Charles Darwin showed that extinction of a species is part of the evolutionary process. There is no reason at all to be concerned about the disappearance of a species. In fact, a steady rate of extinction is a normal process in the course of evolution, and is called the background rate of extinction. Species have slowly evolved and disappeared throughout geologic time because of climatic changes and the inability to adapt to survive competition and predation.
But people who make this argument have overlooked one very important fact: since the 1600s, humans have rapidly accelerated the rate of extinction because of population growth and resource consumption. Today, most of the world's habitats are changing faster than most species can evolve, or adapt to such changes. The current global extinction rate is estimated at about 20,000 species per year, exponentially greater than the background extinction rate. Many biologists believe that we are in the middle of the greatest mass extinction episode since the disappearance of the dinosaurs 65 million years ago.
From what is known about present-day populations and from evolutionary theory, the change in either the physical or the biological environment is the key to extinction. But the vulnerability of a species depends on a wide variety of factors, such as its total population size, geographical distribution, reproductive ability, ecological relations with other species, and genetic characteristics. For example, more emphasis is put on the greater vulnerability of species that reproduce slowly as contrasted with those that reproduce rapidly. Other factors, such as food-plant specialization, may make many fast reproducers more vulnerable than species that reproduce more slowly. No matter how fast an insect species that depends on a certain plan can reproduce, it will still go extinct if that plant's habitat is destroyed.
It has been estimated that about one half billion species have lived at one time or another, and today's existing species are only 2% of those that have ever evolved. The other 98% have either died out or evolved into something sufficiently different to be called a new species.
 Basic Causes
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