Re: [asa] (fruit flies???) A Message from the RTB Scholar Team (fwd)

From: Keith Miller <>
Date: Sat Apr 19 2008 - 21:50:52 EDT

With regard to speciation, I wrote the following several years ago.
It is somewhat out of date, but provides some references to
speciation documented in the field. Speciation by several different
mechanisms is well documented.



First, a definition of "species" -- A species is a population of
interbreeding individuals that are reproductively isolated from other
such populations under natural conditions. This reproductive
isolation can occur in a variety of ways. It is often behavioral in
that while individuals from different species are genetically capable
of interbreeding they rarely if ever do because of behavioral
differences. Isolation may also be genetic, where the DNA of the two
species are incompatible preventing successful fertilization or
development. Anatomical differences may also preclude interbreeding.
Another cause is simple geographic or ecological separation.

Speciation is the formation of a new reproductively (or genetically)
isolated population. The formation of new species has been
documented both in nature and produced through human selective
breeding. Plants in particular speciate quite easily. One of the
more common ways is by a process called polyploidy in which
chromosomes are duplicated. This results in the polyploid forms
being genetically incompatible with their parent populations.
Because polyploids have duplicate sets of genes the genome becomes
more "plastic" and can more easily accumulate mutations. Polyploidy
is accompanied by accelerated evolution. This has been documented in
numerous angiosperm plants. Polyploidy also occurs in animals, but
much more rarely.

Observing a complete speciation process for animal species in the
wild is much more difficult than with plants. However, the
geographic distribution of animal populations and a knowledge of
their genetics provides crucial information. Scientists have
observed populations of individuals ranging from those completely
reproductively and genetically isolated, to those with distinct
traits but continued gene flow between them. A complete gradation
from intraspecies variation to distinct species is thus observed,
giving "snap shot" views of the process of speciation. Another
interesting observation is the occurrence of species "clines" in
which the features of a widely distributed species change
substantially over its geographic range. In some cases populations of
the same species occupying opposite ends of its range will be
reproductively isolated when they come into contact with each other.
These are called "ring species."


Soltis, D.E. and Soltis, P.S., 1999, Polyploidy: recurrent formation
and genome evolution: Trends in Ecology and Evolution, v. 14, p.
This paper lists 45 species of angiosperms and ferns, as well as
ostracode and tree frog species, for which speciation by polyploidy
has been documented. Multiple episodes of speciation of goatsbeard
(a flowering prairie plant) have occurred within the past 60-70 years.

Petit, C., Bretagnolle, F., and Felber, F., 1999, Evolutionary
consequences of diploid-polyploid hybrid zones in wild species,
Trends in Ecology and Evolution, v. 14, p. 306-311.
This paper discusses the origin of reproductive isolation in hybrid
zones in 9 plant species and 3 animal species.

Orr, M.R. and Smith, T.B., 1998, Ecology and speciation: Trends in
Ecology and Evolution, v. 13, p. 502-506.
Thompson, J.N., 1998, Rapid evolution as an ecological process:
Trends in Ecology and Evolution, v. 13, p. 329-332.
These two papers discuss numerous documented examples of rapid
evolution in the wild.

The book The Beak of the Finch (Vintage Books, 1994) by Jonathan
Weiner provides an excellent account of evolution among the Galapagos
finches as well as other examples.

Otte, D. and Endler, J.A. (eds.), 1989, Speciation and its
Consequences: Sinauer Assoc., Inc.
This book, although a little out of date, provides a comprehensive
discussion of speciation. It is an edited volume with many research
papers covering many aspects of the speciation process. A wide range
of species are used as examples of different types of speciation
mechanisms and different stages in the speciation process. The papers
include both controlled experiments and field observations of
mollusks, insects, fish, reptiles, birds and mammals.

Stanley's 1979 book Macroevolution (pages 40-47) mentions a number of
examples of recent speciation, including Hawaiian moths. Five
species of banana-eating moths from Hawaii belong to a closely-
related group of species. The other members of the group feed on
palms, although they will also feed on bananas. The new exclusively
banana-eating species must have evolved since humans first introduced
the plant only about 1,000 years ago.

Prior to the voyages of exploration, rats did not live on the Island
of Mauritius. Some rats deserted the first ships that landed there.
Today, the rats of Mauritius have a chromosome count and type that is
unique. Nowhere else in the world do we find rats with this
chromosomal arrangement. There are similar cases of new rodent
species evolving on other isolated islands.

A great many observed examples of speciation in the wild and
laboratory can be found at <
speciation.html> and <>.

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Received on Sat Apr 19 21:55:21 2008

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