Selection Pressures | HL IB Biology Revision Notes 2025

Selection Pressures: Abiotic Factors

In Biology, ‘abiotic’ means non-living
An abiotic factor is a non-living factor within an ecosystem
- Some abiotic factors are density-independent
  - E.g. temperature or rainfall
Abiotic factors can act as selection pressures
- They affect the survival of individuals in a population, causing the population size to fluctuate

Sexual selection is another type of selection pressure that can affect the evolution of animal species
It can be defined as:
- A form of selection that occurs due to the preference of one sex for certain characteristics in individuals of the other sex
In order for a selection pressure to have an effect, there needs to be variation within the population
- Within the male cohort of a population, there will be variation in physical and behavioural traits which are visible to the female cohort (and vice versa)
These differences are sometimes viewed as indicators of overall fitness by the females within the population
As a result, they can affect an individual's success in attracting a mate and drive the evolution of an animal population

Sexual selection within a population can cause:
- Reproductive isolation
- Sexual dimorphism

Reproductive isolation occurs when changes in the alleles and phenotypes of some individuals in a species prevent them from successfully breeding with other individuals that don't have these changed alleles or phenotypes
For example, certain alleles may result in changes in male courtship behaviour meaning they are no longer attractive to females
- The females no longer choose to mate with these males, creating reproductive isolation

Sexual dimorphism is the distinct difference in size or appearance between the sexes of an animal species
Sexual dimorphism is commonly seen in Birds of Paradise
- The male birds are brightly coloured and can perform intricate courtship displays
- The female's plumage consists of greys and browns

A physical and/or behavioural trait within the male phenotype is used as an indicator of fitness by the females in the population
- E.g. a male peacock with vibrant, healthy tail feathers is likely to have a lower disease burden compared to a male with dull, sparse tail feathers
Females are more likely to be attracted to and mate with males that display the desired traits
Over time, the genes associated with these traits are inherited by subsequent generations and they become more prominent within the species
As a result, sexual selection can impact mating success, driving the evolution of an animal population

Natural selection occurs due to competition for resources whereas sexual selection occurs due to competition for mates
- On islands where resources are plentiful and predators rare, females are often the primary selection pressure which determines how males evolve
Sexual selection results in animals with enhanced mating success whereas natural selection tends to result in populations of individuals that are well-adapted to their environment
Sexual selection does not result in individuals that are well-adapted to their environment
- E.g. Peacocks possess iridescent tail feathers with a specific eye-spot pattern which are used heavily during courtship displays to females
- Over time, sexual selection in peacocks has resulted in males with longer tail feathers and more elaborate patterns
- These traits actually make the bird more prone to predation, reducing their chances of survival
  - A long tail reduces agility, ability to fly and makes the bird easier to spot
Sexual selection can be a more prominent evolutionary force than natural selection as variation in mating success can:
- Amplify selection
- Maintain new genetic variation among individuals
Both of which can result in rapid evolutionary change

A male peacock proudly displays his elaborate tail feathers in an attempt to attract a female mate

The concept of sexual selection is viewed as Darwin's second-greatest insight
Darwin was aware that the existence of traits that were not favoured by natural selection needed a reasonable explanation