Revision - GCSE Biology - evolution and selective breeding


Evolution is the development of new species from existing ones, and this evolution is driven by natural selection which acts on the variations within a population. A species is; a group of individuals of the same type who are able to breed together and have fertile offspring

Darwin’s theory went against the church when it was published, but now it is a cornerstone of biology and genetics. He noted four important facts

  • All organisms produce more offspring that can survive
  • However, population numbers remain constant
  • Organisms within a species show a wide variety of characteristics due to genes
  • Some of these characteristics are passed on to the next generation

Ideas which support his theory are

  • Similar animals exists on different continents (distribution)
  • Homologous structures exist in animals, like skeletal structure and limbs that are similar animal to animal
  • Fossils of animals similar to everyday animals but with slight differences exist

He then came to the conclusion that all organisms must be in a struggle for survival, and the ones which survive pass on their gene – survival of the fittest.


Mutations which help animals in natural selection are good

An example of this is rabbits. They used to have short ears, but a mutation caused one to get longer ears. These long ear rabbits could escape from prey more easily, so their numbers increased.

Another is cockroaches which have become flatter and thinner in order to hide and live in the domestic kitchen. The thin ones escape and have offspring, while the fat ones get squashed

Bacteria are also a good example, because occasionally antibiotics will be used and 0.1% will have a resistance to the antibiotics. They then survive and live on, which means that next time the antibiotics won’t be effective at all. Wild creatures live in a very harsh world, and a farm is a much safer environment for them

The theory also says that life began from simple organisms 3 000 million years ago which have slowly evolved into the animals we know today. Fossils provide us with evidence and information, and we can see how animals have evolved. However, most animals don’t become fossils, so the records are not complete. The horses’ records are fairly complete. We know that it evolved from a dog size animal, an as it grew taller its middle toe got longer. It probably got taller because the taller animals could see predators first, and could run faster. However, species can become extinct, and this happens because of one of three different events:

  • There could be a fast environmental change
  • A new predator could kill them
  • Disease could destroy them
  • They cant compete with a new species for food, and starve

As the environment changes animals adapt to it and it will be better suited to others. However, if the environment changes too quickly an entire species could be killed.


Fossils can be formed in three ways

  • The animal dies, the soft parts decay, but the hard parts (calcium based) remain and are buried. The calcium compounds from the hard parts are then replace by silica compounds and form a different rock in the shape of the fossil
  • Where the soft parts decay very slowly, so slowly that they are “petrified” i.e. the minerals in them are replaced by stone. This is very rare.
  • The animal is preserved in an environment where no decay occurs, e.g. in amber, in ice and even in water logged bogs where the environment is too acidic for decomposition (in these examples decay cannot occur because there is no oxygen or water or heat)

Fossils tell us what the animal looked like, and how long ago it lived, as rock can be dated beforehand from previous knowledge. Generally, the deeper the rock, the older it is

Selective breeding

Asexual reproduction Sexual Reproduction
Only one parent cell or organism is required Two parent cells and organisms are needed
Mitosis meiosis
Offspring are clones of parents Offspring show variation

Selective breeding (or artificial breeding) is the process of breeding the animals with the characteristics you want with other animals with those characteristics to produce more animals with good characteristics over many generations. This must be continued over many generations, taking offspring and breeding them with the ones you already have, and you will get many animals with the desired traits.

Selective breeding helps us make

  • Better beef – Selecting for the best texture, appearance etc
  • Better wool/leather - The finer the leather or wool, the more profit the farmer will earn
  • Better milk – Choosing cows which give highest yield
  • Better chickens – Bigger eggs means more profit
  • Better wheat – Growing disease resistant wheat
  • Better flowers – Choosing the biggest and most colourful flowers

However, there are drawbacks

  • Farmers breed animals that are resistant to disease and provide high quality meat or eggs or milk
  • Because of this it is the same genes that are being selected each time, which mean the gene pool is reduced
  • If a new disease comes along and one of the animals dies, then there is a high risk all the others will die because they are all related, and all share very similar genes
  • In breeding causes the problems, and is often done with pedigrees because you can get the very best looking animals

Pedigree dogs are also in bad health because of the small gene pool, where dogs are bred for appearance, ignoring health. Because of this, mongrels are often better natured and healthier than pedigrees.


Clones are genetically identical organisms, and cloning is widely used in farming. The problem is that the prize cow can only produce one offspring a year, so…

  • Sperm is taken from the prize bull and checked for genetic defects
  • The prize cow is given fertility treatment so she produces lots of eggs
  • She is then artificially inseminated

the embryos are then taken, checked and split (as cells haven’t begun to specialize yet)

  • The split embryos are then inserted into lots of different cows so you get lots of prize calves

This is great because you have hundreds of ideal offspring, and the prize cow can keep working all year. However, there is the reduced gene pool problem.

  • Plants can be cloned by taking a tissue sample and growing it in a growth medium (e.g. jelly and growth hormone), and it grows like a new plant
  • This produces lots of plants very quickly, regardless of the weather or time of year
  • Little space is needed
  • The new plants are disease free
  • However, there is still the gene pool problem associated with clones

Genetic engineering is the idea of taking sections of DNA and putting it into another organism so it produces useful things. We make insulin like this, and human growth hormone.

The DNA code we want to be produced is cut out of the human DNA with restricting enzymes. These same enzymes are then used to cut the bacteria DNA (which is circular and called a plasmid) at exactly the same place to produce two “sticky ends”. The useful DNA section is then stuck into the bacterium DNA (plasmid) using DNA ligase (i.e. enzyme glue). This new plasmid is reinserted into the bacteria, which is then cultured in bio-reactors to produce millions which produce loads of the substance you want. The substance is then purified and can be used.

The same thing can be done with animals, for example, creating sheep’s milk which gives off a drug. This would then be a very cheap way of producing a drug. Mosquitoes are also being tampered with so they produce the malaria vaccine. If they are so good at spreading it they should be good at spreading the vaccine too.


  • Some people think manipulating nature for our own benefit is wrong
  • Others (probably the same people actually, bloody hippies) say that it is cruel to breed (for example) cows which die if you don’t milk them because they produce too much milk, or pigs which are too fat to stand up.
  • However others say we need good food at good prices (farmers, I suspect)
  • There are no laws against selective breeding. Yet. (Bloody hippies will change that, burn ‘em all, I say…)
  • People want to clone human embryos to get replacement organs and tissue for those who need it
  • This treatment would stop deaths from organ rejection
  • However many countries have banned human cloning because they think it is morally wrong
  • People say making a life for spare parts then killing it is wrong, even though abortions are done at that stage.
  • Designer babies could be a problem, as parents see the need to make their children more and more perfect
  • Changing the genetic make up or organisms could produce unexpected results
  • Seed companies could make money by producing plants which only work with one (i.e. their) type of fertilizer

However, it might mean we are able to produce crops in areas we weren’t before

Also See

Here are the other comprehensive GCSE Biology notes by Prometheus: