variation and evolution

• individuals in a population are usually similar to each other, but not identical.
• Some of the variation within a species is genetic, some is environmental - the conditions in which they have developed and some is a combination of both.
• Children generally look a little like their mother and their father, but are not identical to either. They inherit their features from each parent's DNA.
• Every sperm and egg cell contains half of the genetic information needed for an individual. Each sex cell is known as haploid, which has half the normal number of chromosomes. 
• When the chromosomes fuse during fertilisation, a new cell is formed, which is known as a zygote. It has all the genetic information needed for an individual, which is known as diploid and has the full number of chromosomes.
• Examples of genetic variation in humans include blood group, skin colour and natural eye colour.
• Biological sex is also an inherited variation - whether you are male or female is a result of genes you inherited from your parents.
• Characteristics of animal and plant species can be affected by factors such as climate, diet, accidents, culture and lifestyle.
• For example, if you eat too much you will become heavier, and if you eat too little you will become lighter
• A plant in the shade of a big tree will grow taller to reach more light.
• Other examples of features that show environmental variation include:

  • scars
  • language and accent
  • flower colour in hydrangeas as these plants produce blue flowers in acidic soil and pink flowers in alkaline soil
• Some features vary because of a combination of genetic and environmental causes. 
• For example, tall parents will pass genes to their children for height. Their children have the genetic potential to also be tall. However, if their diet is poor then they will not grow very well: their environment also has an impact on their height.
• Extensive genetic variation is contained within any species. This is clearly visible in the domestic dog species.
• Variation within genes leads to different genotypes, and this can be seen by a different phenotype
•  Genetic and environmental variation combine together to produce these different phenotypes. All variants arise from mutations and most have no effect on the phenotype.
• A mutation is a change in a gene or chromosome. Mutations arise spontaneously and happen continually
•  A mutation rarely creates a new phenotype, but if the phenotype is suited to a particular environment, it can lead to rapid change in a species.
• For example, if a mutation leads to a change, such as feather colouring in birds, this new change may allow those individuals to reproduce more frequently, due to them being more attractive and seen as a more desirable mate. This would result in this phenotype being passed on more successfully than the birds of the same species without the new phenotype.
• Natural selection is a process where organisms that are better adapted to an environment will survive and have more offspring. This means their genes are passed on to the future generations. This process is fundamental to the process of evolution.
• charles Darwin was a famous English naturalist, who during his life came up with a theory of evolution. He is associated with the term 'survival of the fittest' which describes how natural selection works, by selecting the best examples of an organism to survive. For example, individuals that are best adapted to their environments are more likely to survive and therefore reproduce.
• During the nineteenth century, pollution killed off some of the lichens and soot deposits caused the bark on trees to appear darker. Light coloured moths were no longer camouflaged and were eaten by birds. The dark moths had a better camouflage.
• As a result, dark moths had a greater chance of reproducing and passing on the phenotypes, and through this the alleles that made them dark. This led to a gradual increase in the proportion of dark moths until light moths became very rare in industrial areas.
•  this change was not due to pollution making the moths darker. The dark variety had always existed, but had an advantage when the environment changed.
• Selective breeding or artificial selection is when humans breed plants and animals for particular genetic characteristics. Humans have bred food crops from wild plants and domesticated animals for thousands of years.
• Selective breeding takes place over many generations.
• These are the main steps for both plants and animals involve:

  1. Decide which characteristics are important enough to select.
  2. Choose parents that show these characteristics from a mixed population. They are bred together.
  3. Choose the best offspring with the desired characteristics to produce the next generation.
  4. Repeat the process continuously over many generations, until all offspring show the desired characteristics.
• Farmers selectively breed different types of cows with highly desirable characteristics in order to produce the best meat and dairy.
• Characteristics can be chosen for usefulness or appearance:
• Desired characteristics in plants:

  • disease resistance in food crops
  • wheat plants that produce lots of grain
  • large or unusual flowers
• Desired characteristics in animals:

  • animals that produce lots of milk or meat
  • chickens that lay large eggs
  • domestic dogs that have a gentle nature
• The new varieties may be economically important. For example, they may provide more or better quality food, or allow farmers to feed more people.
• Because of selective breeding, future generations of selectively bred plants and animals will all share very similar genes which will reduce variation. Genes and their different alleles within a population are known as its gene pool. 
•  Inbreeding can lead to a reduced gene pool, making it more difficult to produce new varieties in the future. This also makes organisms prone to certain diseases or inherited defects.
• Benefits of selective breeding include:

  • new varieties may be economically important, by producing more or better quality food
  • animals can be selected that cannot cause harm, for example cattle without horns
• Risks of selective breeding include:

  • reduced genetic variation can lead to attack by specific insects or disease, which could be extremely destructive
  • rare disease genes can be unknowingly selected as part of a positive trait, leading to problems with specific organisms, eg a high percentage of Dalmatian dogs are deaf
  • can create physical problems in specific organisms, eg large dogs can have faulty hips due to not being formed correctly
• Genetic engineering is also called genetic modification or GM. It involves modifying the genome of an organism by introducing a gene from another organism to result in a desired characteristic.
• Genetic engineering involves these steps:

  1. selection of the desired characteristic
  2. the gene responsible for the characteristic is 'cut out' of the chromosome
  3. the gene is transferred and inserted into another organism
  4. replication of the modified organism.
• Plant crops have been genetically engineered to be disease resistant or to produce bigger fruits.
• Diabetes is a disorder in which the body's blood glucose levels remain too low or too high. It can be treated by injecting insulin. The extra insulin allows the glucose to be taken up by the liver and other tissues, which results in cells receiving the glucose they need, and blood glucose levels stay normal.