Difficulties with classification
There are multiple challenges when it comes to accurately classifying organisms into the hierarchy of taxa described above; such difficulties include:
Morphology
Taxonomic rank
- In the hierarchy system described above, each level of classification fits into an established taxonomic rank, i.e. kingdom, phylum, class etc.
- Classification can be complicated if a group of organisms falls across taxa, or needs to be moved from one taxon to another
Species
- The point at which two populations are classified as different species can be highly subjective
- The fertile offspring of a cross between two species may then go on to breed only with members of one of its parent species; this is known as introgression
- Introgression demonstrates how difficult it can sometimes be to neatly apply species classification; the resulting offspring after several generations do not fit completely into either species, but neither does it seem to make sense to classify them as a new species
- E.g. hundreds of thousands of years ago an early human bred with a Neanderthal and the offspring of this cross then went on to breed only with early humans; the result of this is that some groups of modern humans have some Neanderthal genes in their genomes
Introgression diagram
Several generations after an interbreeding event has occurred, the species X individuals with some genes from species Y do not fit perfectly into either species X or Y, but cannot correctly be considered a new species; this is introgression