How to define species in order to recog­nise the bound­ary between two species? Biol­o­gists have been debat­ing the best def­i­n­i­tion for a very long time.

The Bio­log­i­cal Species Con­cept (BSC), pro­posed by the evo­lu­tion­ary biol­o­gist Ernst Mayr in the mid-​20^th cen­tury, is per­haps the most widely accepted species con­cept (see Ency­clo­pe­dia of Life). Accord­ing the BSC species are groups of actu­ally or poten­tially inter­breed­ing nat­ural pop­u­la­tions which are repro­duc­tively iso­lated from other such groups. This includes the pos­si­bil­ity that organ­isms may appear to be alike and be dif­fer­ent species or organ­isms may look dif­fer­ent and yet be the same species (see Under­stand­ing Evo­lu­tion). This def­i­n­i­tion of a species as a group of inter­breed­ing indi­vid­u­als can­not, how­ever, be eas­ily applied to organ­isms that repro­duce only or mainly asexually.

Within the BSC, a species rep­re­sents a set of indi­vid­u­als con­nected by gene exchange (“gene flow”) that is genet­i­cally iso­lated from all other such sets of indi­vid­u­als. There is gene flow among indi­vid­u­als within a species, but not between dif­fer­ent species. This lack of gene exchange means that dif­fer­ent species can evolve independently.

Other approaches to define species focus on lin­eages in branch­ing evo­lu­tion­ary trees, but besides that the two per­spec­tives on the spe­ci­a­tion process (the ori­gin of new species) are not mutu­ally exclu­sive, they also not explain all issues that appear when try­ing to iden­tify all dif­fer­ent species in nature using a uni­ver­sal definition.

There­fore most biol­o­gists today are more inter­ested in under­stand­ing the process of spe­ci­a­tion than in try­ing to find a strict species def­i­n­i­tion that is always applic­a­ble. Spe­ci­a­tion is usu­ally a grad­ual process, so it is not unusual to encounter pop­u­la­tions that are only partly repro­duc­tively iso­lated. This means that indi­vid­u­als from diverged lin­eages may still exchange genes to a lim­ited degree, per­haps even to the extent that they will merge again. These sit­u­a­tions are chal­leng­ing for both the bio­log­i­cal species con­cepts and lin­eage species con­cepts. Although some peo­ple may wish for a black-​and-​white cri­te­rion for defin­ing species, this is unrealistic.

Recently, in March 2017, a novel intrigu­ing and con­tro­ver­sial idea was pub­lished: the mitonu­clear com­pat­i­bil­ity species con­cept, which is based on the fact that the mito­chon­dr­ial geno­type tends to be very good at show­ing species bound­aries between birds.

Appear­ances can help and deceive
Although, fig­ur­ing out the rela­tion­ships among species is much more com­plex than sim­ply group­ing by sim­i­lar­ity in appearance:

For exam­ple, two organ­isms that are not closely related (such as a cac­tus and a euphorb) may resem­ble each other because they have evolved sim­i­lar adap­ta­tions for sim­i­lar envi­ron­men­tal con­di­tions (this phe­nom­e­non is known as ‘con­ver­gent evo­lu­tion’). Alter­na­tively, two indi­vid­u­als that appear mor­pho­log­i­cally iden­ti­cal may nev­er­the­less belong to dis­tinct repro­duc­tively iso­lated species — with repro­duc­tive iso­la­tion based, for exam­ple, on dif­fer­ent sex pheromones, songs or mat­ing behav­iours (dis­tinct species that are mor­pho­log­i­cally indis­tin­guish­able, or nearly so, to human sci­en­tists are known as “cryp­tic species”).

The East­ern mead­owlark (left) and the West­ern mead­owlark (right) appear to be iden­ti­cal, and their ranges over­lap, but their dis­tinct songs pre­vent interbreeding.

Fur­ther­more, many plants, and some ani­mals, form hybrids in nature. Hooded crows and car­rion crows look dif­fer­ent, and largely mate within their own groups — but in some areas, they hybridize. Should they be con­sid­ered the same species or sep­a­rate species?

In prac­tice care­ful analy­sis of phys­i­cal char­ac­ter­is­tics has been suc­cess­fully used to infer which groups are prob­a­bly repro­duc­tively iso­lated and rep­re­sent inde­pen­dently evolv­ing lin­eages, lead­ing to field guides. So, with­out actual knowl­edge of gene exchange and genetic dif­fer­ences an edu­cated guess could be quite accu­rate. But when, in recent decades, DNA sequence data became avail­able a more pre­cise assess­ment could be made of the evi­dence of gene exchange in recent gen­er­a­tions and esti­mat­ing the evo­lu­tion­ary rela­tion­ships among liv­ing organisms.

(Source: web­site Under­stand­ing Evo­lu­tion; web­site Ency­clo­pe­dia of Life; Wikipedia; naturesongs​.com/​b​i​r​d​s)