Usually natural selection tends to keep things much the same. Species that are living today have evolved to be suited to their environment. However, if the environment changes, a population will need to adapt if it is to survive in the new conditions. Two examples of how this can happen are the response of a moth population to pollution, and the emergence of new strains of bacteria following the introduction of antibiotics.
Industrial melanism
The peppered moth (Biston betularia) is a night-fl ying moth that rests during the day on the bark of trees, particularly on branches that are covered with grey–green lichen. It is a light speckled grey, and relies on camoufl age against the tree branches to protect it from predatory birds.
In Britain in the mid-19th century, a black form of the moth was noticed (Figure 5.15, overleaf ). The appearance of this new colour
Sexual reproduction promotes variation
Mutations in genes cause new variations to arise, but sexual reproduction increases variation in a population by forming new combinations of alleles.
•
During meiosis, crossing over at prophase I and random assortment in metaphase I produce genetically diff erent gametes (see pages 71–75).•
Diff erent alleles are alsobrought together at fertilisation, promoting more variation.
In species that reproduce asexually, variation can arise only by
mutation.
coincided with the period of the industrial revolution when many factories were built and contributed to growing pollution in the atmosphere. This pollution killed the lichens that grow on the bark of trees, which became blackened with particles of soot.
The colour of the moth is due to a single gene, which can be present in two forms. The common recessive form gives rise to a light speckled colour. The much less common dominant form gives rise to the black, melanic moth.
In the polluted areas, the speckled form was no longer camoufl aged on the blackened tree bark, and was easily seen by birds that ate speckled moths. The black moths were better suited to the changed environment as they were camoufl aged. Black moths survived and bred and the proportion of black moths with the dominant allele grew in the population.
In 1956, the Clean Air Act became law in Britain and restricted air pollution. Lichen grew back on trees and their bark became lighter. As a consequence, the speckled form of the peppered moth has increased in numbers again in many areas, and the black form has become less frequent (Figure 5.16).
Antibiotic resistance
Antibiotics are drugs that kill or inhibit bacterial growth. Usually, treating a bacterial infection with an antibiotic kills every invading cell. But, because of variation within the population, there may be a few bacterial cells that can resist the antibiotic. These individuals will survive and reproduce (Figure 5.17). Because they reproduce asexually, all off spring of a resistant bacterium are also resistant, and will also survive in the presence of the antibiotic. The resistant bacteria have enormous selective advantage over the normal susceptible strain, and quickly out-compete them.
Treating a disease caused by resistant strains of bacteria becomes very diffi cult. Doctors may have to prescribe stronger doses of antibiotic or try diff erent antibiotics to kill the resistant bacteria.
The problem of antibiotic resistance is made more complex because bacteria frequently contain additional genetic information in the form
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Figure 5.16 The distribution of the light and melanic forms of the peppered moth in Britain in the early 1960s. The ratio of dark to light areas in each circle shows the ratio of dark to light moths in that part of the country.
Figure 5.15 Light and melanic forms of peppered moths on light and dark tree bark.
5 ECOLOGY AND EVOLUTION 119 of plasmids, which they can transfer or exchange with other bacteria,
even those from diff erent species. Genes for enzymes that can inactivate antibiotics are often found on plasmids, so potentially dangerous bacteria can become resistant to antibiotics by receiving a plasmid from a relatively harmless species. Many bacteria are now resistant to several antibiotics, so pharmaceutical companies are constantly trying to develop new antibiotics to treat the multiple resistance forms of bacteria.
Antibiotic resistance and so-called ‘superbugs’, such as MRSA and Clostridium diffi cile, are bacteria resistant to many antibiotics. They have arisen partly as a result of overuse of antibiotics. Antibiotics used incorrectly or too frequently, help to ‘select’ the resistant individuals, which then increase in numbers. Patients failing to take a complete course of medication can also encourage the survival of slightly resistant bacteria that might have been killed if the antibiotic had been taken properly.
22 Defi ne ‘evolution’.
23 Why is sexual reproduction important for evolution?
24 Individuals in a population are often said to be ‘struggling for survival’.
What is the key fact that causes this struggle?
25 If an environment changes, individuals with particular combinations of genes are more likely to survive. What is the name given to this phenomenon?
26 Give two examples of evolution in response to environmental change.
5.5 Classifi cation
Figure 5.17 The grey areas on the agar jelly in this Petri dish are colonies of the bacterium Escherichia coli. The white card discs are impregnated with different antibiotics. This strain of E. coli is resistant to the antibiotics at the bottom left and has been able to grow right up to the discs.
Assessment statements
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Outline the binomial system of classifi cation.•
List seven levels in the hierarchy of taxa – kingdom, phylum, class, order, family, genus and species – using an example from two diff erent kingdoms for each level.•
Distinguish between the following phyla of plants, using simple external recognition features: bryophyta, fi licinophyta, coniferophyta and angiospermophyta.•
Distinguish between the following phyla of animals, using simple external recognition features: porifera, cnidaria, platyhelminthes, annelida, mollusca and arthropoda.•
Apply and design a key for a group of up to eight organisms.Biological classifi cation attempts to arrange living organisms into groups that enable them to be identifi ed easily and that show evolutionary links between them. The system of classifi cation we use today has its origins in a method devised by the Swedish scientist Carolus Linnaeus (1707–1778).