IB Biology HLNatural SelectionPaper 1 & 2~12 min read
Selection Pressures
Something has to do the selecting. A selection pressure is any factor that makes some individuals more likely to survive and breed than others — and one of the strongest ones is not a predator or the weather at all. It is the opposite sex.
📚 What you need to know
A selection pressure is any factor that affects an organism’s chance of surviving and reproducing.
Abiotic factors are non-living: temperature, rainfall, light, soil pH, wind, oxygen and carbon dioxide levels.
Some abiotic factors are density-independent — they hit a population just as hard whatever its size.
Sexual selection is selection caused by the preference of one sex for certain characteristics in the other sex.
Sexual selection needs variation in the population, just like natural selection does.
It can cause reproductive isolation and sexual dimorphism.
Natural selection is competition for resources; sexual selection is competition for mates.
Sexual selection can produce traits that make an animal less well adapted to its environment, such as a peacock’s tail.
What counts as a selection pressure
A selection pressure is anything in the environment that affects survival, and therefore how many offspring an individual leaves behind. It is the thing doing the choosing in natural selection.
Definition
A selection pressure is an environmental factor that affects an organism’s chance of surviving and reproducing, causing some alleles to become more common than others
Pressures fall into two groups. Abiotic pressures come from the non-living part of the ecosystem. Biotic pressures come from other living things: predators, parasites, disease, competitors and mates.
Abiotic factors
In biology, abiotic simply means non-living. An abiotic factor is a non-living factor within an ecosystem, and these factors affect the survival of individuals, which makes the population size fluctuate.
Many abiotic factors are density-independent. A hard frost kills the same proportion of a population whether that population has fifty members or five thousand, because the frost does not care how crowded it is. Contrast that with a disease, which spreads faster in a dense population.
Abiotic factor
How it acts as a selection pressure
Light intensity
Plants need light for photosynthesis. More light means a faster rate of photosynthesis and faster growth, so plants that reach the light out-compete those that do not
Temperature
Affects the rate of photosynthesis and of every other enzyme-controlled reaction, so individuals that tolerate the local range survive better
Water availability
All plants and animals need water, so drought selects strongly for any allele that reduces water loss
Soil pH and mineral content
Different plant species are adapted to different soil pH values and nutrient levels, so soil chemistry decides what can grow where
Wind speed and direction
Wind increases transpiration. Transpiration also carries water and mineral ions to the leaves, so both too much and too little wind cause problems
Carbon dioxide concentration
Carbon dioxide is a raw material for photosynthesis, so its concentration affects the rate at which plants can grow
Dissolved oxygen
Some aquatic animals, such as many fish, can only survive in water with a high oxygen concentration
A question will rarely say “abiotic factor” out loud. It will describe a lake warming up, or a soil turning acidic, and expect you to spot that the factor is non-living, name it, and then run the normal five-step natural selection chain.
Sexual selection
Sexual selection is a different kind of selection pressure, and it only applies to animal species.
Definition
Sexual selection is a form of selection that occurs due to the preference of one sex for certain characteristics in individuals of the other sex
For any selection pressure to have an effect there must be variation within the population. Within the males of a population there is variation in physical and behavioural traits, and those traits are visible to the females (and the other way round). Females often treat these differences as an indicator of a male’s overall fitness, so the traits affect his success at attracting a mate — and that drives the evolution of the whole population.
The two effects you must know
Sexual selection within a population can cause reproductive isolation and sexual dimorphism.
Reproductive isolation
Reproductive isolation happens when changes in the alleles and phenotypes of some individuals prevent them from breeding successfully with the individuals that do not have those changed alleles.
For example, an allele might change a male’s courtship behaviour so that females no longer find him attractive. Those females stop choosing to mate with those males, and the population splits into groups that no longer interbreed — the first step towards two separate species.
Reproductive isolation is where speciation starts. Here the barrier is behavioural rather than a river or a mountain range.
Sexual dimorphism
Sexual dimorphism is a distinct difference in size or appearance between the sexes of an animal species. Birds of paradise are the classic example: the males are brightly coloured and perform intricate courtship displays, while the females are greys and browns.
How does that difference build up? A physical or behavioural trait in the male phenotype is used by females as an indicator of fitness. A male peacock with vibrant, healthy tail feathers probably carries a lower disease burden than a male with dull, sparse feathers, so females are more likely to mate with him. The alleles associated with the showy trait are inherited by the next generation, and over time the trait becomes more prominent — in the males only, because only males are being judged on it.
Natural selection and sexual selection are not the same thing
Natural selection is driven by competition for resources. Sexual selection is driven by competition for mates. That difference has one surprising consequence: sexual selection does not produce individuals that are well adapted to their environment.
Peacocks have iridescent tail feathers with an eye-spot pattern that they use heavily in courtship displays. Sexual selection has made those feathers longer and the patterns more elaborate over time. But a long tail reduces agility, makes flying harder and makes the bird far easier for a predator to spot. The trait that wins mates is the same trait that gets you eaten.
This is why the phrase “survival of the fittest” is misleading. Fitness means offspring produced, and a short showy life can beat a long dull one.
Feature
Natural selection
Sexual selection
Competition for
Resources such as food, water and space
Mates
Who does the selecting
The environment, including predators and disease
Members of the opposite sex
Result
Populations well adapted to their environment
Animals with enhanced mating success
Effect on survival
Improves it
May reduce it, as with a long tail
Typical outcome
Camouflage, tolerance, efficiency
Sexual dimorphism, elaborate displays
Why sexual selection can be the faster force. On islands where resources are plentiful and predators are rare, females are often the main selection pressure deciding how males evolve. Variation in mating success can amplify selection and maintain new genetic variation, and both of those can produce rapid evolutionary change.
Darwin knew he had a problem. Traits that natural selection should have removed — enormous tails, absurd colours, exhausting dances — kept appearing anyway, and they needed a reasonable explanation. Sexual selection was his answer, and it is often called his second-greatest insight.
Worked examples
WE 1
An abiotic factor acting as a selection pressure
A population of small plants grows on a mountainside. Average temperatures fall over several decades. Explain how this acts as a selection pressure. (3 marks)
Point 1: identify the factor and the variation
Temperature is an abiotic, density-independent factor. There is variation in cold tolerance in the population, caused by different alleles.
Point 2: differential survival
Plants with an allele giving frost protection are more likely to survive the cold and go on to reproduce successfully.
Point 3: change in frequency
They pass that allele on, so its frequency increases each generation and the population becomes more cold tolerant.
Cold does not create the allele — it decides which plants get to breedworth adding: if no such allele exists in the population, the species may become extinct in that habitat
WE 2
A trait that lowers survival
Male widowbirds have extremely long tail feathers that make flying harder. Explain how such a trait can become common in a population. (4 marks)
Point 1: the trait is a signal
Females use tail length as an indicator of male fitness, so long-tailed males attract more mates.
Point 2: more mating means more offspring
Those males reproduce more often and pass on the alleles for long tails to their offspring.
Point 3: frequency rises
Over generations the alleles become more common in the male population, producing sexual dimorphism.
Point 4: the trade-off
The tail reduces agility and increases predation risk, but the reproductive advantage outweighs the survival cost, so the trait is still selected for.
Sexual selection can favour a trait that natural selection is working againstthe mark is usually lost by forgetting to say the trait is passed on genetically
WE 3
Distinguish between the two types of selection
Distinguish between natural selection and sexual selection. (3 marks)
Difference 1: what is competed for
Natural selection involves competition for resources; sexual selection involves competition for mates.
Difference 2: the outcome
Natural selection tends to produce individuals well adapted to their environment; sexual selection produces individuals with enhanced mating success.
Difference 3: adaptation
Sexual selection can result in individuals that are poorly adapted to their environment, such as peacocks with long conspicuous tails.
Resources versus mates; survival versus attractiveness“distinguish” means give paired statements — write both halves of each difference
💡 Exam tips
Say abiotic and non-living in the same sentence to secure the definition mark.
Remember that density-independent factors affect the same proportion of a population whatever its size.
Sexual selection questions still need the full chain: variation, preference, more offspring, alleles passed on, frequency increases.
Use the words reproductive isolation and sexual dimorphism by name — they are the two named effects.
If a question describes a costly, showy trait, the answer is almost always a trade-off between sexual and natural selection.
Fitness in biology means reproductive success, not strength or speed.
⚠ Common mistakes
Calling a predator an abiotic factor. Predators are alive, so they are biotic.
Saying sexual selection improves adaptation. It often does the opposite.
Describing sexual dimorphism as just “males and females are different”. Say it is a distinct difference in size or appearance between the sexes.
Assuming reproductive isolation needs a physical barrier. A change in courtship behaviour is enough.
Forgetting variation. Females cannot prefer one male over another if all males look the same.
Writing that the peacock grows a longer tail to attract females. He inherits it; he does not choose it.
Up next: Selection Pressures (Skills). Everything above is theory. Now we look at the guppy experiments that actually tested it, and at how to pull the trend out of the graphs an examiner puts in front of you.
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