Natural Selection and Genetic Modification

What you'll learn

This topic explores how species evolve through natural selection and how humans manipulate genetic variation through selective breeding and genetic modification. You'll understand Darwin's theory of evolution, evidence supporting it, and how antibiotic resistance develops. You'll also learn about modern genetic engineering techniques and their applications in medicine and agriculture.

Key terms and definitions

Natural selection — the process by which organisms best adapted to their environment are more likely to survive, reproduce and pass on their advantageous characteristics to their offspring

Evolution — the gradual change in the inherited characteristics of a population over time through natural selection

Antibiotic resistance — when bacteria develop the ability to survive exposure to antibiotics that would normally kill them or stop their growth

Selective breeding — the process by which humans breed plants and animals for particular desirable characteristics

Genetic modification (genetic engineering) — the process of changing the genetic material of an organism by removing, changing or inserting individual genes

Gene therapy — the insertion of genes into an individual's cells to treat genetic diseases

Plasmid — a small, circular piece of DNA found in bacterial cells, used as a vector in genetic engineering

GMO (Genetically Modified Organism) — an organism whose genome has been altered using genetic engineering techniques

Core concepts

Darwin's theory of natural selection

Charles Darwin developed the theory of evolution by natural selection in the mid-19th century. The theory explains how species change over time through the following steps:

1. Variation exists — individuals within a species show genetic variation due to mutations and sexual reproduction
2. Competition — organisms produce more offspring than the environment can support, so they compete for limited resources (food, water, shelter, mates)
3. Selection — individuals with characteristics best suited to their environment are more likely to survive (survival of the fittest)
4. Reproduction — survivors reproduce and pass on advantageous alleles to their offspring
5. Inheritance — over many generations, advantageous characteristics become more common in the population

Natural selection takes place over very long time periods, often thousands or millions of years for significant changes to occur. However, in organisms with short generation times (like bacteria), evolution can occur much faster.

Evidence for evolution

Several lines of evidence support Darwin's theory:

Fossils provide a record of organisms that lived in the past. The fossil record shows:

• Simpler organisms appear in older rock layers
• More complex organisms appear in younger rocks
• Gradual changes in species over time
• Species that no longer exist (extinction)

Antibiotic resistance in bacteria demonstrates natural selection occurring rapidly:

• Random mutations in bacterial DNA sometimes produce resistance to antibiotics
• When antibiotics are used, non-resistant bacteria die
• Resistant bacteria survive and reproduce
• The resistant population increases rapidly (bacteria reproduce every 20 minutes)
• MRSA (methicillin-resistant Staphylococcus aureus) is a well-known example

This is why completing antibiotic courses is essential — stopping early leaves resistant bacteria alive to multiply.

Development of antibiotic resistance

The development of antibiotic resistance follows the natural selection model precisely:

Stage 1: Mutation

• A random mutation occurs in bacterial DNA
• This mutation might code for an enzyme that breaks down the antibiotic, or changes in the bacterial cell wall that prevents antibiotic entry

Stage 2: Selection pressure

• When antibiotics are present, they kill non-resistant bacteria
• The mutant bacterium survives because it has resistance

Stage 3: Reproduction

• The resistant bacterium reproduces rapidly by binary fission
• All offspring inherit the resistance gene
• Bacteria can also transfer resistance genes between cells via plasmids (horizontal gene transfer)

Stage 4: Population change

• The resistant strain becomes dominant
• The antibiotic is no longer effective against this bacterial population

Preventing antibiotic resistance:

• Only use antibiotics when necessary (not for viral infections)
• Complete the full course of antibiotics
• Avoid using antibiotics in agriculture unnecessarily
• Develop new antibiotics to replace ineffective ones

Selective breeding (artificial selection)

Selective breeding involves humans choosing organisms with desirable characteristics to breed together. This has been practiced for thousands of years in agriculture.

Process of selective breeding:

1. Identify organisms with desired characteristics
2. Breed them together
3. Select offspring with the best characteristics
4. Continue breeding over many generations
5. Desired traits become more common

Examples of selective breeding:

Food crops:

• Wheat bred for high grain yield and disease resistance
• Tomatoes bred for uniform ripening and longer shelf life
• Bananas bred for seedlessness and sweeter taste

Livestock:

• Cattle bred for high meat or milk yield
• Chickens bred for high egg production or meat
• Racehorses bred for speed and stamina

Limitations of selective breeding:

• Reduces genetic variation in the population (gene pool becomes smaller)
• Can lead to accumulation of harmful recessive alleles (inbreeding)
• Takes many generations to achieve desired results
• Cannot introduce completely new characteristics not present in the species

Genetic modification (genetic engineering)

Genetic modification allows scientists to transfer genes between different species, creating organisms with new characteristics. This is faster and more precise than selective breeding.

Basic process of genetic modification:

1. Isolation — identify and remove the desired gene from an organism's DNA using restriction enzymes (biological scissors)
2. Insertion — insert the gene into a vector (usually a bacterial plasmid or virus)
3. Transformation — introduce the vector into target cells
4. Identification — identify which cells have successfully taken up the new gene
5. Cloning — clone the modified cells to produce many copies

Examples of genetic modification:

Medicine:

• Human insulin production — the human insulin gene is inserted into bacteria, which then produce human insulin in fermenters for diabetes treatment (previously insulin came from pigs/cows)
• Gene therapy — inserting functional genes into patients' cells to treat genetic disorders
• Vaccine production — genetically modified organisms produce antigens for vaccines

Agriculture:

• Herbicide-resistant crops — plants modified to survive herbicide spraying, killing only weeds
• Pest-resistant crops — Bt crops contain a bacterial gene producing insecticide, reducing pesticide use
• Golden Rice — rice modified to produce beta-carotene (vitamin A precursor) to prevent deficiency diseases in developing countries
• Longer shelf-life tomatoes — modified to ripen more slowly

Advantages of genetic modification:

• Introduces new characteristics not present in the species
• Faster than selective breeding
• More precise control over characteristics
• Can improve crop yields and nutritional value
• Reduces need for pesticides

Disadvantages and concerns:

• Long-term effects on health unknown
• Modified genes might spread to wild populations
• Reduces biodiversity
• Ethical concerns about manipulating living organisms
• Concern over large corporations controlling food supply through patents
• Religious/cultural objections

Gene therapy

Gene therapy is the insertion of genes into a person's cells to treat inherited disorders caused by faulty genes.

Process:

1. Functional copy of the gene is isolated
2. Gene inserted into a vector (often a disabled virus)
3. Vector introduced into patient's cells
4. Cells produce the correct protein

Types:

• Somatic gene therapy — genes inserted into body cells (not inherited by offspring)
• Germline gene therapy — genes inserted into gametes or embryo cells (would be inherited — currently not permitted in humans)

Challenges:

• Difficult to deliver genes to correct cells
• Body's immune system may attack modified cells
• Effects may be temporary
• Ethical concerns, especially regarding germline therapy

Worked examples

Example 1: Explain how antibiotic resistance in bacteria provides evidence for natural selection. (4 marks)

Mark scheme answer:

• Bacteria show variation/some have mutations that make them resistant to antibiotics (1)
• When antibiotics are used, non-resistant bacteria die but resistant bacteria survive (1)
• Resistant bacteria reproduce and pass on resistance genes to offspring (1)
• Over time, the population becomes mainly resistant bacteria/evolution has occurred (1)

Example 2: A farmer wants to breed cows that produce high milk yields. Describe how the farmer could use selective breeding to achieve this. (4 marks)

Mark scheme answer:

• Select/identify cows that produce the highest milk yields (1)
• Breed these cows together (1)
• From offspring, select those with highest milk yield (1)
• Continue breeding over many generations (1)

Example 3: Scientists have genetically modified bacteria to produce human insulin. Describe the main steps in this process. (5 marks)

Mark scheme answer:

• The human insulin gene is identified and isolated/cut from human DNA (1)
• The gene is inserted into a bacterial plasmid/vector using enzymes (1)
• The plasmid is inserted into bacterial cells (1)
• Bacteria that have taken up the gene are identified/selected (1)
• Bacteria reproduce/are cloned and produce human insulin (1)

Common mistakes and how to avoid them

• Confusing natural selection with evolution — Natural selection is the mechanism by which evolution occurs. Evolution is the outcome (change in population over time).

• Saying organisms "adapt" during their lifetime — Individual organisms do not change their characteristics to suit their environment. Variation already exists through mutation; selection acts on this variation.

• Thinking antibiotic resistance develops because bacteria "get used to" antibiotics — Resistance arises through random mutation, not because exposure causes resistance. The antibiotic kills non-resistant bacteria, allowing resistant ones to dominate.

• Confusing selective breeding with genetic modification — Selective breeding works with existing variation within a species; genetic modification transfers genes between different species.

• Using vague language — Don't say organisms "become resistant" or "change." Be specific: mutations produce variation, selection acts on variation, advantageous alleles are inherited.

• Forgetting the role of reproduction in natural selection — Survival alone isn't enough; organisms must reproduce and pass on alleles for evolution to occur.

Exam technique for "Natural Selection and Genetic Modification"

• Command words matter: "Describe" requires you to state features or processes; "Explain" requires reasons or mechanisms with linking words like "because," "therefore," "so that."

• Use the step-by-step approach for natural selection questions: variation → competition → selection → reproduction → inheritance. Include the cause (mutation) and timescale where appropriate.

• Show you understand the genetic basis: Reference genes, alleles, and DNA where appropriate. Natural selection works on inherited characteristics, so demonstrate understanding that characteristics are coded for by genes.

• In evaluation questions about genetic modification, present balanced arguments covering advantages and disadvantages, then reach a justified conclusion based on the context given in the question.

Quick revision summary

Natural selection explains evolution: variation exists through mutation; individuals best adapted to their environment survive, reproduce and pass on advantageous alleles. Over many generations, characteristics change. Antibiotic resistance demonstrates rapid natural selection in bacteria. Selective breeding involves humans choosing organisms with desirable traits to breed over generations. Genetic modification transfers genes between species using enzymes and vectors, producing GMOs with new characteristics for medicine and agriculture. Both techniques have benefits and limitations.