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Transcript
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 Fig. 25-24 Pigmented cells Pigmented cells (photoreceptors) Epithelium Nerve fibers (a) Patch of pigmented cells Fluid-filled cavity Epithelium Optic nerve Nerve fibers (b) Eyecup Cellular mass (lens) Pigmented layer (retina) (c) Pinhole camera-type eye Cornea Optic nerve (d) Eye with primitive lens 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? Fig. 25-25 Recent (11,500 ya) Equus Pleistocene (1.8 mya) Hippidion and other genera Nannippus Pliohippus Pliocene (5.3 mya) Hipparion Neohipparion Sinohippus Megahippus Callippus Archaeohippus Miocene (23 mya) Merychippus Hypohippus Anchitherium Parahippus WRONG PICTURE Miohippus Oligocene (33.9 mya) Mesohippus Paleotherium Epihippus Propalaeotherium Eocene (55.8 mya) Pachynolophus Orohippus Key Hyracotherium Grazers Browsers 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 Hyracotherium Grazers 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 1TOED GRAZERS – OTHER LINEAGES ARE Fig. 25-25b MULTI-TOED BROWSERS - EXTINCT Recent (11,500 ya) Equus Pleistocene (1.8 mya) Hippidion and other genera Nannippus Pliohippus Pliocene (5.3 mya) Hipparion Neohipparion Sinohippus Megahippus Callippus Archaeohippus Miocene (23 mya) Merychippus Anchitherium Hypohippus Parahippus 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