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

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

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

(a) Patch of pigmented cells (b) Eyecup Fig. 25-24 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 25.24 A range of eye complexity among molluscs Cornea Lens Retina Optic nerve (e) Complex camera-type eye

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

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?

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 25.25 The branched evolution of horses Mesohippus Paleotherium Epihippus Propalaeotherium Eocene (55.8 mya) Pachynolophus Orohippus Key Grazers Hyracotherium Browsers

ONLY MIOHIPPUS PERSISTS TO END OF OLIGOCENE – OTHER BRANCHES DO NOT Fig. 25-25a 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 25.25 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

SURVIVING LINEAGE OF PARAHIPPUS ARE 1-TOED GRAZERS – OTHER LINEAGES ARE MULTI-TOED BROWSERS - EXTINCT Fig. 25-25b 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 25.25 The branched evolution of horses

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.

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

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