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EnglandBiology

3.7 Genetics, populations, evolution and ecosystems

5 dot points across 5 inquiry questions. Click any dot point for a focused answer with worked past exam questions where available.

How do selection, drift and reproductive isolation change allele frequencies and produce new species?

Individuals within a population of a species may show a wide range of variation in phenotype. This variation may be the result of genetic factors, environmental factors or a combination of both. Differential reproductive success and its effect on the allele frequency within a gene pool. Directional selection, for example antibiotic resistance in bacteria, and stabilising selection, for example human birth weights. The role of geographic isolation (allopatric speciation) and reproductive isolation (sympatric speciation) in the production of new species, and the importance of genetic drift in causing changes in allele frequency in small populations, including the founder effect.
A focused answer to the AQA 3.7 dot point on evolution and speciation. Covers differential reproductive success, directional and stabilising selection, genetic drift and the founder effect, and contrasts allopatric (geographic) with sympatric (reproductive) speciation.
12 min answer →

How can we predict the inheritance of one or more genes and test whether observed offspring ratios fit a genetic model?

Genotype is the genetic constitution of an organism. Phenotype is the expression of this genetic constitution and its interaction with the environment. Most phenotypes are affected by more than one gene. The genotype, phenotype and ratio of offspring can be predicted for monohybrid and dihybrid crosses involving dominant, recessive, codominant and multiple alleles, sex-linkage, autosomal linkage and epistasis. The chi-squared (X2) test can be used to test the significance of the difference between observed and expected results.
A focused answer to the AQA 3.7 dot point on inheritance. Works through monohybrid and dihybrid crosses, multiple alleles, codominance, sex-linkage, autosomal linkage and epistasis, then shows how to run and interpret a chi-squared test on offspring ratios.
13 min answer →

How can we calculate allele and genotype frequencies in a population and decide whether they are changing?

A population is a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed. The individuals in a population of a species may show a wide range of variation in phenotype. This is the result of genetic and environmental factors. A gene pool is all the alleles of all the genes in a population. The frequency of an allele in a population is the proportion of organisms carrying that allele. The Hardy-Weinberg principle provides a mathematical model, which predicts that allele frequencies will not change from one generation to the next, given that no mutation, migration, selection or genetic drift occurs and that there is random mating in a large population. Since allele frequencies do change, the conditions required to maintain a Hardy-Weinberg equilibrium are rarely met. Students should be able to use the Hardy-Weinberg principle (p + q = 1 and p2 + 2pq + q2 = 1) to calculate allele, genotype and phenotype frequencies in populations and changes in these frequencies.
A focused answer to the AQA 3.7 dot point on populations and Hardy-Weinberg. Defines populations, gene pools and allele frequency, states the five conditions for equilibrium, and works through allele, genotype and phenotype frequency calculations with the p + q = 1 and p2 + 2pq + q2 = 1 equations.
11 min answer →

What controls the size of a population in an ecosystem, and how do ecologists estimate it?

An ecosystem includes all the living organisms and all the abiotic conditions in a particular area. Within an ecosystem, populations of different species form a community. The population size of any species is limited by the effect of abiotic factors and biotic factors, such as interspecific and intraspecific competition and predation. Population size may vary as a result of the effect of abiotic factors and interactions between organisms; the carrying capacity is the maximum stable population size that an ecosystem can support over a long period. Students should be able to use given data to describe and interpret predator-prey relationships and to investigate populations and estimate the size of a population using randomly placed quadrats, transects and the mark-release-recapture method, including the assumptions made when using this method.
A focused answer to the AQA 3.7 dot point on populations in ecosystems. Defines ecosystem, community, population size and carrying capacity, explains abiotic and biotic limiting factors and predator-prey cycles, and details estimating populations with quadrats, transects and mark-release-recapture (with its assumptions).
12 min answer →

How do communities change over time through succession, and how is conservation managed?

Succession from 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 changes in the abiotic environment result in a less hostile environment and changing diversity. Conservation of habitats frequently involves management of succession. Students should be able to evaluate evidence and data concerning issues relating to the conservation of species and habitats and consider conflicting evidence; and use the concept of succession to explain the management of an ecosystem.
A focused answer to the AQA 3.7 dot point on succession and conservation. Explains primary and secondary succession from pioneer species to climax community, how each stage changes the abiotic environment and diversity, and how conservation manages succession with reference to conflicting evidence.
11 min answer →