Descent with Modification A Darwinian View of Life

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5

(c) Dorudon (fully aquatic)

Pelvis and

hind limb

Pelvis and

hind limb

(d) Balaena

(recent whale ancestor)

Slide 28

Anatomical and Molecular Homologies

Homology is similarity resulting from common ancestry.

Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor.

Slide 29

Homologous Structures

Humerus

Radius

Ulna

Carpals

Metacarpals

Phalanges

Human

Whale

Cat

Bat

Slide 30

Comparative embryology reveals anatomical homologies not visible in adult organisms:

Human embryo

Chick embryo (LM)

Pharyngeal

pouches

Post-anal

tail

Slide 31

Vestigial structures are remnants of features that served important functions in the organism’s ancestors.

Examples of homologies at the molecular level are genes shared among organisms inherited from a common ancestor.

Slide 32

Homologies and “Tree Thinking”

The Darwinian concept of an evolutionary tree of life can explain homologies.

Evolutionary trees are hypotheses about the relationships among different groups.

Evolutionary trees can be made using different types of data, for example, anatomical and DNA sequence data.

Slide 33

Homologies and “Evolutionary Tree”

Hawks and

other birds

Ostriches

Crocodiles

Lizards

and snakes

Amphibians

Mammals

Lungfishes

Tetrapod limbs

Amnion

Feathers

Homologous

characteristic

Branch point

(common ancestor)

Tetrapods

Amniotes

Birds

6

5

4

3

2

1

Slide 34

Convergent Evolution

Convergent evolution is the evolution of similar, or analogous, features in distantly related groups.

Analogous traits arise when groups independently adapt to similar environments in similar ways.

Slide 35

Fig. 22-20

Sugar

glider

Flying

squirrel

AUSTRALIA

NORTH

AMERICA

Slide 36

Biogeography

Darwin’s observations of biogeography, the geographic distribution of species, formed an important part of his theory of evolution.

Islands have many endemic species (found only in that part of the world and nowhere else). Darwin postulated that endemic species are often closely to species on the nearest mainland or island.