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Transcript
Name __________________________________
Block _____
Evolutionary history in a tiny package
March 2012
Scientists discover new species all the time, but usually
these new species are microbes, plants, insects, and
other forms of non-vertebrate life. Few vertebrate
species have thus far evaded the curious gaze of
biologists intent on understanding the diversity of life
on Earth — that is, unless the vertebrate in question
happens to be very, very tiny. Last month, scientists
announced the discovery of not one, but four miniscule
A juvenile of Brookesia micra, the
lizard species. The smallest of these new chameleons,
smallest of the new chameleon
which live in the far north of the African island of
species
Madagascar and inhabit leaf litter, reaches an adult
body size of just two centimeters. Most popular media
stories have focused on the impressively small size of these critters — it's hard not to coo over
an animal that is dwarfed by a matchstick — but the evolutionary story behind the discovery is
equally interesting ...
Where's the evolution?
Evolutionary theory helps us better understand the two central points of this discovery: first,
that the lizards actually represent new species, and second, their unusually small size. While it
might seem that the discovery of a new species would be cut and dried, that's not always the
case. Species are typically defined as populations that actually or potentially interbreed in
nature. To claim that an organism represents a new species, biologists must provide evidence
that supports that idea. The new chameleons looked somewhat different from known species
and from one another (different body sizes, different length tails, etc.), but of course, some
organisms can look quite different from one another and still interbreed — for example,
juveniles, males, and females of the same species often have very different appearances. How
do we know when an unfamiliar-looking organism is actually a new species?
To help determine whether the chameleons constituted new species, a team of biologists
reconstructed their evolutionary history. They sampled the DNA of the new chameleons,
compared it to the genetic sequences of similar, known species, and used this information to
figure out how all those chameleons are related. If the newly discovered animals are indeed new
and distinct species and don't interbreed with one another or with known species, then we'd
expect them each to occupy their own twig of the tree of life. And that's exactly what the
reconstructed evolutionary tree showed. On the tree below, you can see that individuals of the
four new species (Brookesia confidens, B. micra, B. tristis, and B. desperata) are all most closely
related to other individuals of that species and that they form clades distinct from previously
known species.
You can also see from the phylogeny above that one of the newly discovered chameleons, B.
micra, is the tiniest of the bunch. In fact, B. micra seems to be one of the very smallest
vertebrates on Earth — after some frogs and possibly a dwarf gecko. How did this species (and
all the chameleons of the Brookesia clade) wind up so small? Again, evolution helps provide the
answer ...
Evolutionary biologists have noticed an interesting pattern among organisms that immigrate
from a large land mass to an island. Many evolve into miniaturized forms — a phenomenon
known as island dwarfism or insular dwarfism. For example, 11,000 years ago, dwarf elephants
and hippopotamuses the size of large pigs lived on the island of Cyprus. In such cases, island
living seems to have fostered the evolution of small body sizes. However, this pattern doesn't
hold for all organisms: some clades have a tendency to evolve larger body sizes (e.g., common
rats and mice, as evidenced by the foot and a half long rats found on the island of Flores), some
have a tendency to evolve smaller body sizes (many ungulates), and some don't change much
at all. Why are dwarf forms sometimes more likely to evolve on islands and sometimes not?
In different organisms, changes in body
size are likely to have different causes.
For example, biologists have
hypothesized that in large plant-eating
mammals, island dwarfism occurs
because of a trade-off between
offspring numbers and offspring
survival. Small-bodied species generally
produce more offspring than largebodied species, but often have a
An adult male Brookesia micra
disadvantage in terms of competing for
limited resources and avoiding predation. On an island with little competition for resources and
little predation, natural selection favors individuals with a genetic tendency to put more of their
energetic resources into producing many offspring than into growing larger body sizes —
resulting in a tendency for these species to evolve smaller body sizes. However, on islands
where species face competition for resources and predation, individuals with a genetic tendency
to divert energy from reproduction and put it towards body growth may be favored. On the
other hand, carnivores that immigrate to an island are likely to experience natural selection
based on other constraints than those experienced by herbivores. For example, carnivores are
usually most effective at subduing prey if they have larger bodies than the prey. This could
make carnivores less likely to evolve small body sizes on islands where large prey is available.
It is unclear whether the tiny new chameleon species (which are predators — insect-eaters)
represent a case of island dwarfism. They are certainly smaller than their mainland
counterparts; however, the patterns and processes of this evolution are not obvious.
Interestingly, if island dwarfism is a factor in their small body sizes, the smallest of the small, B.
micra, may represent a case of double island dwarfism. This species inhabits a tiny island off the
coast of Madagascar called Nosy Hara — so it is possible that the ancestors of the Brookesia
chameleons immigrated to Madagascar and evolved small body sizes, and that then the
ancestors of B. micra immigrated from Madagascar to Nosy Hara and evolved an even smaller
body size! More research will help clarify the evolutionary trajectory of these species.
Unfortunately, time to do that research may be running out. Madagascar has suffered largescale deforestation, and while recent conservation efforts on the island aim to provide sanctuary
for some species, many others have been lost to extinction. The names given to several of the
new chameleon species reflect their uncertain future. B. tristis (which means "sorrowful") and B.
desperatus (which means "desperate") were so named because, despite living in areas currently
designated as nature reserves, both are threatened by severe habitat destruction.
Primary literature
 Glaw, F., Köhler, J., Townsend, T. M., and Vences, M. (2012). Rivaling the world's
smallest reptile: discovery of miniaturized and microendemic new species of leaf
chameleons (Brookesia) from northern Madagascar. PLoS ONE. 7: e31314.
Evolutionary History In a Tiny Package - Discussion and extension questions
1. Free Write…. State at least two things that you learned from reading the article. Identify
something in your life that this reading reminded you of.
2. Species are typically defined as populations that actually or potentially interbreed in nature.
What line of evidence described in the article above supports the idea that the chameleons are
actually new species? How does it support this idea?
3. What other lines of evidence (not described above) would support the idea that the
chameleons are new species?
4. Use the theory of evolution by natural selection to explain how genes for smaller body sizes
might become more common in an animal species that has just migrated to a small island.
5. Now imagine that a friend reads the article above and says to you, "Of course, they got
smaller. You don't need to be big when you are living on a tiny island!" How would you correct
your friend's misconception and provide a more accurate description of the chameleons'
evolution?
6. Read the information on the next page about the biological species concept and explain
whether or not you think the new chameleons constitute new species by that
definition and why.
Biological species concept
The biological species concept defines a species as members of populations that actually or
potentially interbreed in nature, not according to similarity of appearance. Although appearance
is helpful in identifying species, it does not define species. Organisms may appear to be alike
and be different species. For example, Western meadowlarks (Sturnella neglecta) and Eastern
meadowlarks (Sturnella magna) look almost identical to one another, yet do not interbreed with
each other — thus, they are separate species according to this definition.
The Western meadowlark (left) and the Eastern meadowlark (right)
appear to be identical, and their ranges overlap, but their distinct songs
prevent interbreeding.
Organisms may look different and yet be the same species. Many characteristics can vary within
a single species. For example, the plant hydrangea may have pink "flowers" — they're actually
modified leaves — or blue "flowers." But that doesn't mean that we should classify the two
forms as different species. In fact, you could cause a blue-"flowered" plant to become a pink"flowered" plant just by changing the pH of the soil and the amount of aluminum taken up by
the plant.
Adding to the problem
We already pointed out two of the difficulties with the biological species concept: what do you do
with asexual organisms, and what do you do with organisms that occasionally form hybrids with
one another? Other difficulties include:
 What is meant by "potentially interbreeding?" If a population of frogs were divided by a
freeway, that prevented the two groups of frogs from interbreeding, should we designate
them as separate species? Probably not — but how distantly separated do they have to be
before we draw the line?
 Ring species are species with a geographic distribution that forms a ring and overlaps at
the ends. The many subspecies of Ensatina salamanders in California exhibit subtle
morphological and genetic differences all along their range. They all interbreed with their
immediate neighbors with one exception: where the extreme ends of the range overlap in
Southern California, E. klauberi and E. eschscholtzii do not interbreed. So where do we
mark the point of speciation?
 Chronospecies are different stages in the same evolving lineage that existed at different
points in time. Obviously, chronospecies present a problem for the biological species
concept — for example, it is not really possible (or very meaningful!) to figure out
whether a trilobite living 300 million years ago would have interbred with its ancestor
living 310 million years ago.
7. Explain whether or not you think the new chameleons constitute new species by that
definition and why.