How do mutations create reproductive isolation in species?

[Darwin123]

Unless there was some sort of gradual chromosomal translocation, that's not very likely though.

Originally Posted by Borek View Post
"Perhaps a good example of either/or is a change in the number of chromosomes. This is not a thing that can happen gradually." In terms of number of chromosomes, the change is sudden. For example, a highly asymmetric translocation can fuse two chromosomes together. This would decrease the number of chromosome pairs by one.
In terms of phenotypes, the change is gradual. Because the phenotype change is slight, I would not even call this a saltation. A chromosome fusion can result in one chromosome with two centromeres. This would result in almost no change in phenotype, and only a very small change in interfertility. As long as both centromeres are functional, the fused chromosomes would pair with the two unpaired chromosomes. Meiosis and recombination would largely be unaffected by such a chromosome fusion.
A fusion of chromosomes would not even be noticeable without microscopes and chromosome staining. The change in structure and behavior would largely be insignificant.
If both centromeres were functional, hybrids of fused and unfused chromosomes would have a slight chance of having a birth defect. The chances of survival for the hybrid would be slightly reduced. Later mutations could remove even this disadvantage.
Chromosome fusion may have occurred in the lineage leading from the human species to the most recent common ancestor of chimpanzees and humans.

Here are some links discussing chromosome fusion. These articles indicate that the change caused by chromosome fusion is not a saltation. The effect on anatomy is small if not negligible.

This link is interesting because the article was written by a Creationist. Notice that he explicitly admits that the result of a chromosome fusion can be viable. He provides proof that such fusions are viable. However, he tries to turn this fact around by saying that there was no new “species” created. He ignores the fact that this is consistent with Darwinian evolution. Biologists say that speciation is caused by the accumulation of a large number of very small changes, which is what this is.
http://amazingdiscoveries.org/C-deception-glossary_fusion_inversion_variation#ChromosomeFusion"
“Other populations have been discovered with chromosome numbers varying between 22 and 40. The number of chromosome arms is the same and banding studies reveal the genes to have the same structural features and pattern of genes. Obviously, in terms of their relationship, these different species are all one group.”

Here is a breeding experiment with fruit flies that shows that chromosome fusions can be viable.
http://www.esp.org/foundations/genetics/classical/holdings/Genetics/Genetics-1935-20-4-327.pdf
“In the original experiment, 14 translocations involving the X and fourth chromosomes, and viable in the male, were obtained. In 8 of these the gene string of the X appeared to be intact up to and including the normal allele of bobbed.”

There is the evidence that this happened in the evolution of the human race.
http://www.evolutionpages.com/chromosome_2.htm
“All great apes apart from man have 24 pairs of chromosomes. There is therefore a hypothesis that the common ancestor of all great apes had 24 pairs of chromosomes and that the fusion of two of the ancestor's chromosomes created chromosome 2 in humans. The evidence for this hypothesis is very strong.”

 

[Darwin123]

Thanks for replying. I probably chose a bad example because height is a continuous variable. Perhaps I should have selected an example of any either/or characteristic, such as a reptile with a three-chambered heart evolving into a bird with a 4-chambered heart, or the first time feathers replaced scales. In that case my question puzzles me. If it is a new species then it can only remain a distinct line by not mating with its neighbors, but then I don't understand how it finds a mate to permit it to reproduce.

You wanted to know about how a geographical barrier can effect the genetic distribution of a discontinuous variable. Chromosome number is an intrinsically “discontinuous” variable that is sometimes associated with speciation.
Biologists still believe that in the vast majority of cases evolution is "gradual". By gradual, I mean that the anatomy of the animal is changing in very small increments. Even in the case of changing chromosome number, the animal undergoes very small changes on an anatomical level. One would not notice the changes in each generation except with specialized equipment.
Chromosome number can result in a genetically unviable hybrid. The effect of mismatched chromosomes can be large. For example, an unbalanced translocation can fuse two chromosomes by causing an imbalanced dosage of enzymes.
Changes in chromosome number can also result in a very small difference in phenotypic expression. The degree of change in the phenotype is sensitive to the mechanism by which the chromosomes change number. Hence, chromosome number within a species can vary over a broad geographic range.
Note that the anatomy and behavior of such a mutation may be unnoticeable. A change in chromosome number doesn’t have to result in a saltation. Balance translocation can change the
In animals that with obligate sexual reproduction, mutations that change in chromosome number are usually unviable. However, there are exceptions.
Many examples of the change in chromosome number occur by sympatric speciation. Thus, there is a geographical “umbilical cord” between two populations with differing chromosome number. within the “umbilical cord”, there is continuous hybridization.
One thing a catastrophe can do is “severe” the umbilical cord. If a species is undergoing sympatric variation, the zone of hybridization is limited. If there is a mass extinction, the middle zone is wiped out and the two varieties can evolve their separate ways.
I think this is the main role of catastrophe. I don't think that it causes the speciation. Catastrophes mostly severe the "geographical umbilical cord" by killing off the varieties that originally evolved by sympatric speciation.
I don't think that a saltation has anything to do with it. "Punctuated evolution" is about disasters that cut the geographic umbilical cord.

The horse genus is well known for having a huge range of chromosome numbers. Horses on the opposite sides of this grade can not interbreed. The donkey-horse hybrids are most famous for being infertile. However, there are species within this genus that are interfertile. The only reason that they don’t interbreed is because of geographical distance. Note that in these cases the change in chromosome number is not associated with a discontinuity in anatomy.

The next two links refer to different species of horses that can interbreed even though they have different numbers of chromosomes.
http://en.wikipedia.org/wiki/Przewalski's_horse
“The karyotype of the domestic horse differs from that of Przewalski’s horse by an extra chromosome pair either because of the fission of domestic horse chromosome 5 in Przewalski’s horse or fusion of Przewalski’s horse chromosomes 23 and 24 in the domestic horse. In comparison, the chromosomal differences between domestic horses and zebras include numerous translocations, fusions, and inversions. Przewalski’s horse is known to have the highest diploid chromosome number among all equine species. Przewalski’s horse can interbreed with the domestic horse and produce fertile offspring (65 chromosomes).”

http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1479&context=gradschool_diss
“COMPARATIVE GENE MAPPING FOR EQUUS PRZEWALSKII AND E. HEMIONUS ONAGER WITH INVESTIGATION OF A HOMOLOGOUS CHROMOSOME POLYMORPHISM IN EQUIDAE
The Przewalski.s wild horse (E. przewalskii, EPR) has
a diploid chromosome number of 2n=66, while the domestic horse (E. caballus, ECA) has 2n=64.”

Most zebras are like donkeys. They don’t produce viable hybrids with the domestic horse. However, Grevy’s zebra is an exception. It can produce viable hybrids with the domestic horse. Here is a link.
http://www.princeton.edu/~dir/pdf_dir/2009_Cordingley_AnimConser.pdf
“Both species are sympatric in the Laikipia ecosystem of northern Kenya, where we have observed purportedly hybrid individuals. Using mitochondrial and Y chromosome DNA, we confirmed the hybrid status of the morphologically identified hybrids and demonstrate conclusively that all first-generation hybrids are the offspring of plains zebra females and Grevy’s zebra males.
…
Plains and Grevy’s zebra have distinct chromosomal structure, morphology, ecology and social systems. Genetically, the Grevy’s zebra has 46 chromosomes; while plains zebra has 44 (Ryder, Epel & Benirschke, 1978). There is a close chromosomal correspondence between the two species, with karyotypic differences in only eight chromosomes (Musilova et al., 2007). In captivity, viable male and female hybrid offspring produced from a plains zebra female and Grevy’s zebra male had 45 chromosomes (Benirschke, 1977).”


This link describes the varying genomes of a species of gopher with varying chromosome number. Or is it two species of gopher? The law hasn’t decided yet.
http://www.nsrl.ttu.edu/personnel/RJBaker/Publications/223-Hybrid%20gophers%20and%20ESA-Jones%20et%20al-1995.pdf
“Hybrid Pocket Gophers and Some Thoughts on the Relationship of Hybrids to the Rules of Nomenclature and the Endangered Species Act
The one animal that was karyotyped possessed a diploid number of 72 (Table 1)…The karyotype was indistinguishable from the karyotype of G. b. major, but distinct from those of G. knoxjoni and G. texensis, both of which have possesses 2n=72.”

 

[Darwin123]

Thanks for replying. I probably chose a bad example because height is a continuous variable. Perhaps I should have selected an example of any either/or characteristic, such as a reptile with a three-chambered heart evolving into a bird with a 4-chambered heart, or the first time feathers replaced scales.

It is important as a science teacher that you don’t teach your students a misrepresentation of evolutionary theory. I am primarily addressing your skepticism that evolution is gradual. I have been posting a number of messages, with links, to show that evolution generally proceeds in a series of very small steps even in animals where sexual reproduction is obligatory. Evolution seldom, if ever, proceeds by a saltation in anatomy.
You claim that scales and feathers can are discontinuous. However, your definition of scale is rather general. Extant reptiles have a flat scales. However, some extant reptiles also have follicles (i.e., tubes) in addition to flat scales. Follicles may have preceded scales.
Please note that extant birds still have flat scales in addition to feathers. This is especially pronounced in birds that haven’t been domesticated until recently. Look at the legs of a turkey, for example. However, feathers did not evolve from these flat scales. What feathers evolved from were follicles such as some lizards and crocodilians have.
Hypothetically, feathers evolved from follicles in a series of very small steps. Maybe flat scales also evolved from follicles. The diversity of dinosaur follicles shows that feathers did not have to develop from follicles in one generation. A continuous variety of follicle forms existed in the Mesozoic. In fact, there are a variety of follicle forms

1)
http://www.mcorriss.com/Prum_&_Brush_2002.pdf[/URL]
“Whatever their function, feathers evolved by selection for a follicle that would grow an emergent tubular appendage. Feathers are inherently tubular structures. The homology of feathers and scales is weakly supported. Feathers are composed of a suite of evolutionary novelties that evolved by the duplication, hierarchical organization, interaction, dissociation, and differentiation of morphological modules. The unique capacity for modular subdivision of the tubular feather follicle and germ has fostered the evolution of numerous innovations that characterize feathers. The evolution of feather keratin and the molecular basis of feather development are also discussed.”

2)
[url]http://icb.oxfordjournals.org/content/40/4/631.full[/url]
“It is likely that feathers evolved from a conical shaped tubercle rather than a plate-like structure. Although the morphology of the presumably most primitive feather is unknown, minimal conditions for its production include the cellular capacity to synthesize feather proteins (=ϕ-keratin) which provides the molecular phenotype, and a follicular mechanism for production and assembly of molecular and gross structure. Once the minimal structural element, presumably recognizable as a barb, existed, a variety of phenotypes followed rapidly. A tubercular growth center of appropriate size could produce a simple barb-like element, with cortex and medulla. This might be recognized externally as a bristle, but need never existed as a separate morphological unit.”

3)
[URL]http://www.nature.com/nature/journal/v464/n7293/full/nature08965.html?free=2[/URL]
“In combination with the wide distribution of proximally ribbon-like pennaceous feathers and elongate broad filamentous feathers among extinct theropods, this find suggests that early feathers were developmentally more diverse than modern ones and that some developmental features, and the resultant morphotypes, have been lost in feather evolution.”

One of the most common lies is that evolution requires saltations. Let me show you an article which presents such a lie regarding the evolution of feathers.
4)
http://darwinism-watch.com/index.php?git=makale&makale_id=1228
“Evolutionists claim that feathers evolved from the scales on birds’ alleged ancestors—reptiles. The fact is, however, that scales are folds in the skin, whereas feathers emerge from follicles in the skin, in a similar manner to hairs. Feathers consist of barbs, barbules and hook. Furthermore, the places where barbules and scales emerge from are very different.
…
Evolutionists suggest random mutations for that alleged mechanism. However, it is a known fact that mutations do not add new genetic information to living things, and can therefore have no evolutionary effect.
…
That advantage deals a fatal blow to the evolutionist scenario that the transition to flight took place after thermal insulation. According to that scenario, feathers assumed to have evolved initially for insulation should have a contoured structure, in which case only feathers which provide greater thermal insulation, in other words more contoured feathers, will be selected. For that reason, the hypothetical progress from a contoured structure to an aerodynamic one will be eliminated.
...
According to the theory of evolution, the genetic information for feathers, which did not at that time yet exist in nature, must have been new, and furthermore this information must have been added to the reptile DNA by a natural causes-based mechanism. Evolutionists suggest random mutations for that alleged mechanism. However, it is a known fact that mutations do not add new genetic information to living things, and can therefore have no evolutionary effect.
…
The irreconcilable differences between scales and feathers clearly reveal the invalidity of the evolutionists’ claims. In addition, the oldest known bird, Archaeopteryx, has an asymmetrical feather structure identical to that of modern flying birds. In other words, the oldest known bird emerged with the most perfect feather structure, whereas the theory of evolution would expect a “primitive” structure. The researchers L. Martin and S. A. Czerkas state that, “The oldest known feathers … are already modern in form and microscopic detail.”

“Evolutionists” now believe that feathers evolved from follicles (link #1). This scenario does not require that the feathers initially evolved for insulation should have a contoured structure, since the property of being contoured is a continuous variable.
The feathers are said to have first evolved in dinosaurs, not birds (links 1, 2 and 3). Since dinosaurs had follicles, this is not a problem. Archaeopteryx is no longer the oldest known bird, as there are fossil birds that seem to have come before. Birds and dinosaurs were not very different, so calling Archaeopteryx a “bird is a matter of sophistry. Some dinosaurs even had fused clavicles, something not mentioned in link #4.
The author’s entire argument is based on a tautology. A bird is a dinosaur with contoured feathers. If Archaeopteryx had merely barbed follicles that were not contoured, then it would not be called a bird.
Feathers can be contoured and still serve as insulation. A bird that flies to a tropical island wouldn’t need insulation anyway. The assumption here is that a dinosaur would have to fly perfectly well to evolve a feather. Obviously, it isn’t true.
Also note that the "known fact" that "mutations do not contain information". This is two lies. It is not a fact. No where does any experimenter or theorist say that mutations do not contain information. However, the bigger lie is "that it is a known fact". There is no reference, let alone links, to this known fact. It is a rumor, not a fact. I bet that you can't find a reputable study where this is explicitly proven. I bet you can't even find a study where biological information is quantitatively defined. How can this be a "known fact" when "information" is not quantitatively defined?
If you have other objections to the theory of evolution, that is fine. I just want to refute the urban legend that evolution takes place in discontinuous steps. Evolution is a gradual process.