3.7 Genetics, populations, evolution and ecosystems (A-level only)

The theory of evolution underpins modern Biology. All new species arise from an existing species. This results in different species sharing a common ancestry, as represented in phylogenetic classification. Common ancestry can explain the similarities between all living organisms, such as common chemistry (eg all proteins made from the same 20 or so amino acids), physiological pathways (eg anaerobic respiration), cell structure, DNA as the genetic material and a ‘universal’ genetic code.

The individuals of a species share the same genes but (usually) different combinations of alleles of these genes. An individual inherits alleles from their parent or parents.

A species exists as one or more populations. There is variation in the phenotypes of organisms in a population, due to genetic and environmental factors. Two forces affect genetic variation in populations: genetic drift and natural selection. Genetic drift can cause changes in allele frequency in small populations. Natural selection occurs when alleles that enhance the fitness of the individuals that carry them rise in frequency. A change in the allele frequency of a population is evolution.

If a population becomes isolated from other populations of the same species, there will be no gene flow between the isolated population and the others. This may lead to the accumulation of genetic differences in the isolated population, compared with the other populations. These differences may ultimately lead to organisms in the isolated population becoming unable to breed and produce fertile offspring with organisms from the other populations. This reproductive isolation means that a new species has evolved.

Populations of different species live in communities. Competition occurs within and between these populations for the means of survival. Within a single community, one population is affected by other populations, the biotic factors, in its environment. Populations within communities are also affected by, and in turn affect, the abiotic (physicochemical) factors in an ecosystem.

Inheritance (A-level only)


Opportunities for skills development

The genotype is the genetic constitution of an organism.

The phenotype is the expression of this genetic constitution and its interaction with the environment.

There may be many alleles of a single gene.

Alleles may be dominant, recessive or codominant.

In a diploid organism, the alleles at a specific locus may be either homozygous or heterozygous.

The use of fully labelled genetic diagrams to interpret, or predict, the results of:

  • monohybrid and dihybrid crosses involving dominant, recessive and codominant alleles
  • crosses involving sex-linkage, autosomal linkage, multiple alleles and epistasis.

Use of the chi-squared ( ) test to compare the goodness of fit of observed phenotypic ratios with expected ratios.

AT h

Students could investigate genetic ratios using crosses of Drosophila or Fast Plant®

MS 0.3

Students could use information to represent phenotypic ratios in monohybrid and dihybrid crosses.

MS 1.4

Students could show understanding of the probability associated with inheritance.

MS 1.9

Students could use the test to investigate the significance of differences between expected and observed phenotypic ratios.

Populations (A-level only)


Opportunities for skills development

Species exist as one or more populations.

A population as a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.

The concepts of gene pool and allele frequency.

The Hardy–Weinberg principle provides a mathematical model, which predicts that allele frequencies will not change from generation to generation. The conditions under which the principle applies.

The frequency of alleles, genotypes and phenotypes in a population can be calculated using the Hardy–Weinberg equation:

where is the frequency of one (usually the dominant) allele and is the frequency of the other (usually recessive) allele of the gene.

AT k

Students could collect data about the frequency of observable phenotypes within a single population.

MS 2.4

Students could calculate allele, genotype and phenotype frequencies from appropriate data using the Hardy–Weinberg equation.

Evolution may lead to speciation (A-level only)


Opportunities for skills development

Individuals within a population of a species may show a wide range of variation in phenotype. This is due to genetic and environmental factors. The primary source of genetic variation is mutation. Meiosis and the random fertilisation of gametes during sexual reproduction produce further genetic variation.

Predation, disease and competition for the means of survival result in differential survival and reproduction, ie natural selection.

Those organisms with phenotypes providing selective advantages are likely to produce more offspring and pass on their favourable alleles to the next generation. The effect of this differential reproductive success on the allele frequencies within a gene pool.

The effects of stabilising, directional and disruptive selection.

Evolution as a change in the allele frequencies in a population.

Reproductive separation of two populations can result in the accumulation of difference in their gene pools. New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring. In this way, new species arise from existing species.

Allopatric and sympatric speciation.

The importance of genetic drift in causing changes in allele frequency in small populations.

Students should be able to:

  • explain why individuals within a population of a species may show a wide range of variation in phenotype
  • explain why genetic drift is important only in small populations
  • explain how natural selection and isolation may result in change in the allele and phenotype frequency and lead to the formation of a new species
  • explain how evolutionary change over a long period of time has resulted in a great diversity of species.

MS 1.5

Students could apply their knowledge of sampling to the concept of genetic drift.

PS 1.2

Students could devise an investigation to mimic the effects of random sampling on allele frequencies in a population.

AT l

Students could use computer programs to model the effects of natural selection and of genetic drift.

Populations in ecosystems (A-level only)


Opportunities for skills development

Populations of different species form a community. A community and the non-living components of its environment together form an ecosystem. Ecosystems can range in size from the very small to the very large.

Within a habitat, a species occupies a niche governed by adaptation to both abiotic and biotic conditions.

An ecosystem supports a certain size of population of a species, called the carrying capacity. This population size can vary as a result of:

  • the effect of abiotic factors
  • interactions between organisms: interspecific and intraspecific competition and predation.

The size of a population can be estimated using:

  • randomly placed quadrats, or quadrats along a belt transect, for slow-moving or non-motile organisms
  • the mark-release-recapture method for motile organisms. The assumptions made when using the mark-release-recapture method.

Ecosystems are dynamic systems.

Primary succession, from colonisation by pioneer species to climax community.

At each stage in succession, certain species may be recognised which change the environment so that it becomes more suitable for other species with different adaptations. The new species may change the environment in such a way that it becomes less suitable for the previous species.

Changes that organisms produce in their abiotic environment can result in a less hostile environment and change biodiversity.

Conservation of habitats frequently involves management of succession.

Students should be able to:

  • show understanding of the need to manage the conflict between human needs and conservation in order to maintain the sustainability of natural resources
  • evaluate evidence and data concerning issues relating to the conservation of species and habitats and consider conflicting evidence
  • use given data to calculate the size of a population estimated using the mark-release-recapture method.

AT k

Students could:

  • investigate the distribution of organisms in a named habitat using randomly placed frame quadrats, or a belt transect
  • use both percentage cover and frequency as measures of abundance of a sessile species.

AT h

Students could use the mark-release-recapture method to investigate the abundance of a motile species.

AT i

Students could use turbidity measurements to investigate the growth rate of a broth culture of microorganisms.

MS 2.5

Students could use a logarithmic scale in representing the growth of a population of microorganisms.

Required practical 12: Investigation into the effect of a named environmental factor on the distribution of a given species.

AT a and k