Speciation
• Process by which a daughter species evolves from
a parent species
Genetic Divergencewhen populations become reproductively isolated
Species-population of individuals who can
interbreed under natural conditions producing
fertile offspring
• Does evolution always result in a new species?
2 Concepts on Speciation
• Evolutionary species concept  every
species has its own evolutionary
history, part of which is in the fossil
record, and diagnostic traits
• Biological species concept 
reproductive isolation rather than trait
differences define a species
Reproductive Isolating Mechanisms
• Any heritable feature of body form, functioning or behavior
that prevents interbreeding between species
• Pre-zygotic and Post-zygotic
Pre-Zygotic
Mechanisms
•
•
•
•
•
Behavioral
Temporal
Mechanical
Habitat isolation
Gametic mortality
Post-Zygotic
Mechanisms
• Hybrid mortality lower in fitness, early
death
• Hybrid sterility
• Would natural selection favor pre- or
post-zygotic mechanisms?
Types of Speciation Processes
• Allopatric Speciation-species form due to
physical barriers that prevents gene flow;
mountain range, river, island
A
BARRIER
B
Species A becomes distinct from species B;
cannot mate with each other
Allopatric Speciation
• Salamanders in Central
California
• Washington Sockeye
salmon in lakes vs rivers
Allopatric Speciation: Adaptive
Radiation
• A single ancestral
species gives rise to a
variety of species, each
adapted to a specific
environment
• Hawaiian honeycreepers
• Occurred after dinosaur
extinction  mammals
diversified in only 10
million yrs!
Types of Speciation Processes
• Sympatric speciation-species forms within the
home range of an existing species in absence
of physical barrier
Species A-blue
Species B-purple
A
B
Species A and B cannot mate
Sympatric
Speciation
• Polypoidy in plants
• Autopoidy  diploid plant
produces diploid gametes
• Alloploidy hybrid plant
doubles it chromosome #
Models of Macroevolution
• Gradual Model-new
species formed through
gradual changes in traits
over time; most
consistent with fossil
record
Models of Macroevolution
• Punctual Model-new
species formed in a short
amount of time; occurs
through
bottlenecks/founder
effects, mutations,
directional selection
(natural selection)
Developmental Genes and Macroevolution
“…all animals share the same control switches for development…”
• Development of eyes is
controlled by Pax6 gene
– It did not matter if it was a
lens or compound eye
• Loss of gene results in
failure of eye to develop in
mice, humans
Developmental Genes and Macroevolution
• Development of overall
shape is controlled by
Hox genes
– Control number and
appearance of repeated
structures
• Differential expression
in chickens vs snakes
“…animal diversity is due to variation in the expression of ancient genes…”
Evolution of the Modern Day Horse
“Macroevolution is not goal-oriented”
• Does not fit a gradual
model perfectly
• Trends observed
– Increase in size
– Toe reduction
– Change in tooth
size/shape
• Modern horse
evolved 4 mya
adapted for living in
open plains
“…speciation, diversification, and extinction are common occurances…”
Darwin’s drawing in
The Origin of Species