Loose, practical definition: A group of individuals that can be recognised on the basis of shared morphological character traits and that resemble each other more than that they resemble inividuals with similar character traits.
One biological definition: A group of actually or potentially interbreeding populations that are reproductively isolated from other such groups (Ernst Mayr, 1963).
There are many definitions of species, many relating to the field they are applied to (identification, ecology, phylogeny, evolution, etc.). Many of the different definitions would give different results as to the species status of some populations of organisms: a population might be considered a species in the view of a evolutionary species concept but not in the view of a biological species concept. In addition it should be noted that a definition applied to one groups of organisms might not be applicabale to another group of organisms; for example groups of different evolotionary origin (bacteria, plants, vertebrates, arthropods, etc.) or groups wiht different life histories (sexually interbreeding, parthenogenetic, etc.).
I can no longer send private messages (and maybe not receive?), even though my inbox and outbox are not yet full. I write, click "send" and they disappear and never show up in the outbox.
Tried to attach an image to a forum post. jpg, 32kB, 72dpi, no blanks, ... File name is correctly displayed, but when I click "Preview Thread" it just vanishes. Help!