Download Lecture #21 Date ______ Macroevolution

8/3/2009
Lecture #21
Date ________
• Chapter 24 ~
The Origin of
Species
Macroevolution: the origin of new taxonomic groups
• Macroevolution refers to
evolutionary change above the
species level
• Speciation: the origin of new species
– Ex: Monkey flower (Lewis v. Cardinal
hybrids) speciation has shown mutations in
as few as 8 genes can result in speciation
• Two basic patterns of evolutionary
change can be distinguished
– 1- Anagenesis (phyletic evolution):
accumulation of heritable changes
– 2- Cladogenesis (branching evolution):
budding of new species from a parent
species that continues to exist (basis of
biological diversity)
Macroevolution
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What is a species?
• Biological species concept
(Mayr): a population or group
of populations whose members
have the potential to interbreed and
produce viable, fertile offspring
(genetic exchange is possible and
that is genetically isolated from
other populations)
Reproductive Isolation (isolation of gene pools), I
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Prezygotic barriers: impede mating
between species or hinder the fertilization
of the ova
Habitat (snakes; water/terrestrial)
Behavioral (fireflies; mate signaling)
Temporal (salmon; seasonal mating)
Mechanical (flowers; pollination
anatomy)
Gametic (frogs; egg coat receptors)
Reproductive Isolation, II
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Postzygotic barriers: fertilization occurs, but
the hybrid zygote does not develop into a
viable, fertile adult
Reduced hybrid viability (frogs; zygotes
fail to develop or reach sexual maturity)
Reduced hybrid fertility (mule; horse x
donkey; cannot backbreed)
Hybrid breakdown (cotton; 2nd
generation hybrids are sterile)
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Modes of speciation
(based on how gene flow is interrupted)
• “Modes of speciation” address the
origins of reproductive isolation.
• Allopatric:
– populations segregated by a geographical
barrier
– genetic divergence arises largely thru
natural selection, mutation, and genetic drift
– believed to be the most common mode of
speciation
– may result in adaptive radiation (island
species)
• Sympatric:
– reproductively isolated subpopulation in the
midst of its parent population (change in
genome)
– Genetic divergence arises largely through
diversifying selection and polyploidy.
– polyploidy in plants; cichlid fishes
Parapatric:
– no specific extrinsic barrier to gene flow
– population is continuous but the
population does not mate randomly
– form of speciation that occurs due to
variations in the mating habits of a
population within a continuous
geographical area.
Peripatric:
– reproductive isolation evolves in a small
population, isolated from its parent
population.
– is a special version of the allopatric
speciation mode and happens when one of
the isolated populations has very few
individuals
– also known as founder effect speciation.
– genetic divergence arises largely through
genetic drift and natural selection.
– proposed explanation for the rapid
speciation of Hawaiian Drosophila.
Hawaiian fruit fly
Drosophila setosimentum
• 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.
2n = 6
Gametes produced
by flowers on this
branch will be diploid.
Offspring with tetraploid
karyotypes may be viable
and fertile—a new
biological species.
2n
4n
4n = 12
Figure 24.8
• An allopolyploid is a species with multiple sets of
chromosomes derived from different species
• Ex: Triticale (has 6 chr. sets) is a hybrid of wheat (4 chr. sets)
and rye (2 chr. sets)
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Unreduced gamete
with 7 chromosomes Viable fertile hybrid
(allopolyploid)
Species A Meiotic error;
2n = 4 chromosome
number not
reduced from
2n to n
2n = 10
Normal gamete
n= 3
Normal gamete
n= 3
Species B
2n = 6
Figure 24.9
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Triangle of U: Overview of genetic relationships between various
members of the genus Brassica. (a type of mustard plant) Chromosomes
from each of the genomes A, B and C are represented by different colours.
The cartoon shows the origin of the AABB, AACC and BBCC
species which have chromosome sets from their AA, BB and CC ancestors.
• In cichlid fish sympatric speciation (Speciation takes
place in geographically overlapping populations ) has
resulted from nonrandom mating due to sexual selection
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
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.
CONCLUSION
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.
Punctuated equilibria
• Tempo of speciation:
– gradual vs. divergence in
rapid bursts
– Niles Eldredge and Stephen
Jay Gould (1972) helped
explain the non-gradual
appearance of species in the
fossil record
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