'Mongrel'

dogs, donkeys, etc. are generally
more robust than the animals we breed specially
for appearance or performance
(though not for har.diness).  But we mongrel*
humans seem to become less and less robust
and more and more prone to new and rare
conditions and diseases
the more that we keep on breeding
and getting more voluminous and bigger.

Go figure.

* mongrel is closely related to mingle.