Species differences

Species differences are the anatomical, physiological or behavioral differences that are used to differentiate one species from another.

Difficulty of defining "species" and identifying particular species
It is surprisingly difficult to define the word "species" in a way that applies to all naturally occurring organisms, and the debate among biologists about how to define "species" and how to identify actual species is called the species problem.

Most textbooks follow Ernst Mayr's definition of a species as all the individual organisms of a natural population that generally interbreed at maturity in the wild and whose interbreeding produces fertile offspring.

Various parts of this definition are there to exclude some unusual or artificial matings:
 * Those which occur only in captivity (when the animal's normal mating partners may not be available) or as a result of deliberate human action.
 * Animals which may be physically and physiologically capable of mating but do not normally do so in the wild, for whatever reason.
 * Animals whose offspring are normally sterile. For example, mules and hinnies have rarely produced further offspring (only one documented case for hinnies, and seven for mules) when mated with a creature of the same type (a mule with a mule, or a hinny with a hinny).

The typical textbook definition above works well for most multi-celled organisms, but there are several types of situations in which it breaks down:
 * By definition it applies only to organisms that reproduce sexually. So it does not work for asexually reproducing single-celled organisms and for the relatively few parthenogenetic multi-celled organisms. The term "phylotype" is often applied to such organisms.
 * Biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding.
 * There is considerable variation in the degree to which hybridization may succeed under natural and experimental conditions, or even in the degree to which some organisms use sexual reproduction between individuals to breed. Some hybrids, e.g., mules, hinnies, ligers and tigons, apparently cannot produce offspring when mated with one of their own kind (e.g. a mule with a mule), but sometimes do produce offspring when mated with members of one of the parent species (e.g. a liger with a lion). Usually in such hybrids the males are sterile, so one could improve the basic textbook definition by changing "... whose interbreeding produces fertile offspring" to "... whose interbreeding produces offspring in which both sexes are normally fertile".
 * In ring species, members of adjacent populations interbreed successfully but members of widely-separated populations do not.
 * In a few cases it may be physically impossible for animals that are members of the same species to mate. For example, a Great Dane and a Chihuahua are both dogs and therefore members of the same species, but cannot mate because of the great difference in size and weight (physical build).

Horizontal gene transfer makes it even more difficult to define the word "species". There is strong evidence of horizontal gene transfer between very dissimilar groups of procaryotes, and possibly between dissimilar groups of single-celled eucaryotes; and Williamson argues that there is evidence for it in some crustaceans and echinoderms. All definitions of the word "species" assume that an organism gets all its genes from one or two parents which are very like that organism, but horizontal gene transfer makes that assumption false.