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Feature Article
EVOLUTION
OF
K. VENKATARAMAN
SEX
Why did sex evolve? From the evolutionary point of view,
sex is definitely an inefficient way to reproduce, but it acts
as a safeguard against extinction
O
NE of the baffling questions in
evolution is the origin of sex. Sex
is the queen of problems in
biology. Even Dar win, the father of
evolution and Mendel, the father of
genetics, could not explain the origin of
sex. Yet evolutionists contend that the
reproductive system has selectively
evolved this elaborate, ubiquitous and
expensive method of reproduction.
Two questions arise with reference to
the origin of sex: i) in what kind of organisms
did sex first arise? and ii) what was the
adaptive advantage that caused sexual
reproduction to become predominant in
higher organisms? Sex would not have
evolved and been retained, unless it had
some advantage.
The asexual mode of reproduction is
common in lower organisms like
invertebrates. Binar y fission, budding,
sporulation, and fragmentation are all
different ways of asexual reproduction. As
if there is a connecting link between sexual
and asexual reproduction, some
organisms adopted alternation of
generations.
Asexual forms can produce
numerous offspring in a short time. They
can produce without needing to find a
mate. In a stable environment they
perpetuate with successful genotypes.They
also save energy and time. For animals
that possessed delicate bodies and no
apparent means of defence and needed
a speedy way of multiplication, the
asexual method of reproduction is best
suited.
SCIENCE REPORTER, MARCH 2012
But, if asexual reproduction is simple
and efficient, why then did animals have
to invent the sexual method, which is a
complex and difficult process?
In asexual reproduction the major
disadvantage is that the organisms
produce clones (genetically same
individuals) and do not produce genetic
diversity. If all organisms continue to
reproduce asexually, the genetic
variations of its species as a whole will slowly
grind to a halt and it becomes likely that a
parasite that can kill one member of the
species can wreak havoc on the entire
population, which will not be able to get
rid of the harmful effects of mutation.
For the origin of sex, it is essential that
the process of meiosis evolve to halve the
chromosome number and this ability
occurs only in gametic cells and not in
somatic cells. In paramecium, a process
of conjugation occurs that can be
considered to be the origin of sex. Two
individuals that are similar in structure
(Isogametes) undergo conjugation where
only nuclear materials are interchanged.
They undergo both mitosis and meiosis.
Isogametes are equal sized and both have
equal energy level. They have low
searching capacity and do not have any
particular attraction towards each other
to fuse and form a zygote.
In some organisms, anisogametes
are produced. The larger gametes are
specialized for storage of nutrients and
development
of
e m b r y o.
The
individuals producing larger gametes
are called females, but the gamete
number is small. Weismann proposed
a theor y, that evolution of anisogamy
from isogamy happened due to the
selection process, like increased
efficiency in finding a mating partner
in anisogamy and to supply the zygote
with sufficient amount of reserve food
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Binory Fusion
for development of embr yo. Thus, the
origin of sexual dimorphism produced
competition and increased the
chances of fusion and also created a
healthy sexual selection.
There are theories that explain why
sex exists but do not explain the origin of
sex. There is no naturalistic explanation for
the origin and maintenance of sex. We
never learn exactly when or how
independent male and female sexes
originated. How could nature evolve a
female member of a species that
produces eggs and is internally equipped
to nourish a growing embryo, while at the
same time evolving male member that
produces motile sperms? When somatic
cells maintain normal number of
chromosomes, gametic cells have only
half the original number by a process called
Darwin
Geaorge Williams
Feature Article
Mendel
John Maddox
meiosis. There is not an evolutionist on the
planet who has been able to come up
with an explanation as to how somatic cells
reproduce by mitosis while gametic cells
are produced by meiosis; also why
somatic cells die whereas germ cells are
immortal.
Promotion of gametic variation, novel
genotypes, increased resistance to
parasites and clearance of deleterious
mutations are some of the advantages of
sexual reproduction.
At the same time, sexual reproduction
takes much longer times and requires
more energy than asexual reproduction.
Sexually reproducing organisms not only
have to produce gametes but also have
to maintain them. In addition various kinds
of incompatibility factors like Rh factor pass
along more “costs” that are inherent in this
expensive means of reproduction. The
problem of tissue rejection can also arise
between mother and the embryo as the
embryo carries half of the male’s genetic
information.
Many authors have tried to explain
the origin of sex through some theories.
One such theory is the “Lottery principle”
of George C. Williams. According to him,
asexual reproduction would be like
buying a large number of lottery tickets
but giving them all the same number.
Sexual reproduction, on the other hand
would be like purchasing a small number
of tickets but giving each a different
number. Since sex introduces variability,
organisms would have a better chance
of producing a range of types rather than
merely more of the same. In the absence
of
genetic
variability,
asexually
reproducing offspring are all exact
duplicates (clones) of their parents. This
principle suggests that sex would be
favoured by a variable environment. But
distribution of sexually reproducing
The ‘Red Queen Hypothesis’
organisms is common in stable
environments like the tropics. In contrast,
where the environment is unstable (high
altitude) asexually reproducing organisms
are more.
According to the ”Tangled bank
hypothesis” where there exists intense
competition for space, food and other
resources, a tendency to show
diversification occurs. Tangled bank,
the phraseology mentioned by Darwin
in his book Origin of Species, refers to
a wide assortment of creatures, all
competing for light and food on a
“tangled bank”. To cite an example, in
a forest canopy, the space is divided
into different niches, where different
skills are needed for normal survival. A
clone,
derived
from
asexual
reproduction, specialized for one
niche, can give birth only to offspring
that can handle only the same niche.
But sexual reproduction leads to
genetic diversity and the offspring may
handle different niches.
“Red Queen hypothesis”, also referred
to as Red Queen race or effect, is an
evolutionary hypothesis. The term is taken
from the Red Queen’s race in Lewis Carroll’s
Through the Looking Glass. The Red Queen
said, “It takes all the running you can do,
to keep in the same place.” From an
evolutionar y
system,
continuing
adaptation is needed in order to maintain
its fitness relative to the systems it is coevolving with. This hypothesis emphasises
that sexual reproduction persists because
it enables host species to evolve new
genetic defences against parasites.
Parasites invent keys; host change the
locks. This hypothesis seems to be the
favourite of evolutionists worldwide in
attempting to explain the “why’ of sex.
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According to the” DNA repair
hypothesis”, the primary function of sex is
to repair the genetic material of the germ
cell. In asexual reproduction, mutation that
occurs in one generation will be passed
on automatically to the next. If an original
matter is xeroxed and the xerox copy is
xeroxed and if it continues, the quality of
later copies will be no good. In the same
way, if an animal goes on reproducing
asexually, it continues to accumulate
mutations and finally becomes unviable.
In sexual reproduction, the chance of
elimination of mutation is more or at least
the mutated gene may not be expressed.
Thus, sexual reproduction makes it easier
for an evolving organism to get rid of
deleterious changes. This theory explains
why sex exists, but it does not explain the
origin of sex.
Under certain conditions, the
asexually reproducing cells are capable
of the exchange and repair of DNA,
through a process called transformation.
It is the transfer of a fragment of DNA from
a donor cell to a recipient cell followed
by the recombination in the recipient
chromosome. This process is considered
as an early version of sex.
So, from the evolutionar y point of
view, sex is definitely an inefficient way to
reproduce, but it acts as a safeguard
against extinction. If everything remained
the same there would be no evolution. Sir
John Maddox, who was a distinguished
Editor of Nature, stated that despite
decades of speculation, we do not know
when sexual reproduction evolved.
Dr. K. Venkataraman is a retired Reader and Head,
Department of Zoology, Madura College. He has a
teaching experience of 35 years. Address: A-T-2
Porkudam Apartments, Bypass road,
Madurai-625 010
SCIENCE REPORTER, MARCH 2012