Life on Earth has existed for something over 3 billion years, and the current number of species (not counting all of those which have become extinct) is roughly 50 million.

http://hypertextbook.com/facts/2003/FelixNisimov.shtml

Whether or not a given species gives rise to a new species depends on its environment. A species that is ideally adapted to a stable environment is unlikely to change much, since any genetic change is far more likely to reduce its fitness for that environment than improve it. On the other hand, a changing environment results in changing selection pressures; different traits may become more advantageous, and the genome of the species changes as a result. Over time, the cumulative effect of these changes is enough to constitute a new species.

The process of evolution involves several interrelated mechanisms, and the concept has been developed well beyond what Darwin was capable of given the limited knowledge of biology at the time. Darwin's original theory was based on observed species variation, and subsequent research has demonstrated the genetic and environmental links between species variation and species differentiation.

http://www.talkorigins.org/faqs/faq-speciation.html

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