Selective Pressures
On the Galápagos island of Daphne Major, researchers measured every finch for decades. In the drought of 1977, the small soft seeds ran out and only big tough seeds remained. Within a single dry season, the average beak depth of the survivors rose measurably, drought had become a selective pressure, and the population shifted before the watching scientists' eyes. This lesson sorts the forces that do the selecting into two kinds: the non-living and the living.
Orient yourself
Make a prediction, scan the goals and the key terms, and see which misconception this lesson is built to fix.
Before You Read
warm-upThink about what could kill a young rabbit before it ever reproduces: a hard frost, a fox, not enough grass to eat, a disease.
Before reading: sort those four threats into two groups based on whether they are "living" or "non-living". What single word would you use to name each group?
Learning Intentions
goalsKnow
- The difference between abiotic and biotic factors
- Examples of each acting as a selective pressure
- That a selective pressure favours some variants over others
Understand
- Why the same factor can pressure different traits in different species
- How abiotic and biotic pressures often act together on one population
- Why selective pressure determines the direction of natural selection
Can Do
- Classify a given factor as abiotic or biotic
- Use a named example to show how a factor selects for a trait
- Predict which variant a stated pressure would favour
Scan these before reading
vocabMatch each term to its meaning: use the Key Terms card above.
- Abiotic factor
- Biotic factor
- Selective pressure
- Population
- Predation
- A group of individuals of one species in the same area at the same time
- A non-living component of the environment, such as temperature or salinity
- An interaction in which one organism kills and eats another
- Any factor that decides which individuals survive and reproduce
- A living component of the environment, such as a predator or competitor
Misconceptions To Fix
watch outTwo Kinds of Pressure
Abiotic vs biotic, the non-living and the living
Core Content
Two Kinds of Pressure
+5 XPAbiotic vs biotic, the non-living and the living
Every selective pressure comes from the environment, and the environment splits cleanly into two halves: the things that are alive and the things that are not.
Abiotic factors are the non-living parts of the environment, temperature, water availability, light, salinity, soil pH, oxygen levels, that can act as selective pressures by favouring variants better able to tolerate them. A cold winter, for example, selects for animals with better insulation.
Biotic factors are the living parts of the environment, predators, prey, competitors, parasites, pathogens and potential mates, that can act as selective pressures by favouring variants better able to deal with other organisms. Predators, for example, select for prey that are faster or better camouflaged.
The two categories are not rivals: a single population usually faces both at once. A desert plant is pressured by an abiotic factor (water scarcity) and a biotic one (herbivores eating its leaves) at the same time.
Pause, copy the highlighted definitions of abiotic and biotic factors into your book.
A non-living environmental factor such as temperature or salinity is described as an _____ factor.
Abiotic Pressures at Work
Worked examples: drought, temperature and salinity
Abiotic Pressures at Work
+5 XPWorked examples: drought, temperature and salinity
We just defined abiotic factors. That raises a question: what does it look like when one actually drives selection? This card answers it → three documented examples you can quote in an exam.
An abiotic pressure changes which variants survive by setting a physical challenge that some individuals are better built to meet.
Text description
- Number of finches (up) against beak depth (across).
- Before the drought (solid): the population's spread of beak depths, with its mean marked.
- After the drought (dashed): fewer finches survive, and the mean shifts to deeper (larger) beaks.
Abiotic selective pressures include drought/water availability, temperature, light, salinity and pH. Each favours the variants physically able to tolerate or exploit the condition, e.g. drought favoured deep-beaked finches; cold favours well-insulated mammals.
Pause, copy the highlighted abiotic examples into your book.
Which of these is an example of an ABIOTIC selective pressure?
Apply it: Classify and Justify
Work the activity in your own book before you read on. This is the first point where you are on your own.
Activity 1
Classify and Justify
Pattern C, Sort, then explain
For each factor below, state whether it is abiotic or biotic, then name one trait it would select for in a named population: (a) increased frequency of bushfire, (b) introduction of a fast-running predator, (c) rising soil salinity, (d) competition for nesting sites. Write your answers as a short labelled list.
Working Scientifically: this is BI-11WS-06, using scientific processes to solve a problem. For each factor, state the trait you predict will increase in frequency AND the observation that would confirm your prediction. A prediction that names no observable outcome cannot be tested.
Biotic Pressures at Work
Predation, competition and disease as drivers of selection
Biotic Pressures at Work
+5 XPPredation, competition and disease as drivers of selection
We just saw the non-living half. That raises a question: how do other organisms drive selection? This card answers it → the three big biotic pressures, each with a worked example.
Living things in the environment pressure a population by competing with it, eating it, or infecting it, and each interaction favours particular variants.
Predation: predators remove the prey that are easiest to catch, selecting for prey that are faster, better camouflaged, or better defended. The peppered moth (Lesson 1) is a predation example.
Competition: when food, water, territory or mates are limited, individuals best able to secure them reproduce more. Among Galápagos finches, competition for seeds reinforced the divergence of beak sizes.
Disease and parasites: a pathogen sweeps through a population and the individuals carrying resistance alleles survive, selecting for resistance, exactly as antibiotics do for bacteria.
An Australian worked example. The myxoma virus was released into Australian rabbit populations in 1950 and killed more than 99 per cent of the rabbits it reached in the first epidemics. The virus was a biotic selective pressure: the rare rabbits carrying alleles for resistance survived and bred, and within about seven years field trials showed markedly higher survival in rabbits from previously exposed populations (Fenner and Ratcliffe, Myxomatosis, 1965). Genome work has since tracked the same resistance alleles rising in Australian, British and French rabbits under the same pressure (Alves and colleagues, Science, 2019). Note what the virus did and did not do: it selected among resistance alleles that were already present at low frequency, it did not create them.
Biotic selective pressures include predation (favours faster/camouflaged/defended prey), competition for limited resources (favours better competitors), and disease/parasites (favours resistant individuals). Abiotic and biotic pressures usually act on the same population together.
Add the three biotic examples, including the myxomatosis case, to your notes before the check below.
Predators are an abiotic selective pressure because they affect survival.
A single population can be under abiotic and biotic selective pressures at the same time.
Competition for limited mates or food can act as a selective pressure even with no predator present.
See where it lands in the real world
Why this matters: predicting climate winners and losers
Why this matters: predicting climate winners and losers
Conservation biologists use the abiotic/biotic framework to predict which species will cope with climate change. A rising average temperature is an abiotic pressure: species with heat-tolerant variants in their gene pool may shift and survive, while those without that variation cannot be "selected" into tolerance and risk decline. Knowing whether a threatened population already carries the needed variation is now a central question in planning which species can be saved, a direct application of everything in this lesson.
Odd one out: three of these are correct statements about the climate example above. Which one is wrong?
Distinguish between abiotic and biotic selective pressures, using one Australian example of each to show how the pressure acts on a population.
Distinguish asks for the difference between two things, stated on a shared criterion. It is not satisfied by describing each separately and leaving the contrast implied.
5 marks. State the dividing criterion in one sentence, then give one worked example on each side, then say what the two have in common so the distinction is not overdrawn.
Both are selective pressures, meaning environmental factors that cause differential survival and reproduction in a population. They differ in their source: an abiotic pressure comes from a non-living component of the environment, such as temperature, water availability, fire, salinity or light, while a biotic pressure comes from another living organism, through predation, competition, disease or parasitism.
In Australian sclerophyll woodland, fire is a recurring abiotic pressure. Within a plant population there is heritable variation in traits such as bark thickness, the presence of protected buds beneath the bark, and whether seed is stored in woody fruits that open after heat. Individuals with thicker insulating bark or heat-triggered seed release are more likely to survive a fire or to regenerate afterwards, so they leave more offspring, and the alleles for those traits increase in frequency in fire-prone country.
The arrival of the introduced red fox imposed a biotic pressure on populations of small and medium-sized native mammals. Within such a population there is heritable variation in traits like body size, nocturnal activity, wariness and burrowing or shelter use. Individuals that shelter in dense cover or in burrows during the fox's main hunting period are less likely to be taken and more likely to breed, so those traits become more common, while populations lacking any such variation have in many cases declined severely or disappeared.
The two therefore differ in source but not in mechanism. Fire acts on the plant population whatever the plants do, and its intensity is unaffected by how the population responds. The fox, being alive, can itself change in response: as native mammals become harder to catch, selection also acts on the fox population, so a biotic pressure can produce reciprocal change in both species over time in a way an abiotic pressure cannot.
Abiotic and biotic factors are both selective pressures, meaning environmental factors that cause differential survival and reproduction within a population, and they differ in their source: an abiotic pressure comes from a non-living component such as temperature, water, fire, salinity or light, while a biotic pressure comes from another living organism through predation, competition, disease or parasitism. Fire in Australian sclerophyll woodland is an abiotic example. Plant populations there carry heritable variation in bark thickness, in whether buds are protected beneath the bark, and in whether seed is held in woody fruits that open after heat, and individuals with these traits are more likely to survive a fire or regenerate after it, so they leave more offspring and the alleles concerned rise in frequency in fire-prone country. Predation by the introduced red fox is a biotic example. Populations of small and medium-sized native mammals vary heritably in body size, activity period, wariness and use of burrows or dense cover, and individuals sheltering during the fox's main hunting period are less likely to be taken and more likely to breed, so those traits become more common; where such variation was absent, populations have declined severely or been lost, which shows a selective pressure does not guarantee adaptation. The two differ in source but not in mechanism, and the sharpest difference is this: fire acts on the plant population regardless of how the plants respond and its intensity is unaffected by them, whereas the fox is itself alive and subject to selection, so as native mammals become harder to catch the fox population changes too. A biotic pressure can therefore produce reciprocal evolutionary change in both species, and an abiotic pressure cannot.
A prolonged drought begins in a grassland where a native grass population varies heritably in root depth. Predict how the population will change over several generations, and justify your prediction.
Predict asks you to state a likely outcome and support it with reasoning from a known principle. A prediction with no mechanism behind it is a guess, and it does not earn the justification marks.
5 marks. State the prediction in one clear sentence first, then justify it step by step, then state one condition that would change the outcome.
Over several generations the mean root depth of the grass population is predicted to increase, and the proportion of individuals carrying alleles for deeper roots is predicted to rise.
Drought reduces the water available in the upper soil layers, so water becomes the limiting factor for survival and reproduction. This is an abiotic selective pressure: it causes differential survival, because individuals differ in their ability to reach water.
The population already varies heritably in root depth. As the upper soil dries, shallow-rooted individuals lose access to water first and are more likely to wilt, fail to set seed, or die before reproducing. Deeper-rooted individuals can still draw water from lower in the profile, so a higher proportion of them survive to flower and set seed.
Because root depth is heritable, the seed produced during the drought comes disproportionately from deeper-rooted parents, so the next generation starts with a higher frequency of alleles for deeper roots. Repeating this over several generations of drought shifts the mean root depth of the population upward, which is evolution by natural selection.
The prediction depends on conditions that may not hold. If the population contains little heritable variation in root depth, no shift can occur however severe the drought. If the drought is severe enough to kill nearly all individuals before any set seed, the population may be lost rather than adapted. And because deeper roots cost energy that could go to growth or seed production, the advantage may reverse once the drought breaks, so the shift is not necessarily permanent.
Over several generations the mean root depth of the grass population is predicted to increase, with a rising proportion of individuals carrying alleles for deeper roots. The justification is as follows. Drought reduces water in the upper soil layers, making water the limiting factor for survival and reproduction, so it acts as an abiotic selective pressure causing differential survival. The population already varies heritably in root depth, and as the upper soil dries, shallow-rooted individuals lose access to water first and are more likely to wilt, fail to set seed or die before reproducing, while deeper-rooted individuals can still draw water from lower in the profile and a higher proportion survive to flower. Because root depth is heritable, the seed produced during the drought comes disproportionately from deeper-rooted parents, so the next generation begins with a higher frequency of the relevant alleles, and repeating this across several generations shifts the population mean upward. That shift is evolution by natural selection. The prediction rests on conditions that may not hold: if there is little heritable variation in root depth no shift can occur however severe the drought; if the drought kills nearly all individuals before any set seed the population may be lost rather than adapted; and because deeper roots cost energy that could otherwise go to growth or seed production, the advantage may reverse once the drought breaks, so the change is not necessarily permanent.
Practise independently
Answer every question in your own words first. The model answers are for checking against, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 12 marks
+5 XPApplyBand 5(4 marks) 1. Using one abiotic and one biotic example, demonstrate how factors in the environment exert selective pressures on a population.
1 mark: valid abiotic factor identified · 1 mark: explains how it selects a trait · 1 mark: valid biotic factor identified · 1 mark: explains how it selects a trait
UnderstandBand 3(3 marks) 2. Classify each of the following as an abiotic or a biotic selective pressure, and in each case name the trait it would be expected to favour: a prolonged drought on a grassland; the arrival of an introduced fox; a rise in soil salinity after land clearing.
1 mark: all three correctly classified · 1 mark: a sensible favoured trait named for at least two of them · 1 mark: each favoured trait is heritable and plausibly linked to survival under that pressure
AnalyseBand 5-6(5 marks) 3. The myxoma virus killed more than 99 per cent of the Australian rabbits it reached in 1950, but within a few years survival rates in exposed populations had risen sharply. Explain this change as the action of a biotic selective pressure, and explain why the same outcome would not have occurred if the original rabbit population had contained no resistance alleles.
1 mark: identifies the virus as a biotic selective pressure · 1 mark: states that resistance alleles pre-existed at low frequency · 1 mark: survivors reproduce, so resistance allele frequency rises across generations · 1 mark: explains that with no variation there is nothing for the pressure to favour · 1 mark: concludes that selection acts on existing variation rather than creating it
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answer
Q1 (4 marks): An abiotic factor such as drought exerts a selective pressure by reducing the availability of soft seeds; in Galápagos finches this meant only birds with deeper, stronger beaks could crack the remaining tough seeds, so they survived and reproduced more, shifting the population's mean beak depth upward. A biotic factor such as predation exerts a selective pressure because predators preferentially catch the most visible prey; in peppered moths on darkened bark, paler moths were eaten more often, so dark moths survived and reproduced more, increasing the frequency of the dark allele. In both cases the environmental factor caused differential reproductive success, which is the essence of a selective pressure.
Q2 (3 marks): A prolonged drought is an abiotic pressure, favouring plants (or animals) with heritable water-conserving features such as deeper roots or a reduced leaf surface area [1]. An introduced fox is a biotic pressure, favouring prey that are faster, more alert or better able to shelter in burrows [1]. A rise in soil salinity is an abiotic pressure, favouring plants able to tolerate or exclude salt at the root [1]. In every case the favoured trait must be heritable, or the pressure cannot change the population.
Q3 (5 marks): The myxoma virus acted as a biotic selective pressure: it is a living component of the environment (a pathogen) and it made survival highly unequal across the rabbit population [1]. Before the virus was released, a small proportion of rabbits already carried alleles conferring some resistance, which had arisen earlier by chance mutation [1]. When the virus swept through, susceptible rabbits died and the rare resistant ones survived and bred, passing their resistance alleles on, so the frequency of those alleles rose sharply over successive generations and survival in exposed populations increased [1]. If the original population had contained no resistance alleles at all, every rabbit would have been equally susceptible and there would have been nothing for the pressure to favour, so no shift in survival could have occurred, no matter how intense the epidemic [1]. This shows that a selective pressure acts on heritable variation that already exists rather than creating the variation it needs [1].
Retrieve and reflect
Do the spaced-review set from memory first, then look back at the prediction you made before you read anything.
Check what actually stuck
A short spaced-review set drawn from this lesson and the lessons before it. Answer from memory first, that is what makes it stick.
Lock it in
bossYou can now sort selective pressures into abiotic and biotic and give a worked example of each. Next lesson looks at a third source of pressure that is reshaping populations worldwide: human activity.
↩ Back to focus-area hubHow did your thinking change?
You sorted four rabbit threats into living and non-living groups. The frost and lack of grass are abiotic; the fox and the disease are biotic. The single words that name those groups are exactly the technical terms of this lesson: abiotic (non-living) and biotic (living) factors. All four can act as selective pressures, even though only two involve another organism attacking the rabbit.
If you grouped "lack of grass" with the living threats because grass is alive, note the distinction: the shortage of a resource is treated as the pressure, and competition for that limited resource is a biotic interaction, but the underlying availability driven by climate is abiotic. Exams accept either framing if your reasoning is explained.