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213

Unless substitutes can be found, this implies a diminishingupper KK and consequentially a

diminishing population.

Both

rr- and

upper KK-selection lead to diminished flexibility: respectively, in

progenesis, by structural simplification caused by the loss of adult genes; and by

overspecialization.

A single species in a new, pristine environment simply proliferates until that niche

is filled (rr-selection). It also explores neighbouring genomes, and if these allow it

to more successfully exploit some part of the environment (e.g., at the periphery

of the zone colonized), a new species may result. Each new species itself makes

the environment more complex, creating new niches for yet more species, and the

environment is thereby transformed into one governed by upper KK-selection.

14.9.5

Homeotic Genes

Homeotic genes regulate homeotic transformations; that is, they are involved in

specifying body structures in organisms, homeosis (or homoeosis) being a shift in

structural development. Homeotic genes encode a protein domain, the homeodomain,

which binds to DNA and regulates mRNA synthesis; that is, it is a transcription

factor. The part of the gene encoding the homeodomain is known as the homeobox,

or Hox gene (in vertebrates). It is a highly conserved motif about 180 bases long. Hox

and Hox-like genes (in invertebrates) are arranged consecutively along the genome,

and this order is projected onto, for example, the consecutive arrangement of body

segments in an insect. Although considerable work has been done on elucidating

the molecular details of homeotic transformations, it is not presently possible to

encapsulate this knowledge in an algorithm for development.

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