1. Concept 25.6: Evolution is not goal oriented
Evolution is like tinkering—it is a process in which new forms arise by the slight modification of existing forms

2. Evolutionary Novelties
Most novel biological structures evolve in many stages from previously existing structures
Complex eyes have evolved from simple photosensitive cells independently many times
Exaptations are structures that evolve in one context but become co-opted for a different function
Natural selection can only improve a structure in the context of its current utility

3. Evolutionary Novelties
Most novel biological structures evolve in many stages from previously existing structures
Complex eyes have evolved from simple photosensitive cells independently many times
Exaptations are structures that evolve in one context but become co-opted for a different function
Natural selection can only improve a structure in the context of its current utility

4. (a) Patch of pigmented cells (b) Eyecup
Fig
Pigmented cells
(photoreceptors)
Pigmented
cells
Epithelium
Nerve fibers
Nerve fibers
(a) Patch of pigmented cells
(b) Eyecup
Fluid-filled cavity
Cellular
mass
(lens)
Cornea
Epithelium
Optic
nerve
Pigmented
layer (retina)
Optic nerve
(c) Pinhole camera-type eye
(d) Eye with primitive lens
Figure A range of eye complexity among molluscs
Cornea
Lens
Retina
Optic nerve
(e) Complex camera-type eye

5. Evolutionary Trends
Extracting a single evolutionary progression from the fossil record can be misleading
Apparent trends should be examined in a broader context

6. Evolutionary Trends
The evolution of a species is treelike and many of the branches do not survive.
When tracing the evolutionary history of a species consider all the evidence.
There is no drive toward a particular outcome (phenotype – physical attributes due to genes)
Does the evolutionary history of horses really show an evolutionary trend toward large size, reduced toe number, teeth for grazing?

7. WRONG PICTURE Fig. 25-25 Figure 25.25 The branched evolution of horses
Recent
(11,500 ya)
Equus
Hippidion and other genera
Pleistocene
(1.8 mya)
Nannippus
Pliohippus
Pliocene
(5.3 mya)
Hipparion
Neohipparion
Sinohippus
Megahippus
Callippus
Archaeohippus
Miocene
(23 mya)
Merychippus
Anchitherium
Hypohippus
WRONG PICTURE
Parahippus
Miohippus
Oligocene
(33.9 mya)
Figure The branched evolution of horses
Mesohippus
Paleotherium
Epihippus
Propalaeotherium
Eocene
(55.8 mya)
Pachynolophus
Orohippus
Key
Grazers
Hyracotherium
Browsers

8. ONLY MIOHIPPUS PERSISTS TO END OF OLIGOCENE – OTHER BRANCHES DO NOT
Fig a
ONLY MIOHIPPUS PERSISTS TO END OF OLIGOCENE – OTHER BRANCHES DO NOT
Miohippus
Oligocene
(33.9 mya)
Mesohippus
Paleotherium
Epihippus
Propalaeotherium
Eocene
(55.8 mya)
Pachynolophus
Orohippus
Key
Figure The branched evolution of horses
Grazers
Hyracotherium
Browsers
Hyracotherium – 55 mya; size of a large dog; 4 toes on front feet, 3 on back; teeth for browsing bushes and trees

9. SURVIVING LINEAGE OF PARAHIPPUS ARE 1-TOED GRAZERS – OTHER LINEAGES ARE MULTI-TOED BROWSERS - EXTINCT
Fig b
Recent
(11,500 ya)
Neohipparion
Pliocene
(5.3 mya)
Pleistocene
(1.8 mya)
Hipparion
Nannippus
Equus
Pliohippus
Hippidion and other genera
Callippus
Merychippus
Archaeohippus
Megahippus
Hypohippus
Parahippus
Anchitherium
Sinohippus
Miocene
(23 mya)
Figure The branched evolution of horses

10. Evolutionary Trends
Does the evolutionary history of horses really show an evolutionary trend toward large size, reduced toe number, teeth for grazing?
NO. The evolutionary history of horses is a tree or bush with many branches. Most branches/species did not survive to the present.

11. Evolutionary trends can result from natural selection.
During the mid-Cenozoic climate became drier and grasslands spread
Parahippus lineage favored – grass eaters (grazers) that could run fast (one-toe)
This trend was “driven” by environmental change – grassland

12. Species Selection Model - trends may result when species with certain characteristics endure longer and speciate more often than those with other characteristics (Stanley)
EVOLUTION IS NOT “GOAL ORIENTED”
appearance of evolutionary trends do not imply some intrinsic drive toward a particular phenotype
CUMULATIVE EFFECT OF ONGOING INTERACTIONS BETWEEN ORGANISMS AND ENVIRONMENT IS RESPONSIBLE FOR BIODIVERSITY