¶ … Gould's and Mayr's biological species concept, as presented by Stamos (2008). According to these scientists and to this species concept, two populations are termed as belonging to the same species if they are capable of interbreeding. So, as Stamos further exemplifies, the chimpanzees form a single species, because they are capable of interbreeding, they are producing offspring. Human beings and chimpanzees are not capable of producing offspring together, so they belong to two different species. Furthermore, two races of dogs belong to the same species: they are capable of interbreeding and the resulting dog is also fertile. However, a horse and a donkey belong to different species: while they are able to interbreed, their resulting offspring is not itself fertile.

To sum up, my understand is that humans belong to the same species because of reproductive compatibility. At the same time, the human species is formed of different races, whereby the races show physical differences between individuals that belong to the same species. This would be why the Asian and the Caucasian races are different from one another.

A scenario that could potentially lead to a second human species would be in-breeding in the same population and reproductive isolation over a long period of time (although it is hard to estimate what a long period of time would actually mean in terms of years). Basically, the human species evolved over time through interbreeding, but particularly through breeding...

If this was no longer possible, the isolated population would eventually lead to different species. This is probably what occurred with the Neanderthals, who were separate from the Homo Sapiens as a species.
2. The answer here is nuanced: studies show that the situation is dangerous, but it can still be reversed. To answer this question, a study by Barnosky et al. (2011) was used. Their conclusions were that the loss of species, the main factor that would characterize a situation as being close to a new mass extinction, is "dramatic and serious," but that the current status is not yet equivalent to a mass extinction, namely one that would have the same characteristics as the previous Big Five.

However, they do emphasize the fact that the situation is one of concern: loss of species in the critically endangered, the endangered and the vulnerable categories could lead to mass extinction occurring in the next hundred of years. Even more so, one needs to look at some of the similarities of factors that led to the previous extinctions. Most of these were caused by climatic dynamics (overheating or periods of cooling off for the planet, leading to the disappearance of species), atmospheric composition or ecological stressors.

Drawing the parallel to the present time, climatic dynamics could be equivalent to the climate changes that occur today. Ecological stressors could be similar to the actions of humans on…