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Chapter 24 Notes
The Origin of Species
The Origin of Species
There is more to evolution than just
explaining how adaptations evolve in a
Evolution must also explain
macroevolution: the origin of a new
taxonomic group
Speciation: development of a new
The Origin of Species
The fossil record displays two patterns of
speciation: anagenesis and
Anagenesis: the accumulation of changes
associated with the transformation of
one species into another
Cladogenesis: the budding of new species
from a parent species that still exists.
The Origin of Species
Concept 24.1
Species: means “kind” or “appearance”
- we distinguish plants and animals by
differences in appearance
Concept 24.1
The biological species concept
emphasizes reproductive isolation
- Ernst Mayr (1942)
Biological species concept: defines a
species as a population whose members
have the potential to interbreed in
nature and produce viable, fertile
Concept 24.1
Prezygotic and postzygotic barriers isolate
the gene pools of biological species
- any factor that impedes two species
from producing fertile hybrids
contributes to reproductive isolation
- reproductive isolation prevents
populations form interbreeding even if
their ranges overlap
Concept 24.1
Prezygotic barriers: impede mating
between species or hinder the
fertilization of the ova
Habitat isolation: two species live in
different habitats; low occurrence of
Concept 24.1
Behavioral isolation: special signals
that attract mates, along with elaborate
behaviors unique to a species
Temporal isolation: two species breed
at different times (day, seasons, years)
Concept 24.1
Mechanical isolation: anatomically
Gametic isolation: gametes rarely fuse
to form a zygote; sperm might not be
able to travel to the ova
Concept 24.1
Postzygotic barriers: prevent the
hybrid zygote from developing into a
viable, fertile adult
Reduced Hybrid Viability: abort
hybrid at some embryonic stage
Concept 24.1
Reduced hybrid fertility: hybrids are
completely or largely sterile
- Donkey + horse = mule
Hybrid breakdown: the first generation
of hybrids are fertile, but as those mate
with one another, the offspring of the
next generation are sterile
Concept 24.1
Evolutionary biologists have proposed
several alternative concepts of species
Ecological species concept: defines a
species in terms of its ecological niche,
the set of environmental resources it
- ex. A parasite may be defined by is
adaptations to its host
Concept 24.1
Pluralistic species concept: the factors
that are most important of the cohesion
of individuals as a species vary
Morphological species concept:
characterize species in terms of
structural features
Concept 24.1
Genealogical species concept: defines
a species as a set of organisms with a
unique genetic history
Concept 24.2
There are two modes of speciation based
on how gene flow among populations is
initially interrupted
Allopatric speciation: speciation takes
place in populations with geographically
separate ranges
Concept 24.2
Concept 24.2
Concept 24.2
Concept 24.2
Adaptive radiation: the speciation of
organisms through an island chain; the
islands are far apart to permit
populations to evolve in isolation, but
close enough together for the
occasional dispersion event to occur
Concept 24.2
Concept 24.2
Sympatric speciation: speciation takes
place in geographically overlapping
- chromosomal changes and nonrandom
mating reduce gene flow
Concept 24.2
Concept 24.2
The punctuated equilibrium model has
stimulated research on the tempo of
- paleontologists rarely find gradual
transitions in fossil forms
- long periods of stasis (equilibrium)
punctuated by episodes of speciation
Concept 24.2