Chapter 18 - Populations and Evolution

Key Words: 

• Gene pool - All the alleles of all the genes within a population at one time
• Allelic frequency - the number of times an allele occurs within a gene pool
• Stabilising selection - selection against the extreme phenotypes
• Directional selection - selection for one extreme phenotype
• Disruptive selection - favours individuals with extreme phenotypes rather than around the mean of the population
• Speciation - the evolution of a new species from existing ones
• Species - a group of individuals that share the same genes and can interbreed to produce fertile offspring

The Hardy-Weinberg Principle 

• Provides a mathmatical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population
• The principle makes the assumption that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next and 5 conditions are met
• There can be no mutations
• The population is isolated
• There is no selection - all alleles have an equal chance at going to the next generation 
• The population is large
• Random mating

p + q = 1.0

• p = dominatant allele
• q = recessive allele
• 1.0 as the probability of one allele plus the other is 100%

p^2 + 2pq +q^2 = 1.0

• This equation is used to express it as a probability

Variation in Phenotypes…