Evolution, speciation and conservation conflictsEdexcel International A Level Biology: Revision notes
Section 1
Evolution as a change in allele frequency
A gene pool is all the alleles of all the genes in a population. Evolution is a change in allele frequency in a gene pool over successive generations. Allele frequency = number of copies of the allele ÷ total number of alleles of that gene in the population. For a diploid population of individuals there are alleles of each gene, so in a sample of 100 flies with 124 copies of allele A, the frequency is 124 ÷ 200 = 0.62.
Two processes produce the change. Mutation creates new alleles at random, and most are neutral or harmful but some give an advantage in a particular environment. Natural selection acts on the variation: individuals with advantageous alleles are more likely to survive and reproduce, pass the alleles on, and so those alleles increase in frequency. Individuals do not evolve: populations do.
Do not write that organisms 'adapt because they need to'. Mutations arise at random; selection then favours those that already give an advantage.
Section 2
Speciation and reduced gene flow
A species is a group of organisms that can interbreed to produce fertile offspring. Speciation is the formation of a new species. It happens when gene flow between populations is reduced, so that their allele frequencies diverge through different mutations and different selection pressures until they are reproductively isolated.
In allopatric speciation populations are separated by a geographical barrier such as a river, mountain range or sea, and the separate populations in different environments evolve differently. In sympatric speciation new species arise in the same area without a geographical barrier. Gene flow is reduced by other isolating mechanisms, for example differences in breeding time, mating on different host plants, or differences in courtship behaviour or food source. Whichever type, the stages are the same: reduced gene flow, divergence of allele frequencies by selection and mutation, and finally the inability to interbreed to produce fertile offspring.
Name the type from the barrier. A physical barrier means allopatric; the same area with different feeding, breeding times or behaviour means sympatric.
Section 3
Conclusions on controversial issues depend on who reaches them
Climate change is a controversial issue because its causes and solutions affect economic and political interests. The same evidence can lead to different conclusions depending on who interprets it. Vested interests, such as the funding source of a study, may bias interpretation, the selection of data, or the way results are presented. Different groups may make different assumptions or use different boundaries, for example whether to include the carbon dioxide from land clearance in a biofuel calculation. Different priorities (economic growth, environment, food security) change how evidence is weighed. Correlations may be presented as proof of causation.
To judge reliability, scientists look for peer review, where independent experts check the methods and conclusions before publication; published methods and data so results can be repeated; declared funding and conflicts of interest; sufficient data; and agreement with independent studies.
Section 4
Reforestation, sustainable resources and biofuels
Reforestation is the replanting of areas where forest has been cleared. Growing trees take up carbon dioxide by photosynthesis and store carbon in wood and soil, so forests act as a carbon sink. Reforestation also restores habitats, reduces soil erosion and regulates water cycles. Limits: it takes decades for new forest to store large amounts of carbon, and a single-species plantation supports less biodiversity than a natural forest.
A sustainable resource is one used so that it meets present needs without compromising future needs, so it is replaced as fast as it is used. Biofuels, such as ethanol from fermenting sugar cane and biodiesel from oil crops, are renewable because the crop can be regrown. They are described as nearly carbon neutral, because the carbon dioxide released on burning was taken up by the crop as it grew. The benefit is smaller if fossil fuels are used to grow, process and transport the crop, and is lost if forest or peat is cleared to grow it, or if land used for food is diverted to fuel crops.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Evolution, speciation and conservation conflicts
- A population of beetles lives on a recently formed island of dark volcanic rock. Beetle colour is controlled by one gene with a dominant allele B for dark colour and a recessive allele b for pale colour. Birds feed on the beetles and find pale beetles easier to see against the rock. The frequency of allele B was 0.30 in the first generation studied and 0.55 five generations later.Explain why the frequency of allele B increased over the five generations.2 marks
- A river changed course and split a large population of ground squirrels into two populations living on opposite banks. The squirrels cannot swim the river, and the habitat on one bank is dry grassland while on the other bank it is wet woodland. After several thousand years, scientists have found that the two populations differ in body size, in alleles at several genes and in the calls used in courtship.Explain how the geographical separation of the squirrel populations led to the formation of new species.2 marks
- Apple maggot flies lay their eggs on fruit, and the larvae feed in the fruit. Originally the flies fed only on hawthorn fruit. After apple trees were introduced to the same area, some flies began to lay eggs on apples, which ripen earlier than hawthorn fruit. The flies mate on the fruit on which they developed. Both groups of flies live in the same orchards. A scientist genotyped 100 flies from each group for one gene. Of the 200 alleles in the hawthorn flies, 124 were allele A, and of the 200 alleles in the apple flies, 70 were allele A.Explain how the two groups of flies living in the same orchards could become separate species by sympatric speciation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).