Theory of Evolution
Evolution has taken place over millions of years and is responsible for all the living things on Earth today. There are different views and theories about the origin of life and the evolutionary process.
How did life start?
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 [DNA: The material inside the nucleus of cells, carrying genetic information. DNA stands for Deoxyribonucleic Acid. ] 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.
Darwin’s theory of evolution
Charles Darwin (1809 - 1882) 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 bacteria [bacteria: Single-celled micro-organisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium. ] to the first modern humans.
Darwin’s finches
Darwin studied the wildlife on the Galápagos Islands - a group of islands on the equator almost 1,000 kilometres west of Ecuador. 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. For example, their beaks were different depending on the local food source. 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.
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 [genes: The basic units of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production. ]
· 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.
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. We then breed together a male and female that show 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 that have the characteristics we are looking for. Over a large number of generations, this can produce some surprising results.
Farmers have used selective breeding for centuries to increase milk yield in cattle, produce larger eggs from chickens and obtain more grain from wheat.
How scientists developed explanations for evolution
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
Lamarck
A giraffe stretches its neck to reach food high up
Darwin
A giraffe with a longer neck can reach food high up
Lamarck
The giraffe’s neck gets longer because it is used a lot
Darwin
The giraffe is more likely to get enough food to survive and to reproduce
Lamarck
The giraffe’s offspring inherit its long neck
Darwin
The giraffe’s offspring inherit its long neck
Evidence for Darwin’s theory of evolution
Fossils
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.
The horse
One of the few animals for which we have a fairly complete evolutionary record is the horse because 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.
The evolution of the horse
time period height
Modern horse 1 million years ago 1.6 metres
Pliohippus 10 million years ago 1.0 metres
Merychippus 30 million years ago 1.0 metres
Mesohippus 40 million years ago 0.6 metres
Eohippus 60 million years ago 0.4 metres
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.
Theory, not a law
Evolution is difficult to observe because it usually occurs over many years. This is one reason why the theory of evolution is still a theory, not a law. Since no one was around millions of years ago to make observations, take notes or carry out experiments, evolution cannot be proven in the usual scientific sense. However, rapid changes in species have been observed that support the theory.
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.
Antibiotic-resistant bacteria
Micro-organisms such as bacteria and viruses reproduce 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.
Extinction
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
Some animals such as the quagga are extinct because of the direct activity of humans.
The last quagga alive in the wild was shot in the late 1870s.
The quagga was the first extinct species to have its DNA studied. The DNA showed that the quagga was not a separate species as had been previously thought, but was a type of zebra.
The 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 in the Mauritius, 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, it was new competitors brought to the islands, such as pigs, cats and rats, which ate the dodos' eggs and young, that were finally responsible for the extinction of the dodo.
Biodiversity
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.
How did life start?
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 [DNA: The material inside the nucleus of cells, carrying genetic information. DNA stands for Deoxyribonucleic Acid. ] 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.
Darwin’s theory of evolution
Charles Darwin (1809 - 1882) 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 bacteria [bacteria: Single-celled micro-organisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium. ] to the first modern humans.
Darwin’s finches
Darwin studied the wildlife on the Galápagos Islands - a group of islands on the equator almost 1,000 kilometres west of Ecuador. 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. For example, their beaks were different depending on the local food source. 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.
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 [genes: The basic units of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production. ]
· 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.
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. We then breed together a male and female that show 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 that have the characteristics we are looking for. Over a large number of generations, this can produce some surprising results.
Farmers have used selective breeding for centuries to increase milk yield in cattle, produce larger eggs from chickens and obtain more grain from wheat.
How scientists developed explanations for evolution
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
Lamarck
A giraffe stretches its neck to reach food high up
Darwin
A giraffe with a longer neck can reach food high up
Lamarck
The giraffe’s neck gets longer because it is used a lot
Darwin
The giraffe is more likely to get enough food to survive and to reproduce
Lamarck
The giraffe’s offspring inherit its long neck
Darwin
The giraffe’s offspring inherit its long neck
Evidence for Darwin’s theory of evolution
Fossils
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.
The horse
One of the few animals for which we have a fairly complete evolutionary record is the horse because 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.
The evolution of the horse
time period height
Modern horse 1 million years ago 1.6 metres
Pliohippus 10 million years ago 1.0 metres
Merychippus 30 million years ago 1.0 metres
Mesohippus 40 million years ago 0.6 metres
Eohippus 60 million years ago 0.4 metres
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.
Theory, not a law
Evolution is difficult to observe because it usually occurs over many years. This is one reason why the theory of evolution is still a theory, not a law. Since no one was around millions of years ago to make observations, take notes or carry out experiments, evolution cannot be proven in the usual scientific sense. However, rapid changes in species have been observed that support the theory.
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.
Antibiotic-resistant bacteria
Micro-organisms such as bacteria and viruses reproduce 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.
Extinction
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
Some animals such as the quagga are extinct because of the direct activity of humans.
The last quagga alive in the wild was shot in the late 1870s.
The quagga was the first extinct species to have its DNA studied. The DNA showed that the quagga was not a separate species as had been previously thought, but was a type of zebra.
The 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 in the Mauritius, 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, it was new competitors brought to the islands, such as pigs, cats and rats, which ate the dodos' eggs and young, that were finally responsible for the extinction of the dodo.
Biodiversity
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.