How species depend on each other, theory of evolution, the importance of biodiversity



There are similarities and differences between living things on Earth. Many species of animal and plant have become extinct while others have survived. Many different species are dependent on each other.


Similarities and differences between living things on Earth are used to put them into groups. This process is called classification. You may have met the first big division of living things in the classification system called the five kingdoms in Key Stage 3.


A species is a group of organisms that can breed together to produce fertile offspring.

A horse (

A horse

A donkey. Credit: Andrew Dernie  (

A donkey

The most common example to show this is with horses and donkeys. A horse and donkey can breed together: the offspring is called a mule. The mule is infertile, which shows that the horse and donkey are a different species.

A mule. Credit: Dorling Kindersley (

A mule


Adaptations are features that help an organism survive in the environment it lives in.

Adult Australian Pelican. Credit: Andrew Parkinson  (

Pelicans have stretchable pouch underneath to hold fish caught underwater.

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Food Webs

A food web is simply a number of food chains joined together. It shows how the loss of one organism has an effect of other organisms in the food web. This is called the interdependence of living things.

An example of a food web (

An example of a food web

Considering the food web above as an example, what would happen if the population of slugs decreased?

Slugs, rabbits and insects all eat grass. If there were fewer slugs there would be more grass for the rabbits and insects. With more food the populations of rabbits and insects would increase. However, the thrushes would have to eat more insects to maintain their population, so it is also possible that the population of insects could decrease. This is turn may reduce the populations of voles and frogs.

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Living organisms are dependent on the environment and other species for their survival. When the environment changes, organisms are not as well adapted to it.

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 become extinct.

Here are some factors that can cause a species to become extinct:

  • rapid changes to the environment, such as the climate
  • new diseases
  • new predators
  • new competitors.

Because we are all part of a giant food web, changes in one part of the web can have a big impact on other parts of the web. When one food source disappears, a predator will have to find another food source in order to survive.

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

The quagga

An image of a quagga, a type of zebra. The quagga has a horse/zebra like face and stripes similar to a zebra. (

A Quagga

The last quagga alive in the wild was shot in the late 1870s. Some animals - such as the quagga - are extinct because of the direct activity of humans.

The quagga was the first extinct species to have its DNA studied. The results showed that the quagga was not a separate species as had been previously thought, but was a type of zebra.

Ideas about science


The DNA result from the quagga is a good example of how information from new data can conflict with a previous scientific explanation.

The dodo

The Dodo, a distant relative of the pigeons, is heavily-built, roughly the size of a swan and flightless (

Engraving of the extinct dodo

Other animals such as the dodo are extinct because of the indirect activity of humans. The dodo was a large flightless bird that lived on Mauritius, one of a group of islands in the Indian Ocean. These islands were uninhabited, and the dodo had no natural predators. Then Mauritius were colonised by the Dutch in 1638. Dodos were hunted for food and easy to catch, because they were not afraid of people. However, the Dutch also brought new competitors to the islands, such as pigs, cats and rats, which ate the dodos' eggs and young and were finally responsible for the extinction of the dodo.

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The Sun

Nearly all organisms are dependent on energy from the Sun.

Plants harness light energy to drive food production. By the process ofphotosynthesis, organic compounds like glucose are made from carbon dioxide and water using this energy. Plants only absorb a small percentage of the Sun’s energy for the process of photosynthesis. The energy is absorbed in chemicals that make up the plants’ cells.

Energy transfer

Animals cannot make their own food so they have to eat. This is one way in which energy is transferred between organisms in an ecosystem. The energy is used for a number of life processes.

In a food chain only around 10 per cent of the energy is passed on to the next level. The rest of the energy passes out of the food chain in a number of ways:

  • via heat energy
  • is used for life processes (for example movement)
  • uneaten parts that pass to decomposers
  • is excreted and passes to decomposers.

As less energy is transferred at each level of the food chain, the number of organisms at each level gets smaller.

Percentage efficiency of energy transfer

An example of energy flow through an ecosystem is shown below.

Energy flow through an ecosystem (

Energy flow through an ecosystem

If we want the percentage efficiency of energy transfer between producers and primary consumers.

1. Calculate the total energy that came into the level of the food chain:

4,500kJ + 500kJ = 5,000kJ

2. Identify how much energy is transferred to the next level:


3. Calculate the efficiency of this transfer using the equation:

(Energy transferred to next level ÷ Total energy in) X 100

(500kJ ÷ 5,000kJ) X 100 = 10%

This shows the percentage efficiency of energy transfer between producers and primary consumers in this ecosystem is 10 per cent.

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The Carbon Cycle

All cells - whether animal, plant or bacteria - contain carbon, because they all contain proteins, fats and carbohydrates. Plant cell walls, for example, are made of cellulose - a carbohydrate.

Carbon is passed from the atmosphere, as carbon dioxide, to living things, passed from one organism to the next in complex molecules, and returned to the atmosphere as carbon dioxide again. This is known as the carbon cycle.

Removing carbon dioxide from the atmosphere

Green plants remove carbon dioxide from the atmosphere by photosynthesis. The carbon becomes part of complex molecules such as proteins, fats and carbohydrates in the plants.

Returning carbon dioxide to the atmosphere

Organisms return carbon dioxide to the atmosphere by respiration. It is not just animals that respire. Plants and microorganisms do, too.

Passing carbon from one organism to the next

When an animal eats a plant, carbon from the plant becomes part of the fats and proteins in the animal. Microorganisms and some animals feed on waste material from animals, and the remains of dead animals and plants. The carbon then becomes part of these microorganisms and detritus feeders.

The slideshow should help you to understand how the cycle works.

Step 1 - carbon in the atmosphere can come from the respiration of plants and animals, and combustion (burning of fuels) (

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Materials from living things decay because they are digested by microorganisms. This process happens faster in warm, moist conditions with plenty of oxygen. Decay can be very slow in cold, dry conditions, and when there is a shortage of oxygen.

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The Nitrogen Cycle

Plants and animals need nitrogen to make proteins. But they cannot get nitrogen directly from the air because, as a gas, nitrogen is fairly unreactive. Plants are able to take up nitrogen compounds such as nitrates and ammonium salts from the soil.

Nitrogen fixation

Making nitrogen compounds from nitrogen in the air is called nitrogen fixation.

Nitrogen fixation happens in three ways:

  • The energy in lightning splits nitrogen molecules into individual nitrogen atoms. These react with oxygen to form nitrogen oxides. Nitrogen oxides are washed to the ground by rain, where they form nitrates in the soil
  • Nitrogen-fixing bacteria found in the soil and in the root nodules of leguminous plants, such as peas, beans and clover, fix nitrogen gas intonitrogen compounds
  • The Haber process is used by industry to produce ammonia from nitrogen and hydrogen. Ammonia is used to make nitrogen compounds that are used as fertiliser by farmers.

Nitrogen compounds

Nitrogen compounds in living things are returned to the soil through:

  • excretion and egestion by animals
  • the decay of dead plants and animals.

Denitrifying bacteria present in soil break down nitrogen compounds and release nitrogen gas into the air.

Key features of the nitrogen cycle

As a result of these processes, nitrogen is cycled continually through the air, soil and living things. This is called ‘the nitrogen cycle’.

This animation summarises the main features of the nitrogen cycle.

Diagram showing the 3 different ways of nitrogen fixation (

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How did Life Start

Evolution has taken place over millions of years and scientists believe this is the reason why all living things on Earth exist today. There are different views and theories about the origin of life and the evolutionary process.

How did life start?

planet earth, as seen from space (


It is not possible to be certain how life on Earth began. We do know that:

  • the Earth is about 4,500 million years old
  • there is evidence that living things existed on Earth at least 3,500 million years ago
  • no one was there to record how life began
  • early Earth was hotter and the atmosphere consisted mostly of carbon dioxide (with other gases such as ammonia and methane).

Scientific theories

Through the study of simple organisms and the chemistry of living things, we can develop scientific theories about how life on Earth began. The main theory is that living things developed from molecules that could replicate, or copy, themselves, rather like DNA does.

There are two possible origins for these replicating molecules:

  • they were produced by the conditions on Earth at the time
  • they came from somewhere else, such as another planet in our Solar System, or further out in space.

Over many millions of years these molecules joined with other molecules, becoming gradually more complex and dependent on each other. The process of evolution by natural selection eventually led to all of the different living things that we see on Earth today.

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Darwin's Theory

Charles Darwin was an English naturalist who studied variation in plants and animals during a five-year voyage around the world in the 19th century. He explained his ideas on evolution in a book called, 'On the Origin of Species', published in 1859.

Darwin's ideas caused a lot of controversy, and this continues today, because they can be seen as conflicting with religious views about the creation of the world and the creatures in it.

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 3 billion years ago (the Earth is about 4.5 billion years old). The timeline below shows some of the key events in the evolution of life on Earth, from the first bacteriato the first modern humans.

You can see a more detailed history of life timeline on BBC Nature.

Natural selection

The theory of evolution states that evolution happens by natural selection. The key points are that:

  • individuals in a species show a wide range of variation
  • this variation is because of differences in their genes
  • individuals with characteristics most suited to the environment are more likely to survive and reproduce
  • the genes that allow these individuals to be successful are passed to their offspring.

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.

You need to remember that variation can be caused by both genes and the environment. But it is only variation caused by genes that can be passed on to the next generation.

Conditions on Earth

Life on Earth today exists because of the conditions that were present when life was evolving. If, in the past, conditions had been different eg hotter, colder, or the Earth more or less massive, then evolution by natural selection could have produced some very different results. Our bodies would have changed to suit the Earth's conditions.

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Selective Breeding

Natural selection and selective breeding can both produce changes in animals and plants. The difference between the two is that natural selection occurs in nature, but selective breeding only occurs when humans intervene.

Selective breeding is a process where we choose the characteristics we want in an animal or plant. We then breed together a male and female showing some of those characteristics. From the offspring produced we select those that show the characteristic the most, and breed them together.

This process is repeated over many generations, each time selecting and breeding together those animals or plants that have the characteristics we are looking for. Over a large number of generations, this can produce some surprising results.

Image showing dogs sitting in a line. They are all bred from a common ancestor (

All of these dogs were selectively bred from a common ancestor

Farmers have used selective breeding for centuries to increase milk yield in cattle, produce larger eggs from chickens and obtain more grain from wheat.

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Causes and effects

Mutations are changes that can occur in genes. These changes are random and can be caused by background radiation and chemicals that we come into contact with, for example the chemicals in cigarette smoke.

Sometimes these changes can be so severe that the cell dies, sometimes the cell can divide uncontrollably and become cancerous, and sometimes the changes are small and the cell survives. Very rarely the changes may even be beneficial to us and produce new and useful characteristics.

Passing on mutations

If these changes occur in normal body cells, the changes are lost when we die. But if the changes occur in our sex cells such as sperm and ova, there is the possibility that the changes in the gene will be passed onto the next generation.

It is when these changes are passed on to the next generation that natural selection can either ensure that they are selected if they are useful, or disappear from the gene pool if they are not.

New species

The combined effect of these mutations, environmental changes, and natural selection, can sometimes produce changes in the organism that are so great that a new species is produced. This does not happen very often and only occurs when the mutated organism can no longer breed with the original species and is capable of producing fertile offspring.

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Explanations For Evolution

Origins of life Living organisms on Earth today and the chemicals which work together to keep them alive, are extremely complex, even in single-celled microorganisms. Some people find it hard to accept that such complexity could have evolved through natural selection. Some religious people believe that all living things on Earth were made by God, or that life was begun by God but then evolved through natural selection. We will probably never be absolutely certain about how life began, as no one was there to observe it. But scientists must base their theories on evidence. Lamarck Darwin was not the only person to develop a theory of evolution. Jean-Baptiste Lamarck was a French scientist who developed an alternative theory at the beginning of the 19th century. His theory involved two ideas. These are: the law of use and disuse the law of inheritance of acquired characteristics. His theory stated that a characteristic which is used more and more by an organism becomes bigger and stronger, and one that is not used eventually disappears. Any feature of an organism that is improved through use is passed to its offspring. This table summarises the two different explanations for giraffes having long necks. Summary of explanations for giraffes having long necks LamarckDarwin A giraffe stretches its neck to reach food high up A giraffe with a longer neck can reach food high up The giraffe's neck gets longer because it is used a lot The giraffe is more likely to get enough food to survive and to reproduce The giraffe's offspring inherit its long neck The giraffe's offspring inherit its long neck Lamarck's theorycannot accountfor all the observations made about life on Earth. For instance, his theory would predict that all organisms gradually become complex, and simple organisms disappear. Darwin's theory can account for the continued presence of simple organisms.

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Developing Explanations

Just like Darwin and Lamarck, when presented with information about alternative views about the origin of life and the evolutionary process, you need to be able to:

  • Identify statements that are data versus statements that offer anexplanation. For example, data is produced from an investigation and often but not always includes numbers. However, an explanation is a hypothesisthat can be used to explain the data. If a scientist grows a microorganism and counts the number of microorganisms present every 30 minutes, the numbers he or she collects is data. If the scientist notices that the number doubles every 30 minutes, he or she can use the data to produce anexplanation, eg that bacteria divide every 30 minutes and the numbers increase exponentially.
  • Recognise data or observations that are accounted for, or conflict with, anexplanation. For example, fossils can be dated by looking at the age of the rock in which they are found. The fossil record shows how organisms have changed over a period of time. Analysis could also take place on the similarities and differences in the DNA of organisms. The data supports the explanation of the theory of evolution. However, when the fossil data is compared with the theory that all life on Earth started just a few thousand years ago, the data does not support the explanation.
  • Identify imagination and creativity in an explanation. For example, sometimes the data collected from an investigation does not immediately lead to an explanation. When Alexander Fleming noticed that bacteria were not growing next to the penicillium mould it took imagination and creativity for him to think that maybe the mould was producing a chemical that was killing the bacteria. That is how antibiotics were discovered.
  • Identify a scientific question for which there is not an agreed answer. For example, did life evolve on planet Earth or did it arrive on a meteorite from outer space? (Because no one was there at the time it is impossible to give an exact answer).
  • Suggest why scientists involved in the same research sometimes disagreewith each other. For example, many years ago, when scientists were first trying to date how old a fossil was, they sometimes disagreed. It was only when more evidence became available that they began to agree on the fossil's age.

Contradictory evidence - Higher tier

You will also need to be able to suggest why some scientists are reluctant to change their minds and explanations, even when new evidence is found that contradicts them. This is often due to the fact that scientists devote their lives to their research. To realise that their theory was wrong after all the years they have spent on it is very difficult to accept.

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Evidence For Darwin's Theory



Fossils of ammonites - sea creatures that became extinct about 65 million years ago

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 many fossils are destroyed by the movements of the Earth, or have simply not been discovered.

Peppered moths

Before the Industrial Revolution in Britain, most peppered moths were of the pale variety. They were camouflaged against the pale birch trees they rested on. Moths with a mutant black colouring were spotted easily by birds and eaten. This gave the white variety an advantage, making them more likely to survive and reproduce.

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


The pale peppered moths camouflage well against the pale birch tree


The darker mutant peppered moths camouflage well against the blackened birch tree

Antibiotic-resistant bacteria


E. coli bacteria (Photo from Rocky Mountain Laboratories, NIAID, NIH)

Microorganisms such as bacteria and virusesreproduce 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 (to the bacteria). For example, it may allow resistance to certain antibiotics. When those antibiotics are present, the resistant bacteria have an advantage over the bacteria that are not resistant. Antibiotic-resistant strains of bacteria are an increasing problem in hospitals.


Scientists can now examine the DNA from different species of organism and use the data produced to see how closely related the two species are to each other. By collecting a lot of this data, scientists can compare the results with conventional ideas about how organisms have evolved. What they found was that DNA data supported the conventional theory of evolution.

Ideas about science

Developing explanations - Higher tier

You need to be able to provide an explanation that links together things that were previously thought to be unrelated. For example, DNA data being used to support the theory of evolution. Also people had noticed that there was a wide variety of different things living on the planet. Darwin noticed that different islands in the Galapagos had different types of finches adapted to eating the type of food found on each island. It was only when Darwin produced his theory of evolution by natural selection that these two unrelated facts were linked together.

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classification of Different Levels

Millions of different living things inhabit the Earth. This wide variety of life is called biodiversity. Biodiversity is important and so we must use the Earth in a sustainable way to ensure we maintain a good level of biodivesity in living things and species.

Classification at different levels

Organisms are classified into different groups. This classification is done according to similarities and differences in characteristics including:

  • physical features
  • DNA.

Organisms are classified at different levels. These levels can be arranged in an order, progressing from:

  • a large group containing many organisms with a small number of characteristics in common, such as a kingdom
  • a small group containing fewer organisms with more characteristics in common, such as a species.

This classification of both living and fossil organisms helps scientists to make sense of the diversity of organisms on Earth, as well as to suggest evolutionary relationships between them.

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Biodiversity means having as wide a range of different species as possible. Maintaining biodiversity is an important part of using the environment in a sustainable way. Indiscriminate use of the environment, for example cutting down large areas of the rain forest to grow crops such as soya, results in a large number of species becoming extinct and reduces biodiversity.

It is only now that we are beginning to realise the potential medicines and crops that we can obtain from a wide range of different organisms. Only by maintaining biodiversity can we be sure that these benefits will be available for future generations.


Monoculture is the continuous production of one type of crop that is often genetically uniform. Doing this in a large field means harvesting by machinery can be done efficiently.

There are disadvantages to monoculture. If pests and disease attacked the crop it could harm it easily, so farmers use a lot of chemical pesticides. This can harm the environment and so is not viewed as sustainable. Using large fields and pesticides reduces the variety of species. This hinders biodiversity.

Another disadvantage is that if a natural disaster were to occur, the whole crop could be wiped out.

Improving sustainability

Packaging can be a major environmental problem. The use of biodegradable packaging is an option to tackle this problem. When put in a landfill site it should decompose quickly, unlike a lot of currently used packaging.

However, to decompose in a way that only produces carbon dioxide oxygen needs to be present, but in landfill oxygen is often not present. This results in the production of a powerful greenhouse gasmethane being produced if decay occurs at all.

Another possible solution is to recycle the packaging. The materials and energy used in production, energy used in transport and pollution created are all considerations when trying to improve sustainability.

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