|Feb27-12, 05:29 PM||#18|
Reproduction and Human evolution
@bobze: I think I understand where i went wrong. I was getting hung up on the number of chromosomes and not realizing that the actual genetic information was all still there. The illustrations helped big time. Thank you very much for the education.
|Oct24-12, 08:07 AM||#19|
The number of chromosomes is the equivalent to the number of characters that you can send in an SMS. With the same space, you can transfer different kind of messages depending on the words that you use. That is the reason why you can have different apperances from the same number of chromosomes. The DNA contained in the chromosomes of a person is different from one person to another.
Tigers and lions can interbreed.
|Oct24-12, 06:49 PM||#20|
There was probably a time when our ape-like ancestors were a population of individuals with varying chromosome number. This situation would be similar to the deer in the link that I posted below. The individuals with a fused chromosome freely crossed with the individuals with the unfused chromosomes.
The consequences of crossing with a dissimilar individual were very small. Hybrids were just as healthy and fertile as the homozygous individuals. There may have been a small penalty in the grandchildren of the hybrid. However, most of the grandchildren were probably healthy. The likelihood that an individual would be maladapted due to the fusion mutation was very small.
Hybrids of parents with different numbers of chromosome pairs can be viable even in species which with obligatory sexual reproduction. Meiosis does not "count" chromosomes. Meiosis "counts" genes and centromeres.
A lot depends on the structure of the chromosomes that make up the difference in chromosome number. If there is a chromosome fusion that preserves both the genome and the centromeres, then the hybrid will be healthy and fertile.
Here is an example where these different varieties (species?) of deer were studied. One of the varieties had a different species number, yet could cross productively with the other varieties.
“It was shown that compared with conventional cattle the Bos frontalis has a homozygous, species specific 2/27 centric fusion which reduced the diploid chromosome number from 60-58. This provided further proof that Robertson translocation-type rearrangements have been the major source of interspecies karyotype differences in the evolution of the Bovidae. There was also a report on the cytogenetics of twin offsprings from an interspecies cross in marmosets (Callitrichinae, Platyrrhini), resulting from a pairing between a female Common marmoset (Callithrix jacchus, 2n = 46) and a male Pygmy marmoset (Cebuella pygmaea, 2n = 44). Both hybrid individuals had a karyotype with a diploid chromosome number of 2n = 45. These genomic imbalances were confined to centromeric and telomeric heterochromatin, while euchromatic chromosome regions appeared balanced in all species investigated”
There are other examples that I can find in the literature involving populations of organisms with varying chromosome number. Horse species vary quite a lot in chromosome number. There are other horse species with differing chromosome number that freely cross.
Even the sterility of the mule and hinny isn't strictly true all the time. However, the occasionally fertile mule crosses with either a horse or a donkey. Thus, the grandchildren end up looking like either a horse or a donkey. There are herds of horses with varying chromosome numbers in the population.
There are also a population of extant gophers in New Mexico with varying chromosome number.
Note that the balanced fusion of two chromosomes actually has very little effect on the phenotype of the mutant. If the fusion occurred in a balanced way, then the mutant can cross freely with other individuals with very small fitness penalty. Hybrids are not always sterile, even if the parents have different number of chromosome pairs.
When biologists say that only small mutations contribute to evolution, they are talking about mutations with a small effect on the phenotype. Admittedly, chromosome fusion has to occur in one step. However, the effect on the body of the mutant is sometimes very small. This balanced type of mutation is improbable on the time scale of decades, but is highly probable on the time scale of megayears.
Most polyploidy hybrids are sterile. However, there is occasionally a mutation that changes the number of chromosomes without changing the genes in the total sum of chromosomes. This mutant can cross with an unchanged individual and have offspring that has nearly the same fecundity as the unchanged offspring. The phenotype in such an individual is almost unchanged. Such a mutation is rare but it still happens.
This type of mutation is small in the sense of having a small effect. There is no intermediate step in any individual when a change in chromosome number occurs. However, there is an intermediate population that has individuals with differing chromosome number. Eventually, even a small difference in fecundity can split such a population into two or more varieties.
Here is a simple article with pictures. It shows how parents with different chromosome numbers can have fertile offspring if the genes and centromeres of the parental chromosomes are the same.
“This happens with a low frequency, too, and has been observed many times (hint: look up Robertsonian fusions on the web.) I think the key issue to understand here is that chromosome number changes are typically going to represent nothing but reorganizations of the genes — the same genes are just being moved around to different filing cabinets. This has some consequences, of course — you increase the chances of losing some important file folders in the process, and making it more difficult to sort out important information — but it’s not as drastic as some seem to think, and chromosome numbers can change dramatically with no obvious effect on the phenotype of the organism. These really are “small adaptations over time”, or more accurately, “small changes over time”, since there is no necessary presumption that these are adaptive at all.”
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