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Variation and Evolution

Part of 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.

What You Need to Learn

Explain why members of a species differ and how inherited differences can change a population over generations. Compare natural selection with the ways people choose, modify or copy organisms for useful characteristics. Weigh benefits against risks; Higher Tier work also follows the transfer of a gene into cells using a vector.

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).
  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, often forming a bell-shaped distribution (e.g. height, weight). Discontinuous variation: distinct, separate categories with no intermediate values (e.g. blood group A, B, AB or O).

Mutations are changes in DNA that create new genetic variants. Most populations contain extensive inherited variation. Most mutations do not affect phenotype, some influence it and very few produce a major new characteristic. A rare beneficial variant can spread relatively quickly when the environment changes in its favour.

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.

All present-day species share ancestry with simple organisms that existed more than three billion years ago. If populations become sufficiently different that they can no longer interbreed to produce fertile offspring, they are separate species. The formation of new species is explored in the development of evolutionary ideas.

Change the climate in the model below and watch which inherited fur types become more common over generations. The food dots and survival rules simplify many interacting pressures; they are not measurements of a real population. Explain why a population changes through differing survival and reproduction, rather than individuals developing the fur they need. Open full interactive.

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.
  • Ornamental plants: large or unusual flowers.

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 — Higher Tier (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.

A vector carries the selected gene into the target cells; a plasmid or a virus can be used. In plants and animals, gene transfer at an early stage of development allows the organism to develop with the desired characteristic.

For Higher Tier, follow a gene into a plasmid vector in the model below. Enzyme cuts and joining are drawn as simplified steps, rather than a full laboratory protocol. Explain how the vector helps a bacterial cell acquire the instructions to make human insulin. Open full interactive.

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).
  • Effects on wild flowers and insects, for example when herbicide-resistant crops allow spraying that removes plants supporting wildlife.
  • Questions about long-term effects must be evaluated using evidence, rather than assuming that all modified foods have the same risks.

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 mammal cloned from an adult body cell. Earlier mammals had been cloned from embryonic cells.

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 loop of DNA in bacteria, separate from the main chromosome.
  • 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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