Jonathan Weiner, "The Beak of the Finch", pp. 89 - 95:
"What the Grants are to Darwin's finches, Endler is to guppies.  His
guppies are not the variety the sell in pet stores (he considers those
trash fish).  His guppies live in northeastern South America, in small
streams that zigzag down the mountains of Venezuala, Margarita Island,
Trinidad, and Tobago, flashing through steep, undisturbed green forests
and then the broad spreads of the old cacao and coffee plantations, on
their way to the Caribbean Sea and the Atlantic.
The male guppies wear black, red, blue, yellow, green, and iridescent
spots in varying hues and combinations.  In fact, their spots vary so much
that they are like fingerprints: no two guppies are alike.
These spots, like the beaks of Darwin's finches, are heritable.  Although
the exact placement and arrangements of spots is unique, each guppy
inherits its particular palette of colors, and also the general size and
brightness of the ensemble, from its parents.  The spots only show up on
the males (they can be made to appear on the female guppies with
testosterone treatments).

Like minute variations in the beaks of finches, the spots on a guppy are
the sorts of details that one might imagine are beneath the notice of
natural selection.  Nature may scrutinize the slightest variation, but
there are some things even Darwin's process cannot see.  Design could not
possibly govern a thing so small.

In the 1970's, while Peter and Rosemary Grant were watching the finches of
the Galapagos, Endler began watching the guppies of Venezuala's Paria
Penninsula, and Trinidad's Northern Range.  There the streams run down the
mountains roughly parallel, as if in a series of vertical stripes.  The
streams are clear, swift, and clean, deeply shaded by tropical evergreens
and punctuated by waterfalls.  Their beds are lined with brilliant, many
colored gravel, much like the floors of the fish tanks in the pet stores.

It is obvious to anyone who has ever tried to watch a school of guppies
against the parti-colored sands and pebbles of a streambed that the spots
are excellent camouflage.  In fact you could watch one of these clear
streams for quite a while before you noticed the guppies at all, because
they tend to swim close to the gravel while the sun is out.

The fish need this camouflage because they have seven enemies: six species
of fish and one freshwater prawn.  All seven of these enemies hunt guppies
from dawn till dusk.  The most dangerous is Crenicichla alta, a cichlid
fish, which eats about three guppies in one hour; the least dangerous is
rivulus hartii, which eats one guppy in about five hours.

Endler found guppies and at least a few of their enemies in almost every
section of almost every stream, from the headwaters near the summit of
each mountain to the plains and plantations below.  Neither the guppies
nor the guppy eaters can swim up a waterfall, and the population of each
section of stream tends to stay put.  (Sometimes a few fish get swept
downstream, but none of them can get back up.)
2
High up near the headwaters of each stream, the only enemy the guppies
have is the comparatively mild-mannered rivulus hartii.  But moving
downstream, section by section, the population of guppies lives and dies
in the company of more and more of its enemies, until down near the base
of each mountain, the stream is loaded with all seven of the guppy
eaters.  So a graph of risk and danger runs with the current.  For the
guppies, the higher in the stream, the lower the risk; the lower in the
stream, the higher the risk.  In stream after stream the intensity of
natural selection is graduated the same way: gentle pressure among the
guppies at the top, violent pressure among the guppies at the bottom.

Endler saw that the streams would make a wonderful natural laboratory for
the study of natural selection.  He developed standardized methods of
measuring guppy spots, as careful and ritualized as the Grants' methods
with Darwin's finches.  He learned to anesthetize and photograph each
guppy he caught.  (Like Darwin's finches, the guppies have met very few
human beings , so they are easy to catch.)  From the photographs he
recorded the color and position of each spot of each and every male guppy,
dividing each guppy into dozens of sectors to make a standardized guppy
map that is easy to read, to tally, and to enter into a computer.

When Endler analyzed his surveys he discovered a pattern.  The spots on
each guppy look chaotic, but the spots of all of the population of guppies
in a stream, taken together, from the headwaters down to the base, have a
kind of order.  The spots on each population of guppies bear a simple
relationship to the number of guppy eaters in their part of the stream. 
The more numerous the guppies' enemies, the larger and brighter the spots.

The lucky guppies in the headwaters wear sporty coats of many colors, and
each color is represented by big clownish splotches.  Many of their spots
are blue.  These blue spots are iridescent, like the Day-Glo patches
cyclists wear; they flash as the fish swim, and they can be seen a great
distance through the clear water.

Meanwhile, the guppies downstream tend to wear conservative pin dots of
black and red.  The spots are almost vanishingly small.  Most wear only a
tiny amount of blue.

Endler looked at his data from stream after stream.  In every one of them,
the size and number of spots ran steeply downhill.  And Endler drew the
same sort of conclusion that Lack did when he noticed the patterns of
beaks in the Galapagos.  Endler thought that he could see the hand of
natural selection at work among the guppies.  The greater the pressure
from predators, the more camouflage they wear; the less the pressure, the
slighter the camouflage.

Of course, that interpretation did not explain why the guppies are
colorful at all.  If they are in some danger everywhere, even in the
headwaters, then why doesn't natural selection favor the best-camouflaged
guppy everywhere?

The answer is that the male guppy has more to do in life than merely
survive.  It also has to mate.  To survive it has to hide among the
colored gravel at the bottom of its stream and among the other guppies of
its school.  It has to elude the eyes of the cichlid or the prawn while
catching the eye of the female guppy.
3
The gaudier the male, the better his sex life.  He is more popular among
females, and he gets many chances to pass on his gaudy genes - as long as
he lives.  In a quiet spot near the base of the stream his life is likely
to be long and happy and he may father innumerable gaudy children.  But in
a spot near the base of the stream he may not father a single guppy before
he vanishes down the gullet of a cichlid.

The quieter the colors of a male, the less luck he has in courting
females.  On the other hand, he is likely to have more time to try,
because the less he stands out among his own kind, the less he stands out
among his enemies.
...
Back in the 1940s, Lack made his selectionist argument about Darwin's
finches without trying to measure it in the field to see if he was right. 
But Endler went the extra step: he decided to test the predictions of his
theory by trying to detect these processes in action.  He built ten ponds
in a greenhouse at Princeton University.  Four of the ponds were about as
wide, deep, and long as the low-water territories of Crinicichla alta. 
The other six ponds were about the size of the headwater streams with the
comparatively mild-mannered Rivulus hartii.  Endler put black, white,
green, blue, red, and yellow gravel in the bottom of his artificial ponds
and pumped water through them to give the current, like the streams in the
wild.

Meanwhile, Endler gathered guppies from up and down a dozen streams in
Trinidad and Venezuala.  In some places, he took guppies that lived with
just one predator, in some places guppies that lived with two predators,
and so on up to the maximum, seven.  He wanted stocks of wild guppies that
had evolved under the whole spectrum of guppy menace, that were coping in
the wild with every level of danger.  He bred each stock in a separate
aquarium.

When the artificial streams were ready for his guppies, Endler took five
pairs at random from each stock and put them all together into two of his
ponds to let them breed and mingle and get used to their new homes.

He let these guppies breed in their new streambeds for months.  Then he
added a few of their natural enemies to the streams, according to a
careful plan.  The evolutionary experiment had begun.

According to his prediction, the guppies should now evolve rapidly.  The
guppies in each tank should begin to look more like guppies that live with
that same set of predators in the wild, they should come to look more like
the gravel in their particular stream, and those in the most dangerous
tanks should come to mimic the gravel more closely than those in the safer
tanks.

After five months, Endler took his first census.  He drained each stream,
counting every male's spots and noting their position, anesthetizing them,
photographing them, as he had done in the wild, and then starting up the
stream again.  Nine months later he took a second census.  By that time
nine or ten generations had passed in the lives of his guppies.
4
Some of the guppies were safe, with no enemies.  These guppies got gaudier
between the foundation of the colony and the first census, and they got
gaudier still by the time of the second census.  The males evolved more
and more spots, bigger spots, wilder and wilder palettes of spots.

Meanwhile males in tanks wit the dangerous cichlids evolved fewer and
fewer, smaller and smaller spots.  They were still visible to females, but
they got less and less visible to the cichlids, who strike from 20 to 40
centimeters away.  These guppies mostly dropped the blue and the
iridescent spots, their Day-Glo patches, just like guppies that live witch
cichlids in the wild...

Each tank had a different bottom: different mixes of gravel colors and
different gravel sizes.  In the pools with no predators the guppies did
not change their spots to match the gravel - the opposite.  Their spots
evolved to be smaller than the big gravel and larger than the small
gravel, making the males easier and easier to see, like chameleons in
reverse.  They carried more iridescent spots, and a wider palette of
colors per fish, and generation after generation they looked less and less
like their background, all of which is just what we would expect if they
were competing for attention.  Sexual selection was operating to make
males as different from the gravel bottom as possible.

If only one force or the other had been operating, just natural selection
or just sexual selection, the guppies would not have evolved in this
remarkable way.  Without natural selection all of the fish would have
gotten gaudier.  Without sexual selection none of them would have gotten
gaudier...

The fish had evolved in Endler's greenhous until they replicated the
patterns that they display in nature, and they had done so in a very short
time.  Of course, Endler's streams were artificial.  He had not seen
natural selection in the wild.  A skeptic [that's you Bill] could still
argue that Endler was wrong about his explanation for the pattern in the
wild.  So Endler figured out a way to run the same sort of evolutionary
experiment in nature.

Early in his fieldwork he had found a Trinidad stream that contained the
guppy eater Rivulus hartii, but no guppies.  About 2 kilometers away was a
second stream that contained both guppy eaters and guppies.  Endler took a
random sample of about two hundred guppies from one of the high-danger
zones in the second river.  He measured each and every one, as usual, and
then he transferred them to the safe place in the first river.  He took a
sample of their descendents more than a year later, after a passage of
fifteen generations.

The males in the safe stream were now much gaudier than their immediate
ancestors, who were still living in the stream next door and coping with
many enemies.  The immigrant males wore bigger spots, and more of them,
and each male sported a wider assortment of colors.  Natural selection had
acted just as predicted.  Evolution had run as fast in the wild as in the
greenhouses."

Bowen Simmons              bowen@netgate.net
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