Welcome to Variation and Evolution!
In this chapter, we are going to explore the story of life on Earth. We’ll look at why every living thing is a bit different (variation), how humans can "design" animals and plants (selective breeding and genetic engineering), and the incredible process of evolution that has been happening for billions of years.
Don’t worry if some of the names sound like a lot to take in—we’ll break them down into simple steps!
1. Variation: Why are we all different?
Variation refers to the differences in characteristics between individuals of the same species. There are three main causes for these differences:
- Genetic causes: The instructions (genes) you inherit from your parents. Example: Eye colour or blood group.
- Environmental causes: The conditions in which you live and grow. Example: A scar from an injury or the language you speak.
- A combination of both: Most things are a mix of genes and environment. Example: You might have the genes to be very tall, but if you don't eat a healthy diet, you won't reach your full height.
The Role of Mutations
Mutations are tiny, random changes in our DNA. They happen all the time!
- Most mutations have no effect on how an organism looks or functions.
- Some influence the phenotype (physical appearance).
- Very rarely, a mutation creates a brand new phenotype that gives an organism an advantage. If the environment changes, this "lucky" mutation can lead to a quick change in the species.
Quick Review: Variation is caused by genes, the environment, or both. Mutations are the source of all new genetic variation.
2. Evolution and Natural Selection
Evolution is the change in the inherited characteristics of a population over time. Scientists believe all species today evolved from simple life forms that first developed more than three billion years ago.
How does Natural Selection work?
Think of it as "survival of the best-suited." Here is the step-by-step process:
1. Individuals in a species show variation because of different genes.
2. Individuals with characteristics best suited to their environment are more likely to survive and breed successfully.
3. The genes that allowed these individuals to survive are passed on to the next generation.
New Species (Speciation)
If two populations of the same species become so different that they can no longer interbreed to produce fertile offspring, they have formed two new species.
Did you know? Charles Darwin is the most famous scientist associated with this theory. He published his ideas in a book called "On the Origin of Species" in 1859.
3. Selective Breeding
Selective breeding (or artificial selection) is when humans choose which animals or plants to breed so that the offspring have specific, desirable traits.
How it’s done:
1. Choose parents with the desired trait from a mixed population.
2. Breed them together.
3. Choose the "best" offspring and breed them together.
4. Repeat this over many generations until all offspring show the trait.
Common Examples:
- Disease-resistant food crops.
- Cows that produce more milk or meat.
- Domestic dogs with a gentle nature.
- Large or unusual flowers.
The Downside: Selective breeding can lead to inbreeding. This makes some breeds more likely to have inherited defects or diseases because the "gene pool" is so small.
4. Genetic Engineering
This is a modern process where scientists "cut and paste" a gene from one organism into another to give it a new characteristic.
- GM Crops: Plants can be engineered to be resistant to insects or herbicides (weed killers). They usually have higher yields.
- Medicine: Bacteria have been genetically engineered to produce human insulin to treat diabetes.
The Steps of Genetic Engineering (Higher Tier Only)
1. Enzymes are used to isolate (cut out) the required gene.
2. This gene is inserted into a vector (usually a bacterial plasmid or a virus).
3. The vector inserts the gene into the required cells.
4. This is done at an early stage of development (like an embryo) so that the organism grows with the desired characteristics.
Memory Aid: Think of the Vector as a delivery van that brings the new gene to the cell!
5. Cloning (Biology Only)
Cloning creates genetically identical copies of an organism.
Plant Cloning
- Cuttings: A simple, old-fashioned method where a piece of a plant is cut off and planted to grow a new one.
- Tissue Culture: Using small groups of cells from a plant to grow thousands of identical new plants. Great for preserving rare species.
Animal Cloning
- Embryo Transplants: Splitting apart cells from a developing animal embryo before they become specialised, then transplanting them into "host mothers."
- Adult Cell Cloning:
1. The nucleus is removed from an unfertilised egg cell.
2. The nucleus from an adult body cell (like a skin cell) is inserted into that egg.
3. An electric shock stimulates the egg to divide and form an embryo.
4. The embryo is put into the womb of an adult female to grow.
6. History of Genetics and Evolution (Biology Only)
The theory of evolution was not accepted immediately. Here is why:
- It challenged the idea that God made all animals and plants.
- There wasn't enough evidence at the time.
- The mechanism of inheritance (how traits pass down) wasn't understood for another 50 years.
Darwin vs. Lamarck
Jean-Baptiste Lamarck had a different idea. He thought that if an organism changed itself during its lifetime (like a giraffe stretching its neck), it could pass that change to its children. We now know this is incorrect in almost all cases.
Gregor Mendel
In the mid-19th century, a monk named Mendel carried out breeding experiments on pea plants. He realised that "units" (which we now call genes) were passed on to descendants. People didn't realise how important his work was until after he died!
7. Evidence for Evolution: Fossils and Antibiotics
We know evolution happened because of two main things: Fossils and Antibiotic Resistance.
Fossils
Fossils are the "remains" of organisms from millions of years ago. They form in three ways:
1. Parts of an organism do not decay (e.g., trapped in ice or amber).
2. Parts are replaced by minerals as they decay.
3. Preserved traces like footprints, burrows, or rootlet traces.
Why is the fossil record incomplete? Many early life forms were soft-bodied (no bones), so they left very few traces. Also, many fossils have been destroyed by geological activity (earthquakes and volcanoes).
Antibiotic Resistance in Bacteria
Bacteria evolve very quickly because they reproduce so fast.
1. A mutation makes one bacterium resistant to an antibiotic.
2. When you take the antibiotic, the normal bacteria die, but the resistant one survives.
3. It reproduces, and soon the whole population is resistant.
Example: MRSA is a "superbug" that is resistant to many antibiotics.
Key Takeaway: To stop this, doctors shouldn't prescribe antibiotics for viral or minor infections, and patients must finish their whole course of medicine!
8. Classification of Living Organisms
Scientists group living things so they are easier to study. This is called Classification.
The Linnaean System
Developed by Carl Linnaeus, this system groups organisms by their structure. The hierarchy is:
- Kingdom
- Phylum
- Class
- Order
- Family
- Genus
- Species
Mnemonic: King Philip Came Over For Good Soup!
Every organism has a two-part name (the Binomial system) made of its Genus and Species. Example: Humans are Homo sapiens.
The Three-Domain System
As we got better microscopes and learned about biochemistry, Carl Woese proposed three "Domains" above the Kingdom level:
- Archaea: Primitive bacteria usually living in extreme environments.
- Bacteria: True bacteria.
- Eukaryota: Includes protists, fungi, plants, and animals.
Final Review: Classification has changed over time as our technology has improved. Evolutionary trees are used to show how we think different species are related to each other based on fossils and DNA.