Definition of a Species

The classic definition of a species is based on reproductive isolation. There are many reasons for reproductive isolation, but fundamental to this concept is that NO viable, fertile offspring are produced even if two species could physically mate.

Definitions are often general and are not definitive. A problem with the definition of a species noted above is that isolation does not necessarily mean that "different" organisms can't interbreed.

Think about lions and tigers. One is found in Africa and the other is found in Asia. You would think that they are separate species - they look different and they are definitely geographically isolated. However, they do mate in zoos and produce offspring. The progeny are called tions or ligers depending on who the father is.

• Do you think that tigers and lions different species by the above definition?

There are basically two major classes of reproductive isolation. These include geographic prezygotic and postzygotic isolation.

• What is the difference between prezygotic and postzygotic isolation?
• There are two basic modes of speciation: allopatric and sympatric. Describe each mode of speciation and find an example of a plant or animal that exhibits each mode of speciation.
• There are two examples in plants of new species arising because of polyploidy. What are they?
• What does nondisjunction have to do with polyploidy?
• Why do you think that polyploidy is not typically found in animals?
• Can you identify the one species of animal that is triploidy (3N)?

Reproductive isolation can occur in many different ways.

• Which reason from the list below may explain why a cheetah is a separate species?
  • Prezygotic isolation or not being able to form a fertilized egg with another type of cat
  • Postzygotic isolation where a zygote is formed when two cats breed, but the zygote does not survive.
  • Geographic isolation where the cats can't meet to breed.