How many number of generations (or time) does it take for a new species to emerge from an old one ? I understand this can vary and am specifically looking for examples in the multicellular species.
Theoretically there is no lower limit. Hybridization can result in a population unable to breed with either parental form in one generation.
It appears that House Mice in Madeira may have evolved into at least six genetically distinct speciesin the five hundred years since they were introduced there from the mainland.
This is not typical, however. I would suggest around 10,000 years/generations as a pretty short time for speciation to occur. More typical would be 100,000 to 1 million years.
The population at the tip of South America has been isolated from that in Polynesia or Siberia for 12,000 or more years, still breeds just fine. (25 years/generation => 480 generations?)
Ditto for the aborigines in Australia or Tasmania, isolated up to 40,000 years and still “compatible” with Europeans genetically.
However, apparently Indians from the high Andes have evolved moderate adaptions (lung capacity, etc.) for thin air; Eskimos have evolved adaptions for extreme cold (compact bodies, insulating fat.) The degree of selective pressure anywhere in the world does not seem to driven genetic change enough to prevent “crossbreeding”.
Creossbreeding itself is a poor criteria, cosidering you can get moderately passable crossbreed between donkeys and horses, occasionally lions and tigers or some other close but distinct species.
Thank you for the replies. What would be a good criteria ? I believe we are both talking about measuring the “speed of evolution” ?
You can’t really call these populations “isolated” unless there is no gene flow whatsoever with other, nearby populations. If there is even a small amount of gene flow, that will serve as a stabilizing force. It’s sort of like the often-cited example of dogs: a mastiff may not be able to mate with a chihuahua, but you can get gene flow between the two breeds by successive breeding with intermediate-sized breeds. Similarly, gene flow between disparate human populations keeps the species stabilized.
That is an excellent question that biologists wrestle with. The problem is that nature is not nicely divided up into species through either time or space and has many different methods of reproduction (how do you define species in populations that do not reproduce sexually?)
I believe the current definition for sexually reproductive species is that two populations that do not reproduce with each other when given a chance. So, that could either be because they are not genetically compatible, are not morphologically compatible (tab A does not fit in slot B), or because they are not behaviorally compatible (tab A does not want to go in slot B).
So, being as we humans bone just about anything that looks like another human whenever populations encounter each other we are all one species, while there are genetically compatible populations of finch that are considered different species as they will not reproduce with each other.
As to how long it takes, well that is incredibly variable. A very large population that spans a large territory with constant gene flow can remain nearly the same for an incredibly long time.
Thanks for the good answers. Mods - you may close the thread, if you’d like.
The theory of punctuated equilibrium (its Wikipedia page has lots of info and links concerning speed of evolutionary change) was developed to explain what we see in the fossil record: species remaining apparently unchanged for a long time, then abruptly disappearing and being replaced by somewhat different species. The short and crude explanation of punctuated equilibrium is that it proposes that species remain more-or-less stable (in equilibrium) for long periods of time, then change relatively rapidly when selection pressures change (as random mutation allows, of course). It may turn out that evolutionary change is substantially less gradual than we have tended to assume.
It also conforms to the most common model for speciation, Allopatric Speciation, in which most evolutionary change takes place in small, isolated populations. Such populations are unlikely to leave a trace in the fossil record.
Then, when that variation encounters the larger population again, the changes quickly dominate the previous standard, resulting in what appears to be a quick change to the entire species from the fossil record, but actually took quite a while to initially develop in the isolated group.
This active thread is very similar to yours. You might find some interesting info there.