What is a species? Its not as straightforward a question as most believe. Speciation Evolution creates (and destroys) new species, but … These are members of different species - eastern (left) and western (right) meadowlark.

What is a Species? There is only one existing human species.

What is a Species? And these are all members of a single species.

Determining What Is and What Isn’t a Distinct Species Can Have Economic Consequences Northern spotted owl (left) and barred owl (right).

What is a Species? The definition we’ll use is this: A species is a group of individuals capable of interbreeding to produce fertile offspring in their natural environment. This is the biological species concept. Like all attempts to define a species, it has many problems.

How Many Species Are There? We don’t know. About 2 million species have been described. Estimates of existing species number range from 4 million to 100 million (with million being a more commonly considered upper estimate).

Two Patterns of Speciation

How Do Species Arise? Geographic Isolation The key to speciation is reproductive isolation of populations.

Two species of ground squirrel are postulated to have descended from a common ancestral population that was separated by formation of the Grand Canyon. Harris’ antelope squirrel White-tailed antelope squirrel

(different habits within an overlapping range)

Courtship rituals, like these, are critical for mating within a species, but ineffective for attracting members of other species.

Speciation Occurs at Widely Differing Rates A slow rate of speciation evidenced by a living horseshoe crab (13 extant species) and a 300 million year-old fossil species A rapid rate of speciation evidenced by Galapagos finches which have diversified into 13 species within the last 100,000 years.

Speciation Rates Generalists, like the horseshoe crab, tend to remain as stable species. Specialists, like the Galapagos finch, tend to be unstable as species. Speciation also becomes rapid when, as occurred with Galapagos finches, new niches become available.

Species Come and Go Best estimates from the fossil record indicate that greater than 99% of species that have existed are now extinct. A typical “lifetime” for a species is about 1 million years.

Gary Larson The Cretaceous/Tertiary Mass Extinction