BIOL 112 Lecture 10
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Chapter 24: The Origin of Species
Evolution of Body Form
Genomic changes that cause new forms or function (new adaptations)
Key developmental genes:
- Spatial patterning genes: where the structure arises along body axis[1] (better understood)
- Heterochronic genes: when structures develop
Spatial Patterning Genes
where the structure arises along body axis[2] (better understood)
These genes are better understood than #Heterochronic genes
In animals, these are called Hox genes
- Hox genes encode transcription factors (TFs)
- TFs contain a homeodomain that binds to the promotor region of the DNA during trascription
- 1 hox gene TF / homeodomain can bind to several (20–30) target genes
- Target genes create proteins that form a structure on the axis
Several hox genes responsible for corresponding parts of body: grouping called a hox gene cluster.
Hox genes are homologous genes that came from a common ancestor (Fig. 21.17):
Ancestor had 1 hox gene
- gene duplication events lead to multiple hox genes that are homologous within same species (paralogs)
- paralogs may not have same function due to divergent evolution and mutation
- another species could also have the same genes (orthologs)
Hetrochronic Genes
Determines when a structure develops or how long it continues to grow (allometric[3] growth)
Some animals have very different juvenile/adult forms (e.g. caterpillar → butterfly)
(Fig. 25.19)
In humans, head seems large in proportion to body; arms and legs grow faster than head.
Jaw of chimpanzees grow much faster than that of humans.
Exaptation
Structures used for something different from original purpose Evolution is not goal-oriented:
- Dinosaurs and early reptiles had scales for insulation/decoration
- Scales developed into feathers for improved insulation (archaeopteryx)
- Modern-day birds use feathers for flight in addition to insulation/decoration