A few months back, I was reading a textbook called "Speciation" by Coyne and Orr. Speciation is the process by which new species arise from pre-existing species. It is the heart of evolution. I got sidetracked by the gardening season, and have not looked at it again until very recently.
The recurring question is whether two new species can arise from an old species without the old species first being split geographically into two or more isolated populations. I find it difficult to envision how a new species could arise in the midst of its parents' population. So far, the book has not shown me any mechanisms for this that I find convincing.
When the old species is first split into two isolated subpopulations, from which new species arise independently of one another, the term used is ALLOPATRY. When a new species arises in the midst of the old species and the two become separate and distinct species in the same geographic area, the term is SYMPATRY.
Allopatric speciation seems to have to be the rule, and if two closely related species now occupy ranges that overlap, then they cannot both have developed in that current range.
When two related species extend their ranges so that they overlap, several things can happen.
1. One is that they can be unable to hybridize. The possible reasons for this are many, from behavioral (for plants, behavior of pollinators) to chromosomal incompatibilities.
2. A second possibility is that their hybrid offspring have poor or no fertility. The hybrids will be at a great reproductive disadvantage compared to the parents, but will be competing against the parents for resources. The hybrids will eventually lose that commpetition.
In either case, the two species are very probably going to remain separate and distinct from each other over the long term.
3. Or they may interbreed freely.
If they can hybridize and if their hybrid offspring are fertile and vigorous, one of the two species is likely to eventually be swallowed up by the other. By "eventually," I mean over several thousand years or several tens of thousands of years.
The whole of North America is a hotbed of evolution because the last glaciers left North America only about 12,000 to 14,000 years ago. That is a short time in the life of a species, but the glaciers have come and gone from here many times over the last 2 million years or so. The constant cycling of environments and climates has probably accelerated the rates of change for the species that live and have lived on this continent.
Here in central Indiana, we were very near the southern edge of the glacier 20,000 years ago, but we were still well under the ice. None of the plants in our woods and fields were growing here 20,000 years ago. They were somewhere else, most likely well south of here. Where they were from 120,000 to 14,000 years ago and how they got back to here after the ice melted would tell the story of their recent evolution.
I'm interested in the species and the recent (last 100,000 years) of the evolution of things like Trillium, Hymenocallis, and Zephyranthes that are native to eastern North America, among other places. I wonder how they got to where they are growing today. I wonder whether they are old species or brand new species. I wonder where they are going.