Natural selection

Populations of organisms have the potential to produce large numbers of offspring, yet their numbers remain fairly constant year after year.




Natural selection occurs as populations have the capacity to produce many offsprings --> compete for resources --> individuals best adapted to survive breed and pass on their alleles.
Variation means some individuals in a population will have features which give them an advantage in the 'struggle for existence'.

Environmental factors
- biotic: caused by other organisms
    e.g.: predation, food competition, infection by pathogens
- abiotic: caused by non-living components of the environment
    e.g.: water supply, nutrient level of soil

Selection pressures control the chances of some alleles being passed on to the next generation.
    e.g.: predators - individuals that can better camouflage themselves survive more, pass on alleles
The effects of such selection pressures on the frequency of alleles in a population is called natural selection. The frequency of advantageous alleles increase, the frequency of disadvantageous alleles decrease.

Types of selection
Stabilising selection: the status quo is maintained because the organisms are already well adapted to their environment
- acts against extremes
- favours the environment
- e.g.: birth weight


Directional selection: the most common varieties of an organism are selected against --> change in the features of the population
- favours variants of 1 extreme when new allele appears or new environmental factor occurs
- e.g.: peppered moths



Disruptive selection: favours the survival of individuals at 2 different points within the range of variation, resulting in 2 different phenotypes
- conditions favour both extremes --> maintain different phenotypes in the population
- e.g.: Galapagos finches

Genetic drift
- a change in allele frequency of a small population
- occurs by chance, because only some of the organisms of each generation reproduce.
The founder effect occurs in small, isolated populations
- results from the colonization of a new location by a small number of individuals
- further genetic drift occurs in the small population
- evolution of this population may take a different direction from the larger population

The Hardy-Weinberg principle
- when a particular phenotypic trait is controlled by 2 alleles of a single gene A/a
- genotypes: AA, Aa, aa
- calculate proportion of these genotypes in a large, randomly mating population

p= frequency of dominant allele A

q = frequency of recessive allele a

Total of whole population = 1

• chance of offspring inheriting dominant allele = px p= p²
• chance of offspring inheriting recessive allele = qx q= q²
• chance of inheriting both dominant and recessive allele = 2 (px q) = 2pq

17.2 Natural and artificial selection Populations of organisms have the potential to produce large numbers of offspring, yet their numbers remain fairly constant year after year.  Humans use selective breeding (artificial selection) to improve features in ornamental plants, crop plants, domesticated animals and livestock. a) explain that natural selection occurs as populations have the capacity to produce many offspring that compete for resources; in the ‘struggle for existence’ only the individuals that are best adapted survive to breed and pass on their alleles to the next generation  b) explain, with examples, how environmental factors can act as stabilising, disruptive and directional forces of natural selection  c) explain how selection, the founder effect and genetic drift may affect allele frequencies in populations  d) use the Hardy–Weinberg principle to calculate allele, genotype and phenotype frequencies in populations and explain situations when this principle does not apply  e) describe how selective breeding (artificial selection) has been used to improve the milk yield of dairy cattle  f) outline the following examples of crop improvement by selective breeding:  • the introduction of disease resistance to varieties of wheat and rice  • the incorporation of mutant alleles for gibberellin synthesis into dwarf varieties so increasing yield by having a greater proportion of energy put into grain  • inbreeding and hybridisation to produce vigorous, uniform varieties of maize