Under equilibrium conditions, the emergence rate of new species should roughly equal the extinction rate. The total number of species on Earth is unknown, but 50 million is a mid-range estimate, and the background extinction rate as determined from the fossil record is about one in a million species per year. Therefore, on average, about 50 new species would be expected to emerge annually while 50 other species become extinct. At the present time, however, the global extinction rate is about one every 20 minutes. The baseline annual total of 50 is reached in less than 17 hours, and the annual total is over 500,000 species, most of which have never been catalogued.

http://hypertextbook.com/facts/2003/FelixNisimov.shtml
http://www.pbs.org/wgbh/evolution/library/03/2/l_032_04.html
http://www.sciencedaily.com/releases/2002/01/020109074801.htm

The "new" species that continue to be discovered aren't necessarily new in the sense of having recently evolved. Many parts of the world are still relatively unexplored in terms of their biodiversity, and many species are highly localized in distribution, so a thorough sampling almost anywhere can turn up a previously unknown species. (A group of students in my undergraduate parasitology class found one in the local shark they were assigned to dissect.) Science Daily has a "New Species News" page with information on the latest discoveries.

http://www.sciencedaily.com/news/plants_animals/new_species/

The basic definition of a species is a group of organisms which interbreed and produce fertile offspring in the wild. However, this definition is somewhat vague and does not entirely take into account modern genetics. If a species becomes split into two isolated populations by some natural disaster, when do the separate groups become separate species? As soon as the natural barrier forms, the groups are unable to interbreed in the wild, but they may be genetically identical until differing environmental factors have selected for different traits in the two groups. The Wikipedia article actually gives a pretty good discussion of the "species problem."

http://en.wikipedia.org/wiki/Species
http://en.wikipedia.org/wiki/Species_problem

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