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Microevolution
• Microevolution: changes in allele frequencies and physical
traits within a population and species
• So we know that alleles that allow cheetahs to run faster will
be favoured
• But how did the cheetah arise as a new species?
What is a Species?
• All members of a population with a common
gene pool
• These members are able to interbreed under
normal conditions and produce fertile
offspring
How Species Arise
• In the formation of a new species, one constant is
always seen:
Distinct features arise that isolate the new species
reproductively (and therefore genetically) from other
species
How Species Arise
• For a variety of
reasons, members
of the new species
are not able to
produce fertile
young with other
species
• This is known as
Reproductive
Isolation
Mechanisms of
Reproductive Isolation
• Any behavioural, structural, or biochemical
trait that prevents individuals from
reproducing successfully
• Two types
– Prezygotic Mechanisms
– Postzygotic Mechanisms
Prezygotic Mechanism
• Prezygotic Mechanism: A reproductive
isolating mechanism that prevents
fertilization and zygote formation
Prezygotic Mechanism
• These are traits between species that prevent
them from being able to create a zygote
(fertilized egg)
• These species may live in the same region, but
these factors prevent breeding from occurring
• There are five types of prezygotic mechanisms
1. Behavioural Isolation
• Different species
use unique
courtship rituals
to find a mate
Example: Birds of
Paradise
2. Temporal Isolation
• Different species breed at different times
Example: Field Crickets
3. Ecological Isolation
• Similar species may inhabit different habitats
within a region
Example: Eastern and Mountain Bluebirds
4. Mechanical Isolation
• Differences in physical features that make two
species unable to mate
Example: Damselflies
5. Gamete Isolation
• Male gametes
are unable to
recognize and
fertilize egg cells
of another
species
Example: Marine
Animals
Postzygotic Mechanism
• Postzygotic Mechanism: An isolating
mechanism that prevents maturation and
reproduction in the offspring of interspecies
reproduction
Postzygotic Mechanism
• In this case, species are able to mate and
create a zygote
• The zygote is either unable to make it to term,
or the offspring is sterile
Postzygotic Mechanisms
• There are three types of postzygotic
mechanisms:
1. Zygotic Mortality
• Mating and fertilization are possible
• Genetic differences result in a zygote that is
unable to develop
Example: Sheep and Goats
2. Hybrid Inviability
• A hybrid individual
develops, but cannot
survive to maturity
– May die before birth
– May die before
reaching maturity
Example: Tigers and
Leopards
3. Hybrid Infertility
• Hybrid offspring
remain healthy
and viable but
are sterile
– Unable to
reproduce and
have their own
offspring
Example: Mules
Allopatric Speciation
• Allopatric Speciation: the formation of a new species
as the result of evolutionary changes after a period of
geographic isolation
• Physical isolation means that these species are no
longer able to exchange genetic info
– Mutations that arise in one population will not be shared
with the other
Allopatric Speciation
• After some time has passed in geographic
isolation, a reproductive isolating mechanism
will develop
Mechanisms of Allopatric Speciation
• How does this
occur?
– Isolated on
remote islands
– Mountain ranges
form between
populations
– Continental drift
Sympatric Speciation
• Sympatric Speciation:
The formation of new
species within the
same geographic area
• This occurs as
populations become
genetically isolated
from the larger
population
Sympatric Speciation
• This can occur through
human action
• The Hawthorne Fly:
– Apples were introduced to
North America from
Europe between 1800 and
1850
– Some flies started laying
eggs on apples
– Today there are two
species:
• one whose maggots feed
mostly on apples
• One whose maggots feed
mostly on hawthorn
• This is an example of disruptive
selection!
Sympatric Speciation
Instant Sympatric Speciation
• Occurs in species that can reproduce asexually
• A random mutation in an individual may
eventually lead to the formation of a new
species
• This has probably happened many times in
plants – many are able to sexually and
asexually reproduce
Polyploidy
• Remember meiosis?
• cells should have 2n
chromosomes: one
from each parent
• Problems during
meiosis can lead to
cells with more than 2n
chromosomes:
– A cell may actually end
up with 3 or 4 copies of
a chromosome
Polyploidy
• Polyploids can successfully breed with other
polyploids but not with the original 2n
ancestor
Polyploidy
• Incredibly important in the evolution of plants
Polyploidy
• Eastern Treefrog has four
sets of chromosomes
– nearly identical to the
Cope’s Gray Treefrog,
which has two sets of
chromosomes
– Suggests that the Eastern
Treefrog has evolved from
the Cope’s Treefrog
Homework Q’s
• PG. 340 #2, 5, 6
• PG. 372 #20, 37