1. Concept 25.5: Major changes in body form can result from changes in the sequences and regulation of developmental genes
How can we understand life’s diversity?
Fossils – evidence of past biodiversity
Continental drift, mass extinction, adaptive radiation – environmental changes influence biodiversity
Genetic Change – changes in DNA sequences and regulation modify bodies/cells

2. Evolutionary Effects of Development Genes
Developmental genes guide the formation of the body from embryo to adult
Even a small change in rate, timing, and spatial pattern can produce major morphological differences between species
Let’s look at some examples!

3. Changes in Rate and Timing
Heterochrony - evolutionary change in the rate or timing of developmental events
Ex. human and chimpanzee skull differences due to small changes in relative growth rates

4. (b) Comparison of chimpanzee and human skull growth
Fig b
Chimpanzee fetus
Chimpanzee adult
Figure Relative growth rates of body parts
Human fetus
Human adult
(b) Comparison of chimpanzee and human skull growth

5. Changes in Rate and Timing
Different parts of our bodies grow at different rates

6. Changes in Rate and Timing
Paedomorphosis – retention of juvenile traits features in the adult
This adult salamander retains the gills of the larval form – adults usually have lungs
Development of reproductive organs accelerates compared to other organs

7. Changes in Spatial Pattern – placement and organization
Homeotic genes – control placement and spatial organization of body parts
Ex. Where do legs develop, where does the head form, how are the parts of a flower arranged
They are master switch genes which activate/regulate other genes needed for formation of body structures
Hox genes provide positional information in animal embryos

9. “Scarce as hens teeth”

10. Changes in Spatial Pattern – placement and organization
The transition from invertebrate to vertebrate may have been influences by alterations of Hox genes
In particular duplication of hox genes may have played an important role

11. Hypothetical vertebrate ancestor (invertebrate)
Fig
Hypothetical vertebrate
ancestor (invertebrate)
with a single Hox cluster
First Hox
duplication
Hypothetical early
vertebrates (jawless)
with two Hox clusters
Second Hox
duplication
Figure Hox mutations and the origin of vertebrates
Vertebrates (with jaws)
with four Hox clusters

12. The Evolution of Development
Changes in developmental genes can result in new morphological forms
This may answer the puzzle of the Cambrian Explosion
WE JUST DISCUSSED GENE DUPLICATION – BE SURE TO LOOK AT Figure 25.22
NEXT CHANGES IN GENE REGULATION
First we need to see how you think about how genes are expressed in cells!

13. OR

14. ANOTHER ANALOGY
EVERYDAY – NO MATTER WHAT THE WEATHER – I PUT ON SOCKS AND UNDERWEAR BUT………………..
But weather conditions influence how I select the rest of my clothing

15. Changes in Gene Regulation
Changes in the form of organisms are often by changes in the regulation of developmental genes instead of changes in their sequence
For example three-spine sticklebacks in lakes have fewer spines than their marine relatives
The gene sequence remains the same, but the regulation of gene expression is different in the two groups of fish

16. Stickleback Fish and a Gene Called Pitx 1
Why do marine stickleback have spines on their lower surface while freshwater stickleback have none (or few)?
Hypothesis A: Developmental gene Pitx 1 had changed (nucleotide sequence changed)
Test – compare DNA for Pitx 1 in both kinds of fish
Hypothesis B: Regulation of the gene Pitx 1 had changed
Test – monitor expression of Pitx 1 in developing embryo

17. Differences in the coding sequence of the Pitx1 gene? Result: No
Fig
RESULTS
Test of Hypothesis A:
Differences in the coding
sequence of the Pitx1 gene?
Result: No
The 283 amino acids of the Pitx1 protein
are identical.
Test of Hypothesis B:
Differences in the regulation
of expression of Pitx1 ?
Pitx1 is expressed in the ventral spine
and mouth regions of developing marine
sticklebacks but only in the mouth region
of developing lake stickbacks.
Result:
Yes
Marine stickleback embryo
Lake stickleback embryo
Figure What causes the loss of spines in lake stickleback fish?
Close-up
of mouth
Close-up of ventral surface

18. Marine stickleback embryo Lake stickleback embryo
Fig a
Marine stickleback embryo
Lake stickleback embryo
Close-up
of mouth
Figure What causes the loss of spines in lake stickleback fish?
Close-up of ventral surface

19. Regulatory Genes
Pitx 1 gene
Pitx 1 protein

20. CONCLUSION
LOSS OF SPINES DUE TO CHANGE IN REGULATION OF GENE NOT THE NUCLEOTIDE SEQUENCE OF THE GENE ITSELF