Before sexual reproduction evolved, there were several ways for
organisms to accomplish the evolutionary task of spreading their
DNA around. There was the divide-and-conquer strategy: wrap
DNA in single cells that busily eat nutrients until they grow large
enough to split in half, leaving each half to grow and split in turn.
Bacteria are the masters of this technique, capable of doubling
their populations every few minutes, but vulnerable to mass
extermination through perils such as toothbrushes and soap.
There was also the cloning-factory strategy: grow a body with
billions of cells, and then assign the task of DNA-spreading to a
privileged minority of those cells, which bud off to make new,
genetically identical bodies. Many fungi reproduce this way,
epitomizing the rustic virtues of simplicity and fecundity. Yet this
strategy, though successful in the short term, stores up trouble for
the long term. Once a harmful mutation arises, as it sooner or
later will, there is no means of expunging it. This propensity to
accumulate damaging mutations makes such asexual species quite
unsuited to evolving much sophistication. This is because bodily
and mental sophistication require a great deal of DNA, and the
more DNA one has, the more trouble mutations cause.
In the last few hundred million years, an increasing number of
species have turned to a third way of spreading their DNA
around—the fashionable new method called sexual reproduction,
with improved mutation-cleansing powers. One grows a trillioncelled
body to produce packets of DNA, makes sure those DNA
packets find complementary DNA packets from suitable others, and
permits the DNA to combine with that of another individual to produce offspring that bear traits from both parents. Of the 1.7
million known species on our planet, most engage in sexual
reproduction. Sexual species include almost all plants larger than
a buttercup and almost all animals larger than your thumb. It
includes most insects, all birds, and all mammals, including all
primates.
organisms to accomplish the evolutionary task of spreading their
DNA around. There was the divide-and-conquer strategy: wrap
DNA in single cells that busily eat nutrients until they grow large
enough to split in half, leaving each half to grow and split in turn.
Bacteria are the masters of this technique, capable of doubling
their populations every few minutes, but vulnerable to mass
extermination through perils such as toothbrushes and soap.
There was also the cloning-factory strategy: grow a body with
billions of cells, and then assign the task of DNA-spreading to a
privileged minority of those cells, which bud off to make new,
genetically identical bodies. Many fungi reproduce this way,
epitomizing the rustic virtues of simplicity and fecundity. Yet this
strategy, though successful in the short term, stores up trouble for
the long term. Once a harmful mutation arises, as it sooner or
later will, there is no means of expunging it. This propensity to
accumulate damaging mutations makes such asexual species quite
unsuited to evolving much sophistication. This is because bodily
and mental sophistication require a great deal of DNA, and the
more DNA one has, the more trouble mutations cause.
In the last few hundred million years, an increasing number of
species have turned to a third way of spreading their DNA
around—the fashionable new method called sexual reproduction,
with improved mutation-cleansing powers. One grows a trillioncelled
body to produce packets of DNA, makes sure those DNA
packets find complementary DNA packets from suitable others, and
permits the DNA to combine with that of another individual to produce offspring that bear traits from both parents. Of the 1.7
million known species on our planet, most engage in sexual
reproduction. Sexual species include almost all plants larger than
a buttercup and almost all animals larger than your thumb. It
includes most insects, all birds, and all mammals, including all
primates.
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