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HEADING north out of Panama City, the landscape changes abruptly from
concrete monoculture to moist tropical rainforest. Only the cola cans and burger
boxes strewn along the roadside remind us of the city we have just left behind.
Chris Jiggins takes a left turn and the road becomes a narrow track of pink,
sticky mud as we go deeper into the jungle.

Suddenly, a red flash shoots in front of the windscreen and Jiggins hits the
brakes, bringing the 4WD to an abrupt halt. Picking up a net, he jumps out of
the car and disappears into a wall of green foliage. Fifteen solitary minutes
later, he reappears with a broad grin on his face, carefully clutching the
butterfly net and his valuable prize.

Jiggins, from University College London but currently based at the
Smithsonian Tropical Research Institute in Panama, describes himself as an
evolutionary biologist. But perhaps “genetic plumber” is a more accurate
description of his trade. Jiggins has spent most of his career in the jungles of
Latin America looking for evidence of genes “leaking” from one species into
another. Hybrids—the living evidence of genetic leaks—are Jiggins’s
speciality, and he has become adept at tracking them down in his favoured group,
Heliconius butterflies.

Hybrid vigour

“The good thing about butterflies,” says Jiggins, “is that it’s easy to
identify hybrids between species, because different species have dramatically
different colour patterns.” In effect, Heliconius butterflies wear
their genes on their wings. Jiggins shows me his catch, a hybrid of
Heliconius cydno and Heliconius melpomene. What’s in front of me
is a long way from the caricature of the sterile and deformed hybrid, commonly
depicted in biology textbooks. Indeed, many of the Heliconius hybrids
can mate and produce fertile offspring with other hybrids or with either of the
parental species.

About a quarter of all Heliconius species hybridise in the wild.
It’s a staggering figure, if only because it rubbishes the traditional view of
how nature should behave. Isn’t a species supposed to keep its mating habits “in
house”? That, after all, is how we define species—as groups of individuals
sharing a common gene pool. But gene pools are not as watertight as we might
like to think.

Evolutionary biologist Jim Mallet is doing more than most to raise the public
profile of hybrids. A senior colleague of Jiggins at University College London,
and a fellow expert on neotropical butterflies, Mallet can serve up some
striking statistics. He points out that about a quarter of all species of birds
of paradise, ducks, gamebirds and pheasants also hybridise in the wild. European
butterflies register a more modest 12 per cent; coral reef fish, 20 per cent.
Blue whales hybridise with fin whales. And even humans may have played away with
Neanderthals, if recent fossil evidence is anything to go by. As a conservative
estimate, 10 per cent of all animal species and 20 per cent of plant species
flout traditional species boundaries. Intriguingly, it is in species that are
evolving most rapidly where you often find a plentiful supply of genetic
leaks.

In bringing these leaks out into the open, Mallet hopes to wash away some of
the monoliths of biological tradition. What is at stake is nothing less than a
completely new way of looking at the world. “It castrates the idea that species
are pure and unsullied by gene flow,” he says.

For the past 70 years genetic leaks have been virtually ignored by mainstream
biological opinion, creating, Mallet believes, a myopic view of evolution. He
blames the biological species concept (BSC) for the current predicament. Dreamt
up in the 1930s by Ernst Mayr and Theodosius Dobzhansky, two eminent
evolutionary biologists, the BSC parcels up all living things into distinct,
reproductively isolated bundles—species in other words. Mayr and
Dobzhansky saw this notion of “species” not merely as a workable definition, but
as encapsulating the underlying truth. The term “species”, which had been
abstract and nebulous since Darwin’s day, acquired a concrete reality.

The BSC caught on quickly. It is what Richard Dawkins would call an extremely
good meme because just about everyone, biologists included, bought it hook, line
and sinker. A whole new biological vernacular helped reinforce the idea of
species as self-contained entities. Species now had “genetic integrity”, which
was “protected” by “reproductive isolating mechanisms”. In this new world order
hybrids became simply irritations in a world of comfortable certainties. Because
species were now “real”, hybrids served not just to blur, but to violate natural
boundaries. In short, hybrids were deemed unnatural. “Hybrid”, like “mutant”,
became a word associated with deformity and derision.

“It is probably no accident,” Mallet remarks, “that this species idea arose at
around the same time that pure race ideals and fascism became prevalent in Europe.”

Prejudice against hybrids persists to this day. It is glaringly obvious in
the “Hybrid Policy”, a 1977 amendment to the US Endangered Species Act of 1973,
which specifically excludes hybrids from protection. In northeast Texas, for
example, the threatened red wolf receives no protection just because it has a
few coyote genes in its genetic recipe. Further evidence of bias can be found by
flicking through the butterfly or bird guide on your bookshelf. You will
struggle to find a mention, let alone a picture, of a hybrid.

Mallet believes that biodiversity must be liberated from its human
straitjacket by abandoning the BSC and accepting hybrids as an integral part of
the biological picture. That’s not to say that terms like species, genus and
family don’t have a place in our efforts to classify and catalogue the
biological world, but they also muddy the waters and get in the way of our
understanding. Natural selection works on individuals or genes, not on species.
Hybrids may face handicaps as a result of genetic incompatibility, but
essentially they are playing the same evolutionary game as any other individual.
The fittest survive, regardless of species boundaries. In Mallet’s view, nature
is full of different kinds of things, and there are degrees of difference
between these. In other words, life is a genetic continuum.

Hill or hummock?

This view of biodiversity can, perhaps, best be visualised in a mathematical
context rather than a biological one. Imagine an abstract mathematical space and
an undulating surface or landscape within it, complete with mountains, hills and
valleys. The height at any point on the landscape represents the number of
individuals with a particular set of genetic characteristics. So peaks on the
surface represent clusters of individuals sharing similar genes—species,
if you like—and the valleys equate to rarer combinations of genes such as
those found in hybrids. This entire landscape is itself just a small part of a
much larger landscape that represents the coarser similarities and differences
between organisms. At each scale of biological diversity, deciding whether a
cluster of individuals is or isn’t a species, a genus, or any of the other
taxonomic categories, is an arbitrary exercise akin to deciding whether a peak
is a hill or a hummock.

But here’s a puzzle. If genetic leaks are part and parcel of the biological
world why have all the peaks on the landscape not been eroded, leaving one vast
plateau? Why do we see so many different kinds of plants and animals? First, the
figures on hybridisation need to be clarified. “Although hybridisation is quite
common on a per species basis,” says Mallet, “if you look at a pair of closely
related species living in the same area, usually less than one in a thousand
individuals will be hybrids.” Animals only rarely choose a mate from a different
species.

Another reason why genetic leaks do not dissolve species differences is
because natural selection usually holds the leaks in check. Hybrids may be
sterile or die before birth. They may not even make it past conception. But
selection against hybrids can also take more subtle forms, as Jiggins discovered
while working in Ecuador a few years ago.

Jiggins wanted to know how two Heliconius species, H. erato
and H. himera could remain distinct from one another despite evidence
of substantial genetic leaks. The ranges of the two species overlap in a narrow
zone, about 5 kilometres wide, that runs along the ridges of the Ecuadorian
Andes. Within that zone, hybrids make up an impressive 10 per cent of the total
number of individuals. The hybrids seemed just as fit and healthy as the two
parental species. On the basis of number of eggs laid, larval survival,
development time, and a host of other life history characteristics, the hybrids
could not be distinguished from their parents.

So how was the leak being contained? Why was a wave of hybridisation not
spreading out from the zone in all directions, gobbling up the species
differences? Jiggins believes that warning coloration is the key. Predators
recognise the different colour patterns of the two unappetising parental species
and have learnt to avoid them. But the hybrids have a novel intermediate design
unfamiliar to predators, who don’t realise that they are in for a bad case of
indigestion. This makes them much more vulnerable to attack. So the steady
stream of genetic leaks created by hybridisation is continually mopped up by
predation.

But selection doesn’t seal all genetic leaks. To find a good example, look no
further than the spiritual home of Darwinian evolution—the
Galápagos Islands. There, evolutionary biologist Peter Grant from
Princeton University has spent the past 30 years monitoring the remarkable
changes taking place among populations of Darwin’s finches.

Ahead by a beak

Hybrids between Darwin’s finches are relatively common. Like the butterflies,
finch hybrids seem healthy enough. They can fly, sing, court and have fertile
offspring. But their beaks—intermediate in shape—are less efficient
feeding tools than the beaks of the parental species. In the competition for
food, hybrids lose out. At least, that was the situation up until 1983, when a
vigorous El Niño event triggered drastic changes in the Galápagos
environment. On Daphne Major for example, one of the smaller islands, the wetter
climate allowed new types of plants to flourish. Seeds—the staple diet of
the finches—became smaller and softer. All of a sudden, hybrid birds found
themselves one step ahead of the competition as the parental species, highly
specialised to feed on traditional seed types, struggled to cope with the novel
seeds.

Grant believes that if the genetic leaks were to continue at their current
rate, the outcome would be complete dissolution of the separate species, perhaps
within 200 years. “However,” he says, “it will not continue indefinitely because
sooner or later the climate will change back to relatively dry conditions.” The
Galápagos climate undergoes both short-term and long-term oscillations.
If and when the climate returns to its pre-1983 state, Grant thinks that natural
selection will operate to re-establish old clusters and discontinuities,
rebuilding the old hills and hummocks from the plateau.

In a sense, hybridisation is loosely analogous to that other force for change
in evolution—mutation. Both usually end in genetic disaster for the
individual concerned. But, just occasionally, hybridisation, like mutation,
comes up with novel solutions to environmental problems. “Hybridisation is
almost always non-adaptive,” says Mallet, “but it does provide genetic variation
that could be useful in adaptive evolution.” This idea has been championed by
Michael Arnold from the University of Georgia who, in his recent book, flouts
biological tradition by promoting the role of hybridisation in the evolution and
diversification of animals and plants.

In effect, hybrids act as vehicles for the transmission of genetic
information between gene pools, leaving natural selection to sort through the
novel genetic combinations that result. Some genes are more leaky than others.
Genes that confer a strong selective advantage against a variety of genetic
backgrounds are the most likely candidates for leaking between species. Genes
for resistance to insecticide and herbicide are perfect examples of such
“useful” genes. Many of these genes are difficult to produce. Millions of deaths
may be necessary before a mutant emerges which is capable of combating a
chemically complex insecticide or herbicide. But once an adequate defence
appears in a population, it becomes a priceless commodity. Natural selection
will take full advantage of any genetic leak, and the gene will spread
rapidly.

It is exactly these sorts of “useful” genes that environmentalists are so
worried about in the debate over the ecological safety of genetically engineered
crops. Fear of genetic leaks between transgenic crops and closely related weeds
in neighbouring fields is a realistic concern. But in the case of herbicide
resistance, the products of resistance genes are energetically costly. Outside
the areas where chemicals are sprayed, plants carrying the gene will be at a
disadvantage: natural selection will seal the leak. Other transgenes, such as
those that confer resistance to specific insect or virus pests, may prove more
useful to wild hybrids that contain them, but these are likely to be held in
check by natural constraints
(see “Running wild”,91av, p 38, 31 October 1998).

Even Mallet, the hybrid’s champion, admits that leaky genes can lead to
ethical dilemmas—particularly when it comes to conservation. The
hybridisation of the white-headed and ruddy ducks is a recent case in point. The
ruddy duck was introduced to Britain from North America in the 1950s, but
subsequently escaped from captivity. Earlier this year, environment minister
Michael Meacher ruled that ruddy ducks should be shot to prevent the endangered
white-headed ducks being hybridised into extinction. Predictably, animal rights
campaigners were outraged.

Is this just the latest in a long line of prejudicial actions against
hybrids? Mallet is uncertain on this point. “Ethically, I’m in a quandary,” he
says. “Whether you should shoot invading birds or let them run their course, I’m
not really sure. I think it’s best to try to prevent things moving in the first
place.” He points to another, similar, example from Africa, where domestic dogs
are interbreeding with the threatened Ethiopian Wolf. “We don’t want our wild
species turning into mongrels,” he says. “Not because we don’t like hybrids, but
because we’re going to lose the wolf. Let’s maintain geographic diversity. It’s
very important.”

Mental hurdle

Nevertheless, Mallet says that preoccupation with the species ideal is a
mental hurdle in conservation efforts. “When trying to conserve something, we
should not consider whether it is or is not a `good’ species. What we need to
think about is its ecology, colour pattern, behaviour and genetics.
Conservationists recognise this, but the law doesn’t. We still have this hybrid
policy, which says that hybrids are not worth conserving. The Endangered Species
Act is just that: a species act.”

Dispensing with the species ideal allows you to take a broader look at the
evolutionary landscape, to understand the significance of the lowlands as well
as the peaks, and to see that the topography is constantly changing. Genetic
leaks are an integral part of that endlessly remoulded landscape. Some are
stopped-up by natural selection, some remain a trickle, producing subtle shifts
in the positions of peaks, while others burst forth, reducing twin peaks to
plateaux.

There’s no denying the convenience of pigeon-holing nature into convenient
species-sized packages. But life is a lot more rebellious than that.