Chapter 24 ~ The Origin of Species

Macroevolution: evolutionary change above
the species level:

 Speciation: the origin of new species
 Results in diversity of life forms
 Can be slow(gradualism) or occur in bursts (punctuated
equilibrium).
 Macroevolution: describes evolution above the level of
the species
Speciation and extinction:

Mass extinctions are often followed by
adaptive radiation
Example: radial expansion of
mammals after the extinction of
dinosaurs (horse)
Two forms of speciation:

1- Anagenesis (phyletic evolution):
accumulation of heritable changes
Transforms a species into a species
with different characteristics
Two forms of speciation:

Cladogenesis:
branching evolution
The splitting of the gene pool into
two or more separate pools, which
give rise to one or more new species
Anagenesis vs.
cladogenesis

Which form of
speciation is the
basis for
biological
diversity?
How does the environment influence speciation?
(physiological responses)

Examples:
Phototropism(plants and planaria) the
response to the presence of light
Photoperiodism(flowering plants,
chickens and eggs) response in change in
the length of the night
More examples:

Circadian rhythms: physiological cycle of
about 24 hours
Present in all eukaryotes
Can persist in the absence of external cues
Diurnal/nocturnal
Seasonal responses- hibernation,
estavation and migration
More examples:

Quorum sensing in bacteria:
regulation of gene expression in
response to cell population and
density
What is a species?

Biological species concept (Mayr):
 a population or group of populations
whose members have the potential to:
Reproductive compatibility;
Interbreeding, producing viable offspring
Cannot produce viable offspring with
members of other populations
Alternatives to the biological species
concept:

 1. morphological species concept
 2. paleontological species concept
 3. ecological species concept
 4. phylogenic species concept
Reproductive Isolation (isolation
of gene pools)

 New species arising when two populations diverge
from a common ancestor and become reproductively
isolated
 Must be maintained for species to remain distinct
Reproductive Isolation (isolation of
gene pools)

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 (cont.)

 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)
Something to think about:

 Can prezygotic and postzygotic reproductive
barriers break down?
 Are there real world examples of this??
Modes of speciation:
(based on how gene flow is interrupted)

Allopatric: populations segregated by a
geographical barrier
 Separated from its parent population
 Adaptive radiation (island species)
 Occurs when potential to interbreed and produce
fertile offspring no longer exists
 Examples: flycatchers
Modes of speciation
(based on how gene flow is interrupted)

Sympatric: reproductively isolated subpopulation in the
midst of its parent population
*no geographic isolation but reproductive barriers
*Examples: polyploidy in plants; cichlids
Modes of speciation
(based on how gene flow is interrupted)
Tempo of Speciation:
Gradualism vs. Punctuated Equilibria

 Gradualism:
 Gradual divergence from a common ancestor
 Change in morphology over a long time as unique
adaptations are acquired
Tempo of Speciation: Gradualism vs.
Punctuated Equilibrium

 Punctuated equilibrium:
 Sudden change
 Evolution in short bursts
 New species bud from parent species and then
changes little for the rest of its existence