Download Species and Speciation

Gene Pool
 All the alleles present in the reproducing members of
an interbreeding population at a given time.
 The gene pool is constantly changing
 Mutations add new alleles to the population
 Immigration also adds new alleles; changes the allele
frequency
 Alleles can be removed by natural selection if
disadvantageous
 Large gene pool
 –a lot of variety in traits (many different alleles)
 Small gene pool
 Little variation in alleles present (common in
inbreeding)
 Allele Frequency: how often a particular allele appears
in a population
(will discuss further in D.4)
Evolution and Alleles
 Gene pools are generally relatively stable over time, but




not always
New alleles can be introduced and old alleles can
disappear
A result of evolution: after many generation of natural
selection, alleles proven to be more advantageous tend
to be more frequent.
Alleles that are disadvantageous to an organism's
survival are not passes on to as many offspring.
Without changes in allele frequency, evolution cannot
occur
What is a species?
 All the members of a population that can interbreed
under natural conditions; share the same gene pool.
 Produce fertile offspring!
 Individuals of different species cannot interbreed
under natural conditions – they are reproductively
isolated from each other and they have different gene
pools.
Species
 A horse (Equus ferus caballus) and donkey (Equus
africanus asinus) can mate to produce a mule
 However, mules are infertile. Horses and donkeys are
of different species.
Horse
+
donkey

mule
What is a Species?
 Members of the same species have similar
physiological/morphological characteristics that can
be measured
 They are genetically distinct from other species (own
identical karyotype)
 They have a common phylogeny (a common ancestor)
Speciation
 The formation of an entirely new species
 For speciation to occur, individuals from the original
species must become reproductively isolated from the
rest of the individuals
 A barrier is created between gene pools within a
population – thus preventing the individuals from
mating. This leads to genetic isolation
Genetic Isolation
 Prezygotic Isolation: prevent fertilization and a zygote
from forming
 Postzygotic Isolation: a zygote is formed but does not
mature into a viable, reproducing adult
 (The 2 species are similar enough to allow their gametes to
combine, but they have different diploid numbers. The
hybrid offspring may not have enough genetic information to
survive-(similar to non-disjunction disorders) or if they do
survive they are infertile because meiosis doesn’t occur since
the individual lacks homologous chromosomes.)
Mechanism
Description
Behavioural Different species used
Isolation
different mating rituals to
find a mate
Temporal
Different species breed at
Isolation
different times of the year
Ecological
Isolation
Species occupy different
habitats within a region
Mechanical Differences in
Isolation
morphological features
make two species
incompatible
Gametic
Gametes are unable to
Isolation
recognize each other to
allow fusion of nuclei.
Example
Many frogs have unique calls that
only attract females of their species
Flowers that release pollen in the
spring are reproductively isolated
from flowers that release pollen in
the summer.
The mountain bluebird lives at high
elevations while the eastern bluebird
lives a lower ones
Male and female genetalia of each
species are uniquely shaped and are
physically incompatible with other
species
Many marine animals release their
sperm and eggs into open water. The
sperm recognize eggs of their own
species through chemical markers.
Mechanisms
Description
Zygotic
Mating and
Mortality
fertilization are
possible, but zygote
doesn’t develop
properly
Hybrid
A hybrid develops but
Inviability
either dies before
birth, or cannot
survive to maturity
Hybrid
Hybrid is born, and
Infertility
lives but is sterile
Example
Some species of sheep and goat
can mate, but zygote doesn’t
live.
When tigers and leopards are
crossed – ends in a miscarriage
or stillborn offspring
Mules are the sterile offspring
of a horse and a donkey cross
Just for fun…. Some hybrids
 Zebra (male) + equine (female)
 zebroid/zorse
(don’t occur in nature)
 Lion + tiger  liger
(male)
(female)
(territories don’t overlap, so only
seen in captivity, however, may
have occurred naturally in the
past)
Photoshopped?
 Liger named Hercules
Just for Fun… Some hybrids
 Sheep + goat
 toast of botswana
•
Camel + llama  cama
Exceptions to the species rule
 Term species only applies to sexually reproducing
organisms.
 Hybrids are sometimes capable of mating with a
parent species and producing fertile offspring.
However, the hybrid is still not considered a member
of the parent species species.
 Ring species – members of adjacent populations
interbreed successfully but members of widely
separated populations do not.
Polyploidy
 When a cell contains three of more sets of
chromosomes
 Human body cells are diploid: 2n
 Triploid= 3n
 Tetraploid = 4n
 Polyploidy is the result of the failure to nuclei to
separate during meiosis
 In plants, polyploidy is much more common than in
animals.
 The extra set of chromosomes leads to more vigorous
plants which produce bigger fruits or storage units, or
are more resistant to disease
 Having extra sets of chromosomes has the
consequence of making errors in replication more
common.
Polyploidy can lead to speciation
 If one plant is triploid and another is tetraploid, they
may not be able to form a zygote
 This can lead to speciation - the evolving population
changes significantly enough so that the production of
offspring becomes impossible
 (Note: organisms with an odd number of sets of
chromosomes (i.e. triploid) are usually sterile b/c
cannot form homologous pairs during prophase I)
 Autopolyploidy: increase in the number of
chromosomes within the same species
 Allopolyploidy: when the chromosomes number in a
sterile hybrid becomes doubled and produces fertile
hybrids
Polyploidy  speciation
 While polyploidy may cause a new species to form, it is
more likely to happen in plants rather than in animals.
 For many animals, have an abnormal number of
chromosomes and extra genes causes developmental
problems.
 Think back to the nondisjunction disorders we discussed
in grade 11.
 Most hybrids a infertile and have reduced lifespans
 That being said, polyploidy can occur in less complex
animals
Polyploidy: the Red Viscacha
Polyploidy: the Red Viscacha
 The red viscacha (Tympanotomys barrerae) is a rodent
native to Argentina.
 It has 102 chromosomes (2n=102) - the highest of any
mammal!
 Its closest living relative is the Andean viscacha-rat
(Octomys mimax) which has 2n=56 chromosomes.
Polyploidy – Red Viscacha
 It is believed that an Octomys ancestor produced
tetraploid offspring (4n=112) that became
reproductively isolated from the parent species.
 With time, some of the additional chromosomes were
lost (naturally selected against)
 Research shows that the Red Viscacha has only 2 of
every autosome pair but there are several genes that
exist in 4 copies.
Polyploidy - Allium
 Allium - includes onion, leeks, garlic, and chives
 Polyploidy has occurred frequently in many species of
Allium
Polyploidy - Allium
 Allium canadense (white onion) is 2n = 14.
 However, variants of 2n=28 exist (such) as Allium
lavendulae)
Allium canadense
Allium lavendulae
Polyploidy - Allium
Allium angulosum
Allium oleracium
 2n =16
 4n= 32
Speciation
 INTRASPECIFIC: Process in which one or more
species arise from a previously existing one
 INTERSPECIFIC HYBRIDIZATION: Process in which 2
different species give rise to a new species
Allopatric Speciation
 When a single species is separated into 2 geographically
isolated populations.
 Once the populations are physically separated they can no
longer exchange genetic info.
 Over many generations, the populations will gradually
become less and less alike.
 Any mutation that occurs in one population, will not be
shared with the other.
Allopatric Speciation….
 Differences in the environments of the 2 populations
can lead to different forms of natural selection.
 With time, it is likely that the 2 populations will have
evolved some sort of reproductive isolating
mechanism.
 The geographical isolation can be caused by
development of mountain ranges, continental drift,
human construction activities that disrupt a habitat….
Allopatric Speciation
Ex: Allopatric Speciation
 2 mya, a thin strip of land
called the Isthmus of
Panama formed to separate
the Caribbean Sea from the
Pacific Ocean and
permanently divide species
such as the wrasse into 2
separate populations.
 Now the species distinct
and cannot successfully
interbreed when placed
with each other.
Speciation on the Galapagos
Islands
 The speciation that occurred on the Galapagos islands
is the result of Allopatric speciation.
 Animals migrated to the islands, and became
geographically and reproductively isolated from parent
species on the main land.
Sympatric Speciation
 When a new species evolves from within a large
population.
 Individuals within the population become genetically
isolated from the larger population.
 Can occur gradually or suddenly.
Ex: Sympatric Speciation
 The hawthorn fly lays its eggs on the fruits of
hawthorn trees. When apple trees were introduced
into its environment, some of these flies began laying
their eggs on apples. Today the species consists of 2
populations.
Divergent Evolution
 The large –scale evolution of a group
into many different forms
 A single parent species is put under
(at least) 2 different selective
pressures.
 With time, 2 or more related species
will develop and become more and
more dissimilar as they adapt to
their environment
 Species that have evolved from divergent evolution
share a common ancestor and often share homologous
traits (i.e. Pentadactyl limb)
Adaptive Radiation
 A type of divergent evolution
 the relatively rapid evolution of a single species into
many new species (that are similar but distinct from
each other)
 Happens because variation in the population allow
certain members to exploit a slightly different niche in
a more successful way
Adaptive Radiation…
 each new species fills a different ecological niche
 occurs when a variety of new resources, that are not
being used by other species become available
 More common in periods of environmental change
Adaptive Radiation
Ex: Darwin’s Finches
 14 species of finches that live on the Galapagos Islands evolved
from a single species.
 Darwin observed that the size and shapes of the finch beak
varied with their diet.
 The original parent species likely lived on the mainland of South
America and had a medium-sized bill used to feed on mediumsized seeds.
 On the mainland, if a finch developed a small bill to eat small
seeds or a large bill to eat large seeds, they likely would have
been in competition with other bird species and would not be
naturally selected for.
 However, if these small and large billed finches lived
on the Galapagos Islands, where there wasn’t other
bird species (yet), there would be no competition for
their ideal food source and they would occupy a new
ecological niche.
 As a result, their bill size would be naturally selected
for.
Convergent Evolution
 When 2 different species evolve to occupy similar
ecological niches.
 As a result they develop, similar traits even though
they do not have a common ancestor with the trait.
 These are analogous structures
Analogous Structures
 Structures that are similar function
but evolved independently in each
species
 The trait is not found in the most
recent common ancestor
 Ex: wings of birds, insects, and bats
Examples of Convergent Evolution
 Cacti and euphorbia
 These are different species of
plant that live in completely
different regions in the world.
 However, they both live in very
dry environments.
 Cacti evolved in the deserts of South America and are
native to the Americas.
 Euphorbia evolved in South Africa and are found in Africa,
Eurasia, and Australia
 As a result, both have evolved adaptations independently
that allow them to survive in their environments.
 Since their environments are so similar, their adaptations
are similar too.
 They both have sharp spines and thick green stems to
perform photosynthesis and store water.
Convergent Evolution – Swimming
Carnivore
 Shark vs. dolphin vs. penguin vs. ichthyosaur (extinct)
 They all have similar body shapes including fins or
flippers and streamlined body shapes that allow them
to move through the water (occupy a common niche)
penguin
bird
Evolutionary Patterns
Divergent Evolution
Convergent Evolution
Parallel Evolution
Gradualism and Punctuated
Equilibrium
 What is the pace of evolution?
 How quickly do new species and entirely new groups
evolve?
 2 Theories to explain the patterns of evolution that
take place over very long periods of time: gradualism
and punctuated equilibrium
Gradualism
 As new species evolve, they appear very similar to the
original species and only gradually become more
distinctive.
 Over long periods of time, the small changes
accumulate, resulting in dramatically different
organisms.
 The fossil record would show many transitional/
intermediate forms.
Punctuated Equilibrium
 Sometimes the fossil record shows a new species
appearing quite suddenly and then remaining little
changed over time.
 3 main assertions of punctuated equilibrium
 New species evolve rapidly
 Speciation occurs in small isolated populations – so few
transitional fossils
 After initial change, additional changes are slow
Gradualism
Punctuated
Equilibrium
Patterns of Natural Selection
 Sometimes abiotic or biotic factors can result in
different patterns of natural selection.
 Can result in directional, stabilizing, or disruptive
selection
Scenario
 Hummingbirds use their bills to feed on nectar from
flowers.
 Hummingbird populations can have varying lengths of
bills (short, medium, long)
Stabilizing Selection
 When the average phenotype within a population is
favoured by the environment.
 Selection pressures remove the extreme varieties
 Ex: What if there were only medium flowers for the
hummingbirds?
 The birds with long bills require more nutrients and energy
 The birds with short bills may not get enough food.
 The medium sized bills are the best and will be selected for.
Directional Selection
 This is when one extreme of a variation is naturally selected
for.
 Ex: If a new habitat has plants with long flowers, birds with
longer bills will be favoured for by the environment.
 These birds will have more food and will be more likely to
live to reproduce and pass their genes on to the next
generation
 Thus, a new mean phenotype is selected for
Disruptive Selection
 Favours individuals with variations at opposite
extremes of a trait over the intermediate variations.
 Ex: If there were both short flowers and long flowers,
this would best suit short bills and long bills for
hummingbirds. The medium sized billed
hummingbirds wouldn’t be selected.
Examples:
 Stabilizing Selection:
 Average birth weight in human babies is favoured over
low birth weight or high birth weight.
Examples:
 Disruptive Selection:
 Red crossbills (Loxia curvirosta)
 Beaks cross on either the left or the right, allowing the
bird to access seeds from conifer cones.
 The intermediate form is a straight bill and it is
naturally selected against.
Examples:
 Directional Selection:
 In giraffes, long necks have be directionally selected
for because they allow the animals to eat leaves from
high branches.
Polymorphism
 The existence of 2 or more forms/phenotypes within a
population
 Remember the peppered moths??? (5.1)
Peppered Moth
Transient Polymorphism
 When one allele is in the process of displacing another.
 Our example of the peppered moth during the Industrial
Evolution is an example of Transient Polymorphism
 Before the industrial revolution, the peppered allele was in
higher frequency
 When the trees turned black, the black moths has a greater
chance of surviving and so the black allele began replacing
the previously common peppered allele
Balanced Polymorphism
 When 2 different forms/varieties coexist in the same
population in a stable environment (because of natural
selection)
 Ex: Sickle cell anemia in Africa
Sickle Cell Anemia
 A recessive allele.
 P 236
Sickle Cell Anemia
 The heterozygous condition causes patients to exhibit
a condition known as sickle-cell trait
 Red blood cells appear normal
 Only ½ the hemoglobin is abnormal
 Produces a mild anemia
 Shorter life span for these red blood cells
The Heterozygote Advantage
 However, it is beneficial because the parasite that
causes malaria cannot complete its life cycle inside the
red blood cells
 A heterozygote advantage
 Some of the homozygous dominants will die because of
malaria
 Some of the homozygous recessive will die because of
sever anemias
 The heterozygotes survive and reproduce
Balancing Selection
 A form of natural selection
 Maintains genetic polymorphism within a population
 Sickle-cell anemia is an example of balancing selection
because the sickle-cell allele frequency is maintained
by the heterozygote advantage.
Co-evolution
 The process in which one species evolves in response
to the evolution of another species.
 (The evolution of one species is linked to the evolution
of another)
 Develops as a result of mutualistic symbiotic
relationships
Ex: Agouti and Brazil Nut
 The brazil nut has a hard
protective shell.
 The Agouti is the only
mammal with jaws and
teeth strong enough to
bite open the shell
Ex: Madagascar long-spurred
orchid and the hawk moth
 The Madagascar long –
spurred orchid has a long
tube called spurs which
contain nectar.
 It is pollinated by a hawk
moth whose tongue is 30
cm long and can reach the
nectar
 In the process it helps the
orchids with pollination by
moving pollen from plant
to plant.
Ex: Dung Beetle and Orchidantha
inouei
 The dung beetle feeds on
dung/feces.
 This particular orchid
produces a foul odour
(similar to poop) to
attract the dung beetle.
 The dung beetle will
unknowingly pick up
pollen from the flower
and deposit it onto
another.
Ex: Orchid bee and bees
 The “orchid bee” Ophrys
apifera is an orchid that
mimics a female bee.
 Male bees will attempt to
mate with the flower.
 Instead they pick up
pollen and deposit it on
the next orchid (aiding
in pollination) as they
attempt to copulate with
it