Transcript Document

Chapter 24
The Origin of
Species
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• Fruit flies
• Horizontal gene flow
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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
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Figure 24.2 Two patterns of evolutionary change
Anagenesis is
evolution within a
lineage.
Cladogenesis is
evolution that
results in the
splitting of a
lineage.
(a) Anagenesis
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(b) Cladogenesis
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.
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Figure 24.4 Reproductive Barriers
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Habitat
isolation
Behavioral
isolation
Temporal
isolation
Individuals
of different
species
Mechanical
isolation
Mating
attempt
HABITAT ISOLATION
TEMPORAL ISOLATION
BEHAVIORAL ISOLATION
(b)
MECHANICAL ISOLATION
(g)
(d)
(e)
(f)
(a)
(c)
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Gametic
isolation
Reduce
hybrid
fertility
Reduce
hybrid
viability
Hybrid
breakdown
Viable
fertile
offspring
Fertilization
REDUCED HYBRID
VIABILITY
GAMETIC ISOLATION
REDUCED HYBRID FERTILITY HYBRID BREAKDOWN
(k)
(j)
(m)
(l)
(h)
(i)
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Figure 24.5 Two main modes of speciation
(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.
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
RESULTS
22
9
8
20
Male
Male
Starch
Maltose
Female
Starch Maltose
Mating frequencies
in experimental group
CONCLUSION
Different
Same
populations population
When flies from “starch populations” were mixed with flies from “maltose populations,”
the flies tended to mate with like partners. In the control group, flies taken from different
populations that were adapted to the same medium were about as likely to mate with each
other as with flies from their own populations.
Female
Different
Same
population populations
18
15
12
15
Mating frequencies
in control group
The strong preference of “starch flies” and “maltose flies” to mate with
like-adapted flies, even if they were from different populations, indicates that a reproductive
barrier is forming between the divergent populations of flies. The barrier is not absolute
(some mating between starch flies and maltose flies did occur) but appears to be under way
after several generations of divergence resulting from the separation of these allopatric
populations into different environments.
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Figure 24.8 Sympatric speciation by autopolyploidy in plants
Failure of cell division
in a cell of a growing
diploid plant after
chromosome duplication
gives rise to a tetraploid
branch or other tissue.
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 24.9 One mechanism for allopolyploid
speciation in plants
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Species A
2n = 4
Unreduced gamete
with 7 chromosomes
Viable fertile hybrid
(allopolyploid)
Meiotic error;
chromosome
number not
reduced from
2n to n
2n = 10
Normal gamete
n=3
Species B
2n = 6
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Normal gamete
n=3
Figure 24.10 Does sexual selection in cichlids result
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 24.11 Long-distance dispersal
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Figure 24.12 Adaptive radiation
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
Figure 24.13 Two models for the tempo of speciation
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.
Figure 24.14 A range of eye complexity among 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.
(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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Figure 24.17 Paedomorphosis
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Figure 24.18 Hox genes and the evolution of tetrapod limbs
Chicken leg bud
Zebrafish fin bud
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Region of
Hox gene
expression
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.
Figure 24.20 The branched evolution of horses
Recent
(11,500 ya)
Equus
Pleistocene
(1.8 mya)
Hippidion and other genera
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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