At what point does a new species emerge?

Naturopathy Atlanta

The definition of species is a population that is from the same gene pool (genome) and they can mate successfully together. Once a group of the population has evolved so mich that it can no longer mate with the source population it is definetley a new species. There are some gray areas inbetween but that is the clearest division between species.

Naturopathy Atlanta

In terms of natural selection, I think it's when that species has something which makes it categorically different to it's decendents, i.e. it has one less leg, or it can breathe in air instead of water.
Like you said though, natural selection has taken a back seat due to our medical advances. In humans, I don't think there will ever be a new species as the genetic alterations in humans all seem to cancel out. In animals, I don't know – maybe those who are best equipped to adapt to new conditions that have arisen due to global warming?
I think when the genetic alterations amount to a new species is very species-specific.

Research:
Several theories about how evolution occurs:

Phyletic gradualism – gradual progressive change
Punctuated equilibrium – sudden changes punctuating long periods of little change.
In general, a stimulus like an environmental change causes the evolution into a new species. The organism must either:
adapt to changing conditions if possible
survive by changing habits or migrating into another area
go extinct
Allopatric speciation
When a population (a small population of a larger species) becomes geographically isolated, it contains a small gene pool. It will be reproductively isolated from the remaining populations of the species. The population experiences genetic drift – it will include a limited range of variability, and will experience unique mutations. The differing environmental conditions in that area will guide natural selection.
When the barriers are removed, the isolated population may have changes so much that it is no longer able to interbreed with the remaining populations of the species. Changes may be in morphology, in function, in developmental timing, or in behavior.

The removal of the barriers and the migration of the isolated population to the larger area will look like the sudden appearance of a new species.

Naturopathy Atlanta

The difference in a species over time is harder to pinpoint than the difference between two species living at the same time.

The easiest process to understand is the branching of a single species into more two or more *separate* species (the process called "speciation"). A single species can get separated into two (or more) subpopulations that are genetically isolated (they are not exchanging genes, either because they are *geographically* separated, or because some members have taken up a different niche). If they are *genetically isolated* from each other, then they will accumulate different genes, different mutations, in response to different pressures from the environment. After many generations (hundreds? thousands? depends on species), the physical differences may reduce or even eliminate the desire for members of the two populations to mate with each other even if they do come into physical contact. This further solidifies the genetic isolation. And after many more generations (hundreds, thousands) they have accumulated so many genetic differences that any offspring, even if produced, will be sterile, still born, or just weak enough that they don't last long. Eventually the two will lose the ability to produce offspring at all. At that point they are clearly *separate species*.

The difference between species *over time* is harder to pinpoint … because we cannot take an ancestor and a descendant separated by thousands of generations (millions of years) and see if they can mate to "test" whether they are the same or a different species.

Remember that the only record we have of what ancestral species existed, are fossils. (That is not to say that this is the only way we know they existed at all … DNA comparisons of modern living species provde the most solid evidence of common ancestry. I'm just saying that fossils are the only way we know what these ancestral species *looked* like.) So paleontologists make judgement calls as to whether (for example) Homo erectus was a different "species" than Homo sapiens. And they will often argue angrily about whether some new specimen should be classified as a "new species" or a "subspecies" of an already known species. Basically it boils down to the number of critical differences.

Again, since we cannot "test" whether two species separated by millions of years could actually mate … we just draw arbitrary lines based on the fossil record so far. New fossil finds will often cause those lines to be redrawn.

Incidentally, the fact that these lines are constantly being redrawn is always seen as an opportunity by Creationists to declare that scientists are "unsure"about evolution! All this really reveals is these Creationists are absolutely CLUELESS about what's being debated.