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Humans are bilateral and have biaxial symmetry. However, small variations in body symmetry occur. One foot may be slightly larger than another and we might never know. Because the eyes are so important in communication the condition called heterochromia iridium is very noticeable. Body plans are very noticeable aspects of the phenotype.

In guppy courtship the male displays himself to the female. Some males are more orange than others and some males have asymmetry with one side being more brightly colored orange than the other side. A pattern in the courtship display is shown in the graphs (Gross et al., Proceedings of the Royal Society B:Biological Sciences, 274, 2007).

This figure is a graph that has a vertical axis labeled Female preference and a horizontal axis labeled Male orange Color. Majority of the orange dots cluster about a quarter into the line and hover closer to it with some of them variously underneath the line and a few orange dots to the upper right corner. The second graph is labeled on the vertical axis Male Display asymmetry and the horizontal line labeled Male orange asymmetry. Most of the dots clutter along the diagonal line.
Figure 34.33

A. Analyze the data to support the claim that the female guppy is responding to information, that the male guppy is aware of this response, and that this communication affects natural selection.

If there were no process that maintained variation in the gene controlling the orange coloration then perhaps all male guppies would symmetrically bright orange. Yet variation remains.

B. A guppy is a small fish and small fish are eaten by bigger fish. Make and justify a claim regarding the effect of allelic variation on the stability of the guppy population.

Phenotype is an expression of the genotype and within a population genotypic variation occurs. For a population to maintain stability in a changing environment genetic variation within the population provides selective advantage at the population level.

C. Make and justify a claim regarding the effect of genetic variation in guppies on the stability of an ecosystem of which the guppy is a member.

The fluctuation of symmetry in body plan of an organism, such as the asymmetry of guppy coloration, is unusual. Large scale fluctuations where the fully developed organism is viable, such as the emergence of an appendage from the eye of the Drosophila, are very rare. The overall body plan is controlled in Drosophila by a cluster of genes called the Hox genes. In the fruit fly there is a single cluster and the arrangement of genes in the genome is a map of the anterior to posterior body plan. In all vertebrate multiple Hox clusters control the development of the body plan and they too show the sequential anterior to posterior arrangement in the genome.

D. During development the transcription factors expressed by the Hox genes initiate and terminate the expression of specialized cells and tissues. Describe the evidence for shared ancestry in this conserved strategy for communication between cells through regulation of transcription factors.

E. Describe one other example of conserved core shared by all domains or within one domain.