Transcript The Origin of Species

The Origin of
Species
Chapter 24
Speciation
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The origin of new species, or speciation
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Evolutionary theory
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Is at the focal point of evolutionary theory, because the
appearance of new species is the source of biological
diversity
Must explain how new species originate in addition to
how populations evolve
Macroevolution
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Refers to evolutionary change above the species level –
brought about by microevolution (change in allele
frequencies in a population over time)
Speciation
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Speciation is the boundary between
microevolution and macroevolution.
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Speciation is the origin of new species.
This is the key process because any genus,
family, or higher taxon originates with a new
species that is novel enough to be the inaugural
member of the higher taxon.
What is a Species?
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Species is a Latin word meaning “kind” or
“appearance.”
Traditionally, morphological differences
have been used to distinguish species.
Today, differences in both body function,
biochemistry, behavior, and genetic
makeup are also used to differentiate
species.
Biological Species Concept
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Defines a species as a population or group
of populations whose members have the
potential to interbreed with one another in
nature to produce viable, fertile offspring,
but who cannot produce viable, fertile
offspring with members of other species.
Other Definitions of Species
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The morphological species concept characterizes a
species in terms of its body shape, size, and other
structural features
The paleontological species concept focuses on
morphologically discrete species known only from the
fossil record
The ecological species concept views a species in
terms of its ecological niche
The phylogenetic species concept defines a species as
a set of organisms with a unique genetic history
Similarity Between Different Species
Figure 24.2a
Reproductive Isolation
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The key to speciation is reproductive
isolation:
Reproductive isolation is the existence of
biological factors that impede members of
two species from producing viable, fertile
hybrids
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Is a combination of various reproductive
barriers
Barriers to Speciation
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Prezygotic:
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Prevent mating or successful fertilization.
Postzygotic:
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Prevent the hybrid zygote from developing into
viable, fertile adult.
Prezygotic Barriers
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Habitat isolation
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Temporal isolation
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Different behaviors to attract a mate
Mechanical isolation
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Breeding at different times
Behavioral isolation
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Living in different habitats – not encountering each other
Two species may attempt to made but physically cannot
Gametic isolation
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Gametes from two different species will not fuse to
become a zygote
Postzygotic Barriers
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Reduced Hybrid Viability: genetic incompatibility
between the two species may abort the
development of the hybrid at some embryonic
stage or produce frail offspring.
Reduced Hybrid Fertility: even if the hybrid
offspring are vigorous, they may be infertile and
cannot backbreed to either parent.
Reduced Hybrid Breakdown: in some cases,
first generation hybrids are viable and fertile, but,
when they mate with either parent or with each
other, the next generation is feeble or sterile.
Limitations to Biological Species
Concept
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Infertility concept does NOT apply to:
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Asexually reproducing organisms
Extinct species
Modes of Speciation
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Speciation can take place with or without
geographic isolation:
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Ways in which speciation CAN OCCUR:
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Allopatric Speciation
Sympatric Speciation
Two Modes of Speciation
Figure 24.6
http://bcs.whfreeman.com/thelifewire/content/chp24/2402001.html
(a) A population forms
a new species
while
geographically
isolated from its
parent population
(b) A small population
becomes a new
species without
geographic
separation from its
parent population
Allopatric Speciation
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Gene flow is interrupted or reduced when a
population is divided into two or more
geographically isolated subpopulations
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New species forms while geographically isolated
from an ancestor.
Modern examples – adaptive radiation on island
chains.
Allopatric Speciation of Squirrels in the Grand
Canyon
Figure 24.7
Adaptive Radiation
 The evolution of many diversely adapted species from a
common ancestor upon introduction to various new
environmental opportunities and challenges is called
adaptive radiation.
 Adaptive radiation typically occurs when a few
organisms make their way to new, often distant areas or
when environmental changes cause numerous
extinctions, opening up ecological niches for the
survivors.
 Fossil evidence indicates that mammals underwent a dramatic adaptive
radiation after the mass extinctions of dinosaurs 65 mya.
• Islands can be living laboratories for the study of species –
and adaptive radiation.
• See figure 24.11
• The Hawaiian archipelago is one of the world’s great
showcases of 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
Figure 24.12
Dubautia scabra
Dubautia linearis
Adaptive Radiation in Darwin’s
Finches
Sympatric Speciation
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Speciation occurs when biological barriers
prevent gene flow in overlapping populations as
seen in
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autopolyploidy
allopolyploidy
mate preference
Etc
Speciation takes place in geographically
overlapping populations
Polyploidy
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Polyploidy
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Is the presence of extra sets of chromosomes in
cells due to accidents during cell division
Has caused the evolution of some plant species
Autopolploidy
• An autopolyploid is an individual that has
more than two chromosome sets, all
derived from a single species
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
Figure 24.8
4n
Allopolyploidy
• An allopolyploid is a species with multiple sets of
chromosomes derived from different species
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
Figure 24.9
Normal gamete
n=3
Habitat Differentiation and Sexual Selection
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Sympatric speciation
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Can also result from the appearance of new ecological
niches
In cichlid fish
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Sympatric speciation has resulted from nonrandom
mating due to sexual selection
Allopatric and Sympatric Speciation: A Summary
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In allopatric speciation
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A new species forms while geographically isolated
from its parent population
In sympatric speciation
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The emergence of a reproductive barrier isolates a
subset of a population without geographic
separation from the parent species
Hybrid Zones
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Where divergent allopatric populations come
back and interbreed.
Biologist look for patterns to study reproductive
isolation.
When closely related species meet in a hybrid
zone, there are 3 possible outcomes:
1.
2.
3.
strengthening of reproductive barriers
weakening of reproductive barriers
continued formation of hybrid individuals
Timing of Evolution
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Gradualism (Lyell, Darwin)
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Slow, steady change over time.
Punctuated Equilibrium (Gould)
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Evolution occurs in spurts and is interspersed
with long periods of stasis.
Punctuated Equilibrium
• Punctuated equilibrium is a pattern of
evolution in which long stable periods are
interrupted by brief periods of more rapid
change
– Typically occurs when new niches become available
following a mass extinction
Gradualism v/s Punctuated Equilibrium
Species descended from a common
ancestor gradually diverge more and
more in morphology as they acquire
unique adaptations.
A new species changes most as it
buds from a parent species, and
then changes little for the rest of its
existence.
Convergent Evolution
• Convergent evolution describes 2
unrelated species that share similar traits.
– These similarities are not due to common
ancestry, but rather a result of similar
environmental factors.
Coevolution
Coevolution
describes the
evolution of one
species in response
to new adaptations
that appear in
another species of
which the first
shares close
interaction
Speciation Flow Chart
Section 17-4
Species
that are
Unrelated
form
Related
in
under
under
in
in
Interrelationshiops
Similar
environments
Intense
environmental
pressure
Small
populations
Different
environments
can undergo
can undergo
can undergo
can undergo
can undergo
Coevolution
Convergent
evolution
Extinction
Punctuated
equilibrium
Adaptive
radiation
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