Summary
A basic unit of classification; organisms that can interbreed and produce fertile offspring.
In depth
Concepts: biological, morphological, phylogenetic, ecological. Problems: hybrids, ring species (such as Ensatina salamanders), asexual organisms and fossils.
The biological species concept fails for asexual organisms. Species are lineages evolving separately. Taxonomists use several lines of evidence.
Example: ring species show gradual divergence around a barrier.
There is no single definition of species. The biological species concept (Ernst Mayr) defines a species as a group of populations that interbreed and are reproductively isolated from other groups; it fails for asexual organisms, fossils and hybridising species.
Other concepts include morphological (based on shape), phylogenetic (smallest diagnosable clade), ecological (occupying a niche) and genetic cluster concepts. Reproductive isolation may be prezygotic (habitat, timing, behaviour, mechanical, gametic) or postzygotic (hybrid inviability or sterility, as in a mule). Speciation is a continuum, so boundaries are often fuzzy.
Example: horses and donkeys can mate, but their offspring, mules, are sterile.
Common mistake: assuming the ability to interbreed is the only test.
Frequently asked questions
What is Species?
A basic unit of classification; organisms that can interbreed and produce fertile offspring.
Which area of biology covers Species, and how advanced is it?
Species is one of the 52 topics in Taxonomy & Biodiversity. It is rated Beginner, so it suits learners who are starting out in biology. Related topics in this area include Species Delimitation, Hybridisation and Species Boundaries, Conservation Biology.
Can you give an example of Species?
Lions and tigers can produce hybrids (ligers) that are typically sterile in males.
What should I learn before Species?
Species is a Beginner topic, so no earlier topic is required. Other Beginner topics in Taxonomy & Biodiversity that pair well with it include Taxonomy, Binomial Nomenclature, Taxonomic Hierarchy.
What topics are related to Species?
The closest topics to Species are Species Delimitation, Hybridisation and Species Boundaries, Conservation Biology, Phylogenetic Classification. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Species?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Species. To practise the whole of Taxonomy & Biodiversity, use the Taxonomy & Biodiversity quiz.
How is Species different from Species Delimitation?
Species: A basic unit of classification; organisms that can interbreed and produce fertile offspring. Species Delimitation: Methods integrating morphology, genetics and ecology to decide where species boundaries lie. Read both side by side and note what each one explains.
What is a common mistake about Species?
Assuming the ability to interbreed is the only test.
How should I study Species step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Species Delimitation and explain the difference aloud. Finally, take the Taxonomy & Biodiversity quiz and come back to this page for any question you miss.