= Solution
The \b[<biological species concept>] groups natural populations that actually or potentially interbreed and are reproductively isolated from other such groups. Its central criterion is <reproductive isolation> and <gene flow>, rather than visible similarity. It is hard to apply directly to <fossils>, organisms reproducing strictly by <asexual reproduction>, and geographically separated populations whose potential interbreeding cannot be tested.
The \b[<phenetic species concept>] groups organisms by overall similarity in measured <phenotypic traits>. A practical <morphological species concept> uses anatomical form as its main evidence; strict phenetic classification can combine many kinds of measured traits. This makes such approaches usable for preserved specimens and much of the <fossil record>, but the decision depends on which traits are measured and how similarities are weighted.
At least four difficulties arise. <Cryptic species> may be reproductively isolated while looking nearly identical, so similarity can lump separate lineages. Conversely, <sexual dimorphism>, juvenile/adult differences and <phenotypic plasticity> can make one interbreeding <species> look like several, causing over-splitting. <Convergent evolution> can make distantly related organisms resemble one another in ecologically selected traits. Finally, continuous geographic or temporal variation supplies no unique gap at which to place a boundary: character choice, sample size, preservation and similarity thresholds can alter the inferred <species delimitation>. These are distinct from the biological concept's practical difficulty of testing actual interbreeding.
Back to article page
