Unit 14 Variation and selection
Key unit competence: Explain the causes and types of variation and how natural and artificial selection lead to adaptation and new species.
Key ideas
- Variation is the differences between individuals of the same species. It is caused by genes (inherited variation), by the environment (diet, climate, disease, exercise: acquired variation, not inherited), or by both.
- Sources of genetic variation: meiosis (crossing over and independent assortment), random fertilisation (any sperm with any egg) and mutation (the original source of new alleles).
- Continuous variation: a range of values with many intermediates, e.g. height, mass, skin colour, milk yield; controlled by many genes plus the environment; shown by a histogram and gives a normal (bell-shaped) distribution.
- Discontinuous variation: distinct categories with no intermediates, e.g. ABO blood groups, tongue rolling, sex, albinism; usually controlled by one or a few genes, little affected by environment; shown by a bar chart.
- A mutation is a change in the DNA (gene mutation) or in chromosome number or structure. Causes (mutagens): radiation (X-rays, UV, nuclear), some chemicals (tar, some pesticides), and random errors in DNA copying. Examples: sickle cell anaemia (gene mutation), Down syndrome (an extra chromosome 21: 47 chromosomes). Most mutations are harmful or neutral; a few are useful.
- Natural selection (Darwin): organisms produce more offspring than survive; there is variation; there is competition (struggle for existence); individuals with features better adapted to the environment survive and reproduce ("survival of the fittest") and pass on their alleles; over many generations the population changes.
- Examples of natural selection: antibiotic resistance in bacteria; insecticide resistance in mosquitoes; sickle cell trait more common where malaria is common (carriers are protected from severe malaria); dark peppered moths in polluted areas.
- Artificial selection (selective breeding): humans choose parents with desired features and breed them over many generations, e.g. high-milk Friesian crosses, disease-resistant cassava and beans, fast-growing chickens, dog breeds.
- Speciation is the formation of new species. A population is split by isolation (geographical, such as a mountain, lake or river; or reproductive); each group meets different conditions and undergoes different mutations and selection; over a long time they can no longer interbreed.
- Adaptations are features that help an organism survive in its environment, e.g. camel (wide feet, stores fat), cactus (spines, thick stem), fish (gills, streamlined), polar bear (thick fur, fat).
Common misconceptions
- "Characteristics acquired during life, such as big muscles, are passed to children." No: acquired characteristics are not inherited; only changes in gametes' DNA are passed on.
- "Bacteria become resistant because antibiotics teach them." No: a few bacteria already have resistance mutations; the antibiotic kills the others, so resistant ones survive and multiply.
- "All mutations are harmful." No: most are neutral or harmful, but some are useful and provide variation for evolution.
- "Natural selection means the strongest animals win." No: "fittest" means best adapted to the environment, not the strongest.
Worked examples
Example 1. Explain why the sickle cell allele is common in some regions where malaria is common.
Solution
People with sickle cell trait (HbᴬHbˢ) are more resistant to severe malaria. In malarial regions they survive and reproduce more than people with normal haemoglobin, so the allele Hbˢ is passed on and stays common: natural selection. In regions without malaria this advantage is lost.
Example 2. Explain how a population of mosquitoes becomes resistant to an insecticide.
Solution
By chance, a few mosquitoes have a mutation giving resistance. When the insecticide is sprayed, non-resistant ones die; the resistant ones survive and breed, passing the resistance allele to offspring. After many generations most of the population is resistant.
Practice questions
Distinguish between continuous and discontinuous variation with two examples of each.
Show answer
Continuous: range of values with intermediates, controlled by many genes and environment (height, mass, skin colour). Discontinuous: distinct categories, usually one gene (blood group, tongue rolling, sex).
Common mistake: Calling height discontinuous.
State three causes of genetic variation.
Show answer
Mutation; crossing over and independent assortment in meiosis; random fertilisation.
Common mistake: Including diet as a genetic cause.
Define mutation and name two mutagens.
Show answer
A change in the DNA or chromosomes of an organism. Mutagens: ionising radiation (X-rays, gamma rays), UV light, some chemicals (tar in tobacco, some pesticides).
Common mistake: Calling vitamins mutagens.
Outline Darwin's theory of natural selection.
Show answer
Overproduction of offspring; variation among them; competition for limited resources; the best adapted survive and reproduce; they pass on favourable alleles; over many generations the population changes.
Common mistake: Saying organisms change because they need to.
Give three differences between natural and artificial selection.
Show answer
Natural: environment selects, slow, features useful for survival. Artificial: humans select, faster, features useful to humans, may reduce fitness in the wild.
Common mistake: Saying both are done by farmers.
Explain how geographical isolation can lead to new species.
Show answer
A barrier divides a population; no interbreeding occurs; each group experiences different mutations and different natural selection; over a long time they change so much that they cannot interbreed: new species.
Common mistake: Leaving out the long time scale.
Give two examples of selective breeding in Rwanda.
Show answer
Crossbreeding local cows with Friesians for more milk (Girinka); improved disease-resistant cassava and bean varieties; high-yield maize; improved chicken breeds.
Common mistake: Giving natural selection examples.
Explain how antibiotic resistance develops in bacteria.
Show answer
A random mutation makes a few bacteria resistant; the antibiotic kills the sensitive ones; resistant bacteria survive, multiply and spread; misuse (not finishing courses, using antibiotics for viruses) speeds this up.
Common mistake: Saying the patient becomes resistant.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Height in humans is an example of: a. adaptation 2. A change in DNA is a: b. X-rays 3. A feature that helps an organism survive in its environment is an: c. continuous variation 4. Which is a mutagen? d. mutation Show answer
1 → c (continuous variation) · 2 → d (mutation) · 3 → a (adaptation) · 4 → b (X-rays)
- There are many intermediates.
- It is the source of new alleles.
- Example: thick fur in cold places.
- Radiation damages DNA.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
mutation · competition · gene mutation · can no longer produce fertile offspring- Two populations are separate species when they .
- The original source of new alleles is .
- Sickle cell anaemia is caused by a .
Show answer
1: can no longer produce fertile offspring · 2: mutation · 3: gene mutation
- Reproductive isolation defines species.
- Meiosis then shuffles them.
- A change in the haemoglobin gene.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. ABO blood groups are an example of: a. an extra chromosome 21 2. Separation of populations by a mountain or lake is: b. geographical isolation 3. The formation of new species is: c. discontinuous variation 4. Down syndrome is caused by: d. speciation Show answer
1 → c (discontinuous variation) · 2 → b (geographical isolation) · 3 → d (speciation) · 4 → a (an extra chromosome 21)
- There are distinct groups.
- Gene flow stops.
- Isolation is usually needed.
- Cells have 47 chromosomes.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
a single gene, giving discontinuous variation · artificial selection by farmers · best adapted individuals · natural selection of resistance- Tongue rolling is controlled by .
- "Survival of the fittest" means survival of the .
- Bacteria that survive antibiotics show .
Show answer
1: a single gene, giving discontinuous variation · 2: best adapted individuals · 3: natural selection of resistance
- People can or cannot roll.
- Fitness means suitability to the environment.
- Resistant bacteria multiply.