7- Genetics , Populations, Evolution and Ecosystems

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 (X₂ 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 p2\*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

Monohybrid inheritance

The inheritance of one gene.

Multiple alleles

When a gene has more than two potential alleles.

Natural selection

The process by which the frequency of beneficial alleles gradually increases in a population’s gene pool over time. This theory was developed by Charles Darwin.

Niche

Describes how an organism ‘fits’ into an ecosystem and its role in that environment.

Phenotype

An organism’s observable characteristics. Due to interactions of the genotype and the environment.

Pioneer species

Species that can survive in hostile environments and colonise bare rock or sand e.g. lichens.

Population

All organisms of the same species living with one another in a habitat at the same time.

Predator

An organism that eats other organisms.

Prey

An organism that is eaten by predators.

Quadrat

A square grid of a known area used in sampling to determine the abundance of organisms in a habitat. There are two types: point quadrats and frame quadrats.

Random sampling

A sampling technique used to avoid bias e.g. creating a square grid and generating random coordinates.

Recessive

Describes an allele that is only expressed in the absence of a dominant allele. Represented by a small letter.

Selection pressures

Environmental factors that drive evolution by natural selection and limit population sizes e.g. competition, predation and disease.

Sex-linkage

The presence of a gene on an X or Y chromosome.

Speciation

The formation of new species due to the evolution of two reproductively separated populations. Two forms: allopatric and sympatric speciation.

Species

A group of similar organisms that are able to breed with one another to produce living, fertile offspring.

Stabilising selection

A type of selection that favours individuals with phenotypes close to the mean (average) and selects against extreme phenotypes.

Succession

Describes changes in the community of organisms occupying a certain area over time.

Sustainable

The ability to maintain something for future generations.

Sympatric speciation

A form of speciation that occurs when two populations within the same area become reproductively isolated.

Systematic sampling

A sampling technique used to determine the abundance and distribution of organisms along an area at periodic intervals e.g. along a belt transect. This is commonly used in ecosystems where some form of gradual change occurs.

Variation

The differences between individuals due to genes, the environment or a combination of both.