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• Chapter 24 ~
The Origin of
Species
Macroevolution: the origin of new taxonomic groups
(change above species level)
Microevolution: changes in a specific gene pool
•
Speciation: the origin of new
species
•
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)
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)
• Usually cannot interbreed with
members of other species
Species
• Gene flow continues through
generations
• Reproductively united - results in
change slowly through time
• Reproductively isolation - results in
speciation
Reproductive Isolation (isolation of gene pools)
(Prezygotic Barriers)
• Prezygotic barriers: impede mating
between species or hinder the fertilization
of the ova
•
- Usually enough to prevent
mating between species
• Habitat -occupation of different niche
(snakes; water/terrestrial)
• Behavioral usually used during sexual
selection (fireflies; mate signaling)
• Temporal - different time
• ex) - salmon; seasonal mating)
• Mechanical - Size/shape of organs]
• ex- flowers; pollination anatomy
• Gametic - gametes are incompatible
• ex. - frogs; egg coat receptors
Reproductive Isolation
- If you are really determined
• Postzygotic barriers: fertilization occurs, but
the hybrid zygote does not develop into a
viable, fertile adult
• Reinforce Pre-mating factors
• 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)
Modes of speciation
(based on how gene flow is interrupted)
• Allopatric:
populations segregated
by a geographical
barrier; can result in
adaptive radiation
(island species)
– Geographical Barriers
– Adaptive Radiation
Geographical Barriers
•
After separation genetic drift may occurdivergence of populations gene pool (that is
microevolution)
•
New species are then formed.
•
The subset of the population may
under-represent the main population.
(May occur along outer edges of large
population that form the fringes of the
parent population.)
• As increase in population occurs,
genetic drift increases the
difference between the original
and new population.
•
May be experiencing
environmental/selective pressures
different from the main group.
Adaptive Radiation
Modes of speciation
(based on how gene flow is interrupted)
• Sympatric:
reproductively isolated
subpopulation in the midst
of its parent population
(change in genome)
• polyploidy (in plants) - caused
by failure to separate in cell
division
• Autopolyploid - more than 2
sets of chromosomes
• Allopolyploid- (hybrid that
became fertile)
How fast does speciation
occur?
(Macroevolution)
• Tempo of speciation:
• Two Schools of Thought
• Gradualism - evolution
proceeds slowly at a
constant rate
• Darwin was a gradualist
• populations slowly diverge
• appearance of species in the
fossil record
Punctuated equilibria
• Niles Eldredge and Stephen Jay
Gould (1972)
• Most of species existence is spent in
stasis (w/o change or in equilibrium)
and a very small amount of time (tens of
thousands of years!) is involved in
speciation.
• changes are brought about by rapid
changes in the environment such as
a mass extinction of species with
few survivors.
(Founders/bottleneck).
• helped explain the non-gradual
appearance of species in the fossil
record
•
Which follows which
school of thought?
Evolution is not Goal
Oriented
•
How could a new species result?