Did You Know ?
Circumstances encourage a form of evolution generally thought impossible, says
Evolution takes place when a small change in an organism confers a survival advantage, and
in time, the evolved population grows apart into a separate species. Masaki Hoso and
colleagues in different institutes in Japan report in the journal, Nature Communications, an
instance of a separate population growing despite a reproductive disadvantage, thanks to the
same feature also keeping predators away.
Handed-ness in snails
The snail is a remarkably evolved creature that carries with it its shell, which is its
habitation and its protection from elements and predators. The snail, in fact, has no bones,
unless we say it has its skeleton on the outside! Many of the snails organs are within the
shell and the whole snail can retract into the shell when desired. For efficiency and
economy, the shell is shaped as a coiled tube - the coil helps keep the dimensions
manageable and grows narrower towards the centre, which gives protection.
But because the shell is coiled, there is the question of which way does the coil go round?
The vast majority of snails have shells that go round clockwise, very few with shells that
go round counter-clockwise. The first kind are called dextral , or right-handed and the
second kind sinistral,or lefthanded.
But this handedness of the snail has a serious consequence, that opposite handed snails
cannot easily reproduce! Right-handed snails, which are the great majority, would thus mate
only with their own kind and the question about left-handed snails, which form effectively a
separate species, is how did they come about and how do they continue to exist?
The emergence of a new species happens when portions of a population accumulate genetic
differences to the point that they can only reproduce within themselves. When species
diverge, it usually happens because a population is divided, physically, by a mountain or a
river, so that there can be no ‘gene exchange’ or when populations migrate and evolve
separately. This is the most common way and it is called allopatric speciation. The word
means ‘other fatherland’ and what happens is that there is a ‘habitat fragmentation’. Over
time, they undergo ‘genetic drift’ and become reproductively incompatible.
Another method is by domesticated animal husbandry. While the mechanisms are not clear, in
the case of domestic sheep, they no longer produce viable offspring with some of their
ancestral species. But domestic cattle can be considered the same species as many varieties
of wild ox, gaur, yak, etc., as they are able to effectively breed with them.
The biologist Diane Dodd was able to show that only 8 generations of dietary separation can
create speciation via mating preferences in fruit flies.
Milder forms of Allopatric speciation are in peripatric and parapatric speciation, where
there is partial separation of populations. But sympatric speciation is when an ancestral
species grows into 2 or more, while still in the same location. While there are instances,
of insect populations that become dependant on different host plants, whether the sympatric
is a separate mechanism or only a form of micro-allopatric speciation, is not decided.
It is in this context that the emergence of the sinistral snail population, within a
predominantly dextral community, presents a paradox.
Genes and evolution
Genetic changes that lead to evolution often come about with changes in only a few genes
that control a large number of features. Handed-ness, which arises in all species, is
controlled by a set of genes called Nodal, which control the multiplication of cells in
different places, to develop as the particular organs. Deficiency in Nodal, for instance,
causes asymmetries and congenital heart defects in about 8 out of 1000 newborns.
Handedness in snails is found to depend on a single maternal gene in at least 4 kinds of
common snails, which have lungs. Genetically engineering snails to select for this gene
could thus generate a separate dextral and sinistral snail populations. Such left to right
reversal, in fact has been observed to arise frequently and repeatedly among land snails,
and not by genetic manipulation. The question is how can such a mutation, when it occurs, be
sustained, when the reverse handedness is a direct reproductive handicap.
What Masaki Hoso and colleagues have discovered is that the reverse handedness, in fact,
confers such a survival advantage, willy-nilly, that it compensates for the mating
disadvantage. The advantage comes about because of the specialized evolution of South East
Asian snakes, to prey more conveniently on right-handed snails!
The pareas Iwasakii is a common, snail eating snake in Japan. As the reptile specializes in
hunting snails, it has evolved apparatus adapted to holding the shell and accessing the
succulent contents. Thanks to the great majority of snails being dextral, the whole family
of pareatidae, or snail and slug eating snakes, has evolved remarkable asymmetry in the
number of teeth in the jaw bone, to enable gripping of the right-spiralling snail shell But
thanks to this specialization, the evolved snakes can rarely get a grip on a sinistral
The hypothesis of Masaki Hoso and others was hence that left-handed snails should hence
emerge more frequently in environment which had the snakes, as compared to other areas.
Anther thing – the emergence of sinistral lines should be more common with flat or larger
snails than with taller snail species. The reason is that it is virtually impossible for
flat snails of opposite handedness to mate, while it is still possible with tall snails
which mate by ‘shell mounting’. And again, it the larger shell that presents the greater
difficulty for gripping, if it is sinister, for the snake. Hence, the larger sinister snails
would have a greater survival advantage.
The scientists carried out a worldwide analysis of populations of dextral and sinistral
snails and pareatid snakes. The study clearly shows that a single gene, which causes
reproductive incompatibility, could generate a new species, given natural selection by
[the writer can be contacted at firstname.lastname@example.org]
Just like with the sinistral snail, congenital inversion of the major organs can occur in
humans. The human body is externally symmetrical – we have 2 eyes, 2 ears, 2 arms and legs,
each side of the body looks the same. But our major organs are not duplicated – the stomach
and spleen are on the left side of the abdomen and the liver and gall bladder are on the
right. The internal structure of the brain and heart is not the same on both sides and the
heart is located to the left, within the thorax.
A rare genetic peculiarity is when all the organs are on the opposite side. This condition
is called situs inversus and is seen less than once in 10,000 births. The positions of the
organs are reversed, even the heart is on the right, except in exceedingly rare cases, where
the heart alone is in the correct position. We can see that for transplant, a situs inversus
recipient will need a similar donor!
Celebrities who were situs inversus include Randy Foye, American basketball player and
Catherine O’Hara, Canadian comedy actress. Dr No in Ian Flemmings’s James bond novel was
also situs inversus and he survived an attempt of murder because the assassin who was
aiming for the heart stabbed in the wrong side!