Rucete ✏ Campbell Biology In a Nutshell
Unit 4 MECHANISMS OF EVOLUTION — Concept 25.5 Major Changes in Body Form Can Result from Changes in the Sequences and Regulation of Developmental Genes
Major evolutionary transformations can occur through changes in genes that control organismal development. These genetic changes influence developmental timing, rates, and spatial patterns, resulting in diverse body forms and structures.
Effects of Developmental Genes (Evo-Devo)
- Evolutionary developmental biology (evo-devo) investigates how small genetic changes during development cause significant morphological differences between species.
- Changes can occur in:
- Gene sequences (mutations altering proteins).
- Gene regulation (when, where, and how strongly genes are expressed).
Changes in Rate and Timing: Heterochrony
- Heterochrony refers to evolutionary changes in the timing or rate of developmental events.
- Example:
- Different skull shapes in humans and chimpanzees result from changes in relative growth rates of skull bones.
- Rapid finger bone growth in bats created wing structures.
- Slowed growth of pelvic bones in whales led to limb reduction.
- Paedomorphosis: sexually mature adults retain juvenile features due to accelerated reproductive development. Example: Axolotl salamanders retaining larval gills.
Changes in Spatial Pattern: Homeotic Genes
- Homeotic genes determine spatial body organization (e.g., where limbs or wings develop).
- Hox genes (a subset of homeotic genes) provide positional information to cells, significantly influencing morphology.
- Example: Changes in Hox gene Ubx expression resulted in insects having fewer legs than their crustacean ancestors.
Changes in Gene Sequence
- New developmental genes, formed by gene duplication, can create new morphological forms.
- Example: Insects' Ubx gene suppresses leg formation; crustaceans’ Ubx gene does not. Experimental studies showed specific nucleotide changes in Ubx were responsible for insects' reduced number of legs.
Changes in Gene Regulation
- Changes in gene regulation (where genes are expressed) often have fewer harmful side effects than altering the gene sequence itself.
- Example: Threespine stickleback fish have reduced or absent spines in lake populations. This morphological difference results not from mutations in the Pitx1 gene itself but from changes in its regulatory regions, altering its expression only in certain body parts.
Evolutionary Novelties and Incremental Changes
- Complex structures (e.g., human eyes) evolved incrementally from simpler forms, gradually modifying existing structures.
- Even simple forms (like light-sensitive patches in limpets) provide clear survival advantages, eventually leading to complex evolutionary novelties.
In a Nutshell
Significant evolutionary changes in organisms’ body forms arise from modifications in developmental genes, either through altering gene sequences or regulation. Evolutionary developmental biology highlights how small genetic differences can dramatically reshape morphology and lead to new species.