D.2 species-and-speciation

Option D.2
IB Biology
Miss Werba

• Allele frequency & the gene pool
• Barriers between gene pools
• Polyploidy & speciation
• Allopatric & sympatric speciation
• Adaptive radiation
• Convergent & divergent evolution
• Pace of evolution – gradualism & punctuated
equilibrium
• Transient & balanced polymorphisms
MISS J WERBA – IB BIOLOGY 2

D.2.3

• A species is often defined as a group of individuals
that actually or potentially interbreed in nature to
produce viable offspring.
• In this sense, a species is the biggest gene pool
possible under natural conditions.


D.2.3

• That definition of a species might seem easy, but it is
not — in nature, there are lots of places where it is
difficult to apply this definition.
• For example, many bacteria reproduce mainly
asexually, by binary fission.
• The definition of a species as a group of
interbreeding individuals cannot be easily applied to
organisms that reproduce only or mainly asexually.


D.2.3

• The commonly accepted definition is also difficult to
apply to :
– hybrids - eg. mules
– Cases where it is physically impossible for
members of the same species to mate – eg. Canis
familiaris


D.2.3

Species

Breeding Ecological Genetic Evolutionary Cladistic

A group of organisms
Group of organisms A group of organisms sharing a unique A group of organisms
Group of organisms
sharing the same with the same collection of structural that shares a common
capable of breeding
ecological niche karyotype & functional ancestor
characteristics


D.2.1

• Modern evolutionists apply concepts in genetics to
explain evolution.
• Individuals who are selected for survival in a species
will reproduce and pass on their genes to the
following generation.
• The alleles present in adapted individuals will
become more common within individuals of that
species.


D.2.1

• A gene pool is the sum of all the individual genes in a
given population.
• Within a gene pool, every allele has a particular ratio
or frequency.
• The frequency of an allele is the number of
occurrences of that allele in that population.


D.2.1

• Gene pools constantly change:
mutations are always occurring and introducing new
genes into the gene pool.
• Genes that confer a disadvantage are (should be) lost
from the pool by natural selection.


D.2.1

• Suppose that the pink
body colour of pigs is
controlled by a single
gene (written as B),
and that a mutation
in this gene results in
brown skin (written
as b).


D.2.1

• There are 10
BB bb
individual pigs in the BB BB
population shown bb
BB
• This means there are Bb Bb

20 alleles.
bb BB
• There are 12 B alleles
and 8 b alleles.


D.2.1

• The frequencies of
BB bb
the alleles are: BB BB
• 12/20 are B bb
BB Bb Bb
This is a frequency of
0.6
bb BB
• 8/20 are b
This is a frequency of
0.4
• The overall frequency should add up to 1.

D.2.2

• Features of a species can change with evolution.
• eg.
• If the habitat for a species of tree-dwelling squirrels
were to change such that there was a decrease in the
number of tree shelters and an increase in the
number of ground shelters, the change
would select for squirrels which could
survive in ground shelters.


D.2.2

• The ground dwelling squirrels would survive to
reproduce and pass on their characteristics.
• Those which could not live in ground shelters were
selected against so that their numbers would
gradually decrease in the following generations.
• Over time, the species would evolve from a
population consisting mainly of tree
dwellers to a population consisting
mainly of ground dwellers.


D.2.2

• The forces of evolution shape and change the
composition of this gene pool and thus the nature of
the population.
• New combinations of alleles produce unique
genotypes.
• When expressed as phenotypes, these combinations
experience natural selection, which determines
which genes are passed on to the next generation.
• There are different types of selection (D.2.9).

D.2.4

• Features of a species can change with evolution.
• eg.
• If the habitat for a species of tree-dwelling squirrels
were to change such that there was a decrease in the
number of tree shelters and an increase in the
number of ground shelters, the change
would select for squirrels which could
survive in ground shelters.


D.2.4

• If a species is somehow separated into two groups by
an isolation mechanism or barrier, one species could
potentially diverge into two.
• If the environments on either side of the barrier are
different, each environment will select for a different
set of features.
• The two isolated groups cannot interbreed, so there
is no gene flow between them.


D.2.4

• After a long period of isolation and selection, the
groups on either side of the barrier may become so
different that they can no longer interbreed when
put together.
• One species has evolved into two.


D.2.4

• The genetic isolation between species can occur in a
number of ways, all the result of reproductive
isolation.
• They can be:
– Pre-zygotic isolation – meaning that the zygotes
are not formed because the gametes never meet
– Post-zygotic isolation – meaning that the zygotes
don’t develop


D.2.4

• The two species may have evolved in such a way that
they are active at different times of day or night.
• They may even evolve different reproductive
seasons.
• Thus isolated in time, the two groups are not likely to
interbreed.


D.2.4

• The two species occupy different habitats in a
similar region.
• May have been separated by an earthquake or river.
• May be the difference between being ground
dwellers or tree dwellers.
• Thus isolated geographically
and ecologically, the two
groups are not likely to
interbreed.

D.2.4

• The two groups may become so different that they
no longer identify with each other’s courtship
behaviour, and therefore cannot interbreed.
• The two groups become so different that they
release slightly different chemical signals
(pheromones), and therefore cannot interbreed.
• Audio and visual mating signals may also change.


D.2.4

Different species of bowerbird construct elaborate bowers and decorate them with
different colors in order to woo females. The Satin bowerbird (left) builds a channel
between upright sticks, and decorates with bright blue objects, while the MacGregor’s
Bowerbird (right) builds a tall tower of sticks and decorates with bits of charcoal.
Evolutionary changes in mating rituals, such as bower construction, can contribute to
speciation.

D.2.4

• The two groups may become so different that they
can no longer physically interbreed.
• If copulation is prevented, there will be no gene flow
between these two groups.

These damselfly penises illustrate just how
complex insect genitalia may be.


D.2.4

• Hybrids are produced but fail to develop to
maturity.
• eg.
– a male horse (2n = 64) and a female donkey
(2n = 62) can mate to produce a mule, but the
mule has 63 chromosomes.
– The chromosomes do not pair up during meiosis
– So the mule is sterile


D.2.4

• Hybrids are produced but fail to produce functional
gametes.

• The F1 hybrids are fertile but the F2 generation fail
to develop or are infertile.


D.2.6

• Speciation is the process by which one or more
species arise from previously existing species.

• A single species may give
rise to a new species
(intraspecific speciation)
or
• Two different species may give rise to a new species
(interspecific hybridisation)

D.2.6

• If intraspecific speciation occurs whilst the
populations are physically separated, it is termed
allopatric speciation.

• If the process of speciation occurs while the
populations are occupying the same geographical
area or range, it is termed sympatric speciation.


D.2.6

• Occurs when a geographical barrier produces a
barrier to gene flow because of spatial separation.
• Organisms are unable to meet and reproduce,
leading to reproductive isolation.
• Adaptations to a new environment will change the
allele and genotype frequencies.
• Prolonged separation of populations will lead to two
genetically isolated populations, even if the barrier is
removed.

D.2.6

• The barriers could be a mountain range, river, etc

• This means that speciation can also occur through
random forces, rather than through natural
selection.

• A famous example of allopatric speciation is that of
Charles Darwin's Galápagos Finches


D.2.6


D.2.6

• Occurs due to variations in the mating habits of a
population within the same geographical area.
• The two species occupy different niches in this
habitat, which can hamper gene flow.
• Prolonged separation of populations will again lead
to two genetically isolated populations, even if the
barrier is removed.


D.2.6

• Occurs due to genetic divergence (through
reproductive isolation) of various populations from a
single parent species.
• The two variants inhabit the same geographic
region.


D.2.6


D.2.5

• A species is often defined as a group of individuals
that actually or potentially interbreed in nature to
produce viable offspring.
• In this sense, a species is the biggest gene pool
possible under natural conditions.


D.2.5

• Condition where the cells of an organism contain
more than two homologous sets of chromosomes.
• eg. eg. salmon
– Triploid (3n)
– Tetraploid (4n)
eg. kiwifruit
– Pentaploid (5n)
– Hexaploid (6n)
– Decaploid (10n) eg. strawberries


D.2.5

POLYPLOIDY
& THE GENE POOL

• Polyploidy is a form of sympatric speciation.
• It doesn’t add new genes to the gene pool, but gives
rise to new combinations of genes.
• It involves a single organism or hybridisation
between organisms of a different species.
• Very common in plants.
• Polyploidy in a species results in very quick changes
to gene structure & gene expression.


D.2.5

AUTOPOLYPLOIDY

• Autopolyploids are polyploids with multiple
chromosome sets derived from a single species.
• Autopolyploids can arise from a spontaneous,
naturally occurring genome doubling (for example,
the potato).
• Others might form following fusion of 2n gametes
(unreduced gametes).
• Bananas and apples can be found as autotriploids.


D.2.5

AUTOPOLYPLOIDY

• Autopolyploids usually have an odd number of sets
of chromosomes.
• They are usually sterile.


D.2.5

ALLOPOLYPLOIDY

• Allopolyploids are polyploids with chromosomes
derived from different species.
• It is the result of doubling of chromosome number in
an F1 hybrid (rare: a fertile hybrid)
• Triticale is an example of an allopolyploid, having six
chromosome sets: 4 from wheat and 2from rye.
• The resulting species is infertile with both parent
species.


D.2.7

• Adaptive radiation describes the situation in which
homologous structures are differentiated to
perform a variety of different functions.
• Homologous structures have the same genetic basis.
• All organisms that belong to a particular taxonomic
class share a number of modified features, which
adapt them to particular ecological habitats.


D.2.7

• eg. Insect mouthparts.


D.2.8

• Divergent evolution:
when one species evolves into several different
species

• Due to different selection pressures in their
environments.

• It is also known as adaptive evolution.


D.2.8

• eg.
Charles Darwin's Galápagos finches proven to have
evolved slightly different beaks due to the nature of
the foods they were eating


D.2.8

• Convergent evolution:
when two species evolve similar features if placed in
similar environments
• Due to similar selection pressures in their
environments.
• The examples are actually pretty cool..... 
• http://en.wikipedia.org/wiki/List_of_examples_of_co
nvergent_evolution


D.2.8

• eg.
The streamlined bodies and fins of sharks (fish) and
dolphins (mammals)


D.2.8

• eg.
ant eaters, aardvarks, echidnas and numbats have all
developed claws and sticky, long tongues to open up
termite nests and eat them


D.2.8

• eg.
These two succulent plant genera, Euphorbia and
Astrophytum, are only distantly related, but have
independently converged on a very similar body
form.


D.2.8

• eg.
The camera eye of cephalopods (e.g. squid) and
vertebrates (e.g. mammals). Their last common
ancestor had at most a very simple photoreceptive
spot, but a range of processes led to the progressive
refinement of this structure to the advanced camera
eye - with one subtle difference; the cephalopod eye
is "wired" in the opposite direction, with blood and
nerve vessels entering from the back of the retina,
rather than the front as in vertebrates

D.2.9

• Evolution is a slow process.
• Most of our ideas about evolution are supported by
fossil records, which are incomplete.
• Darwin thought evolution to be a gradual process, a
series of minor changes which, over time, led to a
distinct difference between the individual and its
ancestors.
• Lately, a new idea has come up which, to a certain
extent, explains the inconsistencies of the fossil
record.


D.2.9

• In 1972, Eldredge and Gould suggested that
evolution may occur in short periods of rapid
change, followed by long periods of no change.
• The idea is that a large population which experiences
different selection pressures will probably not
change much.
• However, a small population, specially one that
experiences a new environment, could undergo rapid
changes due to selection pressures in a certain
direction. This is called punctuated equillibria.


D.2.9

• It is known that some species during certain times
have evolved gradually
(eg. mammals in Africa),
while others seem
to follow the punctuated
equillibria model.
• It is possible that the times of rapid change are
caused by meteor impacts and/or volcanic eruptions
which caused climatic changes.


D.2.9


D.2.9

• Stabilising selection:
– Tends to eliminate extreme phenotypes from
populations.
– Maintains phenotypic stability within a population.

• Directional selection:
– A gradual change in the environment moves the
mean phenotype towards a new mean.
– Will continue until the new mean coincides with the
optimum environmental conditions.

D.2.9

• Disruptive selection:
– Fluctuating conditions in an environment may favour
the presence of 2 phenotypes in a population.
– May split the population into 2 sub-populations
(? speciation)


D.2.10
D.2.11

• If two different variants of a phenotype coexist in the
same population, it is called a balanced
polymorphism.
• eg. Sickle cell anaemia
– Homozygous recessive has SCA, but homozygous dominant
does not
– Heterozygote also shows no symptoms of SCA
– The allele frequency of SCA is maintained by a
heterozygote advantage (can survive malaria)
– Stable frequencies of two or more phenotypic forms are
maintained

D.2.10
D.2.11

• If two different variants of a phenotype are in the
process of replacing each other , it is called a
transient polymorphism.
• eg. Peppered moths
– Light coloured moth was once prevalent
– Industrial revolution lead to increased frequency of dark
variant and the decreased frequency of the light variant


D.2 species-and-speciation

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