What is a Population
A group of organisms of the same species that occupy a particular space at a particular time (Can potentially interbreed)
What is a ‘Gene Pool’
All of the alleles of all the genes of all the individuals in a population at a given time
- Alleles are the same in all cells of an individual, so only one set of alleles is counted per gene, per interval, in the gene pool
What is Allele frequency
The relative frequency of an allele within a population
What is the Hardy-Weinberg Principle
Principle stating that the allele and genotype frequencies within a population remain constant across generations (In the absence of mutations and selection pressures)
- ∴ Allele frequencies can be calculated within a population
What is the Hardy-Weinberg Equation used to determine the allele frequency of a single gene (2 alleles)
P+q = 1
P: Probability of an individual having the dominant allele for a particular gene
q: Probability of an individual having the recessive allele for a particular gene
- Can be used to determine the allele frequency for a particular gene, within a population
What is the Hardy-Weinberg Equation used to determine the allele frequency of a particular gene based on the genotypes of individuals
P^2 + 2Pq + q^2 = 1
P^2: Probability of an individual being Homozygous dominant
2Pq: Probability of an individual being Heterozygous
q^2: Probability of an individual being Homozygous recessive
- Can be used to determine the allele frequency for a particular gene, based on genotypes and observable phenotypes
- P^2 + 2Pq: Probability of an individual expressing dominant phenotype
- q^2: Probability of an individual expressing recessive phenotype
- Derived by considering the the principle for allele probabilities, when two alleles are present in the genotype
- (P+q)^2 = P^2 + 2Pq + q^2 = 1
What assumptions are made for the Hardy-Weinberg principle to be true
- No mutations arise
- Population is isolated, so there is no flow of alleles in/out
- No selection, so all alleles are equally likely to be inherited
- Population is large
- Mating in the population is random
Although most of these conditions are never fully met in naturally populations, the Hardy-Weinberg is still used for determining allele frequencies
How do genetic factors cause variations in phenotype
- Mutations
- Sudden changes to gene and chromosome that are potentially inherited by future generations
- eg. Deletion, substitution, etc
- Mutations are the main source of variation
- Meiosis
- Nuclear division resulting in new combinations of alleles that pass into gametes
- Independent assortment of chromosomes results in genetically unique gametes
- Random fertilisation
- Which gamete fuses with which at fertilisation in sexual reproduction is random, so increases variation in the off-spring
- New allele combinations cause the off-spring to be different to their parents, resulting in variation between generations
How do environmental factors cause variations in phenotype
- Environmental factors affect the way genes are expressed
- The genes set the limits for a particular trait, but the environment influences where within these limits the phenotype will be
- eg. Plant may have genotype to grow tall, but poor environmental conditions such as low soil nitrates, will limit plant growth
What is Natural selection
Process by which individual organisms more suited and better adapt for environmental conditions survive and pass their advantageous genes on to the next generation.
(Leads to Evolution)
What is a selection pressure and how does it lead to Natural selection
Environmental factor that limits the population size of a population
- eg. Predation, Disease, competition, etc.
- Effects allele frequency, as individuals with certain alleles will have advantageous phenotypes under the selection pressure, so are more likely to survive and breed (Natural selection)
What conditions must be met by a population to allow natural selection to occur
- Organisms produce more off-spring than can be supported by the available resources/environment (leads to competition)
- Genetic variation within the population
- Variety of different phenotypes within the population
Why is the ‘Over-production of off-spring’ important for natural selection to occur
- Over production of off-spring leads to there being too many individuals for the available resources
- Results in intraspecific competition between individuals in a population
- ∴ The individuals that survive are the ones more suited to the environment
- ∴ Advantageous alleles are passed on to the next generation (natural selection)
Why is variation within a population important for natural selection to occur
A wide range of different alleles means that when environmental conditions change (selection pressure), it is more likely that at least some individuals will have advantageous traits
- ∴ Less genetic diversity makes the population more vulnerable to extreme selection pressures (eg. diseases)
- Variations in phenotype allows for intraspecific competition to lead to the survival of advantageous individuals (natural selection)
- Variation allows for a ‘continuum’ of traits, giving a normal distribution to allow natural selection to occur
What are the 3 types of selection that can occur as a result of a selection pressure
- Stabilising selection
- Directional selection
- Disruptive selection
What is stabilising selection
Selection that preserves the mean phenotype in a population by favouring ‘average’ individuals and selecting against extremes
What are some examples of stabilising selection
- Mammals with medium length fur survive in an environment with a constant temperature (Long/short hair selected against)
- Babies with a body mass greater or less than average have a higher risk of dying soon after birth, ∴ average weight is advantageous and extremes are selected against
What is Directional selection
Selection favouring individuals with phenotypes in one direction from the mean (selection of one extreme)
What are some examples of directional selection
- Mammals with extremely long fur, in regions with a decreasing temperature, will be at an advantage, so will survive and pass on their genes to the next generation
- Peppered moths: Tree bark was darker due to soot in the industrial revolution, so darker coloured moths were advantageous, ∴ survived and bred
What is Disruptive selection
Selection favouring individuals with both extreme phenotypes (selection against the mean)
What are some examples of disruptive selection
- In regions where the temperature varies throughout the year (summer/winter), mammals with both long and short hair will be advantageous at some point, while the mean is selected against
- Breeding fish: Small fish can ‘sneak’ up to females and breed, and large males win mates through competition, so medium size males (average) are selected against
How do selection pressures affect allele frequency within a population
Selection pressure affect the probability of certain alleles being passed on, so alter the allele frequency in the next generation
- Selection pressures mean that some individuals will be advantageous due to the genetic variation within the population
- These advantageous alleles will be more likely to be passed on (favourable in intraspecific competition) so will be more frequent in the next generation
What is a species
Group of organisms with a common ancestry, that are capable of breeding to produce fertile offspring
What is speciation
The formation of a new species from an existing one, due to isolation and separate processes of natural selection
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Section 1 - Biological molecules: 1. Biological molecules79
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Section 1 - Biological molecules: 2. Nucleic acids62
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Section 2 - Cells: 3. Cell structure74
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Section 2 - Cells: 4. Transport across cell membranes42
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Section 2 - Cells: 5. Cell recognition and the immune system50
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Section 3 - Organisms exchange substances with their environment: 6. Exchange80
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Section 3 - Organisms exchange substances with their environment: 7. Mass transport75
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Section 4 - Genetic information, variation and relationships between organisms: 8. DNA, genes and protein synthesis42
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Section 4 - Genetic information, variation and relationships between organisms: 9. Genetic diversity42
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Section 4 - Genetic information, variation and relationships between organisms: 10. Biodiversity82
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Section 5 - Energy transfer in and between organisms: 11. Photosynthesis34
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Section 5 - Energy transfer in and between organisms: 12. Respiration33
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Section 5 - Energy transfer in and between organisms: 13. Energy and Ecosystems31
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Section 6 - Organisms respond to changes in their environment: 14. Response to stimuli66
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Section 6 - Organisms respond to changes in their environment: 15. Nervous coordination and muscles59
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Section 6 - Organisms respond to changes in their environment: 16. Homeostasis58
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Section 7 - Genetics, populations, evolution and ecosystems: 17. Inherited change37
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Section 7 - Genetics, populations, evolution and ecosystems: 18 Populations and evolution32
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Section 7 - Genetics, populations, evolution and ecosystems: 19. Populations in ecosystems36
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Section 8 - The control of gene expression: 20. Gene expression56
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Section 8 - The control of gene expression: 21. Recombinant DNA technology44
