3 The New York Years (1931–1953)

A Modern Unified Theory of Evolution 217
established by the naturalists (systematists), but it was Mayr’s clear and synthesizing
discussion which convinced the geneticists and zoologists generally of the
biological species concept and of the common occurrence of allopatric speciation:
Either a previously continuous species range is split into two or more parts
(through sea-level changes or climatic-vegetational changes) or a small isolated
founder population is established by dispersal of a few individuals of a parental
species across a barrier. Each isolated population then evolves independently,
gradually diverging from one another. When sufficient genetic differences have
accumulated, the parental and daughter populations may come in contact without
hybridizing–a new (daughter) species has originated. Despite the striking gaps
between the species of a local flora and fauna, the gradual evolution of new species
was no longer a puzzle.
There is no case in birds or mammals that would require sympatric speciation
(without geographical separation of populations) and this may also be true for
many, perhaps the majority of insect families (butterflies, Carabidae, Tenebrionidae,
etc.). To appreciate the effectiveness of allopatric speciation one only needs
to compare the high number of endemic species in island archipelagoes with the
low number of such species in a continental region of comparable size.
Mayr summarized the various stages of the differentiation process during allopatric
speciation in his “dumbbell” model (1940c: 274–275, 1942e: 160) as follows
[with some additions]; see Fig. 5.5:
Stage 1: A uniform species with a large range; followed by
Process 1: Differentiation into subspecies; resulting in
Stage 2: A geographically variable species with a more or less continuous array of
similarsubspecies(2aallsubspeciesareslight,2bsomearepronounced);
followed by
Process 2: (a) Isolating action of geographic barriers between some of the populations;
also (b) development of isolating mechanisms in the isolated
and differentiating subspecies; resulting in
Stage 3: A geographically variable species with many subspecies completely isolated
and some of them morphologically as different as good species
[note that Stage 3 does not require Process 2b, only 2a]; followed by
Process 3 (often in connection with 2): Development of intrinsic isolating mechanisms
for genetical isolation, as well as the development of reproductive
isolation and ecological differentiation, resulting in
Stage 4: Expansion of range of such isolated populations into the territory of the
representative forms; resulting in either
Stage 5a: Noncrossing, that is, new species with restricted range or
Stage 5b: Interbreeding, that is, the establishment of a hybrid zone (zone of secondary
intergradation).
During the course of time, isolated populations differentiate as members of
the following microtaxonomic categories: local population-subspecies-speciessuperspecies-species
group with numerous cases of intermediate differentiation