Introduction to Variation, Evolution and Classification
Welcome to one of the most fascinating parts of Biology! Have you ever wondered why no two people look exactly the same (unless they are identical twins), or how a tiny chihuahua is related to a wolf? In this chapter, we explore how variation leads to evolution and how scientists keep track of millions of different living things through classification. This is a core part of your Biology Paper 2 exam.
1. Variation
Variation refers to the differences in characteristics between individuals of the same species. These differences can be caused by your genes, the environment you live in, or a mix of both.
Genetic vs. Environmental Variation
Differences in the genotype (the genes you have) lead to differences in the phenotype (the physical characteristics you show).
1. Genetic Variation: Things you inherit from your parents, such as eye colour, blood group, and certain inherited disorders.
2. Environmental Variation: Changes caused by the conditions you live in, such as a scar from an accident, your language/accent, or a plant losing leaves because of a lack of light.
3. Combined Variation: Most things, like your height or skin colour, are affected by both your genes and your lifestyle (e.g., your diet).
Mutations
New variations are created by mutations. A mutation is a random change in the DNA. Most mutations have no effect on the phenotype. However, very rarely, a mutation can create a new characteristic that is beneficial to the organism. If the environment changes, this new characteristic might help the organism survive better.
Quick Review: Most variations are caused by a combination of genetics and the environment. Mutations are the source of all new genetic variation!
2. Evolution
The theory of evolution states that all species of living things alive today have evolved from simple life forms that first developed more than three billion years ago.
Natural Selection (The "Survival of the Fittest")
Charles Darwin's theory of natural selection explains how evolution happens. It works in a simple step-by-step process:
1. Variation: Within a population, there is genetic variation due to mutations.
2. Competition: Organisms compete for resources (food, water, mates).
3. Survival: Those with characteristics best suited to the environment are more likely to survive.
4. Reproduction: The survivors breed and pass on their beneficial alleles (versions of genes) to the next generation.
5. Repeat: Over many generations, these advantageous traits become more common.
Analogy: Imagine a group of birds. Some have slightly longer beaks due to a mutation. If the only food available is deep inside a flower, the birds with longer beaks survive better, eat more, and have more babies with long beaks. Eventually, the whole population has long beaks!
3. Selective Breeding
Selective breeding (also called artificial selection) is when humans breed plants or animals for particular genetic characteristics.
How it works:
1. Choose parents with the desired characteristics from a mixed population.
2. Breed them together.
3. From the offspring, choose those with the best desired characteristics and breed them.
4. Repeat this over many generations until all the offspring show the desired trait.
Why do we do it?
Humans have been doing this for thousands of years to produce:
- Food crops with better yields or disease resistance.
- Domestic dogs with a gentle nature.
- Animals that produce more meat or milk.
- Large or unusual flowers.
Warning: The Downside! Selective breeding can lead to inbreeding. This makes breeds more prone to disease or inherited defects because there is less "genetic variety" in the population.
4. Genetic Engineering
Genetic engineering is a modern process that involves changing the genome of an organism by introducing a gene from another organism to give a desired characteristic.
Examples:
- Bacterial cells have been genetically engineered to produce human insulin to treat diabetes.
- GM (Genetically Modified) Crops can be engineered to be resistant to diseases, insects, or herbicides, or to produce bigger fruits.
The Process (HT Only):
In genetic engineering, genes are "cut out" from the chromosome of one organism and transferred to the cells of another organism. This is usually done at an early stage in development (like an embryo) so that the organism develops with the desired characteristics in all its cells.
Key Takeaway: Selective breeding uses natural reproduction over generations; genetic engineering is a direct, fast lab technique that can move genes between different species.
5. Evidence for Evolution
How do we know evolution actually happened? Scientists look at fossils and the antibiotic resistance of bacteria.
Fossils
Fossils are the "remains" of organisms from millions of years ago, found in rocks. They form in three ways:
1. From parts of organisms that have not decayed (because the conditions for decay were missing, like in ice or amber).
2. When parts of the organism are replaced by minerals as they decay.
3. As preserved traces like footprints, burrows, or rootlet traces.
Did you know? We don't have a fossil for every animal that ever lived. Many early life forms were soft-bodied, meaning they decayed completely without leaving a trace!
Resistant Bacteria
Bacteria evolve very quickly because they reproduce fast. MRSA is a famous "superbug" that is resistant to antibiotics. This happens because:
- A mutation makes one bacterium resistant to an antibiotic.
- When the antibiotic is used, the non-resistant bacteria die, but the resistant one survives.
- The survivor reproduces, passing on the resistance gene.
- Soon, the whole population is resistant.
Top Tip: To reduce the rate of resistant bacteria, doctors should not prescribe antibiotics for viral infections, and patients must finish their whole course of antibiotics.
6. Extinction
Extinction occurs when there are no remaining individuals of a species still alive. This can be caused by:
- New diseases.
- New predators.
- New, more successful competitors.
- Changes to the environment over long periods of time.
- A single catastrophic event (like a massive asteroid strike or volcanic eruption).
7. Classification
To make sense of the millions of organisms on Earth, we group them together. This is called classification.
The Linnaean System
Carl Linnaeus famously classified living things into a hierarchy. You need to know the order (from biggest group to smallest):
Kingdom → Phylum → Class → Order → Family → Genus → Species
Memory Aid: "King Philip Came Over For Good Soup"
Organisms are named by the Binomial System (two names). Every organism has a Genus and a Species name. For example, humans are Homo sapiens.
The Three-Domain System
As our understanding of biology improved (thanks to better microscopes and chemical analysis), Carl Woese introduced the Three-Domain System:
1. Archaea: Primitive bacteria usually living in extreme environments.
2. Bacteria: True bacteria.
3. Eukaryota: Includes protists, fungi, plants, and animals.
Final Summary: From tiny mutations to the massive tree of life, biology is constantly changing. Evolution explains the past, and classification helps us map the present!