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Hello, this is David Haig talking on genetic conflicts in human pregnancy.
Pregnancy has traditionally been viewed as
a cooperative enterprise between a mother and fetus.
Recent evolutionary theory has shown that there are also aspects of conflict,
as well as cooperation.
To understand how conflict arises during maternal-fetal relations,
one needs to understand that maternal provisioning of
a fetus is associated with what economists call an opportunity cost.
That is, resources and time committed to
one offspring are unavailable for other maternal activities.
These opportunity costs ultimately translate into
lower expected fitness of the mother, through other offspring.
I would like you to consider
the consequences of a mutation causing extra resources to be transferred to an embryo.
These extra resources have a direct benefit to the embryo
receiving them and the embryo's expected fitness increases.
However there is an indirect cost to
the mother's expected fitness through other offspring.
Let us consider a simple graphical model of the direct benefit to the fetus of resources,
and the indirect costs to its maternal siblings.
In this figure, maternal investment in the fetus is along the horizontal axis.
The indirect cost to other offspring of the mother is represented
by an increasing function that I am representing simply as a straight line here.
As the mother invests more in the current fetus,
the cost to her other reproductive opportunities increases.
The direct benefit to the fetus I am representing
as a simple increasing function under the principle that more is better,
but that this function is subject to diminishing
returns and I'm even allowing the possibility that
beyond some point extra maternal investment in
a fetus may even reduce that fetus's fitness.
Given this simple model,
we can now consider a number of different quantities that
natural selection might be minimizing or maximizing.
For example, if natural selection was
minimizing the cost to the siblings this would be achieved at
point X of zero investment in
the current fetus because that minimizes the cost to siblings.
Alternatively, natural selection might maximize the benefit to the fetus.
This occurs at point Z on the graph,
where the benefit to the fetus is a maximum.
Another possibility is that natural selection is maximizing the profit,
the difference between benefits and cost.
This occurs at point Y on the graph.
And given the simple assumptions that we have made,
the point that maximizes the difference between benefit and
cost is always going to be at a lower level of investment than the point Z,
that maximizes the benefit to the current fetus.
And it is this difference that lies at the heart of
the theory of parent-offspring conflict.