate genes with new functions.”