Ring Species:
Unusual Demonstrations of Speciation
By Darren E. Irwin

A ring of populations encircles an area of unsuitable habitat.
At one location in the ring, two distinct forms coexist without
interbreeding.
Around the rest of the ring, the traits of one species change
gradually through intermediate populations into the second species'
traits

To understand how evolution has produced the diversity of life, we
need to study two fundamental processes:
How a single species changes through time.
How a single species becomes two or more species.


Speciation usually takes too long to observe in one lifetime. The
first process has been observed and studied extensively in many
species, for example in evolving beak sizes of Darwin's finches1 and
in evolving body sizes and developmental rates of guppies.2 The
second process, called speciation, is more difficult to observe
directly, primarily because it usually takes much longer than a
biologist's lifetime to occur.
Speciation can happen when two related species are isolated
geographically.










Geography and speciation

One way to study speciation indirectly is to examine geographical
variation, or how the characteristics of organisms differ between
different locations. We can then infer from the variation how
speciation occurs. During the early 20th century, biologists such as
David Starr Jordan3 and Ernst Mayr4 used this approach and noticed
that, in most cases, two closely related species do not occur at the
same location nor are they distantly separated. Rather, they usually
occur in geographically adjacent regions that are separated by a
geographical barrier such as a mountain range or a body of water.
The biologists concluded from this pattern that:
Speciation often begins when a single species becomes geographically
separated into two populations. Individuals cannot travel between
the populations, preventing the two populations from interbreeding.
Because the two populations cannot exchange genes, and because they
may be subject to different environmental conditions, they slowly
evolve differences.
Eventually the two populations become different enough that they do
not interbreed even if they come into contact (in other words, they
are 'reproductively isolated'), and are therefore separate species.

These conclusions were based on broad patterns in the distribution
and relationships of many species. But determining how speciation
occurs in any particular case can be difficult, because we are
usually only presented with the outcome of the process (two species)
and we often have no record of their common ancestor or the
intermediate forms that occurred during speciation.
Ring species acquire new traits as they move away from the ancestral
home.
Ring species

Ring species provide unusual and valuable situations in which we can
observe two species and the intermediate forms connecting them. In a
ring species:
A ring of populations encircles an area of unsuitable habitat.
At one location in the ring of populations, two distinct forms
coexist without interbreeding, and hence are different species.
Around the rest of the ring, the traits of one of these species
change gradually, through intermediate populations, into the traits
of the second species.

Ring species:
a ring of populations in which there is only one place where 2
distinct species meet. A ring species, therefore, is a ring of
populations in which there is only one place where two distinct
species meet. Ernst Mayr4 called ring species "the perfect
demonstration of speciation" because they show a range of
intermediate forms between two species. They allow us to use
variation in space to infer how changes occurred over time. This
approach is especially powerful when we can reconstruct the
biogeographical history of a ring species, as has been done in two
cases.




California salamanders exhibit ring species traits.

Ensatina salamanders

One well-studied ring species consists of salamanders in the
Ensatina eschscholtzii group, distributed in mountains along the
west coast of North America. In 1949, Robert Stebbins5 described a
fascinating pattern of geographical variation in these salamanders:
Two distinct forms of Ensatina salamanders, differing dramatically
in color, coexist in southern California and interbreed there only
rarely.
These two forms are connected by a chain of populations to the north
that encircles the Central Valley of California, and through this
ring of populations the color patterns of the salamanders change
gradually.

DNA analysis supports a common ancestor for these salamanders.
Stebbins thought that this situation arose when an ancestral
population of salamanders, in northern California, expanded
southward along two fronts, one down the Sierra Nevada mountains,
and the other down the coastal mountains. The two groups gradually
became different as they moved south. When they met again in
southern California, the two expanding fronts were so different that
they rarely interbred, and were therefore different species. More
recently, a team of researchers led by David Wake6-8 has examined
genetic relationships among salamander populations using DNA
sequences and other molecular traits, and the genetic evidence has
supported Stebbins' hypothesis. The geographical variation, when
combined with the inferred history revealed by the molecular traits,
allows us to envision the small steps by which a single ancestral
species in the north gave rise through evolutionary divergence to
two species in southern California.
Greenish warblers, a ring species, are found in parts of Asia and
eastern Europe.










Greenish warblers

Another ring species that has provided valuable insights into
speciation consists of the greenish warblers (Phylloscopus
trochiloides). These small, insect-eating songbirds breed in the
forests of central and northern Asia and eastern Europe. In the
center of Asia is a large region of desert, including the Tibetan
Plateau and the Taklamakan and Gobi Deserts, where the warblers
cannot live. Instead, they inhabit a ring of mountains surrounding
this region, as well as the forests of Siberia to the north. The
warblers have remarkable geographic variation:9-11
In Siberia, two distinct forms of greenish warblers coexist, one in
the west and one in the east, their distributions narrowly
overlapping in central Siberia, where they do not interbreed. These
forms differ in color patterns, the songs that males sing to attract
mates, and genetic characteristics. Also, males of each form usually
do not recognize the song of the other form, but respond strongly to
their own.
The traits that differ between the two Siberian forms change
gradually through the chain of populations encircling the Tibetan
Plateau to the south.
Thus two distinct species are connected by gradual variation in
morphological, behavioral, and genetic traits.

DNA evidence points to an ancestor somewhere in the Himalayas.









Claude Ticehurst,9 who during the 1930s studied variation in museum
specimens of greenish warblers, hypothesized that the present
pattern of variation arose when an ancestral species in the south,
perhaps in the Himalayas, expanded northward along two pathways, one
on the west side of Tibet and the other on the east. The two
expanding fronts gradually became different, resulting in two
distinct Siberian forms. More recently, studies of genetic variation
and song variation have strongly supported this view.10-11

The pattern of song variation is particularly interesting:
Songs are short and simple in the south, but to the north songs
become gradually longer and more complex along both pathways into
Siberia.
However, songs have also become different in structure, resulting in
distinct differences in songs between the Siberian forms.

Song patterns changed as new species emerged.



The birds distinguish between these differences; males respond
aggressively to tape recordings of their own songs, thinking that
another male has invaded their territory, but they do not respond to
songs of the other form. In most species of songbirds, songs play an
important role in mate choice; usually, only males sing, and females
listen to songs when deciding which male to choose as a mate.12
Speciation is essentially the evolution of reproductive isolation
between two populations, and song differences can cause reproductive
isolation. Hence, the geographical variation in songs of greenish
warblers provides a rare illustration of how gradual change in a
trait can cause speciation.


Ring species provide strong evidence for evolution.





Complete geographical isolation is not necessary to produce new
species.

Demonstrations of evolution

Greenish warblers and Ensatina salamanders illustrate three
fundamental ways that ring species can teach us about evolution:
Ring species provide strong evidence for evolution causing the
appearance of new species, demonstrating that many small changes can
eventually accumulate into large differences between distinct
species. Some critics of evolutionary theory think that evolution
can only cause limited change within a species and cannot lead to
the evolution of new species. Ring species show that they are wrong;
variation between species is qualitatively similar, though different
in degree, to variation within a species.
Ring species allow a reconstruction of the history and causes of
divergence during speciation, since spatial variation may illustrate
change through time. Without the rings of populations connecting the
terminal forms, we would have little understanding of the history of
divergence of greenish warbler songs or Ensatina color patterns.
Ring species provide evidence that speciation can occur without
complete geographic isolation. As discussed at the beginning of this
article, the prevailing view of speciation has been that two
populations must become geographically isolated, such that they do
not exchange genes, before speciation can occur (this process is
called 'allopatric speciation'). Ring species, however, show that
the ends of a long chain of interbreeding populations can diverge to
the point that they do not directly interbreed, even though genes
can travel between them through the intermediate populations (in
other words, they are connected by 'gene flow'). This aspect of ring
species has been rather controversial, and critics have argued that
some apparent examples of ring species, such as Ensatina, have
breaks in gene flow.13

There are few clear examples of ring species.


Rarity of ring species

Since we can learn so much from ring species, it is unfortunate that
few examples are known. At least 23 cases have been proposed, but
most of them are not such clear examples as the salamanders and
warblers.14 Most of the proposed cases have major gaps in
distribution in the chain of populations connecting the terminal
forms, and some cases appear to have more than one species boundary
in the ring of populations. However, most of the cases have one
thing in common: in one place, there are clearly two species, while
in another area the boundary between species is difficult to
determine.

One reason ring species are rare is because they require an unbroken
ring of suitable habitat.


Ring species are rare for several reasons:14,15
Their formation requires unusual geographic situations, in which a
species can expand around a geographic barrier through a continuous
ring of suitable habitat. The range expansion must occur slowly
enough that the two expanding fronts have time to diverge before
they meet on the other side of the barrier, and the size of the
barrier must be large compared to the distance that individuals
disperse.
The taxonomic rules that are used by biologists to classify
organisms create a bias against recognizing ring species. Under
these rules, a ring species must be classified either as a single
species or as two species. Both classification schemes conceal the
fact that there is gradual variation between reproductively isolated
forms.
Ring species might be rare because many of them were destroyed
before they could be discovered, as will be described below.

Most conservation is aimed at species rather than within-species
diversity.

Importance of conservation

While ring species teach us about evolution, they also provide
lessons about the importance of habitat conservation. They
demonstrate that species can differ substantially between different
parts of their ranges. Ensatina salamanders and greenish warblers
are each usually classified as a single species, even though each
contains populations that differ at the between-species level. As
the commonly-used phrase 'endangered species' reveals, most
conservation efforts are directed at the species level, and species
are often eliminated from much of their range before they receive
legal protection. However, within-species variation is an important
component of biodiversity. Whenever a species is eliminated from
part of its range, unique traits that were only found in that area
are lost forever. Not only are the species and its traits lost from
that area, but the particular story of evolution that could be
learned from it is lost as well.
Conclusion: If we continue to destroy habitat, we may not be able to
discover and study new ring species.

It is not a coincidence that the best two examples of ring species
are both found in relatively undisturbed mountainous habitat. Ring
species might have existed at one time in habitats such as the
grasslands of central North America, but those would now be
destroyed or severely altered because of the impact of agriculture.
Large areas of greenish warbler habitat are being deforested,
particularly in China, India, and Nepal, and this process might
continue to the point that future biologists would not be able to
recognize greenish warblers as a ring species. Undoubtedly, many
undiscovered ring species are being destroyed now because of human
activity, and many more will be lost in the future if we fail to
protect their habitats.




About the author: Darren E. Irwin, Ph.D., conducted his doctoral
research on the greenish warbler ring species while a graduate
student at the University of California, San Diego. He is continuing
his research on speciation in warblers at the Department of Animal
Ecology of Lund University, Sweden, where he is funded by an
International Research Fellowship from the National Science
Foundation.
http://lucat2000.lu.se/en/normal/docOP120.rbs?
orgid=011012001

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