Download What is a species?

The Origin of Species
Macroevolution
• Macroevolution: the origin of
new taxonomic groups
• Speciation: the origin of new
species
– Anagenesis (phyletic evolution):
accumulation of heritable
changes
– 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
• 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
• Proposed in 1942 by Ernst
Mayer
Prezygotic Reproductive Isolation
• Biological factors impede 2
different species from producing
viable offspring
• Prezygotic barriers: impede
mating between species or hinder
the fertilization of the ova
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Habitat (snakes; water/terrestrial)
Behavioral (fireflies; mate signaling)
Temporal (salmon; seasonal mating)
Mechanical (flowers; pollination
anatomy)
– Gametic (frogs; egg coat receptors)
Postzygotic Reproductive Isolation
• 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)
Reproductive Isolation
Modes of speciation
• Allopatric:
– populations segregated by
a geographical barrier
– can result in adaptive
radiation (island species)
• Sympatric:
– reproductively isolated
subpopulation in the midst
of its parent population
(change in genome)
• polyploidy in plants
• cichlid fishes
Causes of Sympatric Speciation
• Polyploidy
– Autopolyploid
• Individual doubles
chromosome number
(becomes tetraploid)
– Allopolyploid
• Cells have a different
chromosome number due to
successful hybridization
• Habitat Differentiation
– Different food sources
• Sexual Selection
– Females select for
different characteristics in
males
Adaptive Radiation
• Evolution of many diversely adapted species from a common
ancestor
• Introduced to new environments
– Ex: Species on the Hawaiian Islands
Tempo of Speciation
• Gradual change vs.
divergence in rapid
bursts
• Punctuated Equilibrium
– Niles Eldredge and
Stephen Jay Gould
(1972)
– Helped explain the nongradual appearance of
species in the fossil
record
Evo-Devo
Evolution has affected
developmental biology
• Heterochrony
– Rate of developmental
events differs between
organisms
– Allometric growth
(proportioning that gives
body its distinct form)
• Paedomorphosis
– Organism retains
characteristics of larval
stage in adult body
Heterochrony
Paedomorphism
No Goal in Sight
• Evolution is not
goal-oriented
• Branches could
end up in a “trend”
or in a “dead-end”
• Individual species
undergo natural
selection
– “species selection”