INTELLIGENT DEVELOPMENT 153in spatially well- defi ned social groups in which there are many females
and few males. When a male dies or is other wise removed from the group,
one of the females initiates a sex reversal over a period of about two days,
in which she develops the coloration, be hav ior, and gonadal physiology
and anatomy of a fully functioning male.”^12
A wide variety of such inducible phenotypes, responding to environ-
mental signals across the lifespan, are known. In some social insects,
soldier castes may be induced by collateral changes in their prey, such as
the appearance of more defensive phenotypes in aphids. Th ese changes
are akin to rabbits changing to porcupines because wolves have been
smelled in the woods— and then calling on more of their peers to do the
same.
By far the most spectacular example of lifelong plasticity, of course, is
that in the brain and in learning and be hav ior. Such plasticity has already
been mentioned, and chapter 6 is largely devoted to it.
Much developmental plasticity depends on other epige ne tic pro cesses
that alter maternal genes. You may remember from chapter 4 that these
are chemical tags placed on the DNA and/or histones (proteins surround-
ing the DNA and controlling access to genes) during egg formation. It is
known, for example, that the DNA in the egg is heavi ly tagged in this way.
Conversely, there is a massive, but highly selective, detagging in the very
early embryo. Th is is another phenomenon that shapes developmental
futures in the light of current environmental conditions.
I have shown how the princi ples of development in multicellular or-
ganisms are extrapolations of those evolved in single cells. Th ey permit
even greater fl exibility in the face of more changeable environments.
Waddington drew diagrams like that in fi gure 5.6 to illustrate development
and diff erentiation of the hy po thet i cal organism along an epige ne tic
landscape— but here depicted as emergence of attractor states, through
perturbations, in an attractor landscape.
In tracking lifelong, rapid environmental changes, however, these
princi ples have been hugely augmented by adaptable physiology, be hav-
ior, and the brain and cognitive systems. Before turning to them, it is
worth considering some evolutionary implications.
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