topic 7

• Abiotic factors: The non-living aspects of an ecosystem e.g. temperature, light intensity, moisture, soil pH and oxygen levels.
• Adaptation: A feature of an organism that increases its chance of survival in its environment.
• Allele: A version of a gene.
• Allele frequency: The number of times an allele appears within a population’s gene pool.
• Allopatric speciation: A form of speciation that occurs when two populations become geographically isolated.
• Autosomal linkage: When two or more genes are positioned on the same autosome. They are unlikely to be separated by crossing over during meiosis so are often inherited together.
• Autosome: A chromosome that is not an X or Y chromosome
• Belt transect: A line along a sampled area, upon which quadrats are placed at certain intervals to determine the abundance and distribution of organisms in an ecosystem.
• Biodiversity: The variety of genes, species and habitats within a particular area.
• Biotic factors: The living components of an ecosystem e.g. food availability, pathogens and predators.
• Carrying capacity: The average size of a population that can be supported by an ecosystem over extended periods of time. This varies depending on biotic and abiotic factors.
• Chi-squared test: A statistical test used to determine whether a pattern of inheritance is statistically significant
• Climax community: The stable community of organisms that exists at the final stage of ecological succession.
• Codominant: When both alleles for a gene in a heterozygous organism equally contribute to the phenotype.
• Community: All of the populations of different species living together in a habitat.
• Conservation: The maintenance of ecosystems and biodiversity by humans in order to preserve the Earth’s resources. This typically involves the management of succession.
• Degrees of freedom (X 2 test): The number of categories minus one
• Dihybrid inheritance: The inheritance of two different genes, that determine two phenotypes, on two different chromosomes.
• Diploid: Describes a cell with a nucleus containing two sets of chromosomes.
• Directional selection: A type of selection that favours one extreme phenotype and selects against all other phenotypes.
• Disruptive selection: A type of selection that favours individuals with extreme phenotypes and selects against those with phenotypes close to the mean.
• Dominant: Describes an allele that is always expressed. Represented by a capital letter.
• Ecosystem: The community of organisms (biotic) and non-living (abiotic) components of an area and their interactions. It is a dynamic system.
• Epistasis: Describes a relationship between genes where the allele of one gene affects the expression of a different gene.
• Evolution: The gradual change in the allele frequencies within a population over time. Occurs due to natural selection.
• Gene: A length of DNA on a chromosome that codes for the production of one or more polypeptide chains and functional RNA.
• Gene pool: All of the different versions of genes (alleles) in the individuals that make up a population.
• Genetic drift: Variations in allele frequencies in small populations due to chance.
• Genetic variation: Differences in genotypes between members of a population which may occur due to mutations, meiosis, or random fertilisation.
• Genotype: An organism’s genetic composition. Describes all alleles.
• Habitat: The region where an organism normally lives.
• Hardy-Weinberg principle: A model that predicts that the ratio of dominant and recessive alleles in a population will remain constant between generations if the following five conditions are met: no new mutations; no natural selection; no migration; large population; and random mating. It provides an equation for calculating the frequencies of alleles:

  $p 2 +$$2pq +$$q2 =$$1.0$

  where p is the frequency of the dominant allele, and q is the frequency of the recessive allele
• Heterozygous: When someone has two different alleles of a gene e.g. Ff.
• Homozygous: When someone has two identical alleles of a gene e.g. ff.
• Interspecific competition: A type of competition that takes place between members of different species.
• Intraspecific competition: A type of competition that takes place between members of the same species.
• Locus: The position of a gene on a chromosome.
• Mark-release-recapture: A method of estimating the population size of motile organisms. It involves capturing a sample of the population, marking them and releasing them. At a later date, another sample is captured and the number of marked individuals recorded. The population size can be estimated using the following equation:

  estimated population size = (number of individuals in the first sample x number of individuals in the second sample)/number of marked individuals in the second sample
• Monohybrid inheritance: The inheritance of one gene.
• Multiple alleles: When a gene has more than two potential alleles.