The
Holy Wikipedia has this to say about chromosome abnormalities:
Deletions: A portion of the chromosome is missing or deleted. Known disorders in humans include Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4; and Jacobsen syndrome, also called the terminal 11q deletion disorder.
Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material. Known human disorders include Charcot-Marie-Tooth disease type 1A, which may be caused by duplication of the gene encoding peripheral myelin protein 22 (PMP22) on chromosome 17.
Translocations: A portion of one chromosome is transferred to another chromosome. There are two main types of translocations:
Reciprocal translocation: Segments from two different chromosomes have been exchanged.
Robertsonian translocation: An entire chromosome has attached to another at the centromere - in humans these only occur with chromosomes 13, 14, 15, 21, and 22.
Inversions: A portion of the chromosome has broken off, turned upside down, and reattached, therefore the genetic material is inverted.
Insertions: A portion of one chromosome has been deleted from its normal place and inserted into another chromosome.
Rings: A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.
Isochromosome: Formed by the mirror image copy of a chromosome segment including the centromere. |
I would add too that chromosome abnormalities are not the only way that the genome can change. Random changes happen to individual bases along the DNA chains, for example.
In the case of Down Syndrome chromosome 21 is duplicated and the health problems are caused by overproduction of the proteins coded on that chromosome.
If the extra chromosome was inherited and became commonplace in the population (not likely with DS) then selection pressure would apply as random mutations happened in that extra chromosome.
Maybe natural selection would 'find a way' to turn off the genes in one of the chromosome 21 copies, and there would be convergence back to a non-Down human phenotype. Or the genetic material could end up as something else, maybe leading to speciation.
There is a case of this in our genetic history as humans. Following the common ancestor between chimpanzees and humans, two chromosomes fused, which is why the other great apes all have 24 pairs of chromosomes and we have only 23.
The 'new glorious result' you describe is really a strawman argument. There are interesting examples, like sickle cell anaemia. If you have a copy of the sickle cell trait gene and a good, non-sickle cell copy then you have some protection against malaria. Malaria has been the cause of half of all human deaths. So it has been a pretty big selection pressure. The sickle mutation is common in areas of the world where malaria is also common. It's an example of a beneficial mutation! But if you get two copies of the sickle cell trait gene then you will get sickle cell disease, which is life-threatening.
Genomes contain genes that code for proteins, and the proteins make the body. Either the protein becomes tissues or the protein acts as a catalyst for chemical processes that build tissues and regulate them. Genomes also contain switches that turn genes off or on. Sometimes a gene is turned off because it's role in development is no longer needed.
Modern species carry genes that may have been turned off in an ancestor species. The gene for teeth that was present in archosaurs, a common ancestor for birds and alligators, was turned off at some point down the bird line, and the modern bird descendents no longer make teeth. But the genes are still present in the birds and can be
accidentally switched on again.
However, evolution by natural selection is almost entirely about tiny mutations that give a tiny improvement in the ability to survive and reproduce. These tiny changes will be spread throughout the population more successfully over time than the alternatives that aren't quite as good. If the environment changes then there is a change in the needs for survival and reproduction, so selection pressure changes to a different direction, and species will change as a result.
Multiply up the time, and those tiny changes add up to massive differences, given long enough. The range of multi-celled living species on the planet today have had a half a billion years to accumulate differences by natural selection, and that started after single-celled organisms had existed for a few billion years. I don't know about you but I struggle to comprehend 100 years, let alone 1,000,000,000 years. Remember too that more than 99% of the species that have ever existed are now extinct, so it's not as if the mutations that once were beneficial remained beneficial in a changing environment.
Sorry about the long post, but I was keen to give you an idea of how much more complicated things are than you suggested in your post.
Stuart