They usually correlate everything to reproduction as the target which is not correct. The target is "survival of species" and changes in reproduction is only a by product of it.
Survival of the species is not a target of evolution. Traits can be selected for that are good for the individual but bad for the species. For example, a very virulent virus will reproduce very fast, and create tons of progeny. These will go on to do the same. Eventually that virulent phenotype will be the only one left in the virus population. Therefore, evolutionary biologist would say it's been "selected" for. However, if the virus is too virulent, it will destroy the host population it depends on for reproduction. Once this happens, the virus population will die out too. In this way, evolution can lead to the extinction of a species, not the survival of it. There are also examples of the opposite happening, but my point is that survival of a species is not the target and is not the only result of evolution.
The reason evolutionary biologists focus on reproduction is because differential reproduction is how individuals win the evolutionary "game". It's all about the numbers. Those that reproduce the most will have the greater share of the population. If that trend continues, i.e. the offspring of that one individual continue to reproduce more than any others, then they will eventually be the only ones left in the population. The phenotypes that allow an individual to reproduce more offspring than the others in its population will increase in frequency in the population. This is all natural selection is. It's not actually an active force like the term 'selection' would make one think. So, reproduction is a requirement for evolution to occur, but it is also the only metric by which evolutionary 'success' is measured.
Does anybody know when in biology two species are considered separate?
Ah, this is a very deep question, and one that has not been resolved in the biological community. Species is a tricky concept. There are many different species definitions out there, and none has been chosen as the best. The most popular is the Biological Species Concept (BSC) which, to paraphrase significantly, says that two individuals are of the same species if they can produce viable offspring (often the caveat is added that this must also be something that occurs naturally in their usual habitats, not just in zoos or labs). However, as I'm sure you can see, this definition is far from perfect. What about asexual species, like bacteria, which account for more species than all the other types of life combined? What about species that form viable hybrids with other species? There are at least a couple dozen other definitions that you could find on wikipedia, and I won't go through them here. The point is that species does seem to be a real concept in natural world to some degree. What I mean by that is that the concept of species is not purely an artifact of human categorization (like it would be if we considered all organisms of the same general color the same species). However, what we call species tend to fall onto a fine spectrum going from "good" species - those that strictly adhere to a species definition, such as the BSC - to "poor" species - asexual species and those that form hybrids with sister taxa all too readily.
So, when is the exact moment that one species becomes another? Well, there probably is no exact moment, and the more finely one tries to look at the question, the harder it becomes to answer. I find it akin to asking at what point on the light spectrum does red become orange? Wikipedia says at 620 nm. But at exactly 620 nm is it red or orange? Could anyone tell the difference between 619.5 nm and 620.5? The point is that there is probably no exact moment, but it does happen. At some point during speciation, two populations that were once one species are two separate species. This happens due to a whole set of reasons that you can read about elsewhere. The point though, is to emphasize that evolution is not trying to preserve species. Evolution happens because of differential reproduction, and natural selection is measured by determining how differential the reproduction is.
We have many intuitions in our life and the point is that many of these intuitions are wrong. The question is, are we going to test those intuitions?
-Dan Ariely
™ Version 4.2