1. SPECIES & SPECIATION
Option D.2
IB Biology
Miss Werba

2. OPTION D - EVOLUTION D.1 ORIGIN OF LIFE ON EARTH D.2
SPECIES AND SPECIATION
D.3
HUMAN EVOLUTION
AHL D.4
THE HARDY–WEINBERG PRINCIPLE
AHL D.5
PHYLOGENY AND SYSTEMATICS
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3. THINGS TO COVER 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
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4. ALLELE FREQUENCY & THE GENE POOL Command term = DEFINE
Allele frequency: the proportion of all copies of a gene that is made up of a particular variant (allele)
Gene pool: The total collection of different alleles in an interbreeding population
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5. ALLELE FREQUENCY & THE GENE POOL Command term = DEFINEBb BB bb
If a recessive allele b made up 40% of the pig population…
…then the dominant allele B would make up 60%.
The allele frequency for b would be expressed as 0.40 and for B 0.60.
For characteristics determined by two alleles, the sum of the frequency of the recessive allele and the dominant allele will add up to 1.00 (b + B = 1)
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6. ALLELE FREQUENCY & EVOLUTION Command term = STATE
Evolution involves a change in allele frequency in a population’s gene pool over a number of generations.
By natural selection, alleles coding for beneficial adaptations will confer a survival advantage and lead to improved reproduction.
These alleles are more likely to be inherited and thus the population's gene pool will change over generations.
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7. WHAT IS A SPECIES? Command term = DISCUSS
Group of organisms capable of breeding naturally to produce fertile offspring
Breeding
Species
Group of organisms adapted to share the same ecological niche
Ecological
A group of organisms with the same karyotype or gene pool
Genetic
A group of organisms that shares a common ancestor, and have developed a unique collection of structural & functional characteristics
Evolutionary
A group of organisms that shares a common ancestor (parent species goes extinct)
Cladistic
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8. WHAT IS A SPECIES? Command term = DISCUSS
The genetic definition is most widely used.
As always, there are some exceptions…
The species could be reproductively isolated, as it is mechanically not possible for them to mate.
Hybrids can be produced (eg. mules) but are usually infertile. horse (64chroms) + donkey (62chroms) = mule (63chroms)
The genetic definition does not apply to organisms that reproduce asexually.
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9. BARRIERS BETWEEN GENE POOLS Command term = DESCRIBE
If a species is somehow separated 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.
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.
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10. BARRIERS BETWEEN GENE POOLS Command term = DESCRIBE
Circumstances that prevent interbreeding are known as isolating mechanisms or barriers.
These mechanisms can be:
Pre-zygotic barriers
Prevention of mating or fertilisation
Gametes don’t meet  so zygote never forms
Post-zygotic barriers
Prevention of hybrids
Zygote forms
Hybrids are infertile, inviable (not strong) or die in utero
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11. BARRIERS BETWEEN GENE POOLS Command term = DESCRIBE
Pre-zygotic barriers: temporal isolation
the gametes never meet because the two groups are separated in time
eg. diurnal vs noctural
eg. seasonal
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12. BARRIERS BETWEEN GENE POOLS Command term = DESCRIBE
Pre-zygotic barriers: ecological isolation
the gametes never meet because the two groups are separated in their habitats
eg. arboreal vs non-arboreal
eg. akaline vs acidic soils
eg. separated by river
Hello up there!
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13. BARRIERS BETWEEN GENE POOLS Command term = DESCRIBE
Pre-zygotic barriers: behavioural isolation
the gametes never meet because the two groups are separated by different mating and courtship rituals
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14. BARRIERS BETWEEN GENE POOLS Command term = DESCRIBE
Pre-zygotic barriers: reproductive isolation
the gametes never meet because the two groups are separated because of reproductive incompatibility
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15. POLYPLOIDY & SPECIATION Command term = EXPLAIN
Polyploidy: Condition where the cells of an organism contain more than two homologous sets of chromosomes.
eg.
Triploid (3n)
Tetraploid (4n)
Pentaploid (5n)
Hexaploid (6n)
Decaploid (10n)
eg. salmon
eg. kiwifruit
eg. strawberries
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17. POLYPLOIDY & SPECIATION Command term = EXPLAIN
Polyploidy 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.
Two versions of polyploidy are:
autopolyploidy
allopolyploidy
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18. POLYPLOIDY & SPECIATION Command term = EXPLAIN
Autopolyploidy :
Multiple chromosome sets derived from a single species
Result from the fusion of 2n gametes (2n+2n = 4n)
Autopolyploids with odd number ploidys (eg. tetraploid) have trouble reproducing sexually.
But can still be good crops (if they can be propagated asexually).
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19. POLYPLOIDY & SPECIATION Command term = EXPLAIN
Allopolyploidy :
Multiple chromosome sets derived from different species
A sterile F1 hybrid doubles all of its chromosomes and becomes fertile!
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20. POLYPLOIDY & SPECIATION Command term = EXPLAIN
Allopolyploidy :
eg. Triticale
An allopolyploid
A hybrid of wheat and rye
Has 6 sets of chromosomes: 4 from wheat and 2 from rye
The resulting species is infertile with both parent species.
W + R = T
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21. ALLOPATRIC & SYMPATRIC SPECIATION Command term = COMPARE
Speciation: the process by which one or more species arise from previously existing species.
Two different species may give rise to a new species  interspecific hybridisation OR
A single species may give rise to a new species  intraspecific speciation
If it occurs whilst the populations are geographically isolated  allopatric speciation
If it occurs while the populations are occupying the same geographical area or range  sympatric speciation
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22. ALLOPATRIC & SYMPATRIC SPECIATION Command term = COMPARE
Allopatric speciation:
Results from geographical isolation over a long period of time
Gene flow is restricted between the two populations and they can adapt to different circumstances.
They will diverge due to natural selection in their respective environments.
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23. ALLOPATRIC & SYMPATRIC SPECIATION Command term = COMPARE
Allopatric speciation:
A famous example of allopatric speciation is that of Charles Darwin's Galápagos Finches
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25. ALLOPATRIC & SYMPATRIC SPECIATION Command term = COMPARE
Results from reproductive isolation caused by genetic divergence.
The two variants inhabit the same geographic region.
Mating preferences may play a role in the isolation.
Gene flow will be restricted and speciation will occur.
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26. ALLOPATRIC & SYMPATRIC SPECIATION Command term = COMPARE
Parapatric speciation:
Results from ecological isolation.
The two variants inhabit different niches within the same geographical area.
Again, gene flow will be restricted and speciation will occur.
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27. ADAPTIVE RADIATION Command term = OUTLINE
Adaptive radiation: the diversification of a group of organisms into forms filling different ecological niches.
Think of Darwin's Finches again!
They originated from one population of finches that migrated to the Galapagos islands.
They colonised the islands and evolved independently from one another due to geographical isolation.
Their beaks adapted to suit the food on their island.
Their beaks are homologous structures – structures that have evolved from a common ancestor to have different functions.
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28. CONVERGENT & DIVERGENT EVOLUTION Command term = COMPARE
Another way of describing adaptive radiation
When two related species evolve similar features due to their shared ancestry.
These features may have evolved for different functions due to differences in selection pressures in their environments.
Features that come about by divergent evolution are known as homologous structures.
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29. CONVERGENT & DIVERGENT EVOLUTION Command term = COMPARE
eg. The pentadactyl limb of all vertebrates
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30. CONVERGENT & DIVERGENT EVOLUTION Command term = COMPARE
Convergent evolution:
When two unrelated species evolve similar features if placed in similar environments
Due to similar selection pressures in their environments.
Features that come about by convergent evolution are known as analogous structures.
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31. CONVERGENT & DIVERGENT EVOLUTION Command term = COMPARE
Convergent evolution:
eg. The streamlined bodies and fins of sharks (fish) and dolphins (mammals)
eg. ant eaters, aardvarks, echidnas and numbats have all developed claws and sticky, long tongues to open up termite nests and eat them
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32. CONVERGENT & DIVERGENT EVOLUTION Command term = COMPARE
Convergent evolution:
eg. The camera eye of cephalopods (eg. box jellyfish) and vertebrates. 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.
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33. CONVERGENT & DIVERGENT EVOLUTION Command term = COMPARE
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34. CONVERGENT & DIVERGENT EVOLUTION Command term = COMPARE
Convergent evolution
Common ancestor
Different ancestor
Diverge to produce homologous structures
Converge to produce analogous structures
Appearance becomes more different over time
Appearance becomes more similar over time
Species are closely related
Species are unrelated
eg. pentadactyl limb structure in vertebrates
eg. wings in insects, birds and bats eg. streamlined bodies of sharks and dolphins
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35. THE PACE OF EVOLUTION Command term = DISCUSS
Evolution is a slow process.
Most of our ideas about evolution are supported by fossil records, which are incomplete.
There are two ideas about the pace of evolution that need to be discussed:
gradualism
punctuated equilibrium
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36. THE PACE OF EVOLUTION Command term = DISCUSS
Gradualism:
Continuous change at a slow, but steady pace over a long period of time
Punctuated equilibrium:
Species are stable most of the time
Disruptive events prompt periods of rapid change
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37. THE PACE OF EVOLUTION Command term = DISCUSS
Gradualism:
The older of the two ideas
Slow but steady change
Results from the gradual accumulation of mutations / variations
Darwin proposed this concept
He did however recognise that all species do not evolve at the same rate all of the time
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38. THE PACE OF EVOLUTION Command term = DISCUSS
Gradualism:
Supported by intermediate forms found in the fossil record
eg. Fossils found demonstrating the gradual evolution of the modern horse
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39. THE PACE OF EVOLUTION Command term = DISCUSS
Punctuated equilibrium:
Long, stable periods disrupted by events causing periods of rapid change
Stability causes organisms to become well suited to the environment due to natural selection (eliminates extreme phenotypes)
The rapid changes could include volcanic eruptions, meteors, etc
This leads to directional selection towards the mean optimal phenotype.
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40. THE PACE OF EVOLUTION Command term = DISCUSS
Punctuated equilibrium:
Supported by the gaps in the fossil record and the lack of intermediate forms for many species.
Supported by the fact that the fossil record shows that many new species appeared following a mass extinction.
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41. TRANSIENT AND BALANCED POLYMORPHISMS Command term = DESCRIBE
Polymorphism: The existence of two or more different forms of a species
Polymorphisms can be:
Transient
If two different variants of a phenotype are in the process of replacing each other
Balanced
If two different variants of a phenotype coexist in the same population
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42. TRANSIENT AND BALANCED POLYMORPHISMS Command term = DESCRIBE
Transient polymophism:
eg. The peppered moth
Light coloured moth was once prevalent.
Industrial revolution changed the colour of the trees  directional selection.
Lead to increased frequency of dark variant in polluted areas.
Clean air acts have begun to return the trees to their original colour.
Black moths now exist in polluted areas and light moths exist in rural areas.
They are not distinct species – they can still interbreed!
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43. TRANSIENT AND BALANCED POLYMORPHISMS Command term = DESCRIBE
Balanced polymophism:
eg. Sickle-cell anaemia
Sickle-cell anaemia causes a mutant variety of haemoglobin to form
The mutant haemoglobin crystallises at low oxygen concentrations and can block capillaries.
Affected individuals (homozygous recessive) have a low quality of life and a shortened life expectancy
Heterozygotes still have some mutant haemoglobin. They can lead normal lives and are resistant to the malarial parasite.
Unaffected individuals (homozygous dominant) have no sickle cells and no malarial resistance.
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44. TRANSIENT AND BALANCED POLYMORPHISMS Command term = DESCRIBE
Balanced polymophism:
eg. Sickle-cell anaemia
There is a balancing or stabilising selection occurring
Lower survival chances because they are LIKELY TO CONTRACT MALARIA
Lower survival chances because they are SEVERELY ANAEMIC
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45. TRANSIENT AND BALANCED POLYMORPHISMS Command term = DESCRIBE
Balanced polymophism:
eg. Sickle-cell anaemia
There is a balancing or stabilising selection occurring
Heterozygotes have an advantage. They are most likely to survive and reproduce in areas affected by malaria. This maintains the sickle cell allele in the population.
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46. Sample questions
Q1 (6 min)
a) i. Distinguish between transient polymorphism and balanced polymorphism. [2]
ii. State an example of transient polymorphism. [1]
b) Describe an example of a barrier between gene pools. [2]
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47. Sample questions Outline allopatric and sympatric speciation. [4]
Q2 (5 min)
Outline allopatric and sympatric speciation. [4]
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48. Sample questions A1
a) i. transient when one allele gradually replaces another / directional selection;
balanced when allele frequencies do not change / stabilizing selection; [2]
ii. eg. gradual increase in darker peppered moths due to environmental change/industrial melanism [1]
Accept any other valid example.
a) name of type of barrier;
description; [2]
eg. behavioural isolation;
different courtship behaviour so no mating between two populations;
Accept only the first type of barrier in the answer. Accept geographical separation, hybrid infertility, difference in chromosome number or breeding time.
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49. Sample questions Q2
speciation is the formation of a new species by the splitting of an existing species;
allopatric speciation caused by geographical separation;
sympatric speciation occurring within the same habitat caused by different niches / caused by courtship/feeding differences/behavioural differences;
both processes lead to isolation of sub-populations;
isolation favours certain genetic variations (within a species);
over time this leads to genetic barriers/speciation;
Both allopatric and sympatric speciation must be mentioned. [3 max]if only one mentioned. 4 max
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