Speciation is the
evolutionary process by which new biological
species arise
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 mainland) population.
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 genetic differentiation.
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 speciation)
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 settled.
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).
