Chapter 25

Changes in Spatial Pattern

• Changes in genes that control the placement and organization of body parts can drive evolution.

• Hox genes → Class of homeotic genes, provide positional information in animal embryos.

• If the location of Hox gene expression changes, the position of the corresponding body part changes.

  • In crustaceans, a change in Hox gene expression produces a swimming appendage where a feeding appendage should be.

Changes in Gene Regulation

• Many morphological changes are caused by mutations affecting development gene regulation.

  • For example, three-spine sticklebacks in lakes have fewer spines than their marine relatives.

  • Sequence of the Pitx1 developmental gene is the same, but regulation of it’s expression differs between groups.

Concept 25.5: Evolution is not goal oriented

• Evolution is like tinkering— new forms arise by slight modification of existing structures or developmental genes.

Evolutionary Novelties

• Most novel biological structures evolve in many stages from simpler ancestral structures.

• For example, complex eyes have evolved from simple photosensitive cells independently many times.

• Such “simple” eyes are found in molluscs called limpets

Evolutionary Trends

• Fossil record can reveal trends in evolution.

• For example: present-day horse have teeth suitable for gazing, larger body size, and fewer toes per foot compared to their ancestor, Hydracotherium.