IB Biology HLNatural SelectionPaper 1 & 2~13 min read
Selection Pressures: Skills
Guppies are the reason we can say all of this with confidence rather than just telling a good story. In a greenhouse full of ponds, one scientist changed the predators and watched a population change colour within months.
📚 What you need to know
Guppies live in mountain streams in Trinidad and Tobago and show sexual dimorphism: males are brightly coloured, females are dull.
Male spots serve two jobs at once — camouflage against the gravel and attraction of females.
John Endler tested this in the 1970s using ten ponds in a greenhouse, plus a field experiment.
Ponds varied in gravel type (fine or coarse) and predation level (none, weak predator, dangerous predator).
High predation reduced the number of spots; low predation allowed spot number to increase.
In ponds with predators, spot size matched gravel size; with no predators the opposite happened.
Moving dull males into a low-predation stream produced more colourful males within 15 guppy generations.
Guppy colour is a trade-off between natural selection and sexual selection.
Why guppies are such a good model
Guppies are small fish found in mountain forest streams in Trinidad and Tobago. They breed fast, which means you can watch several generations in a single year rather than waiting a human lifetime.
They show clear variation in colour and pattern, and clear sexual dimorphism — males tend to be brightly coloured while females are dull. The spots on the males give them camouflage from predators, because the spots mimic the gravel of the streambed they live over. Some streambeds are fine and sandy, others are coarse and pebbly, and some streams offer far more hiding places than others.
So the same feature, a male’s spots, is being pulled in two directions. Bright spots attract females. Bright spots also attract pike cichlids.
Both fish carry alleles for spots. The difference is which alleles survived long enough to be passed on.
Endler’s experiments
In the 1970s John Endler noticed that guppy colour patterns in Trinidad changed with predation pressure. He then ran both laboratory and field research to investigate how natural and sexual selection shape guppy evolution.
His hypotheses were:
when predators are present, the substrate type of the streambed affects survival and so changes spot brightness;
when the predation rate is low, spot brightness on males increases because of sexual selection;
as predation increases, spot brightness decreases.
The laboratory experiment
🧩 What he actually did
Used a greenhouse to recreate a tropical environment.
Let guppies breed freely in large tanks for six months with no predation, producing a population with a wide range of spot sizes and numbers.
Counted and measured the spots on every guppy — this is the baseline data.
Randomly assigned the guppies to ten ponds: five with coarse gravel, five with fine gravel.
Set three levels of predation across those ponds (see the table below).
Ran the experiment for five months after the predators went in, then counted and measured the spots again.
Ran it for a further nine months and collected data a third time.
Predation level
Example predator
Fine gravel ponds
Coarse gravel ponds
None
no predator added
1
1
Weak predator
killifish
2
2
Dangerous predator
pike cichlid
2
2
Look at the shape of that design for a moment. Two variables are changed on purpose (gravel and predator), everything else — temperature, light, food, starting population — is kept the same, and every combination is repeated. That is what a well designed experiment looks like, and describing it is worth marks in its own right.
What he predicted
A high predation rate would make the populations diverge from each other, so guppies over coarse gravel would end up with different patterns from guppies over fine gravel.
A low predation rate would let males become more conspicuous, because bright spots help them attract females.
What he found
In ponds with a high predation rate, the mean number of spots decreased.
In ponds with a low predation rate, the number of spots continued to increase.
Where predators were present, guppies in coarse gravel ponds had larger spots and guppies in fine gravel ponds had smaller spots — spot size mimicking gravel size, which is camouflage.
In ponds with no predation the opposite happened: fine gravel ponds favoured large spots and coarse gravel ponds favoured small spots. Not matching the background makes a male stand out, and standing out helps him attract females.
If a question hands you an unfamiliar guppy graph, find the predation axis first. The rest of the interpretation follows from it.
The field experiment
Endler also took a number of dull male guppies from an area of high predation and moved them to an area of low predation. He left them for 15 guppy generations — about two years — and then went back to look.
The males he found had more colourful patterning than the ones he had released. With the strong predation pressure removed, sexual selection was free to push colour upwards.
Interpreting the results
Put the lab and field work together and the conclusion is not a tidy one-line answer. Evolution in these guppy populations is a dynamic process of both natural and sexual selection acting at the same time.
Pressure
Where it comes from
Effect on male colour
Natural selection
Competition for food and the need to avoid predators. Predators spot brightly coloured males more easily
Selects for less colourful, less spotted males
Sexual selection
Female choice. Bright, big spots give a reproductive advantage
Selects for more colourful, more spotted males
Net result
Depends on which pressure is stronger in that stream
A trade-off: colour settles at the level that maximises offspring
The honest version. In areas with high predation, brightly coloured males are less likely to survive regardless of their reproductive advantage. Being attractive is no use if you are eaten before you breed.
Reading the graphs in an exam
Endler’s data is usually printed as a block of small graphs, one for each spot colour, and it looks intimidating. It is not.
🧩 A method for any unfamiliar graph
Read both axes and the key before you look at a single bar. Find out what the categories on the x-axis mean.
Find the summary panel if there is one — a total, or an “all colours” graph. That is where the overall trend lives.
Compare only two bars at a time, and say which is higher in words: “males in high predation ponds had fewer spots than males with no predation”.
Check the error bars. If they overlap, the difference may not be significant, and saying so earns credit.
Link the pattern back to the biology: predation up, spots down, because camouflaged males survive to breed.
Worked examples
WE 1
Predict the effect of removing predators
A stream population of guppies has small, dull spots. The predatory fish are removed from the stream. Predict and explain what will happen to the males over the next 20 generations. (3 marks)
Prediction
Males will become more colourful, with more and brighter spots.
Explanation 1: pressure removed
Without predators, being conspicuous no longer reduces survival, so natural selection against bright colour is weak.
Explanation 2: pressure remaining
Females still prefer brighter males, so those males mate more, pass on the alleles for bright spots, and the frequency of those alleles increases.
Remove one pull and the trait moves towards the otherthis is exactly what Endler’s field transfer showed after 15 guppy generations
WE 2
Explain a substrate result
In ponds containing predators, guppies over coarse gravel developed larger spots than guppies over fine gravel. Explain this result. (3 marks)
Point 1: what the spots do
Spots provide camouflage by mimicking the gravel of the streambed.
Point 2: matching the background
Over coarse gravel, males with larger spots blend in better, so they are less likely to be seen and eaten.
Point 3: inheritance
Those males survive to reproduce and pass on the alleles for larger spots, so mean spot size in the pond increases.
Spot size mimics gravel size wherever predators are presentadd the contrast for a stronger answer: with no predators the pattern reversed, because standing out attracts females
WE 3
Evaluate the experimental design
Suggest two reasons why Endler used ten ponds rather than two. (2 marks)
Reason 1: repeats
Several ponds at each combination give replicates, so a result caused by chance in one pond does not look like a real effect.
Reason 2: two variables at once
Ten ponds allow both gravel type and predation level to be tested in every combination, so their separate effects can be identified.
Replicates give reliability; combinations separate the variablesa third mark in longer questions: ponds with no predators act as a control
💡 Exam tips
Learn the direction of both key results: predation up, spot number down; predation down, spot number up.
Remember the reversal — with no predators, spot size stopped matching the gravel.
Use the word trade-off when explaining guppy colour. It is the whole point of the case study.
When describing data, quote a comparison rather than a single value: higher than, lower than, roughly double.
Mention the time scale (five months, then nine more; 15 generations in the field) if a question asks how quickly evolution can happen.
Do not panic at an unfamiliar graph. Axes, then key, then one comparison at a time.
⚠ Common mistakes
Saying the guppies changed colour. Individual fish did not change; the proportions of alleles in the population changed.
Forgetting the no-predation reversal. It is the detail that separates a good answer from a full-mark one.
Only mentioning predators. Sexual selection is half the story.
Confusing the two predators. The killifish is the weak one, the pike cichlid is the dangerous one.
Describing the graph without explaining it. State the trend, then give the biological reason.
Ignoring error bars. Overlapping error bars mean you should be cautious about claiming a difference.
Up next: Gene Pools. So far we have talked about individuals surviving. Now we zoom out to the level evolution actually happens at — the whole set of alleles owned by a population.
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