How can we understand life’s diversity?

1.Fossils – evidence of past biodiversity

2.Continental drift, mass extinction, adaptive radiation – environmental changes influence biodiversity

3.Genetic Change – changes in DNA sequences and regulation modify bodies/cells

Concept 25.5: Major changes in body form can result from changes in the sequences and regulation of developmental genes

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!

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

Fig. 25-19b

(b) Comparison of chimpanzee and human skull growth

Chimpanzee fetus Chimpanzee adult

Human fetus Human adult

Changes in Rate and Timing

• Different parts of our bodies grow at different rates

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

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

“Scarce as hens teeth”

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

Fig. 25-21

Vertebrates (with jaws)with four Hox clusters

Hypothetical earlyvertebrates (jawless)with two Hox clusters

Hypothetical vertebrateancestor (invertebrate)with a single Hox cluster

Second Hox duplication

First Hox duplication

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!

OR

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

Changes in Gene Regulation

• Changes in the form of organisms are often by changes in the regulationregulation 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

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

Fig. 25-23

Test of Hypothesis A:Differences in the codingsequence of the Pitx1 gene?

Result:No

Marine stickleback embryo

Close-up of ventral surface

Test of Hypothesis B:Differences in the regulationof expression of Pitx1 ?

Pitx1 is expressed in the ventral spineand mouth regions of developing marinesticklebacks but only in the mouth regionof developing lake stickbacks.

The 283 amino acids of the Pitx1 proteinare identical.

Result:Yes

Lake stickleback embryo

Close-upof mouth

RESULTS

Fig. 25-23a

Marine stickleback embryo

Close-up of ventral surface

Lake stickleback embryo

Close-upof mouth

Pitx 1 gene

Pitx 1 protein

Regulatory Genes

CONCLUSION

• LOSS OF SPINES DUE TO CHANGE IN REGULATION OF GENE NOT THE NUCLEOTIDE SEQUENCE OF THE GENE ITSELF