from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Random mating within a breeding population.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A situation in which an individual is just as likely to mate with another randomly chosen individual as any other in the population
from The Century Dictionary and Cyclopedia
- n. The principle of cessation or reversion of natural selection.
- n. Indiscriminate crossing of breeds; mongrelism.
Weismann has emphasized this idea in his doctrine of "panmixia," or the withdrawal of selection, which always results in degeneration.
Thus what Weismann terms "panmixia," or free intercrossing, will co-operate with Galton's law of "regression towards mediocrity," and the result will be that, whenever selection ceases to act on any part or organ which has heretofore been kept up to a maximum of size and efficiency, the organ in question will rapidly decrease till it reaches a mean value considerably below the mean of the progeny that has usually been produced each year, and very greatly below the mean of that portion which has survived annually; and this will take place by the general law of heredity, and quite irrespective of any _use_ or _disuse_ of the part in question.
I suppose it is inevitable that one day they will develop the technology to track a fish the size of an eel through thousands of meters of water, and that someone will go down in a submersible and film the giant orgy (referred to by scientists as a panmixia) that is imagined to take place in the Sargasso Sea.
On the other hand, biological isolating mechanisms cannot be perfected, in general, unless panmixia is prevented by at least temporary establishment of geographic barriers.
If we ignore such factors as selection, panmixia, correlation, and the effects of use and disuse during lifetime, and still regard the case of the domestic duck as a valid proof of the inheritance of the effects of use and disuse, we must also accept it as an equally valid proof that the effects of use and disuse are _not_ inherited.
Under domestication there would be a suspension of the previous elimination of reduced breast-bones by natural selection (Weismann's panmixia), and a diminution of the parts concerned in flying might even be favoured, as lessened powers of _continuous_ flight would prevent pigeons from straying too far, and would fit them for domestication or confinement.
Of the inevitability of selection and of its generally adaptive tendencies "there can be no doubt," and panmixia would tend to reduce disused parts; so that there _must always_ remain grave doubts of the alleged inheritance of the similar effects of use and disuse, unless we can accomplish the extremely difficult feat of excluding both natural and artificial selection as causes of enlargement, and panmixia and selection as causes of dwindling.
An apparent increase in this liability might arise from greater attention being now paid to it, or from increased use of harder roads; or a real increase might be due to panmixia and some obscure forms of correlation.
He failed to adequately notice the effect of panmixia, or the withdrawal of selection, in causing or allowing degeneracy and dwindling under disuse; and he hardly attached sufficient importance to the fact that rudimentary organs and other supposed effects of use or disuse are quite as marked features in neuter insects which cannot transmit the effects of use and disuse as they are in the higher animals.
The great reduction in the weight of _female_ jaws _and skulls_ evidently points to sexual selection and to panmixia under male protection.