our perspective on species changes if we shift our attention from one locality to a wider spatial scale -- from individual variation to geographic variation

variation among individuals within one population -- if we study one population (all individuals of one species breeding in one area, for instance, all Hairy Woodpeckers breeding in Orange County), we find that individuals differ in alleles, morphology, and behavior -- usually these differences are small

geographic variation among populations -- if we compare populations over larger areas (such as all of North America), we find that these populations differ in allele frequencies and in averages and ranges of morphological, behavioral, and ecological measurements -- sometimes these differences are large

for instance, the average size of Hairy Woodpeckers increases the farther north we go -- populations along the Gulf Coast do not overlap in size with those in Canada and Alaska -- are they still the same species?

populations of Hairy Woodpeckers on Newfoundland have much less white on their wings and back than those on the nearby mainland -- there is no overlap in coloration -- are they still the same species?

those in the Bahamas and in the mountains of the west (southward into Central America) have much less white on their wings -- in the Bahamas and Central America they are brownish below

populations of woodpeckers closely related to the Red-bellied Woodpecker also vary slightly in size and coloration in different places throughout the southwestern United States and Mexico and on several Caribbean islands

ornithologists have so far agreed that all populations of Hairy Woodpeckers belong to one species -- in contrast, many populations of "red-bellied woodpeckers" are assigned to different species (Golden-fronted, Gila, Cuban Woodpeckers ... and several other species) -- "lumpers" and "splitters" disagree about whether some populations belong to different species or not

allopatric populations (populations occurring in different areas) -- unlike sympatric populations -- are often not easily classified into species

depending on their objectives, systematists (biologists who study the phylogeny of organisms) use different criteria to distinguish species -- and disagree about which criteria are best

some systematists focus on distinct differences between populations -- distinctly different populations are called phylogenetic species

criterion for recognizing phylogenetic species (we need just one!) . . .

(1) populations belong to separate species if they differ distinctly (with almost no overlap and no intergradation (intermediates) in gene frequencies or morphology

advantages
(1) decisions do not require field studies of possible interbreeding where populations meet
(2) same criteria apply to sympatric populations, geographically contiguous allopatric populations (also called parapatric populations), and geographically disjunct allopatric populations

disadvantages
(1) some populations that appear to be different phylogenetic lineages could in fact merge in the future
(2) unless all morphological or genetic differences contribute equally to distinguishing species, we must make judgments about which differences count more than others
(3) we must make an arbitrary decision about how much difference is necessary to distinguish species (molecular analyses often reveal complex variation among populations with virtually indistinguishable morphologies)

some systematists focus instead on the genetic boundaries between populations -- any populations separated by biological limitations to gene exchange are called biological species . . .

criteria for recognizing biological species (we need three different ones!)

(1) sympatric populations belong to different species if they do not interbreed -- absence of interbreeding is called reproductive isolation -- reproductive isolation can occur either because matings do not produce surviving and reproducing progeny (genetic/developmental or postzygotic reproductive isolation) or because individuals do not recognize each other as mates (behavioral or prezygotic reproductive isolation)
(2) contiguous allopatric populations (parapatric) belong to different species if they do not interbreed where they meet -- they belong to the same species if they are connected by gradual intergradation (often called a cline) in their gene frequencies or morphology -- a cline indicates interbreeding between parapatric populations
(3) disjunct allopatric populations belong to different species if their morphological or genetic differences are greater than corresponding differences between populations of related sympatric species or between populations connected by a cline

advantages
(1) clear connections with studies of the biological mechanisms that can produce and maintain genetic differences between populations -- as studied in the fields of ecology, population genetics, and animal behavior

disadvantages
(1) some populations united in one species have already started to separate phylogenetically;
(2) geographic constraints to gene flow are just as effective as biological ones;
(3) decisions often require detailed investigations of zones of contact between populations;
(4) decisions about disjunct allopatric populations still require arbitrary judgements -- how much difference is enough?
(5) decisions about contiguous allopatric (parapatric) populations that hybridize where they meet also require arbitrary judgements -- how much interbreeding (hybridization) is allowable?

when does it make a difference which species concept you use?

for populations that live in disjunct geographical areas and have distinct (even slight) differences -- proponents of phylogenetic species usually call them two separate species -- proponents of biological species often call them separate subspecies of one species (distinct differences but no evidence of reproductive isolation)

populations that form a cline (intergrade continuously) are placed in one species by both phylogenetic and biological species concepts (no distinct differences and no reproductive isolation)

in conclusion, it is important to realize that . . .