Sex is Not a Disease
Traditional definition
To a geneticist, sex is normally defined as
recombination with outcrossing [1].
Recombintaion means that different sets of genes are
combined.
Outcrossing means that the different sets of genes can come
from distantly-related individuals.
Problems
This definition has a problem - it fails to distinguish
between sex and disease.
A DNA sequence that causes its bearer to grow a stalk that
injects DNA into other organisms - in the hope of taking
advantage of the genetic machinery in their cells - would
normally be regarded as a virus.
However - by the traditional geneticists definition of sex -
organisms so infected are sexual - since they wind up
incorporating genes from distantly related organisms into
their own genomes.
Sex and infectious disease are usually regarded as
separate phenomena. They are associated with different
adaptations, and one is considered highly undesirable - while
the other is not.
Here I will argue that a definintion of sex that fails to
distinguish it from infectious diseases is
counter-intuitive, misleading and unfortunate.
Discrimination
How could sex and infectious disease be distinguished from
one another?
Distinguishing sex from disease is not terribly easy.
However, there are two main ways it could be done:
One way is to consider the frequency of the outcrossing. If
all the members of the population do it call it sex. If
only a few do it, call it an infectious disease.
The other approach looks at the effect on fitness. Sex is
usually beneficial, whereas disease is
typically harmful to all the genes - except the
ones that cause the disease.
It is the second approach that I favour.
Benefits
It is easy to see how the donor benefits from sex.
They get their genes copied without having to make any
investment in constructing replication machinery or cell
walls. This benefit is much the same whether the genes
being passed on are useful to the recipient - or simply
exploit the recipient's resources to further their own
replication.
What is less obvious is how the recipient benefits
from incorporating foreign genes into their genome.
I consider whether such a benefit exists to be the key
feature which distinguishes sexual recombination from
infectious diseases.
In the cases where there is no benefit - and the recipient's
genes suffer - then the effect much like an infection - and
it would be reasonable to expect adaptations to arise which
favour disease resistance.
On the other hand, if the recipient's genes benefit from
the encounter, then the effect is best classified as sexual
in nature - and adaptations that assist the process are
the likely result.
Exceptions
Some sexual acts may harm the recipient - but should
still be considered to be sex - rather than disease.
Similarly, no doubt sometimes a disease will benefit an
particular individual - perhaps via some resulting generous
charitable donation which the sick individual would not have
received if they had been healthy.
Rather than looking at each individual act - and classifying
it according to whether it is beneficial, the usual
- or expected case should be considered.
In other words, the key question is whether adaptations on
the part of the recipient of the genes will favour pasting
the genes in, or throwing them out.
The distinction being proposed here is similar to the
one between consensual sex and rape.
Benefits
A classification scheme which distinguishes between
sex and disease seems useful.
One positive aspect about using a definition of sex that
excludes disease is that the origin of sex is no
longer the same as the origin of disease - and can be
considered as a separate issue.
References
- Mitchod, Richard E. "Eros and Evolution"
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