Download 25.6 - Laurel County Schools

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