Grade 11 University Biology – Unit 3 Evolution
Speciation: How Species Form
Section 9.2 Pages 360-373
How are species different? You must examine physiology, biochemistry, physical features,
behaviour and genetics to separate one species from another species.
A biological species is a population or group of populations whose individuals can interbreed to
produce viable, fertile offspring that can also interbreed. Yet, some individuals change so much
that they can no longer produce viable, fertile offspring with other members of the original
population. Thus, a new species has evolved.
Speciation
 Formation of new species from existing species
Two populations may become
reproductively isolated over time…and
become two species if there is little or
no gene flow between the
populations.
There are two types of reproductive
isolating mechanisms: (1) PREZYGOTIC isolating mechanisms and
(2) POST-ZYGOTIC isolating
mechanisms.
Pre-Zygotic Isolating
Mechanisms
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A barrier that either impedes
mating between two species
or prevents fertilization of the eggs of individuals if individuals from different species
attempt to mate (also called Pre-Fertilization Barrier)
There are five kinds of pre-zygotic isolating mechanisms
 Behavioural Isolating Mechanisms are special signals or behaviours that are
SPECIES SPECIFIC preventing interbreeding with closely related species (see
Figure 9.13 on Page 361 of Biology 11).
 Habitat Isolating Mechanisms are two species may live in
the same general area BUT different habitats so that they
rarely encounter each other…and thus, do not interbreed
 Temporal (Time) Isolating Mechanisms are two organisms
occupying of the same habitat but breed at different times.
The diagram shows the time of mating for two frogs…the
two ranges overlap but the two species reproduce at
different times of the day, season or year. Time isolates the
species.
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Mechanical Isolating
Mechanisms are closely
related species that attempt
to mate but fail to fertilize but
they are anatomically
incompatible
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Gametic Isolating Mechanisms prevent the fusion of gametes (egg and sperm)
from different species...even if they do meet.
Post-Zygotic Isolating Mechanisms
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A barrier that prevents hybrid zygotes from developing into viable, fertile individuals (also
called Post-fertilization Barrier)
There are three types: (1) Hybrid Inviability, (2) Hybrid Sterility and (3) Hybrid Breakdown
Hybrid Inviability is genetic incompatibility that prevents the development of a zygotic
hybrid (e.g., humans and apes cannot produce offspring)
Hybrid Sterility is two species mate but the offspring hybrid is sterile (e.g., Horse and
donkey mate but the mule offspring is sterile. Why? The parent chromosome differ in
number and structure)
Hybrid Breakdown occurs when the first-generation hybrids are fertile, but if two hybrids
mate or with an individual from the parent population, the second generation is sterile
and/or weak.
Types of Speciation
Section 9.2 Pages 363-366
Speciation
Species are defined either morphologically (individuals that look like each other) or biologically
(group of interbreeding individuals who are reproductively isolated from other such groups).
SPECIATION is the formation of new species from existing species. This is
MACROEVOLUTION. To work, it requires populations of organisms to become – and largely
remain – genetically isolated from one another.
There are two types of speciation: (1) sympatric and (2) allopatric.
Sympatric Speciation
 Populations in the same geographic area diverge and become reproductively isolated
 Factors such as
chromosomal changes
(plants) and non-random
mating (animals) alter gene
flow
 More common in plants Errors in cell division giving
an extra chromosome
(polyploidy (3n)) which allows
plant to self-pollinate OR two
diploid gametes fuse (2n +
2n) to produce a tetraploid
(4n) which reproduce to
produce diploid
gametes...and self-pollinates
to produce more tetraploids,
The result is a new species
Allopatric Speciation
 Populations are separated by
a geographic barrier, and
consequently, genetically
diverge AND they cannot
interbreed once barrier is
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removed if barrier remains in place long enough for the populations to become
reproductively incompatible
Also called Geographical Speciation
Once populations are reproductively isolated, allele frequencies in the two populations
begin to diverge due to natural selection, mutation, genetic drift and/or gene flow.
Darwin’s finches are an example of Allopartic Speciation. (see Figure 9.17 on Page 366).
Ecological Niche
 The ecological role and physical distribution of a species in its environment
Adaptive Radiation
 To radiate --- spread outward, one
species diversifies to many
 The diversification of a common
ancestral species into a variety of
differently adapted species.
 The most common situations
giving rise to adaptive radiation
occur following mass extinctions
OR migrations into new,
unoccupied regions. The feature
shared by these two situations is
the availability of a variety of
ecological niches.
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Divergent Evolution
 Divergent evolution is when descendants
of a species evolve separate traits.
 A pattern of evolution in which species
that were once similar to an ancestral
species diverge or become increasingly
distinct
It occurs when populations change to adapt to different
environmental conditions
Most people think of evolution as divergent
If different selective pressures are placed on a
particular organism, a wide variety of adaptive traits
result. If only one structure on the organism is
considered, these changes either add to the original
function of the structure or change it completely. Thus,
divergence can lead to speciation (i.e., development of
a new species).
An example is the human foot. Since we no longer
swing in trees and we walk upright, different physical
characteristics required for balance, movement and
speed evolve as the favoured trait.
Convergent Evolution
 Convergent evolution is when to unrelated lineages become more alike by developing
similar traits.
 A pattern of evolution in which similar traits arise because different species have
independently adapted to similar environmental conditions
 Convergent evolution takes place when species of different ancestry begin to share
analogous traits because of a shared environment or other selection pressures.
Environmental circumstances that require similar developmental or structural alterations

for the purposes of adaptation lead to convergent evolution even though the species
differ in descent.
An example is birds, bats and flying insects. Each evolved from a different ancestor; yet,
they fly.
Speed of Evolutionary Change
There are two models of the rate of change: (1) gradualism and (3) punctuated equilibrium. Both
are accepted.
Gradualism
 Evolutionary change is slow and
steady, before and after a divergence
 Big changes occur by the
accumulation of many small changes.
Punctuated Equilibrium
 Suggested by Niles Eldredge and
Stephen Jay Gould, evolutionary
history as long period of stasis
(equilibrium or no change) interrupted
by periods of divergence
 Most species undergo most
morphological change when
divergence from their parents first
occurs. After that, there is relatively
little change.
Mass Extinctions
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Environmental influences create
selective pressures, both positive and
negative. In some cases, the
pressures drive the species to
extinction. After a mass extinction, the
rise in species diversity is slow.
 Overall, biological diversity has
increased since the Cambrian period
(500 MYA). Yet, there have been
periods of sharp decline. These are
mass extinctions. Five mass
extinctions have been identified with
the most severe occurring 250 MYA
when 96% of all species went extinct
(see Figure 9.21 on Page 371).
Cretaceous extinction (65 MYA) was triggered by
The
the impact of a large asteroid with Earth. This led to the extinction of the dinosaur.
Human Activities
Human activities create fragmented habitats (e.g., highways divide forests, land converted to
farms separate woodlands). These geographic barriers may lead to allopatric speciation and
adaptive radiation. Unregulated human change can cause populations to decline (e.g.,
bottleneck effect / genetic drift resulting in loss of genetic diversity).
Homework
 Page 373, Questions 1-4, 7, 8, 11, 13