News Articles on Reproductive Isolation
1) Picky female frogs drive evolution of new species in less than 8,000 years
By Robert Sanders, Media Relations | 27 October 2005 - UCBerkeley News
http://www.berkeley.edu/news/media/releases/2005/10/27_greeneyed.shtml
Summary – The green-eyed tree frog of Queensland was geographically separated. When contact was reestablished (8000
years ago), the males from each population had different calls and the hybrids of the two populations were less viable than
the separate populations. Since then females have selected mates by their call and the two populations have developed
into separate species.
2) Butterfly Unlocks Evolution Secret
By Julianna Kettlewell 24 July 2005 – BBC News
http://news.bbc.co.uk/2/hi/science/nature/4708459.stm
Summary – Two related species of butterflies in Asia look very similar if they are geographically separated. If the
two species share a geographical area, the males of the two species have different coloration. The females select
mates based on the coloration, maintaining the separate species and preventing less viable hybrids.
3) Guppy love under the microscope
No author
21 June 2003 – BBC News
http://news.bbc.co.uk/2/hi/uk_news/scotland/3009170.stm
Summary – Two groups of guppies that have been geographically separated have be reunited in the laboratory. It
appears that the sperm from the native males outcompete those of the foreign guppies. Hybrids are less vigorous.
Female sexual selection does not appear to be effective due to “sneaky” male mating behavior.
4) Male Pregnancy May Spur Seahorse Speciation
Sarah Graham
May 07, 2003
Scientific American
http://www.sciam.com/article.cfm?articleID=000214BD-E52F-1EB7-BDC0809EC588EEDF
Summary – Female seahorses deposit eggs in the male’s pouch, where the eggs are fertilized and carried through
gestation. The mates are chosen based on size. (Similar to the lab exercise --)
5) Biologists Uncover Darwin’s ‘Missing Evidence’ for divergence of Species
http://www.sciencedaily.com/releases/2001/01/010118071334.htm
Summary – A series of 15 populations of greenish warblers in central Asia were studied. They form a ring of
interbreeding populations except at one point in Siberia where the two overlapping populations do not interbreed.
These birds do not recognize each other’s songs.
6) Transplanted sockeye salmon show rapid differentiation
Oct. 19, 2000 University of Washington News
http://www.uwnews.org/article.asp?articleID=3316
Summary – Two populations of sockeye salmon became reproductively isolated due to the body width of the males.
Males that have larger body depths (fin to belly) were more successful mating in deeper water. This resulted in two
populations a shallow-water (river) and a deep-water (beach).
Picky female frogs drive evolution of new species in less than 8,000 years
By Robert Sanders, Media Relations | 27 October 2005
BERKELEY – Picky female frogs in a tiny rainforest outpost of Australia have driven the
evolution of a new species in 8,000 years or less, according to scientists from the University
of Queensland, the University of California, Berkeley, and the Queensland Parks and Wildlife
Service.
The yet-to-be- named species
arose after two isolated populations
of the green-eyed tree frog
reestablished contact less than
8,000 years ago and found that
their hybrid offspring were less
viable. To avoid hybridizing with
the wrong frogs and ensure healthy
offspring, one group of females
preferentially chose mates from
their own lineage. Over several
thousand years, this behavior
created a reproductively isolated
population - essentially a new
species - that is unable to mate
When isolated populations of the green-eyed tree frog
with either of the original frog
(gray and brown) met again 8,000 years ago, they
populations.
found that each had changed in subtle ways. The calls
of the male frogs were different, and more
This example suggests that rapid
importantly, hybrid offspring were less viable. One
speciation is often driven by
population that was cut off from its southern kin (pink)
recontact between long-isolated
found a way to ensure healthy young. Females, who
populations, Moritz said. Random
choose mates based only on their call, began selecting
drift between isolated populations
mates with a the southern call type. Over thousands of
can produce small variations over
years, this behavior exaggerated the pre-existing
millions of years, whereas
differences in call, lead to smaller body size in males of
recontact can amplify the
the "isolated southern population" and resulted in
difference over several thousands
rapid speciation between the two populations of the
of years to generate a distinct
southern lineage (pink and brown). (Nicolle Rager
species.
Fuller/National Science Foundation)
A male green-eyed tree frog
clasps a female during the
mating ritual called
The green-eyed tree frog, Litoria genimaculata, lives in the
amplexus. (Jason MacKenzie
Wet Tropics area of northeast Queensland, a rugged tropical
photo)
region of Australia along the Pacific Ocean's Great Barrier
Reef. The frog, which is green with reddish-brown splotches, is common around streams
and grows to about 2 1/2 inches in length.
Because of geographic isolation that began between 1 and 2 million years ago with the
retreat of rainforest to higher elevations, two separate frog lineages developed in the
northern and southern parts of the species' coastal range - only to be reconnected less than
8,000 years ago as the climate got wetter and warmer and the rainforest expanded.
Hoskin and his colleagues found that the northern and southern calls of the male frog, which
are what females pay attention to in the mating game, had become different from each
other. Yet despite this difference, reflected in the call's duration, note rate and dominant
frequency, the two lineages could still breed with one another.
The southern females, however, were more picky about their mates than the northern
females. And in one area of contact that had become isolated from the southern range, the
southern females were extremely picky, to the extent that they almost never mated with
northern males.
In laboratory breeding experiments, the biologists discovered the reason for this
choosiness: While northern and southern lineages could breed successfully, they apparently
had diverged enough during their million-year separation that offspring of southern females
and northern males failed to develop beyond the tadpole stage. Though crosses involving
northern females and southern males successfully produced frogs, the offspring developed
more slowly than the offspring of pairs of northern frogs.
Field studies confirmed the laboratory results. Researchers could find no hybrid frogs in the
contact zones that were the offspring of southern mothers, judging by the absence of any
southern mitochondrial DNA, which is contributed only by the mother.
Hoskin and colleagues argue that because southern females have the most to lose in such
cross-breeding, there may have been selection pressure to evolve a mating strategy to
minimize dead-end mating with northern males. This appears to have occurred in the
contact region where a population of the southern lineage had become isolated from the
rest of its lineage and had developed a preference for certain male calls. The male frog call
in this population has diverged significantly from both the northern and southern lineage
calls.
"If females have a reason not to get the mating wrong, and they have some way of telling
the males apart - the call - the theory is that this should create evolutionary pressure for
the female choice to evolve so that they pick the right males," Moritz said.
As a comparison, they looked at a second contact zone on the border between north and
south, where frogs were not isolated from either lineage.
"Reinforcement does not appear to occur at the more 'classic' contact between northern and
southern lineages, and we speculate that this may be due to gene flow from the extensive
range of the southern lineage into the contact zone," Hoskin said. "This problem does not
exist at the other contact because the southern lineage population is very small and occurs
primarily within the contact zone."
Because the frogs in the isolated contact area had a distinctively different call, and because
they were effectively isolated from surrounding populations by mating preference, Hoskin
and colleagues concluded that female choice led to this new species.
Interestingly, evolutionary theory would predict that the southern and northern frog
populations would drift apart into two distinct species. In the case of the green-eyed tree
frog, Moritz said, a subpopulation of the southern species drifted away not only from the
northern species, but also from the southern. That was unexpected, he said.
Moritz noted that geographic isolation in this "dinky bit of rainforest in Australia" has split
many species, and that reinforcement at zones of recontact may be generating other new
species.
"In this tropical system, we have had long periods of isolation between populations, and
each one, when they come back together, have got a separate evolutionary experiment
going on. And some of those pan out and some don't. But if they head off in different
directions, the products themselves can be new species. And I think that's kinda cool. It
gives us a mechanism for very rapid speciation."
Butterfly unlocks evolution secret
By Julianna Kettlewell
BBC News science reporter
Why one species branches
into two is a question that
has haunted evolutionary
biologists since Darwin.
Given our planet's rich
biodiversity, "speciation"
clearly happens regularly, but
scientists cannot quite
pinpoint the driving forces
behind it.
Now, researchers studying a
family of butterflies think they
have witnessed a subtle
The butterflies' wings offer clues to speciation
process, which could be
forcing a wedge between newly formed species.
A Russian-US team discovered that closely related species
living in the same geographical space displayed unusually
distinct wing markings.
These wing colours apparently evolved as a sort of "team
strip", allowing butterflies to easily identify the species of a
potential mate.
For me, this is a big discovery just
This process, called
because the system is very beautiful
"reinforcement", prevents
Dr Nikolai Kandul, Harvard University
closely related species from
interbreeding thus driving them further apart genetically and
promoting speciation.
Although scientists have speculated about this mechanism for
years, it has rarely been witnessed in nature.
"The phenomenon of reinforcement is one of the very few
mechanisms that has natural selection playing a role in
speciation," said Nikolai Kandul, who co-authored the Nature
paper with Vladimir Lukhtanov and colleagues.
"It might be very widespread but it is hard to find good
evidence of it," the Harvard University researcher added.
Geographical isolation
For speciation to occur, two branches of the same species
must stop breeding with one another for long enough to grow
apart genetically.
The most obvious way this can happen is through
geographical isolation.
If a mountain range or river
divides a population of
animals for hundreds of
generations, they might find
that if they meet again they
are no longer able to breed.
But geographical isolation is
not enough to explain all
speciation. Clearly, organisms The butterflies choose mates with similar markings
do sometimes speciate even if there is no clear river or
mountain separating them.
The other mechanism that can theoretically divide a species
is "reproductive isolation". This occurs when organisms are
not separated physically, but "choose" not to breed with each
other thereby causing genetic isolation, which amounts to the
same thing.
Reproductive isolation is much hazier and more difficult to
pin down than geographic isolation, which is why biologists
are so excited about this family of butterflies.
Butterfly clue
The Harvard team made the discovery while studying the
butterfly genus Agrodiaetus, which has a wide ranging
habitat in Asia.
The females are brown while the males exhibit a variety of
wing colours ranging from silver and blue to brown.
Dr Kandul and his colleagues found that if closely related
species of Agrodiaetus are geographically separate, they tend
to look quite similar. That is to say, they do not display a
distinctive "team strip".
But if similarly closely related
species are living side-by-side,
the researchers noticed, they
frequently look strikingly
different - their "teams" are
clearly advertised.
This has the effect of
discouraging inter-species
mating, thus encouraging
genetic isolation and species divergence.
"This butterfly study presents evidence that the differences in
the male's wing colouration is stronger [when the species
share a habitat] than [when they do not]," said the
speciation expert Axel Meyer, from Konstanz University in
Germany.
"This pattern would therefore support the interpretation that
it was brought about by reinforcement, hence natural
selection."
The reason evolution favours the emergence of a "team strip"
in related species, or sub species, living side-by-side is that
hybridisation is not usually a desirable thing.
Although many of the Agrodiaetus species are close enough
genetically to breed, their hybrid offspring tend to be rather
weedy and less likely to thrive.
Therefore natural selection will favour ways of distinguishing
the species, which is why the clear markings exist.
"For me, this is a big discovery just because the system is
very beautiful," said Dr Kandul. "As much as we can we are
showing that [reinforcement] is the most likely mechanism."
Guppy love under the microscope
Experts are studying the
"torrid and tangled" sex
life of one of the UK's most popular pet fish as they try
to learn how the battle of the sexes influences
evolution.
Researchers at St Andrews University hope that the guppy's
mating behaviour can help them unravel the mystery of how
new species are formed.
They have put Trinidadian guppies under the microscope
because the fish is in the process of splitting into two new
species.
Two groups that have been living in separate river systems
for about two million years were put together and DNA
fingerprinting is being used to establish the parentage of
baby fish.
The research team discovered a web of sexual conflict,
interbreeding, sneaky mating behaviour and sperm
competition.
Creatures evolved
Professor Anne Magurran said the study had found a number
of ways in which "reproductive isolation" was being
encouraged.
The definition of a species is a group of creatures that can
breed with each other but not with others, even if they look
similar.
The research team believes that this reproductive isolation
was the key to understanding how the world's vast diversity
of creatures evolved.
More than 140 years ago Charles Darwin predicted that a
lack of sexual contact was what would drive species apart.
The Trinidadian guppies have divided into two groups which
never normally meet.
Professor Magurran and her team have been introducing
males and females from the different populations in the
laboratory to gain an insight into how reproductive isolation
develops.
Tools such as DNA fingerprinting have only recently become
available to help scientists test their theories about species
formation.
"One possibility is that female guppies prefer to mate with
native males," said Professor Magurran.
"If animals always mate within their own group reproductive
isolation is very quickly established."
Sneaky mating
A female will usually choose several male guppies to father
her offspring in an attempt to ensure that her babies have
the best genes possible.
However, the researchers found that wily males often
sneaked up on unsuspecting females and mate with them by
surprise.
"This sneaky mating tactic means that there are many
matings between the two groups of guppies and probably
counteracts the effect of female choice on reproductive
isolation," said Professor Magurran.
To test the theory that sperm
from the females' own species
can outcompete sperm from
other species at fertilisation,
the fish were impregnated
with equal numbers of sperm
from a native and a foreign
male.
The guppies live in separate river systems in Trinidad
Researchers are now
performing DNA fingerprint tests - similar to those used in
paternity tests in people - on the baby fish.
The initial results indicate that native males do sire more of a
female's offspring.
The team has also found that hybrid offspring are worse at
reproducing than those produced by parents from the same
group.
"These unfit offspring are another bar to interbreeding, and it
seems that this mechanism is evolving simultaneously with
sperm selection to cause reproductive isolation," said
Professor Magurran.
Male Pregnancy May Spur Seahorse Speciation
By Sarah Graham
No one could accuse a seahorse of being a hands-off father. That's because males are the ones that carry
the young. Now findings published online this week by the Proceedings of the National Academy of
Sciences suggest that male pregnancy not only takes the load off female seahorses, it can also drive the
development of new species.
Prevailing theory holds that new species arise primarily because geographic barriers halt the flow of genes
between different populations. But a number of recent theoretical studies have suggested that so-called
sympatric speciation can occur, in which different populations originate in one geographical area, but do
not interbreed. In the new work, Adam G. Jones of the Georgia Institute and his colleagues studied
seahorses off the coast of Perth, Australia, in which the female deposits her eggs in a male's brood pouch
and he fertilizes and carries the eggs until they hatch. Using genetic analyses the researchers confirmed
that the creatures tend to choose mates of a similar size (a selection process known as assortative mating).
This way, neither female eggs nor male pouch space is wasted. Notes Jones, "in seahorses assortative
mating appears to be a consequence of male pregnancy and monogamy."
The researchers then devised a computer model to test whether this mating regime could lead to
reproductive isolation and subsequent speciation. They determined that if environmental conditions favor
either very small or very large body sizes as opposed to intermediate ones, new species may arise in just
tens or hundreds of generations as a result of assortative mating. Male pregnancy, the authors thus
conclude, "represents an unusual form of parental care with extraordinary evolutionary consequences."
Transplanted sockeye salmon show rapid differentiation
Thomas Quinn, University of Washington
Male sockeye salmon struggles up stream. (For use by news media only.
Photo credit required)
A run of salmon facing new environmental conditions diverged into two populations in as
few as 13 generations -- a time span of only about 60 years -- according to research
conducted at the University of Washington with sockeye salmon in Lake Washington and the
Cedar River near Seattle.
As adaptable as scientists have known salmon to be, this is the first time researchers have
actually demonstrated how quickly salmon can evolve into two separate populations,
according to Andrew Hendry, lead author of a paper in the Oct. 20 issue of Science. He was
a UW graduate student when much of the work was conducted and is now a postdoctoral
researcher at the University of Massachusetts. Previously the fastest-known examples of
such changes have been recorded for certain insects, taking 200 to 400 generations.
In spite of the evolutionary pliability demonstrated in these latest results, scientists cannot
say how or even if salmon might adapt to more dramatic or rapid changes triggered by
climate change or habitats reshaped by development, according to Tom Quinn, professor
with the UW's School of Aquatic and Fishery Sciences, co-author of the Science paper and
Hendry's faculty advisor when he was at the UW. Quinn says the recent results are
consistent with other research, including UW work on chinook salmon introduced to New
Zealand in the early 1900s, that indicated salmon have the capacity for rapid evolution.
The Science paper is based on data collected by UW graduate students and faculty and a
scientist with the Washington Department of Fish and Wildlife. The work revealed how long
it took a run of sockeye salmon with common ancestry to diverge into two populations
genetically different enough that they could no longer spawn with each other as
successfully. When that happens the groups are becoming "reproductively isolated." Being
reproductively isolated is one of the most important benchmarks used to decide if a single
species has diverged into two.
Hendry says that the differences documented are less than those typically used to delineate
separate species. The key focus of this paper is that the processes leading to speciation can
happen much more quickly than anyone had previously supposed, he says.
The sockeye studied were originally from Baker Lake, in northwest Washington state, and
were introduced into Seattle's Lake Washington between 1937 and 1945. Today, runs of
100,000 to 350,000 fish spawn in the Cedar River, which starts in the Cascade Range
southeast of Seattle and flows into Lake Washington. Another smaller group of descendents
was first documented breeding along Pleasure Point Beach on Lake Washington, south of
Bellevue, in 1957.
The divergence scientists detected between these two populations came in response to
conditions that favor different traits. For example, deep-bodied male sockeye -- males that
are wider than average from their top fin to their bellies -- would be more successful mating
in the waters off the beach than in the Cedar River, where deep-bodied fish are more likely
to be stranded in shallow water, eaten by predators or be less maneuverable in fast water.
The reproductive isolation of the two populations was established using genetic analysis
conducted by UW's Marine Molecular Biotechnology Laboratory, which showed that fish
hatched in the river but trying to spawn at the beach had little success. If the immigrants
from the river had been equally successful in producing offspring, then the two populations
would have been homogenous.
The work was funded mainly by the UW from a fund endowed by Seattle businessman and
sports fisherman Mason Keeler. Co-authors with Hendry and Quinn are John Wenburg, a UW
graduate student when this work was conducted and now a postdoctoral researcher at the
University of Montana; Paul Bentzen, UW's director of the Marine Molecular Biotechnology
Laboratory; and Eric Volk, with the Washington Department of Fish and Wildlife.
Real-life Reproductive Isolation Articles
Names ________________
________________
________________
________________
Work as a group to answer the following questions:
 What is the article about? Summarize.
 What kind of reproductive barrier has been made? Explain.
 What has happened to the population as a result of this barrier?
 Has the barrier been a positive factor for the species or a negative factor and why?
Share your group’s findings with the class.
Biologists Uncover Darwin’s "Missing Evidence" For Divergence Of
Species In A Warbler’s Song
ScienceDaily (Jan. 19, 2001) — Biologists at the University of California, San Diego
have demonstrated, in a study of the songs and genetics of a series of interbreeding
populations of warblers in central Asia, how one species can diverge into two.
Their description of the intermediate forms of two reproductively isolated populations of
songbirds that no longer interbreed is the "missing evidence" that Darwin had hoped to
use to support his theory of natural selection, but was never able to find.
"One of the largest mysteries remaining in evolutionary biology is exactly how one
species can gradually diverge into two," says Darren E. Irwin, a biologist at UCSD who
headed the study, detailed in the January 18 issue of the journal Nature. "This process,
known as speciation, is very difficult to study because it can take a great deal of time to
occur."
Biologists have generally learned about the divergence of species by comparing many
different species at various stages of speciation. But in their study of the greenish
warbler, a songbird that breeds in forests throughout much of temperate Asia, Irwin and
his colleagues—Trevor D. Price, a biology professor at UCSD, and Staffan Bensch, a
former postdoctoral student at UCSD now at Sweden’s Lund University—discovered a
rare situation known to biologists as a "ring species."
"Ring species are unique because they present all levels of variation, from small
differences between neighboring populations to species-level differences, in a single
group of organisms," says Irwin, a former student of Price who is in the process of
beginning his postdoctoral work with Bensch at Lund University.
In the case of the greenish warbler, Phylloscopus trochiloides, the scientists discovered
a continuous ring of populations with gradually changing behavioral and genetic
characteristics encircling the Tibetan Plateau, which is treeless and uninhabitable. This
ring is broken by a species boundary at only one place, in central Siberia, where two
forms of the songbird coexist without interbreeding.
"This creates a paradox in which the two co-existing forms can be considered as two
species and as a single species at the same time," says Irwin. "Such ring species are
extremely rare, but they are valuable because they can show all of the intermediate
steps that occurred during the divergence of one species into two."
In their paper, the scientists show how they discovered a gradual variation in the song
patterns, morphology and genetic markers of 15 populations of the greenish warbler. At
each end of the ring of interbreeding populations, which extend around each side of the
Tibetan plateau and through the Himalayas, the scientists found that the two distinct,
non-interbreeding forms of the bird do not recognize each other’s songs, which are
critical in the selection of their mates. They determined this from experiments in which
they played recordings of male greenish warbler songs and judged the response of
other birds in the trees.
"In the greenish warbler, as in most songbirds, males sing to attract mates and to
defend territories," says Irwin. "The greenish warblers living in the Himalayas sing songs
that are simple, short and repetitive. As you go north along the western side of Tibet,
moving through central Asia, the songs gradually become longer and more complex. On
the eastern side of the ring, moving northwards through China, songs also become
longer and more complex, but the structure is different than on the western side. Where
the birds meet in Siberia, their songs are so different that they do not recognize each
other as mates or competitors. They act like separate species, and the genetic evidence
supports that conclusion.
"Apparently, as the birds moved north along two pathways into the forests of Siberia,
their songs became longer and more complex, perhaps because females in the north
rely more strongly on song when choosing a mate. But the forms of complexity differed
between west and east Siberia, because there are more ways to be complex than
simple.
"The greenish warbler is the first case in which we can see all the steps that occurred in
the behavioral divergence of two species from their common ancestor. These results
demonstrate how small evolutionary changes can lead to the differences that cause
reproductive isolation between species, just as Darwin envisioned."