SPECIATION
HOW SPECIES FORM
SPECIATION
The factors that lead to new species being
formed are called speciation
What is a species?
All members of a population that can
interbreed under natural conditions to
produce fertile offspring
Involves the evolution of distinct features that
isolates the new population reproductively and
therefore genetically from other species
REPRODUCTIVE ISOLATING
MECHANISM
Any biological factor that prevents two
populations from interbreeding when living in
the same region
Reproductive isolating mechanisms are either:
Prezygotic mechanisms- prevent fertilization
and zygote formation
Postzygotic mechanisms: prevent a fertilized
egg from being a viable adult
PRE-ZYGOTIC
ISOLATING
MECHANISMS
1. Behavioural Isolation
2. Temporal Isolation
3. Habitat Isolation
4. Mechanical Isolation
5. Gametic Isolation
1. BEHAVIOURAL
ISOLATION
Different species use different
courtship and mating cues to find
and attract a mate
Example: male frogs of different
species have unique calls that
attract only females of their own
species
2. TEMPORAL
ISOLATION
Different species breed at different times
of the year
Example: red and black sea urchins live
in the same location but release gametes
at different times of the year
3. HABITAT
ISOLATION
Very similar species may occupy different
habitats within a region
Example: mountain bluebird (Sialia currucoides)
lives at high elevations, while the eastern
bluebird (Sialia sialis) prefers lower elevations
and does not encounter the mountain species
4. MECHANICAL
ISOLATION
Structural differences in reproductive
organs prevent successful fertilization
Example: male damselflies transfer
sperm during unusual mating flight. The
male and female genitalia of each
species are uniquely shaped and are
physically incompatible with other
species
5. GAMETIC
ISOLATION
Male gametes may not recognize and
fertilize an egg of a different species
Example: many marine animals release
sperm and eggs into the water. The
sperm recognize eggs of their own
species through chemical markers on the
surface of the eggs.
POST-ZYGOTIC
ISOLATING
MECHANISMS
1.Hybrid Breakdown
2.Hybrid Inviability
3.Hybrid Infertility
1. HYBRID
BREAKDOWN
Mating and fertilization are
possible, but genetic differences
result in a zygote that is unable to
develop properly
Example: some species of sheep
and goat are able to mate, but the
zygote is not viable
2. HYBRID
INVIABILITY
A hybrid individual develops but either
dies before birth, or if born alive, cannot
survive to maturity
Example: when tigers and leopards are
crossed, the zygote begins to develop but
the pregnancy ends in a miscarriage or
stillborn offspring
3. HYBRID
INFERTILITY
Hybrid offspring remain healthy and
viable but are sterile
Example: mules are the sterile
hybrid of a horse-donkey cross
Homework
Pg. 363 # 13 - 16, 18
TYPES OF
SPECIATION
There are 2 types of speciation
1. Allopatric speciation
2. Sympatric speciation
1. ALLOPATRIC SPECIATION
Most new species form when a single species is
separated into two geographically isolated
populations
Physical separation prevents the exchange of
genetic information and causes populations, over
many generations, to be less and less alike.
Environmental differences will lead to different
forms of natural selection.
1. ALLOPATRIC SPECIATION
Can occur in a number of ways including:
Populations that live on remote islands
(flightless birds)
Populations split by the formation of
mountain ranges, glaciers, rivers etc.
More generally, continental drift.
2. SYMPATRIC
SPECIATION
Is a process that occurs when individuals
within a population become genetically or
reproductively isolated from the larger
population
Example: feeding behaviours have led the
hawthorn fly to become genetically different
than the apple maggot flies after the
introduction of apple trees to North America
between 1800 and 1850
PATTERNS IN
EVOLUTION
1. Adaptive radiation
2. Divergent evolution
3. Convergent evolution
4. Coevolution
1. ADAPTIVE RADIATION
The rapid evolution of a single species into
many new species (distinct but closely related)
Each new species fills a different ecological
niche (the role of a species in its environment)
Increases biodiversity
1. ADAPTIVE RADIATION
Example: Darwin’s finches
Ground finches migrated from South America
(where they ate medium-sized seeds) to the
Galapagos Islands
Island did not exhibit much competition, thus
the founding population was very successful
Since there were many food sources
available, the ecosystem grew to contain 13
different species which descended from the
founding population
2. DIVERGENT EVOLUTION
Species evolve with significantly different
morphological and behavioural traits due to
genetic drift and other selective pressures
Competition between species is minimized
as ecological niches are filled
Increases biodiversity
2. DIVERGENT EVOLUTION
Example: homologous features such as
tailbones are evident in the embryos of
these organisms.
Example: rodents in Northern Ontario (red
squirrels, chipmunks, deer mouse, flying
squirrel, porcupine, beaver) unique
characteristics of each species has been
successful and selected for by the
environment
3. CONVERGENT
EVOLUTION
When two different species, or taxa, evolve to
occupy similar ecological niches (similar traits in
distantly related species)
Natural selection will favour the evolution of
similar traits in similar environments
Each species will still retain features that
provide evidence of their distinct evolutionary
past
3. CONVERGENT
EVOLUTION
Example: dolphins and sharks
Morphologically similar, but their genetic
history is different
Both are streamlined, fast swimming,
and carnivorous, but a shark is a fish,
and a dolphin is a mammal
3. CONVERGENT
EVOLUTION
4. COEVOLUTION
One species evolves in response to
the evolution of another species
4. COEVOLUTION
Example: Yucca moths and Yucca
flowers
Yucca flowers are a certain shape
so only that tiny moth can pollinate
them. The moths lay their eggs in the
yucca flowers and the larvae
(caterpillars) live in the developing
ovary and eat yucca seeds.
THE SPEED OF EVOLUTIONARY
CHANGE
2 different models
Gradualism
Punctuated
equilibrium
GRADUALISM
A theory that
attributes large
evolutionary
change in
species to the
accumulation of
many small and
ongoing
changes and
processes
PUNCTUATED
EQUILIBRIUM
A theory that
attributes most
evolutionary
changes to
relatively rapid
spurts of change
followed by long
periods of little or
no change
Homework
Pg. 373 # 2, 3, 4, 7, 9