CASE HISTORIES OF SPECIATION I&II
Some of the best examples of speciation are examples of diversification
on archipelagos. These provide clear contexts of allopatry and hence
provide the extrinsic barrier to gene exchange from the source (usually
The most famous are the Galapagos Islands. The islands are young
(some ~ 1 million years), have a volcanic origin providing an opportunity
for new arrivals to "radiate" into open niches and the islands
are quite distant from the mainland. This isolation and context
of primary succession (e.g., development of a flora and fauna on
a "clean slate") will allow for a random element in community
composition. Irrespective of genetic consequences of the founding event,
subsequent evolution of species quite likely will be under dramatically
different selective regime than those in the source population.
Darwin's Finches. Morphological and genetic studies indicate
that they are derived from single ancestral finch, i.e., are monophyletic.
There has been dramatic specialization in ecological roles, each species
having distinct morphologies and associated food items (beak size and shape
associated with seed size, grub feeding, tool use, etc.). Classic examples
of different distributions of beak depths: difference between means is
greater between species when they occur on the same island than when they
occur alone on different islands (see figure below).
Often cited as a clear indication that competition played a role
in the adaptive radiation of the finches. There are obvious alternative
hypotheses to explain these patterns: populations on different island differ
by these amounts as a consequence of drift; different islands have different
plants, insects (food items in general) thus the differences are a result
of food, not competitors). As P.R. Grant concludes in Ecology and Evolution
of Darwin's Finches, Princeton Univ. Press, 1985, patterns of differentiation
and speciation are a combined effect of adaptation to different flora/food
and adaptive responses to competitors. The issue of different plants/food
on different islands just shifts the question to another trophic level:
how did the different islands come to be different in these species.
Another evolutionary paradigm: the Hawaiian islands. Again the
islands are young (< 5 million years old), have a volcanic origin and
an interesting one: convection currents in the earth's mantle generate
a "hot spot" where volcanic activity occurs above. The pacific
plate moves northwest over this spot so the islands' geographical location
is related to their age (Kauai in the north west is ~ 5 million
years old; Hawaii [the big island] in the southeast is ~ 500,000 years
old and still active).
Hawaiian Drosophila show remarkable patterns of colonization and speciation.
At least 700 species of Drosophilids on Hawaiian islands. Not just
typical little fruit flies either: large body size, dramatic "picture
wing" species, some with "hammer-head" shaped heads. Banding
patterns of polytene chromosomes allows phylogeny reconstruction: these
and other data show that patterns of colonization are from older to younger
islands (flies on Hawaii are derived from ancestors on Maui). Most species
are found only on one island (high levels of endemism; more
later in Biogeography). This implies that most new colonization events
have lead to speciation events! This observation lead Hampton Carson
to propose the founder-flush model of speciation.
African cichlid fishes are another remarkable case of "explosive
speciation" (the Hawaiian Drosophila of the fish world). Geology and
geography again plays an important role. African rift lakes: great fresh-water
lakes in east Africa. Formed recently: < 1 million years old. Lake Victoria
colonized by one (??) founder 200,000 years ago(??) now has ~ 200 species
of fish!. Recent study (Meyer et al. 1990, Nature vol. 347, pg.
550 and see pg. 512) used mitochondrial DNA to show that the species in
the lake are indeed monophyletic and that there is very little sequence
divergence between species: confirms short time span. But there
has been remarkable evolution of morphological, ecological and behavioral
variation in these fish: algae grazers, snail crushers, plankton
feeders, paedophages (clamp onto the mouth of a fish brooding her
young in her mouth and force her to spit out here young into the mouth
of the attacker), one fish (in Lake Malawi) plucks the eyes out of other
fish as food. All this diversity in 200,000 years with very little
Another set of important examples of speciation are those that are believed
to have speciated as a result of isolation in Pleistocene refugia.
Glacial advances and retreats during the Pleistocene epoch acted as vicariance
events in areas where glaciers were present (Wisconsin ice sheet).
Dramatic evidence of this is in the North American bird fauna and the clear
faunal break between the east and west, e.g., wood warblers; Peterson's
field guides have an Eastern and Western edition).
Climatic changes associated with the glacial advances and retreats altered
habitats in the tropics resulting in "islands" of habitat that
fluctuated in size and geographic location, leading to fragmentation
of distributions and contribution to speciation. Believed to one explanation
for patterns of speciation in the Amazon. Also a possible explanation
for the Larus ring species complex: genus Larus (seagulls)
fragmented in Siberia during the Pleistocene. Diverged populations of Larus
argentatus (herring gull) colonized eastern Siberia, across the Bering
straits, across North America, Iceland and back to Northern Europe becoming
increasingly diverged at each step. Hybrid zones exist between successive
populations but the ends of the ring are reproductively isolated
implying that speciation has gone to completion (an example of geographic
There have been some controversial examples of sympatric speciation
documented in the literature. The apple maggot fly (Rhagoletis pomonela)
mates and lays eggs on a specific host, originally Hawthorn. In 1864 Rhagoletis
was found on apple trees that had been introduced to regions where
hawthorn grew. In early 1960's Rhagoletis was found on cherry. This
host race formation has been argued as an incipient stage of sympatric
speciation. Advantage of this model is that the temporal framework is reasonably
well documented and the species in question is an agricultural pest
so it is likely that it will receive further study and the issue can be
Model invoking a survival locus (S) and a host selection locus
(H) with each with new mutant alleles that shift survival and selection
to the new host (e.g., apple from Hawthorn).
Allochronic speciation was proposed as a model where species
differentiated in time. Crickets of the genus Gryllus were taken
as an example because species with virtually identical songs and morphology
had evolved as spring adults versus fall adults (overwinter as juveniles
or eggs, respectively). The model may apply but this particular example
was shot down by phylogenetic analysis which showed that the two "allochronic
species" (Gryllus veletis and Gryllus pennsylvanicus
are actually distantly related in the genus (see figures).