Unit 2: Body Systems, Genetics, Microorganisms and Health
Chapter: Variation and Natural Selection
Welcome to one of the most fascinating topics in Biology! Have you ever looked around your classroom and wondered why everyone looks different, even though we are all the same species? Or why some bacteria can survive the medicines doctors give us? In this chapter, you will learn how differences arise in living organisms and how these differences drive the process of natural selection and evolution. Don't worry if some terms feel new at first — we will break down each concept step by step!
1. Understanding Variation
Variation refers to the differences that exist between individuals of the same species. No two humans (except identical twins) are completely identical!
Types of Variation
Variation falls into two main categories depending on whether the characteristic can be measured or placed into distinct groups:
A. Continuous Variation
• Definition: Characteristics that show a gradual change across a continuous range with no distinct categories.
• Key feature: There are no clear cut-off points; individuals can have any value within a range.
• Examples: Height, body mass, hand span, shoe size.
• Causes: Usually controlled by many genes (polygenic) acting together, combined with environmental influences (for example, diet affects height and mass).
• Graph: Plotted as a histogram or a smooth, bell-shaped normal distribution curve.
B. Discontinuous Variation
• Definition: Characteristics that fall into distinct, separate categories with no in-between values.
• Key feature: You either have the trait or you do not.
• Examples: ABO blood groups (\(A\), \(B\), \(AB\), or \(O\)), ability to roll the tongue, attached vs. unattached earlobes.
• Causes: Usually controlled by one gene (or a very small number of genes) with no environmental influence.
• Graph: Plotted as a bar chart with gaps between the bars.
Quick Comparison
• Continuous: Measured in numbers (quantitative), broad range of values, influenced by genes + environment.
• Discontinuous: Grouped in categories (qualitative), distinct groups, influenced by genes only.
Key Takeaway: If you can measure it with a ruler, scale, or tape (like height), it is continuous. If you can only tick a box (like blood group), it is discontinuous.
2. Causes of Variation
Why do individuals differ? There are two primary sources:
1. Genetic Causes:
Differences are inherited from parents through genes. During sexual reproduction, gametes (sperm and egg) combine randomly, creating unique combinations of alleles in the offspring. Examples include natural eye colour and blood group.
2. Environmental Causes:
Differences are caused by conditions in an organism's surroundings, such as diet, climate, physical activity, sunlight, or lifestyle. Examples include scars, tattoos, and language spoken.
3. Combined Causes (Genetics + Environment):
Most characteristics in humans are a mix of both! For instance, your genes determine your potential maximum height, but poor nutrition during childhood can prevent you from reaching that potential.
3. Mutations: The Source of New Variation
A mutation is a random, spontaneous change in the structure of a gene or the number of chromosomes in an organism.
What Causes Mutations?
Mutations occur naturally at a very low rate, but the rate can be increased by exposure to environmental agents called mutagens:
• Ionising radiation: Ultraviolet (UV) rays from sunlight, X-rays, gamma rays, and radioactive emissions.
• Chemicals: Tar in cigarette smoke and certain industrial toxins.
The Effects of Mutations
• Neutral: Most mutations have no noticeable effect on the organism.
• Harmful: Many mutations disrupt essential body processes and cause genetic disorders.
• Beneficial: Occasionally, a mutation gives an organism an advantage (e.g., antibiotic resistance in bacteria or camouflage in animals).
Two Important Genetic Conditions to Know:
1. Down's Syndrome (Chromosome Mutation):
• Normal human body cells have \(46\) chromosomes (\(23\) pairs).
• Down's syndrome is caused by non-disjunction during gamete formation, resulting in an egg or sperm having an extra chromosome.
• The resulting individual has \(47\) chromosomes in their body cells instead of \(46\) (specifically, three copies of chromosome \(21\)).
• Features include characteristic facial features, reduced muscle tone, and varying degrees of learning difficulty.
2. Cystic Fibrosis (Gene Mutation):
• Caused by a recessive allele resulting from a mutation in a specific gene.
• Causes the body to produce thick, sticky mucus that clogs the lungs and blocks the digestive enzymes from the pancreas.
• An individual must inherit two copies of the faulty allele (one from each carrier parent) to suffer from the condition.
Key Takeaway: Mutations are random. While some cause harmful disorders like cystic fibrosis or Down's syndrome, rare beneficial mutations provide the raw material for evolution.
4. Natural Selection and Evolution
Evolution is the gradual change in the inherited characteristics of a population over many generations.
The mechanism that drives evolution was proposed by Charles Darwin (along with Alfred Russel Wallace) and is known as Natural Selection.
The Step-by-Step Process of Natural Selection
To score full marks on exam questions about natural selection, follow these five logical steps:
Step 1: Overproduction & Competition
Organisms produce more offspring than the environment can support. This leads to a struggle for survival as individuals compete for limited resources such as food, water, light, space, and mates.
Step 2: Variation
Within any population, there is natural variation in traits caused by different alleles (arising from mutations).
Step 3: Survival of the Fittest
Individuals with characteristics (phenotypes) best suited to their environment have a selective advantage. They are more likely to survive predators, disease, and harsh conditions.
Step 4: Reproduction
The surviving, better-adapted individuals are more likely to reproduce successfully.
Step 5: Passing on Advantageous Alleles
These survivors pass their favourable alleles on to their offspring. Over many generations, the frequency of these advantageous alleles increases in the population, leading to evolution.
Memory Aid: The 5 Steps of Natural Selection
Remember the acronym V-C-S-R-A:
• V — Variation exists in the population.
• C — Competition for limited resources.
• S — Survival of the best adapted ("fittest").
• R — Reproduction of survivors.
• A — Alleles passed on to the next generation.
5. Real-World Examples of Natural Selection
Example 1: Antibiotic Resistance in Bacteria
This is one of the clearest and most important examples in modern medicine:
• 1. A population of bacteria infects a person. Random mutations produce genetic variation, creating a few individual bacteria that happen to be resistant to an antibiotic.
• 2. The patient takes the antibiotic.
• 3. The antibiotic kills the normal, non-resistant bacteria.
• 4. The resistant bacteria survive the treatment because of their protective mutation.
• 5. With all competition removed, the resistant bacteria multiply rapidly and pass the resistance allele to their offspring.
• 6. Soon, the entire population becomes resistant (forming a "superbug" like MRSA), and the antibiotic is no longer effective.
Did you know? Doctors avoid overprescribing antibiotics and advise patients to always complete their full course of medication to prevent resistant strains from surviving and spreading!
Example 2: The Peppered Moth
• Before the Industrial Revolution, light-coloured peppered moths were common because they camouflaged against pale tree bark and lichens.
• During the Industrial Revolution, soot covered the trees. Dark (melanic) moths, which arose from a mutation, were now camouflaged against the soot-covered trees, while light moths were easily spotted and eaten by birds.
• Dark moths survived, reproduced, and passed on the allele for dark colour. Over time, dark moths became the most common type.
Key Takeaway: Natural selection does not mean organisms "choose" to change; instead, changing environments select which existing variations survive!
6. Selective Breeding (Artificial Selection)
While natural selection happens on its own in nature, humans have used a similar process called selective breeding (artificial selection) for thousands of years to produce domesticated plants and animals with desirable traits.
How Selective Breeding Works:
• 1. Humans choose parents with the desired characteristics (e.g., high milk yield in cows, disease resistance in crops, gentle temperament in dogs).
• 2. Breed these chosen parents together.
• 3. From the offspring, select only those that show the desired traits.
• 4. Breed these selected offspring together.
• 5. Repeat this process over many successive generations until all offspring consistently show the desired characteristic.
Natural Selection vs. Selective Breeding
• Natural Selection: Driven by environmental pressures; features chosen benefit the organism's survival in the wild; happens slowly over very long periods.
• Selective Breeding: Driven by human choice; features chosen benefit human needs (and may even reduce the animal's survival in the wild); happens much more rapidly.
7. Extinction
Extinction occurs when an entire species permanently ceases to exist because there are no surviving individuals left on Earth.
Causes of Extinction:
If an environment changes faster than a species can adapt through natural selection, the species may become extinct. Major causes include:
• Climate change: Rapid changes in temperature or rainfall patterns.
• Habitat destruction: Deforestation, urbanization, and loss of feeding grounds.
• New diseases: Pathogens against which the species has no natural immunity.
• New predators: Introduction of non-native species (invasive species).
• Increased competition: Being outcompeted for food or shelter by better-adapted species.
Quick Chapter Summary & Exam Checklist
Make sure you are confident with these key points before your exam:
• Variation: Can be continuous (measured, range, histogram) or discontinuous (categories, bar chart).
• Sources of variation: Genetic, environmental, or both.
• Mutations: Random DNA/chromosome changes caused by radiation or mutagens. Example: Down's syndrome (\(47\) chromosomes).
• Natural Selection (V-C-S-R-A): Overproduction \(\implies\) Competition \(\implies\) Variation \(\implies\) Survival of fittest \(\implies\) Reproduction \(\implies\) Passing of alleles.
• Antibiotic Resistance: A classic example of natural selection occurring in real time.
• Selective Breeding: Controlled by humans over generations to select useful traits.