I’m reading the _Origin of Species_and Darwin, example after example, never suggests a time span for any specie’s appearance after its predecessor.
Some questions:
1.
Am I such a yutz that I missed that point or disclaimer by Darwin, or that he didn’t want to open another can of worms?
2.
What is the fastest “branching” of a genome?
As part of that question, didn’t some brand new soot-colored moth turn up in the area of coal mines in England, and that was the fastest evolutionary change anyone had ever seen?
Or is the whole shebang determined by isolation, food sources, etc?
AFAIK, Darwin never postulated just how long speciation took other than on a time scale comparible to geologic change. Also, bear in mind that the length of time it takes would be in number of generations, not years, so faster-breeding creatures can change more rapidly.
Two competing theories: first, gradualism, where speciation occurs slowly and constantly, vs. Eldredge and Gould’s punctuated equilibrium, where speciation occurs rapidly, sometimes due to environmental changes, but are otherwise mostly static outside of that time period. Both postdate Darwin I believe. Both have support, and it may be likely that both occur or some intermediate form predominates.
As far as the moth question, this is only a "breed’ or color morph change and not a new species. Humans can probably create new breeds faster, perhaps in something like Drosophila. The key is, as mentioned above, fast-breeding species.
I don’t know if Darwin ever mentioned this subject in other writings, but I can certainly see why he didn’t it in Origin. He really had no way of knowing how many years or generations speciation takes and he surely realized that. The geologic record was not dated at that time and, besides, it was too sparse to show the descent of individual species. With domesticated animals he might have been able to estimate how many generation it took to get, say, one breed of terrier from another, but that wouldn’t really tell him much about how many generations would be needed to get a whole new species in the wild.
Darwin didn’t know about genetics and didn’t know how variation came about. He didn’t know how often mutations would occur or if the rate was anywhere near constant. He would have realized that there is also a great deal of chance involved in the selection process and thus the rate at which populations evolve could be highly variable.
Any attempt by Darwin to put a timescale to the descent of, say, horses and zebras would have been pure speculation. He didn’t need to go out that limb. It would just have made him look silly and would have given fuel to his critics.
Looks like I’m off by several orders of magnitude. I just looked and found some documents that state 120,000 yrs as the bottom end for rate of speciation…not 4000-10,000
Speciation is a process, rather than an event. There is really no theoretical lower limit on how fast speciation can occur (though of course it will take at least one generation).
Speciation can occur very rapidly by polyploidy, or the increase in chromosome number. This mechanism seems to be quite common in plants, but rare in animals.
Speciation by other mechanisms typically takes hundreds of thousands to millions of years; however, in exceptional cases it can probably take place within a few thousand years.
I suspect that research in metagenomics is revolutionizing our understanding of evolution. Instead of having to evolve new genes, organisms can acquire new traits by borrowing genetic material from other species.