What is variation?
The phenotypic differences between individuals of the same species.
What are the factors which produce variation between individuals of the same species?
Genetic and environmental
Which type of variation is due to genetic factors only (generally monogenic) and leads to discrete groups?
Discontinuous variation
What is the cause of heritable variation?
- Crossing over during prophase I of meiosis
- Independent assortment during metaphase I of meiosis
- Mixing of 2 parental genotypes during sexual reproduction
Which type of variation follows a normal distribution curve and is due to the interaction between both genetic and environmental factors?
Continuous variation
Generally polygenic (controlled by many genes)
What is non-heritable variation?
Variation not passed down to offspring
The genetic changes are in somatic cells (body cells) and are not present in gametes
What is the gene pool?
Total of all alleles of all genes within a population
What is genetic drift?
A change in allele frequencies caused by random events
What is allele frequency?
The percentage of the total number of copies of all alleles for that gene
What is a mutation?
A random, spontaneous change to a DNA sequence
Why is genetic variation within a population favourable?
It enables organisms to adapt more readily to a changing environment
What is evolution?
A gradual change in allele frequency in organisms over many generations, giving rise to new species
What is natural selection?
Mechanism driving evolution
Organisms with advantageous phenotypes are more likely to survive, breed and pass on the alleles for those phenotypes to their offspring
What are the steps involved in natural selection?
- There is competition for limited resources
- Random mutations give rise to phenotypic variation
- Certain phenotypes confer a selective advantage
- Individuals are more likely to survive, breed and pass on the allele for that phenotype to their offspring
- The allele frequency in the population increases
What is a selective agent?
An environmental factor that acts on a population, exerting selective pressures on the survival and breeding success of different phenotypes, altering the frequency of alleles
Give examples of selective agents
Predation
Competition for food, light, mates
Disease
Parasitism
Climate change
What is a positive selective pressure?
Certain phenotypes have a selective advantage, resulting in the organism being more likely to survive, breed and pass on the allele coding for the those characteristics
The allele frequency increases
What is a negative selective pressure?
Certain phenotypes have a selective disadvantage, resulting in the organism being less likely to survive, breed and pass on the allele coding for the those characteristics
The allele frequency decreases
What is intraspecific competition?
Competition for resources between members of the same species
What is interspecific competition?
Competition for resources between members of different species
What is the Hardy-Weinberg principle?
The frequencies of alleles (both dominant and recessive) and genotypes in a population remain constant from generation to generation
What is the Hardy-Weinberg equation?
p2 + 2pq + q2 = 1
and p + q = 1.0
where
p2 = frequency of AA (homozygous dominant)
q2 = frequency of aa (recessive)
2pq = frequency of Aa (heterozygote)
What conditions must remain true for the Hardy-Weinberg principle to remain true (population is in Hardy-Weinberg equilibrium)?
Population is large (100+ individuals)
No selection for or against a particular phenotype
Mating is random
No mutations
The population is isolated (no immigration/emigration)
Method for solving Hardy-Weinberg problems
- p2 or q2 given (may need to convert from percentage to decimal)
- Work out p or q (square root)
- Using ‘p + q = 1’ work out other allele frequency
- Calculate p2, q2 and 2pq
