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

(a) Patch of pigmented cells

Opticnerve Pigmented

layer (retina)

Pigmented cells(photoreceptors)

Fluid-filled cavity

Epithelium

Epithelium

(c) Pinhole camera-type eye

Optic nerve

Cornea

Retina

Lens

(e) Complex camera-type eye

(d) Eye with primitive lens

Optic nerve

CorneaCellularmass(lens)

(b) Eyecup

Pigmentedcells

Nerve fibers Nerve fibers

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. 25-25Recent

(11,500 ya)

NeohipparionPliocene(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)

Oligocene(33.9 mya)

Eocene(55.8 mya)

Miohippus

Paleotherium

Propalaeotherium

Pachynolophus

Hyracotherium

Orohippus

Mesohippus

Epihippus

BrowsersGrazers

Key

WRONG PICTURE

Fig. 25-25a

Oligocene(33.9 mya)

Eocene(55.8 mya)

Miohippus

Paleotherium

Propalaeotherium

Pachynolophus

Hyracotherium

Orohippus

Mesohippus

Epihippus

Browsers

Grazers

Key

ONLY MIOHIPPUS PERSISTS TO END OF OLIGOCENE – OTHER BRANCHES DO NOT

Hyracotherium – 55 mya; size of a large dog; 4 toes on front feet, 3 on back; teeth for browsing bushes and trees

Fig. 25-25b

Recent(11,500 ya)

NeohipparionPliocene(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)

SURVIVING LINEAGE OF PARAHIPPUS ARE 1-TOED GRAZERS – OTHER LINEAGES ARE MULTI-TOED BROWSERS - EXTINCT

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