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The Origin of Species
chapter 24
Figure 24.0 A Galápagos Islands tortoise
the origin of species
• The beginning of new forms of
life.
• Speciation, key process.
• Explains, macroevolution, the
origin of new taxonomic groups.
• Two patterns:
1) anagenesis- linear evolution
in which the entire population
changes to be different from and
to replace the ancestral
population. (Lamarckian)
2) cladogenesis- branching
evolution that creates a greater
diversity of sister organisms.
Each branch is called a clade.
Figure 24.24 The branched evolution of horses
populations & species
Populations are groups of
individuals that
a) are the same species
b) live in the same geographical
area at the same time.
A species is the largest unit of
population
a) reproductively compatible
b) Gene flow: possible to
produce viable fertile
offspring
Regardless of geographical
barriers
•
A species can be divided into subspecies, if they
become reproductively isolated.
• Subspecies are different due to pre and/or postzygotic
barriers:
a) Prezygotic reproductive barriers:
impede mating between species or hinder the
fertilization of ova.
b) Postzygotic reproductive barriers:
prevent hybrid zygote from developing into a viable,
fertile adult.
• If subspecies occur together, but remain
reproductively isolated, these subspecies may
eventually become two distinct species.
The biological species concept
is based on infertility rather than physical
similarity. (horse + donkey = mule)
species
•
species is Latin for “kind” or “appearance”
biological species concept:
• Population or group of populations
• members have the potential to interbreed with
one another to produce viable, fertile offspring
• cannot produce viable, fertile offspring with
members of other species.
Figure 24.5 A summary of reproductive barriers between closely related species
prezygotic barriers: Factors that
lead to Reproductive Isolation
A) Ecographic Isolation: (geographic) isolation
ex. Asian and African elephants
B) Habitat Isolation: two species live in different
habitats within the same area.
ex. Garter snakes- one aquatic, one terrestrial
C) Seasonal/Temporal Isolation: two species that breed
during different times of the day, seasons, or years
cannot mix their gametes.
ex. Skunks: S. gracilis mates in late summer;
S. putorius mates in late winter.
D) Behavioral Isolation: signals to attract mates,
elaborate behaviors, courtship rituals differ between
species.
ex. Eastern & Western Meadowlark songs differ
Figure 24.3 Courtship ritual as a behavioral barrier between species
D) Behavioral Isolation: signals to attract mates,
elaborate behaviors, courtship rituals differ between
species.
ex. Eastern & Western Meadowlark songs differ
E) Mechanical Isolation: anatomical incompatibility.
ex. Insect copulatory organs don’t fit together
floral anatomy specialized to one pollinator
F) Gamete Isolation: incompatibility between sperm/egg.
ex. Sperm of one species may not be able to survive
in the environment of the female reproductive tract of
another species.
gamete recognition based on complementary
molecules found on sperm/egg surfaces.
postzygotic barriers: Examples that expend
(waste) energy and lead to reproductive isolation
G) Reduced Hybrid Viability: genetically incompatible hybrid
zygotes abort development at some embryonic stage.
ex. frogs in genus Rana
H) Reduced Hybrid Fertility: results in completely or largely
sterile hybrids. Chromosomal differences (structure or
number) results in malformed gametes during meiosis.
ex. Mule- (sterile) but robust hybrid of a horse and donkey
I) Hybrid Breakdown: first generation hybrids are viable but
second generation offspring are feeble or sterile.
ex. cotton
How do new species arise?
• 1) By geographic
isolation:
• This is the way the
flora and fauna of the
Galapagos Islands
evolved.
• The barrier prevents
gene flow.
• When two different
species arise this way,
it is called allopatric
speciation.
• Greek: allos, other &
patria, homeland
Figure 24.8 Has speciation occurred during geographic isolation?
• The factors that lead to
divergence:
A) size of population
(small)
• The founder effect- genetic
drift attributed to
colonization by a limited
number of individuals
from a parent population.
B) ability of organism to
move about (isolation)
C) harshness/ differences of
new environment.
Allopatric speciation of squirrels
in the Grand Canyon
A famous example of divergent
evolution/speciation: Adaptive radiation
• Adapative radiation is evolution of
many diversely adapted species from
a common ancestor.
Example: Darwin’s Finches
• The 14 species of Finch evolved
from one species of ancestral finch.
• They have adapted to exploit
different food sources with
differently shaped beaks and feeding
behaviors.
• They exhibit character displacement
- evolutionary change driven by
competition among species for a
limited resource (eg. Food)
• Gause’s Law- competitive exclusion
2) If two different species arise from a population
without geographic barriers, it is called sympatric
speciation.
• Examples of sympatric speciation: balanced
polymorphism, polyploidy, hybridization.
• Polyploidy (having more than the diploid number
of chromosomes) and chromosomal change
• This condition is common in plants and less
common in animals.
• It can make offspring reproductively isolated
from their parental species. (post-zygotic
barrier is created in one generation)
• Polyploid population can self-pollinate, mate
with other polyploids, or reproduce by
asexual propagation.
Figure 24.13 Sympatric speciation by autopolyploidy in plants
Figure 24.15 One mechanism for allopolyploid speciation in plants
Causes of Polyploidy:
1) accidents during meiosis (autopolyploidy)
results in the wrong number of sets of
chromosomes in the gametes
2) the contribution of two different species to a
polyploid hybrid (allopolyploidy) nonhomologous chromosomes can’t align during
meiosis.
•
The chemical colchicine induces polyploidy.
summary
• In allopatric speciation, a new species
forms while geographically isolated from its
ancestor.
• Sympatric speciation requires the
emergence of some type of reproductive
barrier that isolates the gene pool of a
subset of a population without geographic
separation from the parent population.
PATTERNS OF EVOLUTION
1.
2.
3.
4.
Divergent Evolution- two or more species originate from a
common ancestor. homologous traits.
Convergent Evolution- two unrelated species that share similar
traits. Arise not from a common ancestor but because each
species has independently adapted to similar ecological
conditions or lifestyles. analogous traits. Ex. Shark,
porpoises, penguins bodies
Ex. Eyes of squids and vertebrates.
Parallel Evolution- two related species making similar
evolutionary changes after their divergence. Ex. Marsupial
and Placental mammals. analogous traits.
Coevolution- tit-for-tat evolution of one species in response to
new adaptation that appear in another species.
ex. Pollinators-Flowering Plants
Figure 25.10 Convergent evolution and analogous structures
analagous structures
convergent evolution
Punctuated Equilibrium
(proposed by Stephen J. Gould)
• A catastrophic event or major genetic change
occurs, rapid evolution and speciation occurs.
• The new population works back toward a long
period of no evolution (few or no transitional
forms.)
• The Cambrian Explosion represents a period in
time(560 MYA) where we see diversification of
animal phyla.
Patterns of macroevolution
• Phyletic gradualismevolution occurs by the gradual
accumulation of small changes.
The intermediate stages of
evolution not represented by
fossils merely testifies to the
incompleteness of the fossil
record.
• Punctuated Equilibriumevolutionary history consists of
geologically long periods of
stasis with little or no
evolution, interrupted or
“punctuated” by geologically
short periods of rapid
evolution.