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Basics of Natural Selection
Basics of Natural Selection
Natural selection is the change in the composition of a gene pool in response to a deferentially selective environmental pressure
- The frequency of one particular phenotype in relation to another will be a product of the type of selection that is occurring
Natural selection can only occur if there is variation among members of the same species
Mutation, meiosis and sexual reproduction cause variation between individuals in a species
Adaptations are characteristics that make an individual suited to its environment and way of life
Species tend to produce more offspring than the environment can support
Individuals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die or produce fewer offspring
Individuals that reproduce pass on characteristics to their offspring
Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species
The theory of natural selection was posited by Charles Darwin (and also Alfred Wallace) who described it as ‘survival of the fittest’
- According to this theory, it is not necessarily the strongest or most intelligent that survives, but the ones most responsive to change
This video introduces natural selection and the three key ingredients through which it effects change over time: the trait must vary in the population, it must be heritable, and individuals with a certain variation must have a reproductive advantage over those that do not.
- Natural selection acts on random variations.
- Variations must pass from parent to offspring.
The process of natural selection occurs in response to a number of conditions:
- Inherited Variation – There is genetic variation within a population which can be inherited
- Competition – There is a struggle for survival (species tend to produce more offspring than the environment can support)
- Selection – Environmental pressures lead to differential reproduction within a population
- Adaptations – Individuals with beneficial traits will be more likely to survive and pass these traits on to their offspring
- Evolution – Over time, there is a change in allele frequency within the population gene pool
The key components to the process of natural selection are:
- Inherited variation exists within the population
- Competition results from an overproduction of offspring
- Environmental pressures lead to differential reproduction
- Adaptations which benefit survival are selected for
- Genotype frequency changes across generations
- Evolution occurs within the population
Types of Selection
Types of Selection
Stabilizing Selection
- Where an intermediate phenotype is favoured at the expense of both phenotypic extremes
- This results in the removal of extreme phenotypes (phenotypic distribution becomes centrally clustered to reflect homogeneity)
- Operates when environmental conditions are stable and competition is low
- An example of stabilising selection is human birth weights (too large = birthing complications ; too small = risk of infant mortality)
Directional Selection
- Where one phenotypic extreme is selected at the cost of the other phenotypic extreme
- This causes the phenotypic distribution to clearly shift in one direction (towards the beneficial extreme)
- Operates in response to gradual or sustained changes in environmental conditions
- Directional selection will typically be followed by stabilising selection once an optimal phenotype has been normalised
- An example of directional selection is the development of antibiotic resistance in bacterial populations
Disruptive Selection
- Where both phenotypic extremes are favoured at the expense of the intermediate phenotypic ranges
- This causes the phenotypic distribution to deviate from the centre and results in a bimodal spread
- This occurs when fluctuating environmental conditions (e.g. seasons) favour the presence of two different phenotypes
- Continued separation of phenotypic variants may eventually split the population into two distinct sub-populations (speciation)
- An example of disruptive selection is the proliferation of black or white moths in regions of sharply contrasting colour extremes
Quite complex Video about types of Natural Selection and Selection Pressures
How does Natural Selection Lead to Evolution
How does Natural Selection Lead to Evolution
It is crucial that you remember Darwin’s steps of how natural selection leads to evolution. Be sure to memorize the following:
(1) overproduction of offspring
(2) variation within the population, as a result of meiosis, sexual reproduction, and mutations
(3) struggle for survival, because there are not enough resources for all members of the population
(4) differential survival, those individuals best fit for their environment tend to survive better
(5) reproduction, those who survive can pass on their genes to the next generation.
It is through these steps that populations evolve. Remember that, even though the changes can be observed in individuals from generation to generation, what is of importance is what happens at the level of populations rather than at the individual level.
Awesome Simulation Video to combine many aspects of Natural Selection, mutation and evolution
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