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The Diversity of Species
• A single species can
diverge to produce several
new species
• Ancestral South American
tortoise split to produce
the giant Galapagos Island
• Speciation is a key par of
the theory of evolution
• 3 broad processes that work together:
– Natural selection - better adapted to environment.
– Population changes over time as genes accumulate small
changes in response to natural selection - microevolution
– Accumulation of so many changes – new species formation
and multiplication of species
– Rapid series of speciation can lead to development of
whole new collection of species, genera, families or higher
classification groups - macroevolution
• Another important process in evolution that
helps us to identify current species via the
fossil record
• Vast disappearance of species over time
allows new species to evolve
The Biological Species Concept
• The biological species model defines a species
as a reproductively isolated group of
• These organisms can be identified through
consistent differences in morphological and
physiological traits as well as genetic
Mechanisms of Speciation
• Isolating mechanisms
Pre-reproductive isolating mechanisms
Post reproductive isolating mechanisms
Allopatric speciation
Sympatric speciation
• Separation of groups, preventing them from producing
fertile, viable offspring
• Can occur before or after reproduction
• After isolation, different phenotypes can occur
Pre-reproductive isolating mechanisms
Pre-reproductive isolating mechanisms
• Summarise the following types of isolating
– Geographic
– Temporal (time)
– Behavioural
– Morphological
Post-reproductive isolating mechanisms
two frogs from different species will be unable to
mate and produce fertile offspring
• Chromosomes in gametes do not line up successfully
at meiosis, so no zygotes formed
• This is an example of post-reproductive isolation.
Reproduction can occur, however no viable offspring
Post-reproductive isolating mechanisms
• Summarise the three types of postreproductive isolating mechanisms
• Explain why hybrid sterility is not a barrier in
plants as it is in animals.
Allopatric Speciation
• Allopatric speciation is when new species arise due
to isolation of a population by geographical barriers.
• Features such as rivers or mountain ranges isolate
populations of animals and plants.
• Movement of land-masses by continental drift led to
geographical isolation millions of years ago.
Sympatric Speciation
• Sympatric speciation is when new species
arise despite occupying the same
geographical area.
• Ecological barriers - although groups are not
geographically isolated from each other they
may be isolated by occupying different
habitats or breeding areas, pH and salinity.
• Behavioural barriers – a population may carry
out complex mating rituals that may create a
barrier to reproduction.
• Different timings, locations or mating dances
may result in members of a population, who
are not geographically separated, not being
able to mate with each other.
• Complete questions on page 200 of your
• Make sure that you answer questions
thoroughly and make notes on anything that
you’re not sure about.
Useful slides
• The following slides are from another source –
I have added them in for some extra notes if
you want to read them.
Isolating Mechanisms
• Since members of the same species
share a common gene pool, in order for a
species to evolve into 2 new species, the
gene pools must be separated into 2
• As new species evolve, populations
become reproductively isolated from
each other
Reproductive isolation
• When the members of 2 populations
cannot interbreed and produce
fertile offspring
Behavioral Isolation
• When two populations are capable
of interbreeding but have differences
in courtship rituals or other
reproductive strategies
Eastern & Western Meadowlark
Geographical Isolation
• When two populations are separated
by geographic barriers such as rivers,
mountains, or bodies of water
Albert & Kaibab Squirrels
Temporal Isolation
• When 2 or more species reproduce at
different times
Rana aurora - breeds January March
Rana boylii - breeds late March - May
Testing Natural Selection in Nature
Q: Can evolution be observed in nature?
The Grants
Testing Natural Selection in Nature
• Darwin hypothesized that finches had
descended from a common ancestor and
overtime, natural selection shaped the
beaks of different bird populations as
they adapted to eat different foods
• The Grants, realized that Darwin’s
hypothesis relied on two testable
1. There must be enough heritable
variation in these traits to provide
raw materials for natural selection
2. Differences in beak size and shape
must produce differences in fitness
that cause natural selection to
• The Grants identified and measured
every variable characteristic of the
birds on the island
• Their data indicated that there is a
great variation of heritable traits
among the Galapagos finches
Natural Selection
• During the…
• Rainy season – enough food for
everyone, no competition
• Dry season – some foods become scarce
• At that time, differences in beak sizes can
mean the difference between life and
• Birds become feeding specialists
Natural Selection
• The Grants discovered that individual
birds with different size beaks had
different chances of survival during a
Speciation in Darwin’s Finches
• Speciation in the Galapagos finches
occurred by founding of a new
population, geographical isolation,
changes in the new population’s gene
pool, reproductive isolation and
ecological competition
Founders Arrive
• Many years ago, a
few finches from
South American
• Species A, flew or
were blown to one
of the Galapagos
Geographic Isolation
• Later on, some birds from
species A crossed to
another island in the
Galapagos group
• The finches then became
unable to fly from island to
island and become isolated
from each other and no
longer share a common
gene pool
Changes in the Gene Pool
• Overtime,
populations on each
island became
adapted to their local
Reproductive Isolation
• Now imagine that a few
birds from the second
island cross back to the
first island
• Q: Will the population A
birds, breed with the
population B birds?
• A: Probably not
Ecological Competition
• As these two new
species live together in
the same environment,
they compete with each
other for available seeds
• The more different birds
are, the higher fitness
they have, due to less
Continued Evolution
• This process of isolation on
different islands, genetic
change, and reproductive
isolation probably repeated
itself time and time again
across the entire Galapagos
island chain
• Over many generations, it
produced the 13 different
finch species found there
Studying Evolution Since
• It is useful to review and critique the strength and
weakness of evolutionary theory
• Darwin made bold assumptions about heritable
variation, the age of the Earth, and the relationships
among organisms
• New data from genetics, physics, and biochemistry
could have proved him wrong on many counts, and
they did not
• Scientific evidence supports the theory that living
species descended with modification from common
ancestors that lived in the past
Limitations of Research
• The Grants data shows how
competition and climate change
affects natural selection
• However, they did not observe the
formation of a new species
Unanswered Questions
• Many new discoveries have led to
new hypotheses that refine and
expand Darwin’s original ideas
• No scientist suggests that all
evolutionary processes are fully
understood. Many unanswered
questions remain
Why Understanding Evolution is
• Evolution continues today
• Drug resistance in bacteria and viruses
• Pesticide resistance in insects
• Evolutionary theory helps us understand and
respond to these changes in ways that
improve human life
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