AQA Core Science: Evolution and environment

Darwin's theory of evolution explains how species of living things have changed over geological time. The theory is supported by evidence from fossils, and by the rapid changes that can be seen to occur in microorganisms such as antibiotic-resistant bacteria. Many species have become extinct in the past and the extinction of species continues to happen.

Charles Darwin was an English naturalist. His ideas caused a lot of controversy, because they can be seen as conflicting with religious views about the creation of the world and creatures in it.

While Darwin studied the wildlife on the Galápagos Islands he noticed that the finches - songbirds - on the different islands there were fundamentally similar to each other, but showed wide variations in their size, beaks and claws from island to island. Darwin concluded that, because the islands are so distant from the mainland, the finches that had arrived there in the past had changed over time. Darwin studied hundreds more animal and plant species. After nearly 30 years of research, in 1858 he proposed his theory of evolution by natural selection.

The basic idea behind the theory of evolution is that all the different species have evolved from simple life forms. These simple life forms first developed more than three billion years ago - the Earth is about 4.5 billion years old. The theory of evolution states that evolution happens by natural selection. Key points:

• Individuals in a species show a wide range of variation.
• This variation is because of differences in genes.
• Individuals with characteristics most suited to the environment are more likely to survive and reproduce.
• The genes that allowed the individuals to be successful are passed to the offspring in the next generation.

Individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed to the next generation. Given enough time, a species will gradually evolve. The evolutionary connections between biological groups can be illustrated in a tree of life diagram.

Most of the evidence for evolution comes from the fossil record. Fossils show how much, or how little, organisms have changed over time. One of the problems with the fossil record is that it contains gaps. Not all organisms fossilise well, and there will be many fossils that have been destroyed by the movements of the Earth, or simply not yet been discovered.

One of the few animals for which we have a fairly complete evolutionary record is the horse. All the main stages of the evolution of the horse have been preserved in fossil form.

Over 60 million years, the horse evolved from a dog-sized creature that lived in rainforests into an animal adapted to living on the plains and standing up to 2 metres high.

In the process its multi-toed feet, that were adapted for walking across the forest floor, evolved into single-toed hooves more suited for running over open country.

Rapid changes in species have been observed, supporting the theory of evolution.

Before the industrial revolution in Britain, most peppered moths were of the pale variety. This meant that they were camouflaged against the pale birch trees that they rest on. Moths with a mutant black colouring were easily spotted and eaten by birds. This gave the white variety an advantage, and they were more likely to survive to reproduce.

Airborne pollution in industrial areas blackened the birch tree bark with soot. This meant that the mutant black moths were now camouflaged, while the white variety became more vulnerable to predators. This gave the black variety an advantage, and they were more likely to survive and reproduce. Over time, the black peppered moths became far more numerous in urban areas than the pale variety.

Microorganisms such as bacteria and viruses reproduce very rapidly and can evolve in a relatively short time. One example is the bacterium E. coli. Its DNA can be damaged or changed during replication, and most of the time this causes the death of the cell. But occasionally, the mutation is beneficial - for the bacteria. For example, it may allow resistance to an antibiotic. When that antibiotic is present, the resistant bacteria have an advantage over the bacteria that are not resistant. Antibiotic resistant strains of bacteria are a large problem in hospitals.

Individuals that are poorly adapted to their environment are less likely to survive and reproduce than those that are well adapted. Similarly, it is possible that a species that is poorly adapted to its environment will not survive and will become extinct. Factors that can cause a species to become extinct include changes to the environment, such as a change in climate, new diseases, new predators and new competitors.

The fossil record shows that many species have become extinct since life on Earth began. Extinction is still happening and a lot of it occurs because of human activities. We compete with other living things for space, food and water, and we are very successful predators.

The dodo was a large flightless bird that lived on Mauritius, an island in the Indian Ocean. The island was uninhabited and the dodo had no natural predators. Then Mauritius was colonised by the Dutch in 1638. Dodos were hunted for food and easy to catch because they were not afraid of people. New competitors were brought onto the island, including pigs, cats and rats, which ate the dodos' eggs and their young. After 80 years the Dodo became extinct.

Human beings compete with other living things for resources and space. As the world’s population continues to increase, and standards of living improve, there is serious danger of a permanent change to the global environment.

Human activities have led to the pollution of the environment, and a reduction in the amount of land available for other animals and plants, which makes it difficult for some species to survive. There is a need to achieve a level of development that also sustains the environment for future generations.

Like all living things, humans exploit their surroundings for resources. Before the beginning of agriculture about 10,000 years ago, small groups of humans wandered across large areas, hunting and gathering just enough food to stay alive. Population numbers were kept low because of the difficulty of finding food.

The development of agriculture led to a population explosion that has accelerated enormously during the past 500 years. Unlike other species, humans can adapt to and survive in almost all habitats and climates

People in the developed world enjoy a high standard of living, with abundant food, cars and comfortable housing. People in the developing world have a lower standard of living, but many countries are catching up quickly.

The world’s human population has passed 6 billion and continues to increase. The growth in the human population and the increase in the standard of living are putting strains on the global environment. Here are some of the ways in which this is happening:

• non-renewable energy resources, such as coal, oil and natural gas, are being used up rapidly
• raw materials are being used up rapidly
• more waste is being produced
• more pollution is being caused

Pollution is the addition of substances to the environment that may be harmful to living organisms. Population growth and increases in the standard of living cause more waste to be produced. If this waste is not handled correctly, it leads to pollution. The most obvious form of pollution is often simply just litter on the ground, but pollution can affect the air and water too.

Most rubbish is buried in landfill sites and not all of it comprises safe materials. Even common household items can contain toxic chemicals such as poisonous metals. Many smoke alarms contain radioactive americium. Industrial waste is also discharged onto the land. Many farmers apply pesticides to improve their crops, but these can damage living things. Toxic chemicals can be washed from the land into rivers, lakes and seas.

Water pollution is caused by the discharge of harmful substances into rivers, lakes and seas. Many aquatic invertebrate animals cannot survive in polluted water, so their presence or absence indicates the extent to which a body of water is polluted.

Some common water pollutants include fertilisers, sewage and toxic chemicals.

The most common source of air pollution is the combustion of fossil fuels. This usually happens in vehicle engines and power stations.

Lichens are plants that grow in exposed places such as rocks or tree bark. They need to be very good at absorbing water and nutrients to grow there, and rainwater contains just enough nutrients to keep them alive. Air pollutants dissolved in rainwater, especially sulfur dioxide, can damage lichens, and prevent them from growing. This makes lichens natural indicators of air pollution. For example: bushy lichens need really clean air, leafy lichens can survive a small amount of air pollution and crusty lichens can survive in more polluted air. In places where no lichens are growing, it's often a sign that the air is heavily polluted with sulfur dioxide.

Smoke deposits soot on buildings and trees, causing them damage. Permeates the air, making it difficult for living creatures to breathe, carbon monoxide produces poisonous gas, carbon dioxide produces greenhouse gases that contribute to global warming and sulfur dioxide contributes to acid rain.

Humans have been cutting down trees for thousands of years. We do this to clear land for farming and building, and for wood to use as a fuel or building material. Forestry is sustainable as long as forests are allowed to replace themselves, or are replanted after felling, but often this is not done. The result is that the world’s forests are steadily shrinking.

This process is called deforestation. It has some important consequences such as forest habitats are destroyed, soil erosion increases, which causes barren land, flooding and land slides and atmospheric pollution is caused when forests are cleared by burning trees.

The term biodiversity refers not only to the number of different species, but also to all the variations within and between species, and all the differences between the habitats and ecosystems that make up the Earth’s biosphere. The loss of forests reduces biodiversity and we run the risk of losing organisms that might have been useful in the future - for example as sources of new medicines. There is also a moral responsibility to look after the planet and its resources.

Methane and carbon dioxide are greenhouse gases. They absorb heat energy and prevent it escaping into space. This keeps the Earth warmer than it would be without these gases. Remember that the Moon and the Earth are the same distance from the Sun. The Moon has no atmosphere and has an average surface temperature of –18ºC, while the Earth has an average surface temperature of 14ºC. So you can see that greenhouse gases are not a bad thing in themselves, but too much of them in the atmosphere leads to global warming. Rice paddy fields produce methane gas, and so do cattle. As the numbers of rice fields and cattle have increased, so has the amount of methane in the atmosphere. Carbon dioxide is produced by burning fossil fuels.

When land is cleared for timber and farms, there are fewer trees to perform the important task of removing carbon dioxide from the atmosphere by photosynthesis. If the fallen trees are burned or left to rot, additional carbon dioxide is released into the atmosphere. These factors are particularly important in tropical areas, where forests might be cleared to make way for cattle farms. Then, not only are there fewer trees left to absorb carbon dioxide, but the graphburning trees release carbon dioxide and the cattle release methane.

As the concentration of greenhouse gases in the atmosphere has increased, so too has the average global temperature. This is what scientists mean when they talk of global warming. An increase of only a few degrees will cause changes to the Earth’s climate, and lead to sea levels rising.

Humans reduce the amount of land and resources available for plants and animals. This happens because of farming, quarrying and dumping waste.

Urbanisation means the growth of cities. About half the world’s population live in cities, and most of the population growth in the future is expected to take place in cities. It is predicted that by 2015, the world’s six largest cities will each have more than 20 million inhabitants. Some of the effects of urbanisation incude increased pollution, increased energy consumption, land no longer used for food production, loss of natural habitats and decline of rural towns and villages as people leave them to live in cities.

Sustainable development means improving our quality of life without damaging the quality of life of future generations. It is important to all of us, not just the other inhabitants of the planet, that sustainable development is achieved. This involves each of us as individuals, and careful planning at local, regional and global levels.

Animals and plants have features that allow them to adapt to their environment. The organisms in a community compete with each other for limited resources, including water and space. Plants also compete with each other for minerals.

Environment: all the conditions that surround a living organism.
Habitat: the place where an organism lives.
Population: all the members of a single species that live in a habitat.
Community: all the populations of different organisms  live together in a habitat.
Ecosystem: a community and its habitat.

For example, a pond ecosystem consists of a pond habitat, inhabited by aquatic plants, waterside plants, micro-organisms, minnows and herons. The organisms together make up a community of living things.

Every organism has certain features or characteristics that allow it to live successfully in its habitat. These features are called adaptations, and we say that the organism is adapted to its habitat. Organisms living in different habitats need different adaptations. Polar bears are well adapted for survival in the Arctic as they have:

• a white appearance as camouflage from prey on the snow and ice
• thick layers of fat and fur for insulation against the cold
• a small surface area to volume ratio, to minimise heat loss
• a greasy coat that sheds water after swimming
• large furry feet to distribute their load and increase grip on the ice

The snowshoe hare has white fur in the winter and reddish-brown fur in the summer. Meaning it is camouflaged from its predators for most of the year.

The Arctic is cold and windy with very little rainfall. Plants in the Arctic often grow very close to the ground and have small leaves. This helps to conserve water and to avoid damage by the wind.

Camels live in deserts that are hot and dry during the day, but cold at night. They are well adapted for survival in the desert. They have large, flat feet to spread their weight on the sand, thick fur on the top of the body for shade, and thin fur elsewhere to allow easy heat loss, a large surface area to volume ratio to maximise heat loss, the ability to go for a long time without water - they don't store water in their humps, but they lose very little through urination and sweating, the ability to tolerate body temperatures up to 42ºC, slit-like nostrils and two rows of eyelashes to help keep the sand out.

 Cacti are well adapted for survival in the desert. They have stems that can store water and widespread root systems that can collect water from a large area. In addition, they have spines instead of leaves. These minimise the surface area and so reduce water loss by transpiration. The spines also protect the cacti from animals that might eat them.

Animals and plants may have specific features that adapt them to their environment. These include barbs and spines, poisons and warning colours that deter predators and herbivores. Some harmless species may even resemble a poisonous or dangerous species to increase their chances of survival.

Habitats have limited amounts of the resources needed by living organisms. Organisms can only survive if they can get enough resources. They must compete with other organisms for these resources. If they are unsuccessful and cannot move to another habitat, they will die.

Some of the resources that animals compete for: food, water and space.

Animals may also compete for mates so that they can reproduce.

Remember that plants make their own food using photosynthesis, so they do not compete for food. Some of the things that plants do compete for are: light, water, space and mineral salts.

Human beings are very successful organisms. We compete with animals and plants all over the world. For example, our farmers compete with animals and plants for water and space, and with animals for food.

The genetic information passed from parent to offspring is contained in genes carried by chromosomes in the nucleus. Sexual reproduction produces offspring that resemble their parents, but are not identical to them. Asexual reproduction produces offspring - clones - which are genetically identical to their parents.

Plants can be cloned artificially using cuttings or tissue culture. Animals can be cloned using embryo transplants or fusion cell cloning. Genetic information from one species can be transferred to another species using genetic engineering. Offspring resemble their parents because they contain genetic information passed on to them by their parents.

A gene is a section of DNA that carries the code for a particular protein. Different genes control the development of different characteristics of an organism. Many genes are needed to carry all the genetic information for a whole organism. Chromosomes, found in the cell nucleus, contain many genes. The number of genes and chromosomes varies from species to species. For example, cells in human beings have 46 chromosomes that carry about 30,000 genes in each cell; and cells in fruit flies have eight chromosomes that carry about 13,600 genes.

Organisms have sex cells called gametes. In human beings, the male sex cells are called sperm and the female sex cells are called eggs or ova.

Sexual reproduction happens when a male gamete and a female gamete join. This fusion of gametes is called fertilisation. Sexual reproduction allows some of the genetic information from each parent to mix, producing offspring that resemble their parents, but are not identical to them. In this way, sexual reproduction leads to variety in the offspring. Animals and plants can reproduce using sexual reproduction.

In human beings, each gamete contains 23 chromosomes, half the number found in the other cells of the body. When the male and female gamete fuse, the new embryo contains the full 46 chromosomes – half from the father and half from the mother.

Asexual reproduction only needs one parent, unlike sexual reproduction, which needs two parents. Since there is only one parent, there is no fusion of gametes and no mixing of genetic information. As a result, the offspring are genetically identical to the parent and to each other. They are clones.

Asexual reproduction in plants can take a number of forms. Many plants develop underground food storage organs that later develop into the following year’s plants. Potato plants and daffodil plants are examples that do this.

Some plants produce side branches with plantlets on them. Busy Lizzy does this. Other plants, such as strawberries, produce runners with plantlets on them.

Asexual reproduction in animals is less common than sexual reproduction. It happens in sea anemones and starfish, for example.

People may want to clone a plant deliberately. Cloning of plants has many important commercial implications. It allows a successful variety of a plant to be produced commercially and cheaply on a massive scale in a short space of time.

The simplest way to clone a plant involves taking a cutting. A branch from the parent plant is cut off, its lower leaves removed and the stem planted in damp compost. Plant hormones are often used to encourage new roots to develop. The cutting is usually covered in a clear plastic bag at this stage to keep it moist and warm. After a few weeks, new roots develop and a new plant is produced. The method is easy enough for most gardeners to do successfully.

Another way of cloning plants is by tissue culture, which works not with cuttings but with tiny pieces from the parent plant. Sterile agar jelly with plant hormones and lots of nutrients is needed. This makes tissue culture more expensive and difficult to do than taking cuttings.

Tissue culture involves the following steps:

1. Small amounts of parent tissue/number of cells are taken
2. The plant material is transferred to plates containing sterile nutrient agar jelly
3. Plant hormones are added to stimulate the cells to divide
4. Cells grow rapidly into small masses of tissue
5. More growth hormones are added to stimulate the growth of roots and stems
6. The tiny plantlets are transferred into potting trays where they develop into plants

People may want to clone an animal deliberately. Just like the cloning of plants, the cloning of animals has many important commercial implications. It allows an individual animal with desirable features, such as a cow that produces a lot of milk, to be duplicated several times, but the process takes much longer than with plants.

A developing embryo is removed from a pregnant animal at an early stage, before the embryo’s cells have had time to become specialised. The cells are separated, grown for a while in a laboratory and then transplanted into host mothers. When the offspring are born, they are identical to each other and genetically related to the original pregnant animal. They are not related to their host mothers because they contain different genetic information. Fusion cell cloning involves replacing the nucleus of an unfertilised egg with the nucleus from a different cell. The replacement nucleus can come from an embryo, but if it comes from an adult cell, it is called adult cell cloning.

'Dolly the sheep' was the first mammal to be cloned using adult cell cloning. She was born in the UK in 1996 and died in 2003. Here’s how she was produced:

1. An egg cell was removed from the ovary of an adult female sheep, and its nucleus removed.
2. The nucleus from an udder cell of a donor sheep was inserted into the empty egg cell.
3. The fused cell then began to develop normally, using genetic information from the donated DNA.
4. Before the dividing cells became specialised, the embryo was implanted into the uterus of a foster mother sheep. The result was Dolly, genetically identical to the donor sheep.

Genetic engineering is also called genetic modification or GM. It is not the same as cloning. Although cloning techniques are used in genetic engineering, the two things should not be confused. The table shows some of the differences.

Cloning produces exact copies and the genes copied within the same species whereas genetic engineering produces a unique set of genes that can be swapped across species.

Certain enzymes can cut pieces of DNA from one organism, and join them into a gap in the DNA of another organism. This means that the new organism with the inserted genes has the genetic information for one or more new characteristics. For example, the organism might produce a useful substance, or be able to carry out a new function. We say that the organism has been genetically modified.

The animation shows how the method can be used to produce bacteria that produce insulin. This is a human hormone and valuable to people with diabetes. Bacteria reproduce quickly, so a lot of insulin can be made quickly.

There are strong arguments for and against cloning and genetic engineering. It is possible to produce genetically modified animals and plants. Sheep that produce human proteins for treating the symptoms of cystic fibrosis - a disease which causes sufferers to produce abnormally thick and sticky mucus in their lungs - have been produced, and even tobacco plants that glow in the dark when they need watering.

Some people are excited by the almost limitless possibilities of cloning and genetic engineering, while some people believe the process is unethical and should be banned. Others are concerned about what might happen in the future.