Variation and Evolution

Individuals within a population show variation in their characteristics - otherwise the world would be a very boring place. The existence of variation is the basis for evolution by natural selection.

Variation

Individuals within a population show a range of phenotypes such as differences in height and hair colour. This is known as variation and may be caused by:

• Genetic causes – the genes they inherited

• Environmental causes – conditions which have developed due to interaction with environmental factors such as sunlight or diet

• A combination of genes and the environment

Within a population of a species there is a huge amount of genetic variation. Different forms of a gene (alleles) arise as a result of gene mutation. A mutation is when a DNA sequence changes, for example adenine replacing guanine within a gene. It occurs rarely and can be inherited in the next generation.

Sometimes the DNA change can have no effect at all. For example, if the codon AAG mutates into AAA, the protein is unaltered because both of these codons both code for the same amino acid. Therefore, this mutation will have no effect on the organism. This is the case for most genetic mutations. Very rarely do mutations have a significant impact on the organism. An example of this is the mutation which causes sickle cell anaemia. A change in DNA sequence of the gene which codes for haemoglobin results in a change to an amino acid in the protein, causing haemoglobin to fold into an awkward shape. The misfolded haemoglobin cannot carry oxygen effectively, resulting in sickle cell anaemia. Sometimes a mutation may result in a phenotype which is advantageous to the organism, making it more suited to its environment. This can lead to a rapid change in the phenotype of the whole species.

Evolution

Charles Darwin proposed his theory of evolution by natural selection after studying animals living on the Galapagos islands in South America in the 19th Century. This theory states that all species of living things have evolved from simple life forms that first developed more than three billion years ago. By observing living animals and fossilised remains, Darwin noticed that features such as the beak size of finches increased over time. His theory can be summarised as follows:

1. Individuals within a population show variation for a particular characteristic.

2. Individuals with characteristics most suited to the environment are more likely to survive and reproduce.

3. They pass on their advantageous genes to their offspring.

4. This increases the frequency of the characteristic in the population

If two populations of a single species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have become two new species. For example, let’s imagine that there is a population of finch living on an island. One group of the population prefers feeding on the nectar in flowers whereas another prefers feeding mostly on seeds with a tough coating. We would expect the two groups of finch to evolve differently. For example, the birds which feed on flowers may develop a longer, thinner beak whereas the one feeding on seeds may evolve a thicker, stronger beak. The two groups of finch may become so physically different that they are now no longer able to reproduce to form fertile offspring – at this point they are no longer the same species and speciation has occurred.

Selective Breeding

Selective breeding is a process which is carried out by humans to improve characteristics in plants and animals, usually for human benefit. As this process is carried out by the intervention of humans, it is sometimes referred to as artificial selection. Organisms can be selectively bred for the following features:

• Increased meat or milk production

• Larger eggs

• Larger fruit

• Resistance to disease

• Increased nutritional content

• Calm temperament

 Selective breeding is a long process which results in gradual changes to a species with each successive generation. It is carried out in the following way:

1. A male and female organism are selected which display the desired characteristic (e.g. high meat production)

2. The parents are bred together to produce offspring

3. The offspring which also display the desired characteristic are selected and bred together.

4. This process is repeated over many generations until all of the offspring show the desired characteristic 

As selective breeding involves a lot of inbreeding, the offspring are genetically similar so are equally vulnerable to the same diseases. Inbreeding can also result in loss of alleles from a population, making it difficult to produce different varieties of plants or animals in the future.

Selective breeding for certain traits can often result in adverse health problems. For example, certain dog breeds have been selectively bred to produce cuter puppies with more exaggerated features. Inbreeding of dog breeds such as pugs and French bulldogs to achieve squashed noses has resulted in blocked airways and breathing difficulties.

Genetic Engineering

Genetic engineering involves taking a gene from one species and placing it in another species to produce useful characteristics in that organism. An organism containing DNA from another species is called transgenic. It can be used in agriculture to produce bigger or more nutritious plants. One of the most common uses is the production of insulin by genetically modified bacteria for the treatment of diabetes.

Restriction enzymes are used to cut DNA at specific sites to extract a gene of interest. The same restriction enzymes are used to cut plasmid DNA, creating complementary sticky ends. DNA ligase joins the two pieces of DNA together so that the gene of interest is now contained within a plasmid. This is called recombinant DNA. This is either placed in a virus, which will infect organisms with the recombinant DNA, or into a plasmid which will be taken up by bacteria. Plasmids and viruses which carry the DNA molecule are called vectors. The gene of interest is transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with the desired characteristics.

This process is used for the production of insulin.

1. The insulin gene is removed from human DNA using restriction enzymes.

2. A plasmid is also cut with restriction enzymes.

3. DNA ligase joins the complementary sticky ends to form the recombinant DNA.

4. The recombinant plasmid is taken up by bacteria.

5. The transgenic bacteria are grown in large fermenters to produce large amounts of insulin, which can then be extracted.

Genes can be inserted into plant species to provide:

• Resistance to insect attack

• Herbicide resistance

• Increased yield

• Higher nutritional content

The beta-carotene gene was added to wild rice, to create a new strain called golden rice with increased nutritional value. Beta carotene is used to synthesise vitamin A which is needed for healthy vision. Golden rice can be used in areas where vitamin A deficiency is common to protect against blindness.

Crops that have had their genes modified in this way are called genetically modified (GM) crops. Many people are opposed to the use of genetically modified organisms and GM-products are heavily regulated by government agencies. The main objections include:

• They may produce unknown health risks (for example allergies)

• The gene may accidentally be transferred to other plants by cross-pollination (gene flow)

• GM crops could reduce biodiversity (e.g. plants which secrete toxins as an insecticide will reduce insect populations and disrupt the associated food chain)

• Religious reasons - do humans have the right to create new lifeforms?

• The seeds for GM crops are patented and may not be affordable for farmers in developing countries

Cloning (triple sciences only)

Cloning is the process of creating an organism which is genetically identical to the parent. Natural clones are common in nature (e.g. asexual reproduction in yeast, human twins). There are various methods of artificial cloning, including tissue culture, cuttings, embryo transplants and adult cell cloning.

Tissue culture

Small pieces of a plant (explants) are grown in vitro. They are grown in a petri dish using sterile agar jelly containing plant hormones, such as auxins and nutrients such as glucose. Once the explants have grown into small plants (platelet), they are removed from the petri dish and planted in compost. This is an important way to preserve a rare plant species. It is also used commercially to produce large numbers of genetically identical plants with desired characteristics and can be done at any time of the year.

Cuttings

Taking cuttings is a very simple way of cloning plants that is used by gardeners. It involves cutting off a section from a plant and dipping the end into rooting powder to stimulate the growth of new roots. The cutting is planted into soil and will grow into a new plant which is genetically identical to the plant from which the cutting was taken.

Embryo transplants

In this process, an embryo forms naturally by sexual reproduction. The developing embryo can be split into individual cells which will multiply into individual embryos and can be implanted into surrogate mothers, to produce multiple clones. They will be clones of each other, but not genetically identical to either of the parents.

The disadvantages associates with embryo splitting are:

• We cannot predict the specific traits of the clones (as the embryos contain a mixture of DNA from both parents).

• It has a low success rate.

Adult cell cloning

Dolly the sheep was the first mammal to be cloned (in the UK in 1996). Adult cell cloning has been used since to create genetically identical organisms for research purposes. It involves taking a body cell, such as a skin cell and extracting the nucleus. We then take an egg cell from another organism and discard the nucleus. The nucleus that was removed from the body cell is placed into the empty egg cell using an electric shock. The cell divides by mitosis to form a ball of cells called an embryo, which is implanted into the uterus of a surrogate mother.

Adult cell cloning has a number of disadvantages:

• Cloned animals can have developmental problems

• Reduces the gene pool - this means there isn’t much genetic differences between organisms, making them equally vulnerable to a certain disease

• Religious/ethical considerations - do humans have the right to create life?

Next Page: Understanding of Genetics and Theory of Evolution