4 The Structure of the Animat
The scheme for expression of morphology from the genetic representation is designed to generate many variations. Previously I had developed a biomorph design with an open-ended structure based on an idiosyncratic scheme for expression of morphology from the genotype. Figure 1 shows three examples from this morphology scheme.
(b)
(c)
Figure 1 Three examples of un-evolved biomorphs from an earlier scheme for morphology - without symmetry.
The black spheres indicate head coordinates.
Through a convoluted embryology, genes affect the number of limbs, branch points of each limb, and branching angles for each limb, resulting in asymmetrical forms. This scheme was implemented as an experiment to see if the pressure to affect locomotion would cause symmetrical structures to emerge from a search space of mostly asymmetrical forms. Many trial runs suggested that the search space was too large for the GA to easily converge on symmetry, and that the representation may not have been designed sufficiently for evolvability. It did not incorporate a "constrained embryology", in which there are few genes yet each gene controls a more powerful feature of the phenotype (Dawkins, 89). In discussing his own search for a useful biomorph, Dawkins refers to the emergence of segmentation in animal morphology in the natural world as a watershed event - as a likely increase in evolvability for those animals which chanced upon it. No such structural scheme existed in my earlier design.
After exploring variations on this open-ended biomorph design, I settled on a generalized bilateral-symmetric structure, which still allowed for variation, and even occasional asymmetric features, but was characterized by a more realistic embryological scheme - with segmentation. Variations in topological connectivity of limbs and angles in the joints give rise to structures resembling spiders, birds, four-legged mammals, human-like shapes, and, of course, a great variety of monsters.
In this morphology scheme, a few basic elements are always present: a central body functioning as a backbone, having from one to four segments, and opposing pairs of limbs (as many as three), each limb having from one to four segments. Thus, the simplest topology is one body segment with a pair of opposing one-segment limbs (Figure 2a). All segment lengths are set to be of equal length. Angles of limb connectivity are specified with a local (yaw, pitch, roll) coordinate system.
Figure 2 shows three typical un-evolved animats. In some cases, asymmetry can arise (Figure 2c). For instance, if the number of body segments is less than the number of limb pairs, extra limb pairs grow out from other limbs. This embryological quirk has been kept in the biomorph design, keeping with the idea that features like this might by chance produce useful strategies for locomotion.
(b)
(c)
Figure 2 Three un-evolved animats illustrating morphological variety in the search space.
A list of effects of the genes for morphology is given below:
number of body segments
pitch angle of joints between body segments
number of opposing limb pairs
pitch angle at the branch point of each limb pair
yaw angle at the branch point of each limb pair
changes in pitch angle at the branch point among consecutive limb pairs
changes in yaw angle at the branch point among consecutive limb pairs
number of segments in each limb
pitch angle of joints between limb segments
yaw angle of joints between limb segments
5 The Head
