These phenomena are compelling in light of the belief that arthropods

These phenomena are compelling in light of the belief that arthropods

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These phenomena are compelling in light of the belief that arthropods (insects, crustaceans, etc.) evolved from annelids (segmented worms; see figure below). One can envision that sequential modification of body segments, through mutations such as those described above, might allow for the evolution of insects from a worm-like ancestor. Suggestive of this is the observation that when the Antennapedia complex and the Bithorax complex are mutated the larval stage of the fruit fly is transformed into a larva with many thoracic segments rather than the wild-type pattern of differentiation into maxillary, labial and abdominal segments (see fig.below). This "throw back" to the ancestral form (i.e., the middle segments of worms are relatively undifferentiated) is called an atavism . In thinking about all possible morphologies one might be able to get with bizarre mutants in flies, and looking out at the incredible diversity of form in the natural world we might best think of this problem in terms
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Unformatted text preview: of the question: why this and not that? Are there forbidden morphologies that development cannot produce?. There is some nice evidence that the different forms seen between species may be the result of the "playing out" of discretely different developmental programs. When the developing limb bud of one salamander is treated with an inhibitor of mitosis, the number and pattern of digits developing resembles that of another species (section reading). This suggests that there are developmental constraints , i.e., that development is constrained to proceed in a certain way. If different developmental programs are carried by different lineages of organisms as they diverge from one another, these developmental constraints become phylogenetic constraints : there is no chance that horses will sprout wings because the lineage of horses (and ungulates in general) are constrained to develop and use their forelimbs in very different ways than bats, lets say....
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