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Chapter 14
The Origin of Species
Mosquito Mystery
• Speciation is the emergence of new species
• How do we know that a distinctly new species
has evolved?
– In London, two populations of mosquitoes
exist with very little overlap in their
respective habitats
– Evidence indicates that the two species did
not diverge from one species
– In the United States the two species appeared
to hybridize into one species, which transmits
West Nile virus
– How could the mosquitoes behave like two
species on one continent and one species on
another?
14.1 The origin of species is the source of
biological diversity
• Microevolution, gradual adaptation of a
species to its environment, does not produce
new species
• Speciation, the origin of new species, is at the
focal point of evolution
• Macroevolution, dramatic biological changes
that begin with the origin of new species, has
led to Earth's great biodiversity
CONCEPTS OF SPECIES
14.2 What is a species?
• Taxonomy is the branch of biology concerned
with naming and classifying the diverse forms
of life
– The binomial system was introduced by
Linnaeus in the 18th century
• Similarities between some species and
variation within a species can make defining
species difficult
• The biological species concept
– Defines a species as a population or group
of populations whose members can
interbreed and produce fertile offspring
• Reproductive isolation of different
species prevents gene flow
– Cannot be used as the sole criterion for
species assignment
• The morphological species concept
– Classifies organisms based on observable
phenotypic traits
• The ecological species concept
– Defines a species by its ecological role
• The phylogenetic species concept
– Defines a species as a set of organisms
with a unique genetic history
14.3 Reproductive barriers keep species
separate
• Reproductive barriers serve to isolate a
species' gene pool and prevent interbreeding
– Prezygotic barriers prevent mating or
fertilization between species
• Temporal isolation: Species breed at
different times
• Behavioral isolation: There is little or no
sexual attraction between species due to
specific behaviors
• Mechanical isolation: Female and male sex
organs or gametes are not compatible
• Gametic isolation: After copulation, gametes
do not unite to form a zygote
– Postzygotic barriers operate after hybrid
zygotes are formed
• Hybrid inviability: Hybrids do not survive
• Hybrid sterility: Hybrid offspring between
two species are sterile and therefore
cannot mate
• Hybrid breakdown: Hybrids that mate
with each other or either parent species
produce feeble or sterile offspring
Video: Albatross Courtship Ritual
Video: Blue-footed Boobies Courtship Ritual
Video: Giraffe Courtship Ritual
MECHANISMS OF SPECIATION
14.4 Geographic isolation can lead to speciation
• In allopatric speciation, a population is
geographically divided
– Barriers include geologic processes such as
emergence of a mountain or subsidence of
a lake
– Changes in allele frequencies are
unaffected by gene flow from other
populations
– New species often evolve, but only after
reproductive barriers develop
LE 14-4
A. leucurus
A. harrisi
Video: Grand Canyon
14.5 Reproductive barriers may evolve as
populations diverge
• Diane Dodd tested the hypothesis that
reproductive barriers can evolve as a byproduct of the adaptive divergence of
populations in different environments
– Fruit flies bred for several generations on a
certain food tended to choose mates that
were raised on the same food
• Reproductive isolation was well under way
after several generations of evolutionary
divergence
LE 14-5a
Initial sample
of fruit flies
Starch medium
Results of
mating experiments
Female
Maltose medium
Female
Same
Different
population populations
Starch
Maltose
22
9
18
15
8
20
12
15
Mating frequencies
in experimental group
Mating frequencies
in control group
• Geographic isolation in Death Valley led to
allopatric speciation of pupfish
– By genetic drift or natural selection, the
isolated populations evolved into separate
species
Video: Galápagos Marine Iguana
LE 14-5b
A pupfish
14.6 New species can also arise within the same
geographic area as the parent species
• In sympatric speciation, new species may arise
without geographic isolation
– Not widespread among animals but
important in plant evolution
• Many plant species have evolved by
polyploidy, multiplication of the chromosome
number due to errors in cell division
– First discovered by Hugo de Vries
– Most polyploid plants arise from the
hybridization of two parent species
LE 14-6a
Parent species
Zygote
Meiotic
error
Offspring
may be
viable and
self-fertile
Selffertilization
4n = 12
Tetraploid
2n = 6
Diploid
Unreduced
diploid gametes
LE 14-6b
O. lamarckiana
O. gigas
CONNECTION
14.7 Polyploid plants clothe and feed us
• 20—25% of all plant species are polyploids
– Most result from hybridization between two
species
– Many of our food and fiber plants are
polyploids
• Bread wheat, Triticum aestivum, is a
polyploid with 42 chromosomes that evolved
over 8,000 years ago
• Today, plant geneticists create new polyploids
in the laboratory

AA
BB
Wild
Triticum
(14 chromosomes)
Triticum monococcum
(14 chromosomes)
AB
Sterile hybrid
(14 chromosomes)
Meiotic error and
self-fertilization

AA BB
T. turgidum
Emmer wheat
(28 chromosomes)
DD
T. tauschii
(wild)
(14 chromosomes)
ABD
Sterile hybrid
(21 chromosomes)
Meiotic error and
self-fertilization
AA BB DD
T. aestivum
Bread wheat
(42 chromosomes)
14.8 Adaptive radiation may occur in new or
newly vacated habitats
• Adaptive radiation: the evolution of many new
species from a common ancestor in a diverse
environment
– Occurs when mass extinctions or
colonization provide organisms with new
environments
• Island chains with physically diverse habitats
are often sites of explosive adaptive radiation
– 14 species of Galápagos finches differ in
feeding habits and beak type
– Evidence indicates that all 14 species
evolved from a single small population of
ancestors that colonized one island
Video: Galapágos Islands Overview
LE 14-8a
Cactus-seed-eater
(cactus finch)
Tool-using insect-eater
(woodpecker finch)
Seed-eater
(medium ground finch)
LE 14-8b
A
B
B
B
B
C
C
CD
C
C
D
D
TALKING ABOUT SCIENCE
14.9 Peter and Rosemary Grant study the
evolution of Darwin's finches
• Peter and Rosemary Grant have documented
natural selection acting on populations of
Galápagos finches
– Finch beaks adapted to different food
sources through natural selection, as
Darwin hypothesized
– Occasional hybridization of finch species
may have been important in their adaptive
radiation
14.10 The tempo of speciation can appear
steady or jumpy
• Gradualism model: New species evolve by the
gradual accumulation of changes brought
about by natural selection
– Darwin's original model
– Not well supported by the fossil record,
because most new species seem to appear
suddenly in rock strata without intermediary
transitional forms
• Punctuated equilibrium model: periods of rapid
evolutionary change and speciation interrupted
by long periods of little or no detectable
change
– Fossil record shows species changing most
as they arise from an ancestral species and
then relatively little for the rest of their
existence
• Most evolutionary biologists now see both
models as having merit
• Current research is focused on the tempo of
evolution
LE 14-10a
Time
LE 14-10b
Time
MACROEVOLUTION
14.11 Evolutionary novelties may arise in several ways
•
Darwin's theory of gradual change can account for the
evolution of intricate structures
– Complex structures may evolve in stages from
simpler versions having the same basic function
• Example: Eyes of molluscs
– Existing structures may be gradually adapted to
new functions
•
Exaptation: a feature that evolved in one context and
was later adapted for another function
LE 14-11
Light-sensitive
cells
Light-sensitive
cells
Fluid-filled cavity
Transparent protective
tissue (cornea)
Cornea
Lens
Eye cup
Nerve
fibers
Nerve
fibers
Layer of
light-sensitive
cells (retina)
Retina
Optic
nerve
Optic
nerve
Optic
nerve
Patch of lightsensitive cells
Eye cup
Simple pinhole
camera-type eye
Eye with
primitive lens
Complex
camera-type eye
Limpet
Abalone
Nautilus
Marine snail
Squid
Animation: Macroevolution
14.12 Genes that control development are
important in evolution
• "Evo-devo" combines evolutionary and
developmental biology
– Studies how slight genetic changes can be
magnified into significant phenotypic
changes
• Many striking evolutionary transformations are
the result of a change in the rate or timing of
developmental changes
– Paedamorphosis: retention in adult of
features that were juvenile in its ancestors
LE 14-12b
Chimpanzee fetus
Human fetus
Chimpanzee adult
Human adult
Animation: Allometric Growth
• Important in human evolution
– Large skull and long childhood provide
humans with more space for brain and more
opportunity to learn from adults
– Juvenile physical traits may make adults
more caring and protective
• Example: "evolution" of Mickey Mouse
14.13 Evolutionary trends do not mean that
evolution is goal directed
• Evolutionary trends reflect the unequal
speciation or unequal survival of species on a
branching evolutionary tree
– Example: lineages of horses that died out
• Evolutionary trends do not imply an intrinsic
drive toward a goal
– If environmental conditions change, an
apparent trend may cease or reverse
LE 14-13
Equus
Hippidion and other genera
Nannippus
Pliohippus
Hipparion Neohipparion
Sinohippus
Megahippus
Callippus
Archaeohippus
Merychippus
Anchitherium
Hypohippus
Parahippus
Miohippus
Mesohippus
Paleotherium
Epihippus
Propalaeotherium
Pachynolophus
Orohippus
Hyracotherium
Grazers
Browsers