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Variation and Evolution

Part of 4.6 Inheritance, Variation and Evolution.

Variation and evolution explain why individuals differ and how populations change over time. Human intervention through breeding and genetic engineering sits alongside natural selection in this topic.

Learning Objectives

ID Official specification wording Main teaching sections
4.6.2-lo-1 4.6.2.1 Students should be able to describe simply how the genome and its interaction with the environment influence the development of the phenotype of an organism.
4.6.2.1 Differences in the characteristics of individuals in a population is called variation and may be due to differences in:
4.6.2.1 • the genes they have inherited (genetic causes)
4.6.2.1 • the conditions in which they have developed (environmental causes)
4.6.2.1 • a combination of genes and the environment.
4.6.2.1 Students should be able to:
4.6.2.1 • state that there is usually extensive genetic variation within a population of a species
4.6.2.1 • recall that all variants arise from mutations and that: most have no effect on the phenotype; some influence phenotype; very few determine phenotype.
4.6.2.1 Mutations occur continuously. Very rarely a mutation will lead to a new phenotype. If the new phenotype is suited to an environmental change can lead to a relatively rapid change in the species.		 Variation and Its Causes
4.6.2-lo-2 4.6.2.2 Students should be able to describe evolution as a change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species.
4.6.2.2 The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.
4.6.2.2 Students should be able to explain how evolution occurs through natural selection of variants that give rise to phenotypes best suited to their environment.
4.6.2.2 If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring they have formed two new species.		 Natural Selection and Evolution
4.6.2-lo-3 4.6.2.3 Students should be able to explain the impact of selective breeding of food plants and domesticated animals.
4.6.2.3 Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics. Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals.
4.6.2.3 Selective breeding involves choosing parents with the desired characteristic from a mixed population. They are bred together. From the offspring those with the desired characteristic are bred together. This continues over many generations until all the offspring show the desired characteristic.
4.6.2.3 The characteristic can be chosen for usefulness or appearance:
4.6.2.3 • Disease resistance in food crops.
4.6.2.3 • Animals which produce more meat or milk.
4.6.2.3 • Domestic dogs with a gentle nature.
4.6.2.3 • Large or unusual flowers.
4.6.2.3 Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects.		 Selective Breeding (Artificial Selection)
4.6.2-lo-4 4.6.2.4 Students should be able to describe genetic engineering as a process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic.
4.6.2.4 Plant crops have been genetically engineered to be resistant to diseases or to produce bigger better fruits.
4.6.2.4 Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes.
4.6.2.4 Students should be able to explain the potential benefits and risks of genetic engineering in agriculture and in medicine and that some people have objections.
4.6.2.4 In genetic engineering, genes from the chromosomes of humans and other organisms can be ‘cut out’ and transferred to cells of other organisms.
4.6.2.4 Crops that have had their genes modified in this way are called genetically modified (GM) crops. GM crops include ones that are resistant to insect attack or to herbicides. GM crops generally show increased yields.
4.6.2.4 Concerns about GM crops include the effect on populations of wild flowers and insects. Some people feel the effects of eating GM crops human health have not been fully explored.
4.6.2.4 Modern medical research is exploring the possibility of genetic modification to overcome some inherited disorders.
4.6.2.4 (HT only) Students should be able to describe the main steps in the process of genetic engineering.
4.6.2.4 (HT only) In genetic engineering:
4.6.2.4 • enzymes are used to isolate the required gene; this gene is inserted into a vector, usually a bacterial plasmid or a virus
4.6.2.4 • the vector is used to insert the gene into the required cells
4.6.2.4 • genes are transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristics.		 Genetic Engineering
4.6.2-lo-5 4.6.2.5 Tissue culture: using small groups of cells from part of a plant to grow identical new plants. This is important for preserving rare plant species or commercially in nurseries.
4.6.2.5 Cuttings: an older, but simple, method used by gardeners to produce many identical new plants from a parent plant.
4.6.2.5 Embryo transplants: splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers.
4.6.2.5 Adult cell cloning:
4.6.2.5 • The nucleus is removed from an unfertilised egg cell.
4.6.2.5 • The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell.
4.6.2.5 • An electric shock stimulates the egg cell to divide to form an embryo.
4.6.2.5 • These embryo cells contain the same genetic information as the adult skin cell.
4.6.2.5 • When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.		 Cloning

Variation and Its Causes

Variation is the differences in phenotype between individuals of the same species. It arises from three sources:

  1. Genetic variation — differences in the genes inherited from parents. Some characteristics are controlled by genes alone (e.g. blood type, tongue rolling ability, natural eye colour).
  2. Environmental variation — acquired characteristics caused by the surroundings (e.g. accent, muscle mass developed through training).
  3. Genetic + environmental — most characteristics depend on both (e.g. height depends on genes but also on nutrition).

Continuous variation: a range of values with no clear categories, forming a bell curve (e.g. height, weight). Discontinuous variation: distinct, separate categories with no intermediate values (e.g. blood group A, B, AB or O; ability to roll tongue).

Mutations — random changes in DNA — are an additional source of new variation within a species.

Natural Selection and Evolution

Evolution is a change in allele frequency within a population over many generations. It operates through natural selection:

  1. There is variation in a population.
  2. There is competition for limited resources.
  3. Individuals with characteristics better suited to the environment are more likely to survive and reproduce.
  4. These individuals pass on the alleles for those advantageous characteristics.
  5. Over many generations, the frequency of those alleles increases in the population.

This process is also called survival of the fittest — though "fittest" means best adapted, not physically strongest.

Selective Breeding (Artificial Selection)

Selective breeding is when humans choose parents with desirable traits to breed together, deliberately increasing those traits in future generations. This is also called artificial selection.

Process:

  1. Identify the desired characteristic in a mixed population.
  2. Select individuals with that characteristic.
  3. Breed those individuals together.
  4. From the offspring, again select those with the desired trait and breed them.
  5. Repeat over many generations.

Examples:

  • Crops: larger fruits, disease resistance, high yield, drought tolerance, extended shelf life.
  • Livestock: increased milk yield in dairy cattle, more meat in beef cattle, larger eggs in chickens.
  • Pets: gentle temperament, specific coat types.

Advantages:

  • Predictably improves desired characteristics.
  • Has produced much of the food we depend on.

Disadvantages:

  • Reduces genetic diversity (gene pool narrows — called inbreeding depression).
  • Accumulation of harmful recessive alleles.
  • Can produce animals with health problems (e.g. breathing difficulties in brachycephalic dog breeds).

Genetic Engineering

Genetic engineering (or genetic modification) involves removing a gene from one organism's genome and inserting it into another organism, giving that organism a new characteristic.

Why bacteria are useful in genetic engineering:

  • Reproduce rapidly.
  • Can make complex proteins.
  • Contain plasmids — small circular pieces of DNA that are easy to modify and transfer.
  • Shared genetic code with all life means human genes can be expressed in bacteria.

Key steps (example: engineering bacteria to produce insulin):

  1. The insulin gene is cut from a human cell's DNA using restriction enzymes — these cut at specific base sequences and leave "sticky ends" of unpaired bases.
  2. A bacterial plasmid is cut with the same restriction enzyme, leaving complementary sticky ends.
  3. The insulin gene is inserted into the plasmid; ligase enzymes join the sticky ends together.
  4. The modified plasmid is inserted into bacteria.
  5. Bacteria that have successfully taken up the plasmid (identified using antibiotic resistance markers) are grown in fermenters to produce insulin in bulk.

Applications:

  • Insulin production — bacteria engineered to produce human insulin, replacing less reliable animal sources.
  • GM crops — e.g. herbicide-resistant soya, pest-resistant maize, Golden Rice (engineered with beta-carotene to combat vitamin A deficiency).
  • Medical research — potential treatments for HIV, sickle cell anaemia, cystic fibrosis.

Concerns:

  • Potential allergens or toxins in GM food.
  • Ethical concerns about "playing God" or releasing GM organisms into ecosystems.
  • Modified genes could spread to wild species (e.g. weed plants gaining herbicide resistance).
  • Unknown long-term effects.

Cloning

Cloning produces organisms or cells that are genetically identical to one another.

Plant cloning:

Cuttings — a piece of stem or leaf is cut from a plant and allowed to grow roots; produces an identical copy of the parent. Simple and inexpensive.

Tissue culture (micropropagation) — very small pieces of plant tissue (explants) are grown on sterile agar containing hormones and nutrients. Produces thousands of identical plants quickly. Useful for conserving rare species or producing commercial quantities of desirable varieties.

Animal cloning:

Embryo transplantation — a fertilised embryo is allowed to develop into a ball of cells, which are then separated and implanted into different surrogate mothers. All offspring are genetically identical.

Adult cell cloning (somatic cell nuclear transfer): 1. The nucleus is removed from an adult (donor) body cell. 2. The nucleus is inserted into an egg cell from which the nucleus has been removed. 3. An electric shock stimulates the egg to divide. 4. The developing embryo is implanted into a surrogate mother. 5. The offspring is genetically identical to the donor of the original nucleus.

This technique produced Dolly the Sheep (1996, Edinburgh) — the first cloned mammal.

Advantages of cloning:

  • Can preserve genetic characteristics of organisms with desirable traits.
  • Could help prevent extinction of endangered species.
  • Medical: can produce tissues for transplantation.

Disadvantages:

  • Reduces genetic diversity → greater vulnerability to disease.
  • Adult cell cloning is technically difficult with low success rates.
  • Ethical concerns about animal welfare and potential application to humans.

Common Confusions

  • Continuous vs discontinuous variation: height is continuous (infinite range); blood group is discontinuous (four distinct categories). The difference is whether there are intermediates.
  • Selective breeding vs natural selection: both involve survival of the fittest in some sense, but selective breeding is driven by human choice, not environmental pressure.
  • Restriction enzymes vs ligase enzymes: restriction enzymes cut DNA; ligase enzymes join it. These are opposite functions.
  • GM vs selective breeding: selective breeding works with existing alleles in a species; genetic engineering transfers genes between species, which cannot happen through breeding.

Key Terms

  • Variation: differences between individuals of the same species.
  • Natural selection: the process by which advantageous inherited characteristics become more common over generations.
  • Selective breeding: choosing parents with desired traits so those traits appear more often in offspring.
  • Genetic engineering: changing an organism by inserting a useful gene from another organism into its DNA.
  • Clone: an organism or cell that is genetically identical to another.
  • Mutation: a random change in DNA that can create a new allele.
  • Restriction enzyme: an enzyme that cuts DNA at specific base sequences; used to isolate genes in genetic engineering.
  • Ligase enzyme: an enzyme that joins DNA strands together; used to insert genes into plasmids in genetic engineering.
  • Plasmid: a small circular piece of DNA in bacteria, used as a vector in genetic engineering.
  • Continuous variation: a range of phenotypes between two extremes with no distinct categories (e.g. height).
  • Discontinuous variation: distinct categories with no intermediates (e.g. blood group).
  • Inbreeding depression: reduced fitness in a population due to the accumulation of harmful recessive alleles from selective breeding.
  • Tissue culture: growing new plants or animal cells from small tissue samples in a sterile growth medium.
  • Adult cell cloning: cloning an organism by inserting an adult cell nucleus into an enucleated egg cell.

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