Biology Reference
In-Depth Information
cell (or multicellular)
structure
natural selection
the genes determine
cell structure
A
DNA (genes)
cell (or multicellular)
structure
natural selection
the cell structure sorts
non-specific molecular
interactions
B
DNA (genes)
F
IGURE
14. Extension of evolutionary synthesis. A:
In the present context of evo-
lutionary synthesis, natural selection is exerted on phenotypes (cells and multicel-
lular organisms). It selects the most adapted variants and thus fashions the
structure of the cells. In this way the genes (mutations) corresponding to the most
adapted phenotypes are also selected. However, the relationship between the genes
and the phenotype is unidirectionally determined. The genes determine the phe-
notypes. The processes of natural selection and ontogenesis are two separate
processes.
B:
The cell structure, produced by natural selection, sorts the molecu-
lar interactions, which means that natural selection is a cause of ontogenesis.
In this case, the relationship between the genes and the phenotype is no longer
unidirectional but bidirectional. The genes provide the proteins while the pheno-
typic structure sorts their interactions from among the possible combinations. The
two processes of natural selection and ontogenesis are but one single process of
ontophylogenesis.
In the nervous system, the construction of neural cell circuits also
seems to occur through 'neuronal selection'. In the first instance
neurons are said to associate randomly owing to the huge number
of combination possibilities for their ends (synapses and dendrites),
creating very many circuits. Later, only the circuits that permit an
adequate response to the stimuli received by the organism are
retained (Changeux
et al
., 1973; Edelman and Mountcastle, 1978).
