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READING PASSAGE 14
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.
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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 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.
