Biology: Unit One

A healthy diet has the right balance of food types. Carbohydrate, fat and protein are used by the body to release energy and to build cells. Mineral ions and vitamins are needed to keep the body healthy. If the diet is unbalance a person can become malnourished.

If you exercise, more energy is used by the body. Exercise increases the metabolic rate, which means that the chemical reactions in cells work faster.

The proportion of muscle to fat in your body and your inherited factors can also affect your metabolic rate.

It is important for good health to get the energy balance correct. If the same energy you take in equals the energy you use then your mass will stay the same. Eating too much food can lead to becoming overweight or obese.

Long term obesity can lead to severe health problems including Type 2 diabetes (high blood sugar). These problems can be reduced by eating less carbohydrate and increasing the amount of exercise.

Some people are unhealthy because they have too little food (starvation). They find it difficult to walk about and may suffer from deficiency diseases due to lack of vitamins and minerals.

You metabolic rate can be affected by the genes you inherit from your parents.

There are two types of cholesterol. You need 'good' cholesterol for your cell membranes and to make vital substances. Small numbers of the population inherit high levels of 'bad' cholesterol, which can lead to heart disease.

Foods rich in saturated fat can also increase blood cholesterol levels.

By exercising regularly a person can increase their metabolic rate and lower cholesterol levels.

Tip (answering questions):
Why does eating too much make you put on weight (mass)?

The food contains energy and any excess energy is stored in the body as fat.

Pathogens cause infectious diseases. Pathogens are tiny microorganisms- usually bacteria or viruses.

When bacteria or viruses enter the body they reproduce rapidly. They can make you feel ill by producing toxins (poisons).

Viruses are much smaller than bacteria and reproduce inside cells. The damage to the cells also makes you ill.

Before bacteria and viruses had been discovered a doctor called Semmelweiss realised that infection could be transferred from person to person in a hospital. Semmelweiss told his staff to wash their hands between treating patients. However, other doctors did not take him seriously. We now know that he was right.

The skin prevents pathogens getting into the body. Pathogens are also trapped by mucus and killed by stomach acid.

White blood cells are part of the immune system. They do three things to defend the body:

• They can ingest pathogens. This means they digest and destroy them.
• They produce antibodies to help destroy particular pathogens.
• They produce antitoxins to counteract toxins that pathogens produce.

Antibiotics kill infective bacteria in the body.

Penicillin is an antibiotic, but there are many others. It was first discovered by Alexander Fleming in 1928. He noticed that some bacteria he had left in a petri dish had been killed by naturally occurring penicillium mould.

Viruses are difficult to kill because they reproduce inside the body cells so any treatment could damage the body cells.

Painkillers and other drugs relieve the symptoms of a disease but do not kill the pathogen.

Your immune system will usually overcome the viral pathogens.

Jenner noticed that milkmaids who had caught the cowpox virus did not normally then catch smallpox. Cowpox was very similar to smallpox but less contagious.

He collected pus from the cowpox blisters on a milkmaid’s hands and purposefully infected a small boy. The boy was taken ill for a short while, but was then resistant to any subsequent infections of the cowpox and smallpox viruses. He tested this by infecting the boy with smallpox. No illness occurred.

Pure cultures of non-pathogenic (safe) bacteria can be used for laboratory investigations.

A culture of microorganisms can be used to find the effect of antibiotics on bacteria. Investigations need uncontaminated cultures of microorganisms. Strict health and safety procedures are used to protect yourself and others.

Contamination might come from your skin, the air, the soil or the water around you. If the culture is contaminated other bacteria could grow, including pathogens.

To culture (grow) miccrorganisms in a laboratory you must:

• Give them a liquid or gel containing nutrients- a culture medium. It contains carbohydrate as an energy source, various minerals and sometimes other chemicals.A culture medium called agar jelly is used.
• Provide warmth and oxygen.
• Keep them incubated at 25 degrees celsius in school laboratories and 35 degrees celsius in industry.

To keep the culture pure you must:

• Kill all bacteria on the equipment- pass metal loops through a flame; boil solutions and agar.
• Dip the sterilised loop in a suspension of the bacteria you want to grow and use it to make zigzag streaks across the surface of tge agar. Replace lid as quickly as possible. Thus preventing microorganisms from the air getting into the equipment.
• Seal it with adhesive tape to prevent microorganisms from the air contaminiating the culture- or microbes from the culture escaping.

Some pathogens, particularly viruses, can mutate (change) resulting in a new form called a mutation. Very few people are immune to these changed pathogens so disease can spread very quickly.

Diseases that spread within a country result in an epidemic. Those that spread across countries result in a pandemic.

Antibiotic-resistant bacteria: The MRSA 'super bug' is a bacterium that has evolved through natural selection. MRSA and other bacteria have become resistant to common antibiotics.

• Mutations of pathogens produce new strains, some are resistant to antibiotics
• Antibiotics kill individual pathogens of the non-resistant strain.
• The resistant bacteria survive and repoduce and a whole population of a resistant strain develops. This is natural selection.
• Antibiotics shouldn't be used for mild infections in order to slow down the rate of development of resistant strains.

Dead or inactive forms of a pathogen are used to make a vaccine. Vaccines can be injected into the body. The white blood cells react by producing antibodies. This makes the person immune. It prevents further infection because the body responds quickly by producing more antibodies.

The antibodies recognise the antigen (the protein shape) on the pathogen.

The MMR vaccination (immunisation) is one of several vaccines. MMR is given to prevent measles, mumps and rubella.

Most people in a population need to be vaccinated to protect society from very serious diseases.

Diseases such as measles and lead to long term damage to the body, such as deafness and occasionally death.

Some vaccines cause side effects which may be mild or serious. So there are advantages and disadvantages of vaccination.

• Overuse of antibiotics can lead to the development of new strains of bacteria.
• Doctors do not prescribe antibiotics for mild infections such as minor sore throats.
• Scientists are always trying to find new ways or treating diseases.

The nervous system has receptors to detect stimuli. The receptors are found in sense organs, the eye, ear, nose, tongue and skin.

Light stimulates receptors in the eye and the electrical impulses then pass to the brain along neurons (nerve cells). Other stimuli include sound, chemicals, temperature changes, touch and pain.

The brain coordinates responses to many stimuli.

The brain and spinal cord form the central nervous system (CNS). Nerves contain neurons. Sensory neurons carry impulses form receptors to the CNS. Motor neurons carry impulses from the CNS to effector organs which may be muscles or glands. The muscles respond by contracting. The glands respond by secreting (releasing) chemicals.

The main steps involved in reflex actions (reflexes) are:

• A receptor detects a stimulus (e.g. a sharp pain)
• A sensory neuron transmits the impulse to the CNS.
• A relay neuron passes the impulse on.
• A motor neuron is stimulated
• The impulse passes to an effector (muscle or gland)
• Action is taken (the response)

At the junction between two neurons is a synapse. Chemicals transmit the impulse across the gap.

The sequence from receptor to effector is a reflex arc.

Follicle stimulating hormone (FSH) is made by the pituitary gland and causes the egg to mature and oestrogen to be produced.

Oestrogen is produced by the ovaries and inhibits (stops) the further production of FSH. It stimulates the production ofLH and also stimulated the womb lining to develop to receive the fertilised egg.

Luteinising hormon (LH) is made by the pituitary gland and stimulates the mature egg to be release from the ovary (ovulation).

The contraceptive pill (oral contraception) may contain oestrogen and progesterone. Some pills are progesterone-only pills and have fewer side effects. The contraceptive pill prevents the production of FSH so no eggs mature.

If a woman cannot produce mature eggs then FSH and LH can be given. This is known as 'fertility treatment'. The FSH causes eggs to mature and the LH stimulates ovulation.

Fertility treatment:

• Advantages: Contraceptive pills have helped to reduce family size which has reduced poverty in some areas. It allows women to plan their pregnancies. Fertility drugs can help infertile couples who are having IVF. IVF helps couples to have a baby.
• Disadvantages: The contraceptive pill can cause side effects. Some people object to its use for ethical or religious reaons. IVF is an expensive process. Some people think it is unethical when older women have babies by IVF. Extra embryos produced may be stored or destroyed.

The body carefully controls its internal environment. Internal conditions that are contr. olled include:

• Water content
• Ion content
• Temperature
• Blood sugar level

Water is leaving the body all the time as we breathe out and sweat. We lose any excess water in the urine (produced by the kidneys). We also lose ions in our sweat and in the urine.

We must keep our temperature constant, otherwise the enzymes in the body won't work properly or not at all.

Sugar in the blood is the energy source for cells. The level of sugar in our blood is controlled by the pancreas

Plants are sensitive to light, gravity and moisture.

• Plant shoots grow towards light. This response is phototropism.
• In a shoot, the shaded side contains more auxin. This means that the shaded side grows longer, causing the shoot to bend towards the light.
• Roots grow down towards gravity. This response is gravitropism.
• Auxins are also involved in gravitropisms. In a root placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow less than the top side, causing the root to bend in the direction of the force of gravity. In a shoot placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow more than the top side, causing the shoot to bend and grow against the force of gravity.
• Roots also grow towards water.
• Auxin is the hormone that controls phototropism and gravitropism.
• Unequal distribution of auxin causes unequal growth. This results in bending of the shoot or root.
• Plant growth hormones can be used as weed killers and to stimulate root growth.

In women:

• Many women use the contraceptive pill to prevent unwanted pregnancies. It helps them to plan when they have a baby.
• Other women use hormones to help them become pregnant.
• Sometimes hormone treatment is used to help older women have babies. This may involve using an egg donor who is given hormones to produce extra eggs.
• Taking hormones for a long time can lead to side effects.

In plants:

• Plant hormones can be used by farmers and gardners.
• Weedkillers are used to kill unwanted plants on lawns.
• When cuttings are taken from plants, hormones are used to encourage roots to grow before the cutting is planted.
• Some hormones are used to encourage fruits to ripen.
• If plant hormones are used incorrectly they can cause damage to the environment e.g. weedkillers may harm other more useful plants.

Large numbers of substances are tested to see if they might cure a disease or relieve the symptoms.The first tests are in scientific laboratories on cells and tissues or organs. If the drug seems to work it is then tested on animals, healthy human volunteers and finally on patients.

Healthy people are given very low doses of the drug to find out if they are safe.

In some trials, with patients, placebos are used. Placebos don't contain a drug. Half the patients have the drug and the other half are given a placebo. This is to check that the drug being tested really does have an effect on the patient.

In a double-blind trial, neither the doctor nor the patient knows who is given a drug.

People taking part in a drug trial are asked to report any side effects.

Thalidomide was developed as a sleep pill, but doctors realised it could control morning sickness in pregnant women. Unfortunately it had not been tested for use in pregnancy.

Some babies were born with limb abnormalities as a result of their mothers taking thalidomide.

The drug was banned and the rules for drug testing were improved.

More recently thalidomide has been used to treat other conditions, including leprosy, but it is never given to pregant women.

Statins are drugs which lower the amount of 'bad' cholesterol carried in the blood. They are given to older people and are taken daily.

Trials using very large numbers of people have shown that the incidence of heart disease and strokes has gone down by over 40%

Some people prefer to take drugs which are not prescribed by doctors. Herbs are often useed instead of prescribed medicines.

A herb called St Johm's Wort is sometimes taken to treat depression instead of anti-depressants such as Prozac.

The only way to be sure that the herb works as well as or better than Prozac is to conduct a double blind test.

Useful drugs, made from natural substances, have been used by indigenous people for a very long time.

When we develop new drugs to help people, we have to test them over a long time to make sure that there are no serious side effects.

Recreational drugs are used by people for pleasure. Heroine and cocaine are examples of recreational drugs. They are very addictive and illegal.

Cannabis is a recreational drug. It is also illegal. Some argue that using cannabis can lead to using 'harder' drugs.

If you try to stop taking addictive drugs you will suffer withdrawal symptoms

Medicinal drugs are developed over many years and used to control diseases or help people that are suffering, Many medicinal drugs are only available on prescription from a doctor.

Recreational drugs are used only for pleasue and affect the brain and the nervous system. They may also have adverse affects on the heart and circulatory system. Recreational drugs include cannabis and heroine, which are both illegal. As recreational drugs affect the nervous system it is very easy to become addicted to them.

Nicotine and caffeine (in coffee and coke) are legal drugs which are used recreationally. Alcohol is also legal for people over the age of 18 in the UK.

There are many health problems associated with legal recreational drugs e.g. alcohol poisoning, addiction to nicotine leading to lung cancer from cigarette smoke.

Some drugs used for medicinal purposes can be used illegally e.g. stimulants used by sports people.

There is evidence that cannabis can cause mental illness in some people. Teenagers who smoke cannabis increase their risk of getting depression.

Cannabis is an illegal drug which must be bought from dealers. This brings cannabis users in contact with hard drugs such as heroine.

Not all cannabis users go on to use hard drugs.

Nearly all heroine users previously smoked cannabis.

Some athletes are tempted to use drugs to improve their performance.

• Steroids are drugs which are used to build up muscle mass. Other drugs may be used to increase stamina.
• Stong pain killers are banned because the athlete may ignore an injury and suffer further damage.
• Many performance-enhancing drugs are very expensive. This gives an unfair advantage to those who can afford them.
• Using performance-enhancing drugs can damage the body permanently and even lead to death.
• Most people think it is unethical to take drugs to enhance performance.

To survive and reproduce, organisms require materials from their surrounding and from other organisms living there. Plants need light, carbon dioxide, water, oxygen and nutrients such as mineral ions from the soil. Animals need food from other organisms, water and oxygen. Different microgranisms need different materials. Some are like plants, other are like animals, and some fo not need oxygen or light to survive.

Special features of organisms are called adaptations. Adaptations allow organisms to survive in a particular habitat even when the conditions are xtreme, e.g. extremely hot, very salty or at high pressure.

Plants are adapted to obtain light and other materials efficiently in order to make food by photosynthesis. Animals may be plant eating (herbivores) of eat other animals (carnivores). Their mouthparts are adapted to eat their diet.

Most organisms live in temperatures below 40 degrees celsius so their enzymes can work. Extemophiles aremicroorganisms which are adapted to live in conditions where enzymes won't usually work because they would denature.

If animals were not adapted to survive in the areas they live in, they would die.

Animals in cold climates (e.g. in the Arctic) have thick fur and fat under tge skin (blubber) to keep them warm. Some animals in the Arctic (e.g. Arctic fox, Arctic hare) are white in the winter and brown in the summer. This means that they are camouflaged and so they are not easily seen.

Bigger animals have smaller surface areas compared to their volume. This means that they can conserve energy more easily but it is also more difficul to cool down.

In hot dry conditions (deser) animals are adapted to conserve water and to stop them getting too hot. Animals in the desert may hunt or feed at night so that they remain cool during the day.

Plants need light, water, space and nutrients to survive.

Plants need to collect and conserve water. They can lose water as water vapour through holes in the leaves called stomata. Water can be collected if the plant has an extensive root system. Water can be conserved if the plant has very small or waxy leaves. A plant might have a swollen stem to store the water.

In dry conditions, e.g. in deserts, plants (such as cacti) have become very well adapted to conserve water. Others (such as the mesquite tree) have adapted to collect water using extensive root systems.

Plants are eaten by animals. Some plant have developed thorns, poisonous chemicals and warning colours to put animals off.

Animals are in competition with each other for water, food, space, mates and breeding sits.

An animal's territory will be large enough to find water, food and have space for breeding.

• Predators compete with their prey, as they want to eat them.
• Predators and prey may be camouflages, so that the are less easy to see.
• Prey animals compete with each other to escape from the predators and to find food for themselves.

Some animals, e.g. caterpillars, may be poisonous and have warning colours so that they are not eaten.

All plants compete for water, nutrients and light. For example, in woodland some smaller plants (e.g. snowdrops) flower before the trees are in leaf. This ensures that they get enough light, water and nutrients.

Plants which group deep roots can reach underground water better than those with shallow roots.

Some plants spread their seeds over a wide area so that they do not compete with themselves.

• Some of these plants use animals to spread their fruits and seeds.
• Some plants use the wind (e.g. sycamore) or mini explosions to spread their seeds (e.g. broom)

Some animals and plants have very unusual adaptations which make them successful competitors. Female fig wasps have specifically shaped heads for getting into fig tree flowers and ovipositors that allow them to place eggs deep inside the flower. Some male fig wasps spend their lives inside the flowers waiting for a female.

The star-nosed mole lives underground and is almost blind, but is very sensitive to touch and smell.

Venus fly traps are insect eating plants. They have sweet, sticky nectar and are bright red inside.

Animals and plants are affected by their environment. If the environment changes the organisms may not be able to live there anymore.

• Non-living factors might change, these include: temperature, rainfall, light and oxygen levels.
• Living factors that might change include: arrival of a new predator or disease, or the introduction of new plants which might provide new food or habitats.

Lichens indicate the level of air pollution, particularly sulfur dioxide. The more species of lichen growing, the cleaner the air. They are an example of an indicator species, which indicate changes in environmental pollution levels.

Freshwater invertebrates indicate the level of water pollution in the same way, in particular the concentration of dissolved oxygen in the water. The wider the range of these invertebrates the cleaner the water in the streams, rivers or ponds. Some freshwater invertebrates will only live in polluted water.

Equipment such as rain gauges, thermometers, pH and oxygen sensors and data loggers can be used to monitor non-living changes in the environment.

Changes in the environment affect the distribution of living organisms.

It is sometimes difficult to determine what is affecting the organism.

Birds may fly further North if the climate gets warmer. Other birds may then have new competitors.

The large fall in the bee population may have been caused by several factors. These include the use of chemical sprays by farmers, a viral disease or possible changes in flowering patterns in plants due to climate change.

Biomass is the mass of living material in plants and animals.

A pyramid of biomass represents the mass of the organisms at each stage in a food chain. It may be more accurate than a pyramid of number. For example, one bush may have many insects feeding on it but the mass of the bush is far greater than the mass of the insects. 

Green plants transfer solar (light) energy to chemical energy which is then oassed through the food chain.

There is energy wastage between each stage of a food chain. This means that not all of the energy taken in by an organism results in the growth of that organism.

• Not all of the food can be digested, so energy is stored in faeces or as urea in urine (waste materials)
• Some of the biomass (food) is used for respiration, which releases energy for living processes. This includes movement, so the more something moves the more energy it uses and the less is available for growth.
• In animals that need to keep a constant temperature, energy from the previous stage of the food chain is used simply to keep the animal at its normal body temperature.
• Much of the energy released in respiration is eventually transferred to the surrounding.

Sankey diagrams show how energy is transferred in a system.

All organisms take up nutrients. If they didn't eventually release them, the nutrients would run out.

Detritus feeders (such as some types of worms) may start the process of decay by eating dead animals or plants and producing waste materials. Decay organisms then break down the waste or dead plants and animals.

Decay organisms are microorganisms (bacteria and fungi). They are called decomposers. Decay is faster if it is warm and wet. Many decomposers also need oxygen.

All of the materials from the waste and dead organisms are recycled, returning nutrient to the soil.

Humans can recycle waste in sewage treatment plants and compost heaps.

• The recycling of carbon involves both photosynthesis and respiration.
• Photosynthesis removes CO2 from the atmosphere.
• Green plants as well as animals respire. This returns CO2 to the atmosphere.
• When humans cut down and burn trees, CO2 is released into the atmosphere.
• Animalls eat green plants and build the carbon into their bodies. When plants or animals die (or produce waste) miscroorganisms release the CO2 back into the aitmosphere through respiration.
• A stable community recycles all the nutrients it takes up

• Waste vegetables and peelings from the kitchen, or grass cuttings and clippings from trees in the garden, contain organic waste. This can be recycled.
• Organic waste can be composted in several ways.
• The most efficient method of composting allow the waste to be mixed with oxygen and moisture. They also allow energy to escape by heating the surroundings.
• Gardeners may add worms and layers of garden soil to composters to speed up the process.
• Councils also collect garden waste and use shredders and large bins to compost the material.

The nucleus of a cell contains thread-like structures called chromosomes. These chromosome threads carry the genes.

In the nuclei of s.ex cells (gametes) there is only a single set of chromosomes, therefore nuclei of male and female s.ex cells contain one set of genes.

The genetic information from the parents is passed on to the offspring during reproduction. So the offspring cells contain two sets of genes, inherited from both parents.

Different genes control the development of different characterists of the offspring.

In most body cells the chromosomes are in pairs. One set came from the female gamete (from the mother) and one set from the male gamete (from the father).

As.exual reproduction does not involve the fusion of gametes (se.x cells). All of the genetic information comes from one parent. All of the offspring are genetically identical to the parent, so there is little variety.

Identical copied produced by ase.xual reproduction are called clones.

Se.xual reproduction involves the fusion of se.x celles (gametes). There is a mixture of genetic information, so the offspring show variation.

In animals, the se.x cells are eggs and sperm.

Offspring produced by sexual reproduction are similar to both parents, but cannot be identical. This is because they have a combination of two sets of genes.

Random mixing of genes leads to variation in the offspring. This is important in survival. Some characteristics may give offspring a better chance of surviving in difficult conditions.

Difference in the characteristics of individuals of the same kind (same species) may be due to:

• Differences in the genes they have inherited
• The conditions in which they have developed
• A combination of both these genetic and environemental causes.

Genes are the most important factor in controlling the appearance of an individual.

• Plants may be affected by lack of light, nutrients or space to grow. The weaker plants may have the same genes as the healthier plants but cannot grow well if deprived of nutrients.
• Human development may be affected during pregnancy. If the mother smokes or drinks a lot of alcohol, the baby may have a small birth weight.
• Once animals are born, too much or too little food can alter their characteristics. For example, genes may determine if someone has the potential to be a good athlete. However, training to develop muscles and eating the correct diet will also alter the althlete's body.

Individuals which are genetically identical to their parent are known as 'clones'. It is much more difficult to clone animals than it is to clone plants. Cloning is used to produce new individuals that are useful in farming and agriculture.

• In plants, the process of cloning can be cheap and effective. Plants can be cloned by taking cuttings and growing them.
• Taking small groups of cells from part of a plant and growing them under special conditions (tissue culture) is more expensive. Tissue culture can be used to reproduce large numbers of a rare or top quality plant.

• Embryo transplants are used to clone animals. In this process and embryo with unspecialised cells is split into smaller groups of cells. Each group of genetically identical cells is transplanted and allowed to develop in a host animal.

Sometimes animals or plants are genetically modified to produce useful substances before they are cloned.

• In adult cell cloning the nucleus of an adult cell, e.g. a skin cell, replaces the nucleus of an egg cell.
• First the nucleus is removed from an unfertilised egg cell. The nucleus is removed from the skin cell and placed inside the 'empty' edd cell.
• The new cell is given an electric shock which causes it to start to divide. The ball of cells is called an embryo.
• The embryo is genetically identical to the adult skin cell.
• Once the embryo has developed into a ball of cells it is inserted into the womb of a host mother.
• Dolly the sheep was produced by this method in 1997.

Advantages:

• Development of cloned animals which have been genetically engineered to produce valuable proteins in their milk. These have uses in medicine.
• Cloing can save animals from extinction.

Disadvantages:

• Concerns about the ethics of cloning.
• Cloning limits the variation in a population (limits the gene pool). This can be a provlem for natural selection if the environment changes.
• Concerns about using the technique to clone humans in the future.

Genetic engineering involves changing the genetic make-up of an organism.

Genes can be transferred to the cells of animals, plants and microorganisms at an early stage in their development.

• A gene is 'cut out' of the chromosome of an organisim using an enzyme. The gene is then placed in the chromosome of another organism.
• The gene may be placed in an organism of the same species to give it a 'desired' characteristic.
• Sometimes genes are placed in a different species, such as a bacterium. For example, the gene to produce insulin in humans can be placed in bacteria. Then the bacteria can produce large quantities of insulin to treat diabetes.

New genes can be transferred to crop plants. Crops with changes genes are called genetically modified (GM) crop plants. GM crops may be insect- or herbicide-resistant and usually have increased yields.

• Cloning cattle can produce herds of cattle with useful characteristics
• Adult cell cloning may be used to make copies of the best animals, e.g. race horses
• If a person has a faulty gene they have have a genetic disorder. If the correct gene can be transferred to the person they could be cured.
• Several medical drugs have been produced by genetic engineering, such as insulin and antibodies.
• GM crops include ones which are resistant to herbicides or to insects.

• GM crops have a bigger yield, but farmers have to buy new GM seeds every year because the crops are infertile.
• Some people are concerned about accidentally introducing genes into wild flower populations.
• Insects which are not pests may be affected by GM crops.
• Many people worry about the effect of GM crops on health.
• Many people argue about whether it's ethical. What will the long-term effects be? Will we create new organisms that we know nothing about? Is it ethically correct?

Life began on Earth an estimated 3 billion years ago. Before the 18th century there were very few ideas about how evolution works.

• Jean-Baptiste Lamark suggested a theory called 'the inheritance of acquired characteristics'. The theory stated than characteristics which develop during an organism's lifetime can be passed on to the next generation. People found this hard to believe. E.g. if two parents were to build up their muscles in the gym, his theory would predict that this characteristic would be passed onto their offspring.
• Charles Darwin suggested the theory of 'natural selection' after he had made a journey to the Galapagos Islands. He recorded many observations about life on the islands.
• Darwin's theory stated that small changes in organisms took place over a very long time. All organisms in a species vary and therefore some are more likely to survive (natural selection). Those that are best adapted breed and pass on their characteristics.
• Darwin did not know about genes. We can now say that the best adapted organisms survive to breed. They are the ones that pass on their denes to the next generation.

Darwin's theory of evolution by natural selection was only gradually accepted for several reasons:

• The theory of natural selection challenged the idea that God made all the animals and plants that live on the Earth.
• Many scientists weren't convinced because they still didn't think there was sufficient evidence for the theory.
• Darwin couldn't explain why there was variety in organisms, or how inheritance worked. Scientists didn't know about genes and genetics until about 50 years later.
• Darwin had tried to show that birds, such as finches on the Galapagos Islands could change over time if they lived under different environmental conditions. During his lifetime he couldn't explain, in terms of genes, why the offspring inherited the useful adaptations.

• Most organisms produce large numbers of offspring. For example, a pair of rabbits may have 800 children, grandchildren and great grandchildren in one 9-month breeding season.
• Undividual organisms will show a wide range of variation because of differences in their genes.
• All the organisms in the population will compete for food, shelter from predators and mates.
• The organisms with the characteristics most suited to the environment will survive. E.g. the best camouflage, the best eyesight to find food, the strongest to build a burrow, the quickest to run from a predator. The 'fittest' organisms survive.
• The organisms which survive are more likely to breed successfully.
• The genes that have enabled these organisms to survive are then passed onto their offspring.
• Sometimes a gene accidentally changes and becomes a new form of the gene. These changes are called mutations. If the mutated gene controls a characteristic which makes the organism better adapted to the environment it will be passed onto the offspring.

Mutations may be particularly important in natural selection if the environment changes. For example, when the rabbit disease myxomatosis killed most of the rabbits in the UK, a few rabbits had a mutated gene which gave them immunity. The rabbits with the mutated gene survived to breed.

• There are millions of different types of living organisms. By putting organisms into groups we dcan make more sense of how closely they are related. Grouping organisms is called classification.
• Biologists study the similarities and differences between organisms in order to classify them. The system is called the natural classification system.
• The easiest system to understand is one which starts with large groups and splits these up gradually into smaller ones. The largest groups are called kingdoms. The main kingdoms are plant, animal and microorganism.
• The smallest group in the classification system is the species. Members of a species are very similar and can breed together to produce fertile offspring.
• Evolutionary trees are models that can be drawn to show the relationships between different groups of organisms.
• When new evidence is found, biologists may modify these evoltionary relationships.
• Ecological relationships tell us how species have evolved together in an environment.