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READING PASSAGE 6
When evolution runs backwards
Evolution isn’t supposed to run backwards - yet an increasing number of examples show that it does and that it can
sometimes represent the future of a species.
The description of any animal as an ‘evolutionary throwback’ is controversial. For the better part
of a century, most
biologists have been reluctant to use those words, mindful of a principle of evolution that says ‘evolution cannot run
backwards. But as more and more examples come to light and modern genetics enters the scene, that principle is
having to be rewritten. Not only are evolutionary throwbacks possible, they sometimes play an important role in the
forward march of evolution.
The technical term for an evolutionary throwback is an ‘atavism’, from the Latin atavus, meaning forefather. The
word has ugly connotations thanks largely to Cesare Lombroso, a 19th-century Italian medic who argued that
criminals were born not made and could be identified by certain physical features that were throwbacks to a
primitive, sub-human state.
While Lombroso was measuring criminals, a Belgian palaeontologist called Louis Dollo was studying fossil records
and coming to the opposite conclusion. In 1890 he proposed that evolution was irreversible: that ‘an organism is
unable to return, even partially, to a previous stage already realised in the ranks of its ancestors. Early 20th-century
biologists came to a similar conclusion, though they qualified it
in terms of probability, stating that there is no
reason why evolution cannot run backwards -it is just very unlikely. And so the idea of irreversibility in evolution
stuck and came to be known as ‘Dollo’s law.
If Dollo’s law is right, atavisms should occur only very rarely, if at all. Yet almost since the idea took root,
exceptions have been cropping up. In 1919, for example, a humpback whale with a pair of leglike appendages over a
metre long, complete with a full set of limb bones, was caught off Vancouver Island in Canada. Explorer Roy
Chapman Andrews argued at the time that the whale must be a throwback to a land-living ancestor. ‘I can see no
other explanation, he wrote in 1921.
Since then, so many other examples have been discovered that it no longer makes sense to say that evolution is as
good as irreversible. And this poses a puzzle: how can characteristics that disappeared millions of years ago
suddenly reappear?
In 1994, Rudolf Raff and colleagues at Indiana University in the USA decided to use genetics
to put a number on the
probability of evolution going into reverse. They reasoned that while some evolutionary changes involve the loss of
genes and are therefore irreversible, others may be the result of genes being switched off. If these silent genes are
somehow switched back on, they argued, longlost traits could reappear.
Raff’s team went on to calculate the likelihood of it happening. Silent genes accumulate random mutations, they
reasoned, eventually rendering them useless. So how long can a gene survive in a species if it is no longer used? The
team calculated that there is a good chance of silent genes surviving for up to 6 million years in at least a few
individuals in a population, and that some might survive as long as 10 million years. In other words, throwbacks are
possible, but only to the relatively recent evolutionary past.
As a possible example, the team pointed to the mole salamanders of Mexico and California. Like most amphibians
these begin life in a juvenile ‘tadpole’ state, then metamorphose into the adult form – except
for one species, the
axolotl, which famously lives its entire life as a juvenile. The simplest explanation for this is that the axolotl lineage
alone lost the ability to metamorphose, while others retained it. From a detailed analysis of the salamanders’ family
tree, however, it is clear that the other lineages evolved from an ancestor that itself had lost
the ability to
metamorphose. In other words, metamorphosis in mole salamanders is an atavism. The salamander example fits
with Raff’s 10million-year time frame.
More recently, however, examples have been reported that break the time limit, suggesting that silent genes may not
be the whole story. In a paper published last year, biologist Gunter Wagner of Yale University reported some work
on the evolutionary history of a group of South American lizards called Bachia. Many of these have minuscule
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limbs; some look more like snakes than lizards and a few have completely lost the toes on their hind limbs. Other
species, however, sport up to four toes on their hind legs. The simplest explanation is that the toed lineages never
lost their toes, but Wagner begs to differ. According to his analysis of the Bachia family tree, the toed species re-
evolved toes from toeless ancestors and, what is more, digit loss and gain has occurred on more than one occasion
over tens of millions of years.
So what’s going on? One possibility is that these traits are lost
and then simply reappear, in much the same way
that similar structures can independently arise in unrelated species, such as the dorsal fins of sharks and killer
whales. Another more intriguing possibility is that the genetic information needed to make toes somehow survived
for tens or perhaps hundreds of millions of years in the lizards and was reactivated. These atavistic traits provided an
advantage and spread through the population, effectively reversing evolution.
But if silent genes degrade within 6 to million years, how can long-lost traits be reactivated over longer timescales?
The answer may lie in the womb. Early embryos of many species develop ancestral features. Snake embryos, for
example, sprout hind limb buds. Later in development these features disappear thanks to developmental programs
that say ‘lose the leg’. If for any reason this does not happen, the ancestral feature may not
disappear, leading to an
atavism.
