IB Biology HLEvolution & SpeciationPaper 1 & 2~12 min read
Types of Speciation
Splitting a species needs gene flow to stop. A mountain range will do it — but so will breeding a month too early, or getting the courtship dance wrong. That gives us the two named types you have to know.
📚 What you need to know
Allopatric speciation happens when two populations are geographically isolated. It is the most common type.
Sympatric speciation happens when the two populations are living in the same area, with no geographical barrier.
Allele frequencies change through different selection pressures and through genetic drift, the accumulation of random changes in allele frequency.
In sympatric speciation, isolation comes from phenotype changes: temporal isolation (different mating or flowering seasons) or behavioural isolation (changed courtship).
A species is a group of organisms with similar characteristics that can interbreed to produce fertile offspring.
A hybrid is the offspring of two different species. Hybrids are rare and usually infertile.
Hybridisation is blocked by incompatible chromosome numbers and by incompatible courtship behaviours.
The two situations
Evolution causes speciation: the formation of new species from pre-existing species over time. There are two different situations in which it can take place.
The mechanism differs but the outcome does not. In both cases alleles stop being shared between the two halves.
🧠
Never mix the names up again
Allopatric — “allo” means other, as in another place. Sympatric — “sym” means together, as in symmetry or sympathy: same place.
Allopatric speciation
Allopatric speciation occurs as a result of geographical isolation, and it is the most common type.
Populations of a species become separated by geographical barriers. The barrier can be natural, such as a body of water or a mountain range, or man-made, such as a motorway.
This creates two populations of the same species between which no gene flow is taking place.
Allele frequencies in the gene pools of the two populations then change in different ways, for two reasons: different selection pressures acting on them, and genetic drift — the accumulation of random changes in allele frequency.
Changing allele frequencies lead to changes in the phenotypes of the two populations.
If enough differences build up, the populations become reproductively isolated and are separate species.
Example: trees divided by a new mountain range
Stage
What is happening
Start
A population of trees lives in a mountainous habitat, with pollen and seed moving freely across it
The barrier
A new mountain range forms and divides the species into two geographically isolated populations
No gene flow
The barrier prevents the two populations from interbreeding, so no alleles pass between them
Differential selection
The two sides experience different environments, so different alleles are more likely to be passed on in each
Divergence
Different alleles become more frequent in each population, and the phenotypes drift apart
Speciation
Over thousands of years the divided populations form two distinct species that are reproductively isolated
Genetic drift is worth a sentence in a top answer. Selection is not the only thing changing allele frequencies — in a small isolated population, which alleles get passed on is partly luck. Two isolated groups will drift apart even if their environments are identical.
Sympatric speciation
Sympatric speciation takes place with no geographical barrier. Isolation instead occurs when random changes in the alleles, and therefore the phenotypes, of some individuals prevent them from breeding successfully with other individuals in the same population.
Type of isolation
The phenotype change
Why breeding stops
Temporal isolation
Some individuals develop different mating or flowering seasons from the rest of the population
Their reproductive timings no longer match up, so they are never sexually active at the same time
Behavioural isolation
Some individuals develop changes in their courtship behaviour
Their methods of attracting a mate no longer work on the rest of the population
The two groups may still live in the same habitat, but they do not interbreed. That lack of gene flow means allele frequencies in the two gene pools change in different ways, phenotypes diverge, and if enough differences build up they become reproductively isolated and are two separate species.
Example: fruit flies in a laboratory
A population of fruit flies exists in a laboratory.
A random allele change caused by mutation divides the species into two populations, because it leads to a change in phenotype — for example a different food preference.
The difference in phenotype prevents the two populations from interbreeding, so there is no gene flow between them.
Different alleles are passed on in each population. This could be due to a difference in selection pressure — certain enzymes being better for digesting one food — or simply due to random passing on of different alleles.
Different alleles become more frequent in each population, and over time the two form two distinct species that can no longer interbreed to produce fertile offspring.
Do not mix the two halves up. The reason gene flow stopped (temporal, behavioural, geographical) is not the same thing as the result (reproductive isolation). The terminology sounds similar, which is exactly why questions target it.
Preventing hybridisation
Definition
A species is a group of organisms with similar characteristics that can interbreed to produce fertile offspring
A hybrid is the offspring of individuals of two different species, and hybridisation is the mechanism that produces one — the mating, fertilisation and development processes. Hybrids are rare, and are usually infertile. Two barriers explain why.
Barrier 1: incompatible chromosomes
Fusion of gametes from different species often produces non-viable zygotes, because the chromosomes of the two species do not match. The gene at a particular locus needs to be the same in both chromosomes of a homologous pair.
Sometimes a viable zygote does form, but it usually develops into an infertile hybrid.
Different species often have different chromosome numbers, so their gametes carry different numbers of chromosomes.
The new diploid cell then contains an uneven number of chromosomes, which cannot pair up into homologous pairs.
Without pairing there is no meiosis, so the individual cannot make gametes and is infertile.
32 plus 31 is 63, and 63 will not divide into pairs. The arithmetic is the whole explanation.
Barrier 2: incompatible courtship behaviours
In many species, successful breeding is preceded by some form of courtship behaviour, which is a ritual that eventually results in mating and reproduction.
It can be very simple, involving a small number of visual, chemical or auditory stimuli.
It can also be a highly complex sequence involving two or more individuals and several modes of communication — many birds of paradise have intricate courtship rituals.
If the courtship rituals of two individuals do not match, no mating occurs and hybridisation is prevented.
Worked examples
WE 1
Naming the type of speciation
In a lake, some fish begin spawning in spring while the rest spawn in autumn. Over time the two groups become separate species. Name and explain this type of speciation. (3 marks)
Point 1: name it
This is sympatric speciation, because there is no geographical barrier and both groups live in the same lake.
Point 2: the mechanism
An allele change altered the phenotype so that reproductive timings no longer match. This is temporal isolation, and it stops gene flow.
Point 3: the outcome
Allele frequencies in the two groups then change in different ways until they are reproductively isolated.
Same lake, different calendars, no gene flowname the isolation type as well as the speciation type — they are separate marks
WE 2
Why the mule is infertile
A horse (2n = 64) and a donkey (2n = 62) can breed to produce a mule. Explain why the mule is infertile. (3 marks)
Point 1: the gametes
The horse gamete carries 32 chromosomes and the donkey gamete carries 31, so the mule has 2n = 63.
Point 2: the problem
63 is an odd number, so the chromosomes cannot all pair up into homologous pairs.
Point 3: the consequence
Meiosis cannot be carried out, so the mule cannot produce gametes of its own and is infertile.
Odd number → no pairing → no meiosis → no gametesshow the arithmetic; 32 + 31 = 63 is often a marking point by itself
WE 3
Two causes of allele frequency change
Two populations of a plant are separated by a new road. Suggest two reasons why their allele frequencies may change in different ways. (2 marks)
Reason 1: selection
Different selection pressures act on the two sides, so different alleles are advantageous and become more frequent.
Reason 2: chanceGenetic drift — random changes in allele frequency accumulate, especially in small populations.
One cause is the environment; the other is pure chancemany students give two versions of selection; the drift mark is easy and often missed
💡 Exam tips
State which type of speciation it is and why — barrier present or absent.
Say that allopatric is the most common type if the question asks you to compare them.
Learn temporal and behavioural isolation as named terms for sympatric speciation.
Include genetic drift alongside selection pressures when explaining changing allele frequencies.
Learn the species definition exactly, especially the words fertile offspring.
For hybrids, the two barriers are chromosome numbers and courtship behaviours.
⚠ Common mistakes
Swapping allopatric and sympatric. Allo = another place; sym = same place.
Confusing the cause with the result. Temporal isolation is the cause; reproductive isolation is the result.
Saying sympatric speciation needs a small barrier. It needs none at all.
Saying a mule cannot be produced. It can be produced — it just cannot reproduce.
Forgetting the word “fertile” in the species definition. That word is the whole test.
Up next: Adaptive Radiation — what happens when one species arrives somewhere empty and splits many times over in a short stretch of evolutionary time.
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