Transcript IB Evolution Option D2

D2: Species and Speciation
5 hours
D.2.1 Define allele frequency, gene pool.
• Allele frequency
– % of certain allele (variation of a gene) in
the population for a certain locus/gene
• Gene pool
– All genetic info of reproducing members of
the population
– Variation
D.2.2 State that evolution involves a change in allele
frequency in a population’s gene pool over a number of
generations.
• Mutation introduces new alleles
• Old alleles “die out”
• Advantageous  survives
• Change in allele freq = evolution
– SEVERAL generations!
D.2.3 Discuss the definition of the term species.
Xref- species- 5.1.1
• Organisms have similar physiological
and morphological characteristics
• Ability to interbreed, produce fertile
offspring
• Genetically distinct from other species
• Common phylogeny
• Not exactly that simple, but good
enough for now!
D.2.4 Describe three examples of barriers between gene
pools. Examples include
• SEPARATE POPULATIONS...MAY LEAD TO SPECIATION
• geographical isolation
– Physical barriers (land, water formation) prevent
males/females from meeting & interbreeding—
populations are separated
• hybrid infertility
– 2nd generation unlikely
– Genetic barrier between species
• temporal isolation
– Incompatible time frames for mating (flowers
blooming/seasons; hibernations; migrations)
• behavioural isolation
– Lifestyle, habits incompatible with other population
– Courtship displays
D.2.5 Explain how polyploidy can contribute to speciation.
Xref- Meiosis- 4.2.3 / 10.1
• Avoid examples involving hybridization as
well as polyploidy, such as the evolution of
wheat.
• 3n, 4n, 5n, etc.
• Cell division mistakes, chromosomes don’t
separate completely
• Common in plants
• Extra chroms  plant more vigorous
• Replication errors more common
• 2 populations could evolve at different rates
b/c of difference in ploidy
• Could lead to speciation
D.2.6 Compare allopatric speciation, sympatric speciation.
• Speciation: the formation of a new species by
splitting of an existing species.
• Sympatric: in the same geographical area—
Temporal, behavioural isolation
– Pheromones, mating calls, etc. may be changed
Snow geese (Chen
slightly, allow for mates or not
SawFly (Tenthredo livida)
Pin and thrumb primroses (Primula vulga
Fertilisat
different
geographical areas—
ion
• Allopatric: in
geographic isolation
– 2 pops separated, evolve separately
– If allowed to mingle in future, may not be able to
interbreed...new species
Genetic
Hybrid inviability
• Hybrid offspring die
• Hybrid infertility
Hybrids survive but
are incapable of
producing gametes
Zedonk
D.2.7 Outline the process of adaptive radiation.
• “Rapid” evolution from 1 (or a few) species
into more
• Slightly diff niches, more successful
• Natural selection
• Speciation event(s) (isolation)
• EX-finches, lemurs
• Lemurs once widespread b/c no competition
(apes, monkeys)
– Lots phenotypic diversity
– Some better adapted to certain niches, led to
adaptive radiation
– Apes, monkeys...outcompete lemurs, so lemurs not
found
Mechanisms of speciation
Isolation of a population
so that it cannot
breed freely with
others is necessary
 Geographic
 Ecological
 Behavioural
 Mechanical/anatomic
al
 Physiological
 Genetic
© 2008 Paul Billiet ODWS
Madagascar Google earth
Ringtailed lemurs
(Lemur catta)
D.2.8 Compare convergent evolution, divergent evolution.
• Convergent
– Not a recent common ancestor
– 2 species or characteristics look similar
– Australia & N America: marsupials vs
placentals
– Wings (bat, bird); bioluminescence
(bacteria, fungi)
• Divergent
– Recent common ancestor
– Less similar over time
D.2.9 Discuss ideas on the pace of evolution including
gradualism and punctuated equilibrium.
• Gradualism: small, continuous, slow change
from one form to another
– Fossil record
– Present day examples
– P. 429 diagrams
• Punctuated equilibrium: long periods without
appreciable change and short periods of
rapid evolution
– Response to change in environment
• Volcanic eruptions and meteor impacts affecting
evolution on Earth
– 65mya...dinosaurs extinct; mammals survived
– Until a big environmental change, little/no change
in fossil record
Evolution of new species
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Two possible ways from gradualism
PHYLETIC TRANSFORMATION
ALLOPATRIC SPECIATION
© 2008 Paul Billiet ODWS
PHYLETIC TRANSFORMATION
Gradual accumulation of small genetic
variations preserved by natural selection
 A whole population imperceptibly to
evolve in to a new species
 Impossible to draw a clear line between
the end of the first species and the
beginning of its descendant species
 There would be a long period of
intermediate forms

© 2008 Paul Billiet ODWS
ALLOPATRIC SPECIATION
Geographical or reproductive isolation of
a part of the population would allow it to
evolve in a different direction
 Possibly more rapidly than the main
population
 If the isolated population is small, it might
be very difficult to find fossils of the
intermediate stages

© 2008 Paul Billiet ODWS
Allopatric speciation
of Species W into
species X due to the
isolation of a small
population of Species
W
Species X
Species Z
Species Y
Species Y
Time
Allopatric speciation
of species Y into
species Z due to the
isolation of a small
population of species
Y
Phyletic transition of species W
into Species Y due to the slow
gradual accumulation of mutations
in Species W
Species W
© 2008 Paul Billiet ODWS
Evolution
THE PUNCTUATED
EQUILIBRIUM MODEL
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They observed that the
fossil record gives a
different picture for the
evolution
They claim that there were
long periods of stasis (410 million years) involving
little evolutionary change
Then occasional rapid
formation of new
species
As little as 5,000 - 50,000
years
© 2008 Paul Billiet ODWS
Stephen J
Gould
Niles Eldredge
Species Y
Species Z
Species X
Stasis
Species W
Stasis
Time
Stasis
© 2008 Paul Billiet ODWS
Rapid
speciation
Rapid
speciation
Evolution
Rapid
speciation
Problems...
• Fossil evidence is the only evidence of
either theory
• Not all characteristics of a species are
present in fossil record
• Not necessarily proof of phylogeny
NATURAL SELECTION AT THE
LEVEL OF THE ALLELE
Sickle cell anemia
Biston betularia
D.2.10 Describe one example of transient polymorphism.
• Polymorphisms = many “shapes”
(phenotypes)
• Industrial melanism: peppered moth
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–
–
–
–
–
–
–
Biston betularia
Peppered (grey)
Melanic (black)
Pre-Industrial Revolution: advantageous to be
grey, camouflaged on tree bark
Ind. Rev.  soot on trees, black bark; black p’types
survived (more fit)
Natural selection!
Clean Air Act…grey more fit
Temporary change  TRANSIENT polymorphism
D.2.11 Describe sickle-cell anemia (SCA) as an example of
balanced polymorphism.
Xref- SCA-4.1.4; malaria
• 2/more alleles of population are not
transient and changing. They’re stabilized by
natural selection.
• Sickle-cell anemia
– Balanced polymorphism
– Heterozygotes have advantage in malarial regions
•
•
•
•
•
HbAHbS
More fit than either homozygote
AA = very susceptible to malaria
SS = sickle cell, but resistant to malaria
AS = some sickled cells, but usually not anemic; Kdeficient sickled cells kills Plasmodium parasite
– “S” Selected for & selected against ... balanced