The History of Life on Earth

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Studying genetic mechanisms of change can provide insight into large-scale evolutionary change.

Genes that program development control the rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adult.

Heterochrony is an evolutionary change in the rate or timing of developmental events.

It can have a significant impact on body shape.

The contrasting shapes of human and chimpanzee skulls are the result of small changes in relative growth rates.

Slide 51

Allometric Growth

(a) Differential growth rates in a human

(b) Comparison of chimpanzee and human skull growth

Newborn

Age (years)

Adult

15

5

2

Chimpanzee fetus

Chimpanzee adult

Human fetus

Human adult

Slide 52

Heterochrony can alter the timing of reproductive development relative to the development of nonreproductive organs

In paedomorphosis, the rate of reproductive development accelerates compared with somatic development.

The sexually mature species may retain body features that were juvenile structures in an ancestral species.

Slide 53

Paedomorphosis - Juvenile Gills Retained by Adult Salamander

Gills

Slide 54

Changes in Spatial Pattern - Hox genes

Substantial evolutionary change can also result from alterations in genes that control the placement and organization of body parts.

Homeotic genes determine such basic features as where wings and legs will develop on a bird or how a flower’s parts are arranged.

Hox genes are a class of homeotic genes that provide positional information during development.

If Hox genes are expressed in the wrong location, body parts can be produced in the wrong location.

For example, in crustaceans, a swimming appendage can be produced instead of a feeding appendage.

Slide 55

Evolution of vertebrates from invertebrate animals was associated with alterations in Hox genes.

Two duplications of Hox genes have occurred in the vertebrate lineage.

These duplications may have been important in the evolution of new vertebrate characteristics.

The tremendous increase in diversity during the Cambrian explosion is a puzzle.

Changes in developmental genes can also result in new morphological forms.

Slide 56

Hox Genes Alterations

Vertebrates (with jaws)

with four Hox clusters

Hypothetical early

vertebrates (jawless)

with two Hox clusters