7. Genetics, Populations, Evolution & Ecosystems: Definitions (A Level only) (AQA A-Level Biology): Revision Notes

7. Genetics, Populations, Evolution & Ecosystems: Definitions (A Level only)

Abiotic factors: The non-living components of an ecosystem, including physical and chemical factors such as temperature, light intensity, moisture content, soil pH, and oxygen concentration.

Adaptation: A characteristic of an organism that enhances its chances of survival and reproduction in its specific environment.

Allele: A particular version or variant of a gene that occupies the same position on homologous chromosomes.

Allele frequency: The proportion of times a specific allele occurs within a population's total gene pool.

Allopatric speciation: The evolutionary process where new species form when populations become geographically separated and evolve independently.

Autosomal linkage: The phenomenon where two or more genes located on the same autosome tend to be inherited together because they are less likely to be separated during crossing over in meiosis.

Autosome: Any chromosome that is not a sex chromosome (not X or Y).

Belt transect: A sampling method using a measured line across a study area, with quadrats placed at regular intervals to assess organism abundance and distribution patterns.

Biodiversity: The total variety of genetic material, species, and habitat types found within a specific area or ecosystem.

Biotic factors: The living components of an ecosystem, including factors such as food availability, presence of pathogens, predators, and competitors.

Carrying capacity: The maximum population size that an ecosystem can sustainably support over extended time periods, determined by available resources and environmental conditions.

Chi-squared test: A statistical method used to determine whether observed inheritance patterns differ significantly from expected patterns, helping to test genetic hypotheses.

Climax community: The final, stable stage of ecological succession where the community composition remains relatively constant over time.

Codominant: A genetic situation where both alleles in a heterozygous individual contribute equally to the observable phenotype.

Community: The collection of all populations of different species that inhabit the same area and interact with each other.

Conservation: The active management and protection of ecosystems and biodiversity by humans to preserve natural resources for future generations, often involving succession management.

Degrees of freedom (χ² test): A statistical parameter calculated as the number of categories minus one, used in chi-squared test calculations.

Dihybrid inheritance: The inheritance pattern involving two different genes that control two distinct characteristics, located on separate chromosomes.

Diploid: Describes cells that contain two complete sets of chromosomes, one from each parent.

Directional selection: A form of natural selection that consistently favours individuals at one extreme of a phenotypic range, shifting the population mean over time.

Disruptive selection: A type of natural selection that favours individuals with extreme phenotypes whilst selecting against those with intermediate characteristics.

Dominant: Describes an allele that is expressed in the phenotype even when only one copy is present. Conventionally represented using capital letters.

Ecosystem: A dynamic system comprising all living organisms (biotic components) and non-living elements (abiotic components) in a specific area, including all their interactions.

Epistasis: A genetic interaction where the expression of one gene is modified or masked by the action of alleles at a different gene locus.

Evolution: The gradual change in allele frequencies within a population over successive generations, primarily driven by natural selection.

Gene: A specific DNA sequence on a chromosome that contains the instructions for producing one or more polypeptide chains or functional RNA molecules.

Gene pool: The complete collection of all different alleles present in all individuals within a population.

Genetic drift: Random fluctuations in allele frequencies that occur in small populations due to chance events during reproduction.

Genetic variation: The differences in genetic composition between individuals in a population, arising from mutations, meiosis, and random fertilisation processes.

Genotype: The complete genetic composition of an organism, describing all the alleles present.

Habitat: The specific environmental area where an organism naturally lives and meets all its survival requirements.

Hardy-Weinberg principle: A mathematical model predicting that allele frequencies remain constant across generations when five specific conditions are met: no mutations, no natural selection, no migration, large population size, and random mating. The equation describes allele frequency relationships, where p represents the dominant allele frequency and q represents the recessive allele frequency.

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

Heterozygous: The genetic condition where an individual possesses two different alleles for the same gene (for example, Ff).

Homozygous: The genetic condition where an individual possesses two identical alleles for the same gene (for example, ff or FF).

Interspecific competition: Competition for resources that occurs between individuals of different species.

Intraspecific competition: Competition for resources that occurs between individuals of the same species.

Locus: The specific physical position of a gene on a chromosome.

Mark-release-recapture: A population estimation technique involving capturing individuals, marking them, releasing them back into the population, then recapturing a sample later. Population size is estimated using the formula:

$\text{estimated population size} = \frac{\text{number in first sample} \times \text{number in second sample}}{\text{number of marked individuals in second sample}}$

Monohybrid inheritance: The inheritance pattern involving a single gene that controls one specific characteristic.

Multiple alleles: The situation where a single gene has more than two possible allele variants within a population.

Natural selection: The evolutionary process where individuals with advantageous characteristics have greater reproductive success, gradually increasing the frequency of beneficial alleles in the gene pool. This concept was developed by Charles Darwin.

Niche: The complete description of how an organism fits into its ecosystem, including its habitat requirements and ecological role.

Phenotype: The observable characteristics of an organism, resulting from interactions between its genotype and environmental influences.

Pioneer species: Early colonising species capable of surviving in harsh, unstable environments and establishing themselves on bare substrates like rock or sand. Examples include lichens.

Population: All individuals of the same species living together in the same habitat at the same time.

Predator: An organism that hunts, kills, and consumes other organisms for nutrition.

Prey: An organism that serves as food for predators.

Quadrat: A square sampling frame of known dimensions used to estimate organism abundance and distribution in ecological studies. Two main types exist: point quadrats and frame quadrats.

Random sampling: A sampling methodology designed to eliminate bias by using random coordinate generation or systematic grid placement.

Recessive: Describes an allele that is only expressed in the phenotype when present in two copies (homozygous condition). Conventionally represented using lowercase letters.

Selection pressures: Environmental factors that influence evolutionary change through natural selection and affect population sizes, including competition, predation, and disease.

Sex-linkage: The inheritance pattern of genes located on the X or Y sex chromosomes.

Speciation: The evolutionary process leading to the formation of new species when populations become reproductively isolated. Two main forms exist: allopatric and sympatric speciation.

Species: A group of similar organisms capable of interbreeding to produce fertile, viable offspring.

Stabilising selection: A form of natural selection that favours individuals with phenotypes close to the population average whilst selecting against extreme variants.

Succession: The predictable sequence of changes in community composition that occurs in a given area over time.

Sustainable: The principle of using resources in a way that meets current needs whilst preserving them for future generations.

Sympatric speciation: The formation of new species when populations within the same geographical area become reproductively isolated.

Systematic sampling: A sampling technique that determines organism abundance and distribution by placing samples at regular intervals along a measured transect. This method is particularly useful in ecosystems showing gradual environmental changes.

Variation: The differences observed between individuals, resulting from genetic factors, environmental influences, or interactions between both.