Please explain the mechanism by which a species that is drifting genetically to become a new species reorganizes its DNA on its chromosomes, creating a new number of chromosomes or putting the information in a new arrangement.
Genome rearrangements occur randomly, and rearranged chromosomes are often lost because they are lethal or interfere with survival or fertility. Genome content changes quantitatively when an entire chromosome or section of a chromosome is deleted or duplicated. Genome content changes qualitatively when the order of genes on a chromosome is shuffled without any net gain or loss of genes, such as when pieces of chromosomes are flipped around (inversion) or moved to a new location (translocation). Likewise, a genome can be reorganized when chromosomes are joined together or split in two.
When a cell divides, its chromosomes are replicated, and each daughter cell inherits one complete set of chromosomes. Loss or duplication of whole chromosomes occurs when a cell divides and erroneously divvies up the chromosomes unequally between the two daughter cells. These sorts of drastic changes in reproductive cells usually do not produce viable offspring. Chromosomal rearrangements such as inversions, translocations, duplications, and deletions result from recombination. Evolutionarily relevant recombination occurs in the germ line, where sperm and eggs are produced. (Rearrangements, losses, and duplications that occur in nonreproductive tissues like muscles and neurons may cause cells to die or become cancerous but cannot be inherited by the next generation.)
In the context of reproduction, recombination is a normal mechanism through which new combinations of alleles are generated and is often referred to as “crossing over.” During crossover, large pieces of chromosomes are exchanged reciprocally between homologous chromosomes. The vast majority of crossover events result in no net loss or gain of genetic material and no rearrangement of gene order. This process generates new combinations of alleles and thus contributes to genetic diversity within a species. The original and recombined chromosomes are said to be “collinear” because they have the same genes in the same order; they differ only in which alleles they carry.
Abnormal recombination occurs during the repair of DNA damaged by factors such as ultraviolet radiation, chemicals, and metabolic stress. The ends of chromosomes are protected by structures called telomeres, so normally there are no free unprotected DNA ends in a cell. Chromosome breakage creates free DNA ends (i.e., double-strand breaks), which the cell detects and repairs. The DNA repair machinery fixes breaks by inserting the broken piece of DNA into an intact chromosome. The repair machinery is usually able to put the broken DNA back in the correct place, but sometimes errors occur. If the broken DNA is inserted into an incorrect chromosomal location, a rearrangement of the genetic material results. Such errant repair events can create new chromosomes by mixing and matching, joining, or deleting parts of the starting chromosomes.
Some researchers have promoted the idea that chromosomal rearrangements can actually drive speciation. This “chromosomal speciation” model predicts that chromosome rearrangement reproductively isolates the animals with the new arrangement and thus causes speciation. Chromosomal rearrangements can cause hybrid sterility, a condition in which individuals produced from interspecies breeding are themselves unable to reproduce. However, hybrid sterility is the result of speciation and not a cause. Because gene flow by recombination between collinear chromosomes is much greater than it is between rearranged chromosomes, retaining a consistent gene order makes reproductive isolation, and therefore speciation, less likely. It seems reasonable, then, to hypothesize that chromosome rearrangements could drive speciation, but this has not been demonstrated in any natural instance and is still vigorously debated.
Chromosomal rearrangements are examples of random genetic variation and provide some of the raw material upon which natural selection acts. If a newly rearranged chromosome can be passed on to future generations without causing sterility or lethality, then this new chromosome could enter the population. If the new genome structure confers some sort of survival or reproductive advantage, then it will undergo positive selection and may become fixed in the population and thus contribute to speciation.
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