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Chapter 24
The Origin of Species
Key Concepts
(1) Biological species concept: reproductive isolation
(2) Speciation: geographic separation or in sympatry
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(3) Macroevolution:
Biology,
Seventh Edition occurs
through accumulation of many
generations
Neil Campbell
and Jane Reeceof change and speciation events
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Figure 24.1 The flightless cormorant (Nannopterum harrisi), one of many
new species that have originated on the isolated Galápagos Islands
Speciation
that occurred
in isolation
Geographic
barrier = ?
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Patterns of Speciation
• Two basic patterns of evolutionary change can
be distinguished
– Anagenesis
– Cladogenesis
Figure 24.2 (a) Anagenesis
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(b) Cladogenesis
The Biological Species Concept
• The biological species concept
– Defines a species as a population or group of
populations whose members have the
potential to interbreed in nature and produce
viable, fertile offspring but are unable to
produce viable fertile offspring with members
of other populations
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Figure 24.3 The biological species concept is based on the
potential to interbreed rather than on physical similarity
(a) Similarity between different species. The eastern
meadowlark (Sturnella magna, left) and the western
meadowlark (Sturnella neglecta, right) have similar
body shapes and colorations. Nevertheless, they are
distinct biological species because their songs and other
behaviors are different enough to prevent interbreeding
should they meet in the wild.
(b) Diversity within a species. As diverse as we may be
in appearance, all humans belong to a single biological
species (Homo sapiens), defined by our capacity
to interbreed.
Potential to interbreed = future offspring
will have genes that come from different
individuals in the same species.
(Dog example: Great Dane vs Chihuahua)
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Figure 24.3 The biological species concept is based on the
potential to interbreed rather than on physical similarity
Problems with this definition:
If two populations are geographically separated, how do
we know if the individuals from one population are able
to breed with individuals from the other population?
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Other Limitations of the Biological Species Concept
• The biological species concept cannot be
applied to
– Asexual organisms
– Fossils
– Organisms about which little is known
regarding their reproduction
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Other Species Definitions (see pg. 476)
Morphological:
species defined by measurable characteristics
Paleontological:
fossil species identified by morphology
Ecological:
species are distinguished by ecological factors
(e.g., habitat, food)
Phylogenetic:
species = organisms with a unique genetic history,
as seen in a phylogenetic tree.
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Reproductive Isolation: characterizes all species.
• Reproductive isolation
– Is the existence of biological factors that
impede members of two species from
producing viable, fertile hybrids
– Is a combination of various reproductive
barriers
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• Prezygotic barriers
– Impede mating between species or hinder the
fertilization of ova if members of different
species attempt to mate
• Postzygotic barriers
– Often prevent the hybrid zygote from
developing into a viable, fertile adult
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Figure 24.4 Reproductive Barriers
Pre-zygotic Isolation (prior to fertilization)
Habitat Isolation ………….…... no mating attempt
Temporal Isolation …………... no mating attempt
Behavioral Isolation …………. no mating attempt
Mechanical Isolation ………… mating attempt fails
Gametic Isolation ……………. mating attempt fails
Post-zygotic Isolation (after fertilization)
Reduced hybrid viability …….. offspring likely to die
Reduced hybrid fertility ……… offspring less likely to reproduce
Hybrid breakdown ……………. first generation is fertile, but
subsequent generations not fertile
See text pgs. 474-475
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Figure 24.5 Two main modes of speciation
Allopatric speciation
Is more common
Change due to
natural selection
and drift
(a) Allopatric speciation. A
(b) Sympatric speciation. A small
population becomes a new species
population forms a new
without geographic separation.
species while geographically
isolated from its parent
population.
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Figure 24.6 Allopatric speciation of antelope
squirrels on opposite rims of the Grand Canyon
A. harrisi
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A. leucurus
Figure 24.7 Can divergence of allopatric fruit fly
populations lead to reproductive isolation?
EXPERIMENT
Diane Dodd, of Yale University, divided a fruit-fly population, raising some
populations on a starch medium and others on a maltose medium. After many generations,
natural selection resulted in divergent evolution: Populations raised on starch digested starch
more efficiently, while those raised on maltose digested maltose more efficiently.
Dodd then put flies from the same or different populations in mating cages and measured
mating frequencies.
Results: Divergent evolution
incipient speciation
Initial population
of fruit flies
(Drosphila
Pseudoobscura)
Some flies
raised on
starch medium
Mating experiments
after several generations
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Some flies
raised on
maltose medium
Figure 24.8 Sympatric speciation by autopolyploidy in plants
Failure of cell division after chromosome duplication
Failure of cell division
in a cell of a growing
diploid plant after
chromosome duplication
gives rise to a tetraploid
branch or other tissue.
Tetraploidy
Diploid gametes
Gametes produced
by flowers on this
branch will be diploid.
Offspring with tetraploid
karyotypes may be viable
and fertile—a new
biological species.
2n
2n = 6
4n = 12
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4n
Figure 24.10 Does sexual selection in cichlids result
(“sik-lids”)
in reproductive isolation?
EXPERIMENT
Researchers from the University of Leiden placed males and females of Pundamilia pundamilia and
P. nyererei together in two aquarium tanks, one with natural light and one with a monochromatic orange
lamp. Under normal light, the two species are noticeably different in coloration; under monochromatic orange
light, the two species appear identical in color. The researchers then observed the mating choices of the fish
in each tank.
Monochromatic
Normal light
orange light
P. pundamilia
P. nyererei
RESULTS
CONCLUSION
Under normal light, females of each species mated only with males of their own species. But
under orange light, females of each species mated indiscriminately with males of both species.
The resulting hybrids were viable and fertile.
The researchers concluded that mate choice by females based on coloration is the main
reproductive barrier that normally keeps the gene pools of these two species separate. Since
the species can still interbreed when this prezygotic behavioral barrier is breached in the
laboratory, the genetic divergence between the species is likely to be small. This suggests
that speciation in nature has occurred relatively recently.
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Figure 24.12 Pattern of Speciation: Adaptive radiation
“ silversword alliance” =
all descended from tarweed
Dubautia laxa
1.3 million years
MOLOKA'I
KAUA'I
MAUI
5.1
million
years O'AHU LANAI
3.7
million
years
Argyroxiphium sandwicense
HAWAI'I
0.4
million
years
Dubautia waialealae
Dubautia scabra
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Dubautia linearis
Timing of speciation events
Time
(a) Gradualism model. Species
descended from a common
ancestor gradually diverge
more and more in their
morphology as they acquire
unique adaptations.
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(b) Punctuated equilibrium
model. A new species
changes most as it buds
from a parent species and
then changes little for the
rest of its existence.
Macroevolutionary changes
Major transformations
resulting from prolonged microevolution.
Evolutionary novelties
mollusc eye complexity
Evolution of genes that control development (Evo-Devo)
Heterochrony= Differential timing of development
Allometric growth
Adult retention of juvenile morphology = paedomorphism
Changes in spatial patterns: Hox genes
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Macroevolutionary changes: eye complexity in molluscs
Pigmented cells
(photoreceptors)
Pigmented
cells
Epithelium
Nerve fibers
Nerve fibers
(b) Eyecup. The slit shell
(a) Patch of pigmented cells. The
mollusc Pleurotomaria
limpet Patella has a simple
has an eyecup.
patch of photoreceptors.
Cornea
Cellular
Fluid-filled cavity
fluid
Epithelium
(lens)
Optic
nerve
Pigmented
layer (retina)
(c) Pinhole camera-type eye.
The Nautilus eye functions Cornea
like a pinhole camera
(an early type of camera
lacking a lens).
Lens
Optic nerve
Optic nerve
(d) Eye with primitive lens. The
marine snail Murex has
a primitive lens consisting of a mass of
crystal-like cells. The cornea is a
transparent region of epithelium
(outer skin) that protects the eye
and helps focus light.
Retina
(e) Complex camera-type eye. The squid Loligo has a complex
eye whose features (cornea, lens, and retina), though similar to
those of vertebrate eyes, evolved independently.
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Figure 24.15 Allometric growth
(a) Differential growth rates in a human. The arms and legs
lengthen more during growth than the head and trunk, as
can be seen in this conceptualization of an individual at
different ages all rescaled to the same height.
Newborn
2
5
15
Age (years)
Chimpanzee fetus
(b) Comparison of chimpanzee and human skull
growth. The fetal skulls of humans and chimpanzees
are similar in shape. Allometric growth transforms the
rounded skull and vertical face of a newborn chimpanzee
into the elongated skull and sloping face characteristic of
adult apes. The same allometric pattern of growth occurs in
humans, but with a less accelerated elongation of the jaw
relative to the rest of the skull.
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Human fetus
Adult
Chimpanzee adult
Human adult
Figure 24.16 Heterochrony and the evolution of
salamander feet in closely related species
(a) Ground-dwelling salamander. A longer time
peroid for foot growth results in longer digits and
less webbing.
Timing of development differs
for certain parts of the body.
(b) Tree-dwelling salamander. Foot growth ends
sooner. This evolutionary timing change accounts
for the shorter digits and more extensive webbing,
which help the salamander climb vertically on tree
branches.
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Paedomorphosis: retention of juvenile characteristics in adult
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Figure 24.18 Hox genes and the evolution of tetrapod limbs
Hox genes affect position
and distance of developing
structures.
Chicken leg bud
Region of
Hox gene
expression
Zebrafish fin bud
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Figure 24.19 Hox mutations and the origin of vertebrates
Hypothetical vertebrate
ancestor (invertebrate)
with a single Hox cluster
First Hox
duplication
1 Most invertebrates have one cluster of homeotic
genes (the Hox complex), shown here as colored
bands on a chromosome. Hox genes direct
development of major body parts.
2 A mutation (duplication) of the single Hox complex
occurred about 520 million years ago and may
have provided genetic material associated with the
origin of the first vertebrates.
3 In an early vertebrate, the duplicate set of
genes took on entirely new roles, such as
directing the development of a backbone.
Hypothetical early
vertebrates (jawless)
with two Hox clusters
Second Hox
duplication
Vertebrates (with jaws)
with four Hox clusters
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4 A second duplication of the Hox complex,
yielding the four clusters found in most present-day
vertebrates, occurred later, about 425 million years ago.
This duplication, probably the result of a polyploidy event,
allowed the development of even greater structural
complexity, such as jaws and limbs.
5 The vertebrate Hox complex contains duplicates of many of
the same genes as the single invertebrate cluster, in virtually
the same linear order on chromosomes, and they direct the
sequential development of the same body regions. Thus,
scientists infer that the four clusters of the vertebrate Hox
complex are homologous to the single cluster in invertebrates.
Evolution Is Not Goal Oriented
Recent
(11,500 ya)
Equus
Hippidion and other genera
Pleistocene
(1.8 mya)
Evolution
of horses
Nannippus
Pliohippus
Hipparion Neohipparion
Pliocene
(5.3 mya)
Sinohippus
Megahippus
Callippus
Archaeohippus
Miocene
(23 mya)
Merychippus
Hypohippus
Anchitherium
Parahippus
Miohippus
Oligocene
(33.9 mya)
Mesohippus
Paleotherium
Epihippus
Propalaeotherium
Eocene
(55.8 mya)
Pachynolophus
Orohippus
Key
Hyracotherium
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