"Edward L. Ferman - Best From F&SF, 23rd Edition" - читать интересную книгу автора (Ferman Edward L)

encountered this asexual form of reproduction, hi connection with fruit trees probably, that such a
one-parent organism of non-sexual origin is called a "twig"; that is, "clone."
And what of multicellular animals?
Asexual reproduction can take place among them as well. The more primitive the animalтАФthat is, the
less diversified and specialized its cells areтАФthe more likely it is that asexual reproduction can take place.
A sponge, or a freshwater hydra, or a flatworm, or a starfish can, any of them, be torn into parts and
these parts, if kept in their usual environment, will each grow into a complete organism. The new
organisms are clones.
Even organisms as complex as insects can in some cases give birth to parthenogenetic young and, in
the case of aphids, for instance, do so as a matter of course. In these cases, an egg cell, containing only a
half set of chromosomes, does not require union with a sperm cell to supply the other half set. Instead,
the egg cell's half set merely duplicates itself, producing a full set, all from the female parent, and the egg
then proceeds to divide and become an independent organism, again a kind of clone.
In general, though, complex animals and, in particular, vertebrates do not clone but engage in sexual
reproduction exclusively.
Why?тАФTwo reasons.
In the first place, as an organism becomes more complex and specialized, its organs, tissues, and
cells become more complex and specialized as well. The cells are so well adapted to perform their highly
specialized functions, that they can no longer divide and differentiate as the original egg cells did. [This is
not mysterious. We see an analogy on the social plane. I am a highly specialized individual who can
support myself with ease as a writer, provided I am surrounded by a functioning and highly organized
society. Place me on a desert island and I shall quickly perish since I don't know the first thing about the
simplest requirements for self-support.]
This seems a terrible disadvantage. Organisms that can clone, reproducing themselves asexually,
would seem to be much better off than other organismsтАФwho must go to the trouble of finding partners
and who must engage in all the complex phenomena, both physical and chemical, involved in sexual
reproduction. Think of all the human beings who, for one slight flaw or another, can't have children тАФa
problem that would be unknown if we could just release a toe and have it grow into another individual
while we grew another toe.
Here comes the second reason, then. There's an evolutionary advantage to sexual reproduction that
more than makes up for all the inconveniences. In cloning, the genetic contents of new organisms remain
identical with those of the original organisms, except for occasional mutations. If the organism is very
efficiently adapted to its surroundings, this is useful, but it is an extremely conservative mechanism that
reduces the chance of change. Any alteration in the environment could quickly lead to the extinction of a
species.
In short, a sexually reproducing species evolves much more quickly than a cloning species, and such
difficult-to-evolve specializations as intelligence are not likely to arise in the entire lifetime of a habitable
planet, without sexual reproduction.
Yet in one specialized way cloning can take place in even the most advanced animals-even in the
human being.
Consider a human egg cell, fertilized by a human sperm cell. We now have a fertilized egg cell which
contains a half set of genes from its mother and a half set from its father.
 This fertilized egg cell cannot become an independently living organism for some nine months, for it
must divide and redivide within its mother's womb and be nourished by way of its mother's bloodstream.
It must develop, specialize, and grow larger until it has developed the necessary ability to live
independently. Even after it emerges from its mother's womb, it requires constant and unremitting care for
a period of time before it can be trusted to care for itself.
Nevertheless, the matter of necessary care is genetically irrelevant The fertilized egg is already a
separate organism with its genetic characteristics fixed and unique.
The first step in the development of the fertilized egg is that it divides into two cells that cling together.