By definition, a biological species is a classification of organisms which interbreed.
The rate of propagation of a biological species, or of a lineage within a species, is determined by the periods of gestation and intergestation for that species, and is proportional to the size of the organism. The smaller the organism, the shorter the period of gestation, and the larger the organism, the longer the gestation period.
There are various theories which describe the average rate at which a biological species evolves. One theory states that the rate of evolution of a species is more or less constant, that selection acts to maintain its characteristics rather than to change them. When a change in the environment occurs, such as an ice age, or the appearance of a new predator, this imposes new selective pressures, and the species responds by a burst of evolution. This is possible without having to wait for new mutations because the species will have generated a large amount of genetic variation while it was at rest.
Another view states that when a single species in an ecosystem makes an advance, other species in the system will experience it as a deterioration, which in their turn will evolve causing deterioration for still other species in the system. The result is that every species will be evolving as rapidly as possible in order to keep up with the others.
The most current theory suggests that the evolution of a species is determined by a combination of causal and random processes, in which many groups, each with a few members, is established within a short period of experimentation, followed by a much longer period of stabilization in which a gradual selection process or general reduction of variation occurs, including the elimination of extremes. This results in the stabilization of a few select patterns from which many slight variations occur.
It has been predicted that a biological species will either survive or die out within the time required for the number of generations to equal the number of individuals in the population.
Difficulty in verifying the rate at which biological species evolve is due to the number of individuals that must be accounted for, and the long observational times necessary to complete a measurement.
Overall, the natural rate of extinction of biological species is about 1 every 13 months.