Encyclopedia of Evolution.pdf - Online Reading Center

speciation
Ensatina eschscholtzii is an amphibian that forms a ring species. Each
subspecies overlaps with other subspecies, but the subspecies at the
southern ends of the distribution are unable to interbreed with one another.
(Klauberi and Croceater)
can raise many generations of fruit flies under laboratory conditions.
Beginning with a single population of flies, they separated
them into two populations, one kept in warm conditions,
the other kept in cool conditions. After many generations, the
two populations of flies had diverged enough that they did not
interbreed when they were placed back together. This may have
occurred partly from the gradual accumulation of genetic differences
but may also have involved symbiogenesis, since one of
the populations lost microbes that had lived in the digestive system
of its ancestors. In another fruit fly experiment, researchers
kept some flies in the dark and others in the light. Not surprisingly,
the flies in the dark lost the genetic ability to use visual
cues for mating; they evolved mating cues based upon touch
and vibration. The visual flies could not interbreed with the tactile
flies.
Reconstructing speciation in the wild. In natural populations
where speciation may be impossible to observe in
action, researchers have frequently found convincing evidence
regarding how the speciation occurred. Researchers Douglas
Schemske and H. D. Bradshaw have found the traits that
were almost certainly responsible for the evolution of two
species of Mimulus from a single ancestor, and Loren Rieseberg
has identified the two parental species of sunflowers
whose chromosomes have come together to form the hybrid
species Helianthus paradoxa. Peter and Rosemary Grant have
carefully measured divergence in beak size in two groups of
Darwin’s finches which, if it continues for a long time, may
result in the formation of new species.
Speciation occurring in the wild. In some cases, two
incipient species (not quite yet differentiated enough to be
considered separate species) can exist in adjacent habitats.
If there is a series of such species, each one grading into the
next, it is possible that the species at the two ends of the
series cannot interbreed. This would constitute a ring species.
Examples include gulls of the genus Larus across the northern
continents, salamanders of the genus Ensatina in the mountains
of California (see figure), and the greenish warbler Phylloscopus
trochiloides around the Himalayas.
Evolutionary scientists have found numerous examples
of speciation that is occurring in wild populations (see
table). Evolution has produced far more species than any
human observer would consider necessary. For example,
there are well over 350,000 species of beetles. If evolution
produced species only in response to environmental conditions,
there would be far fewer species of organisms in the
world: a few species of animals and plants best suited to
moist, cool conditions, a few other species best suited to hot,
dry conditions, etc. Species frequently evolve in response to
one another (see coevolution) and as a result of the population
fragmentation that results from geographic and reproductive
isolation.
A Few Examples of Speciation
Organism Divergence that is occurring
Flowering plants Coevolution with pollinators
Hawaiian plants Coevolution with birds for seed
dispersal
Fireweeds Polyploidy
Soapberry bugs Different host plants
Goldenrod gall insects Different host plants
Heliconius butterflies Different warning colorations
Field crickets Male courtship song
Stickleback fish Bottom-dwelling vs. open-waterdwelling
populations
Sockeye salmon Populations that return from the sea
v. remained in lakes
Blue tits Populations that breed in evergreen
v. deciduous oaks
Bowerbirds Male courtship patterns