Welcome to the Amazing World of Genetics and Evolution!
Have you ever wondered why you have the same eye colour as your parents, or why some dogs are tiny while others are huge? In this chapter, we are going to explore the "instruction manual" for life and find out how animals and plants change over time. Don't worry if it sounds like a lot—we'll take it one step at a time!
1. Inheritance: The Code of Life
Inside almost every cell in your body, there is a control centre called the nucleus. Inside this nucleus is all the genetic information needed to build and maintain an organism.
What is DNA?
Imagine a giant instruction manual for building a living thing. In living cells, that manual is called DNA (deoxyribonucleic acid). DNA is a long, coiled molecule that carries genetic instructions. It looks like a twisted ladder, a shape scientists call a double helix.
The Discovery of the DNA Model
Working out the structure of DNA in the 1950s was one of the greatest scientific breakthroughs in history. It was a team effort involving several key scientists:
- Rosalind Franklin and Maurice Wilkins used X-ray techniques to take detailed photographs showing the shape of DNA.
- James Watson and Francis Crick used these data and built physical models, figuring out that DNA is structured as a double helix.
Chromosomes and Genes
Because DNA molecules are very long, they are tightly packed inside the nucleus:
- Chromosomes: Long strands of coiled DNA. Human body cells have 46 chromosomes (23 pairs: 23 inherited from your mother and 23 from your father).
- Genes: Short sections of DNA found along a chromosome. Each gene carries instructions to produce a specific characteristic, such as eye colour.
Quick Memory Aid: Think of it like a computer:
- The Cell is the computer.
- The Nucleus is the hard drive.
- DNA is the master programming code.
- Genes are the individual apps or programs.
2. Variation: Why We Are All Unique
Variation means the differences between individuals of the same species. There are two causes of variation:
Genetic Variation
This is the variation you inherit from your parents through DNA.
Examples: Eye colour, natural hair colour, blood group, and whether earlobes are attached or free.
Environmental Variation
This is caused by your surroundings and your lifestyle.
Examples: Scars, accents, having dyed hair, or plants growing towards the light.
Combined Variation: Many characteristics are influenced by both genes and the environment. For example, your genes influence your potential height, but your diet and health (environment) determine whether you reach that full height.
Continuous vs Discontinuous Variation
Scientists classify variation into two types based on how it can be measured and graphed:
- Discontinuous variation: Characteristics that fall into distinct, separate categories with no in-between values (for example, blood group: A, B, AB, or O). This type of data is represented using a bar chart with gaps between the bars.
- Continuous variation: Characteristics that can take any numerical value within a range and are measured on a scale (for example, height, body mass, or hand span). This data is plotted on a histogram or frequency graph, often forming a smooth, bell-shaped curve where most individuals are close to the average.
3. Natural Selection and Evolution
Evolution is the change in the inherited characteristics of a population over many generations. The driving force behind evolution is natural selection.
How Natural Selection Works (Step-by-Step):
1. Variation: Individuals within a species show natural differences in their characteristics (e.g. some rabbits have thicker fur than others).
2. Competition: Organisms compete for limited resources, such as food, water, mates, and space to live, and must avoid predators.
3. Survival: Organisms with adaptations best suited to their environment are more likely to survive and compete successfully.
4. Reproduction: The surviving individuals reproduce and pass on the genes for those advantageous features to their offspring.
5. Generations: Over many generations, these helpful characteristics become more common in the population.
Common Mistake to Avoid: Individual organisms do not change their genes on purpose during their lifetime to adapt. Natural selection occurs across populations over generations because individuals with helpful variations survive and reproduce more successfully.
4. Extinction and Preserving Biodiversity
When the environment changes, a species must adapt. If a species cannot adapt quickly enough, individuals die, and if every member dies out, the species faces extinction.
Causes of Extinction
- Environmental changes: Rapid changes in climate or weather patterns.
- New predators or diseases: Introduced predators or new pathogens that the species has no defence against.
- Increased competition: Another species competing more successfully for food or territory.
- Human activity: Habitat destruction, pollution, or overhunting.
Maintaining Biodiversity and Gene Banks
Biodiversity is the variety of different living species in an area or on Earth. Maintaining high biodiversity keeps ecosystems stable and provides potential future resources for medicine and agriculture.
- Gene banks: Special storage facilities that preserve the genetic material of organisms to prevent extinction and maintain genetic diversity.
- Examples of gene banks: Seed banks (storing seeds of wild and crop plants in cold, dry conditions) and cryobanks (storing frozen animal sperm, eggs, or embryos). These allow scientists to breed plants or animals in the future if wild populations are threatened.
Summary: The Big Picture
- DNA is a double helix molecule in the nucleus; its structure was modelled by Watson, Crick, Wilkins, and Franklin.
- Genes are sections of DNA on chromosomes that determine inherited traits.
- Variation can be genetic or environmental, and is recorded as discontinuous (distinct groups on a bar chart) or continuous (numerical range on a histogram/curve).
- Natural selection drives evolution: organisms best suited to their environment compete successfully, survive, and pass on their genes.
- Extinction occurs when species cannot adapt to environmental changes; gene banks help preserve biodiversity.