Classification and evolution

Welcome to Classification and Evolution!

Hi there! Welcome to one of the most fascinating chapters in your Biology A Level. Have you ever wondered why a dolphin looks like a fish but is actually more related to you than a shark? Or why some bacteria can suddenly survive the strongest antibiotics? This chapter is all about how we organise the incredible variety of life on Earth and the processes that allow species to change over millions of years. Don’t worry if it seems like a lot of names and categories at first—we'll break it down step-by-step!

1. The Art of Sorting: Biological Classification

Classification is the process of placing living things into groups. We do this because it makes organisms easier to study and helps us understand how they are related.

The Taxonomic Hierarchy

Biologists use a hierarchy. This means we start with big, broad groups and narrow them down into smaller, more specific groups. There are eight main levels (taxa):

1. Domain (The broadest group)
2. Kingdom
3. Phylum
4. Class
5. Order
6. Family
7. Genus
8. Species (The most specific group)

Memory Aid: To remember the order, use this classic mnemonic:
"Dear King Philip Came Over For Good Soup"

What is a Species?

A species is a group of organisms that are able to reproduce to produce fertile offspring. This means their babies can also have babies of their own. (A mule is not a species because it is the infertile offspring of a horse and a donkey!)

The Binomial System

To avoid confusion across different languages, we use the Binomial System. Every organism has a two-part Latin name:
1. The first name is the Genus (always starts with a Capital letter).
2. The second name is the species (always lower case).
Example: Humans are Homo sapiens.

Quick Review: Classification goes from broad (Domain) to specific (Species). A species must be able to produce fertile offspring.

2. The Five Kingdoms vs. The Three Domains

Originally, scientists used observable features (what an organism looks like) to put them into Five Kingdoms:

1. Prokaryotae: Bacteria (no nucleus).
2. Protoctista: Single-celled eukaryotes (like Amoeba).
3. Fungi: Mushrooms, yeast, moulds.
4. Plantae: Plants.
5. Animalia: Animals.

The Shift to Three Domains

As technology improved, we started looking at molecular evidence (DNA and proteins). We discovered that some "bacteria" were actually very different from others. This led to the Three Domain System:

• Bacteria (True bacteria)
• Archaea (Extremophiles—organisms that live in crazy conditions like volcanoes!)
• Eukarya (Everything with a nucleus: Plants, Animals, Fungi, Protoctista)

Did you know? The shift from 5 Kingdoms to 3 Domains happened mainly because of differences in rRNA (ribosomal RNA). Archaea have rRNA that is actually more similar to humans than to "normal" bacteria!

Key Takeaway: Modern classification relies more on DNA and biological molecules than just looking at an organism's physical appearance.

3. Phylogeny: The Family Tree of Life

Phylogeny is the study of the evolutionary history of groups of organisms. It tells us "who is related to whom."

While classification is about putting things in boxes, phylogeny is about drawing the lines of the family tree. A phylogenetic tree shows how different species evolved from common ancestors. The closer the branches, the more closely related the species are.

Common Mistake to Avoid: Don't assume that because two animals look similar, they are closely related. Sharks and dolphins look similar because they live in the sea, but their DNA shows they belong to very different branches of the tree!

4. Evidence for Evolution

Evolution is the theory that organisms change over long periods of time. Charles Darwin and Alfred Russel Wallace are the two scientists famous for formulating the theory of natural selection.

How do we know evolution is real?

1. Fossils: These show us how organisms have changed over millions of years (the fossil record).
2. DNA Evidence: We can compare the genomic DNA of different species. The more similar the DNA sequence, the more recently they shared a common ancestor.
3. Molecular Evidence: We compare proteins (like cytochrome c) and other molecules. Similar molecules suggest a close relationship.

Key Takeaway: Fossils give us the history, but DNA provides the "smoking gun" proof of how species are linked.

5. Variation: Why We Are All Different

Variation refers to the differences that exist between individuals. There are two main types:

• Interspecific variation: Differences between different species (e.g., a bird vs. a dog).
• Intraspecific variation: Differences within the same species (e.g., you vs. your classmate).

Continuous vs. Discontinuous Variation

1. Continuous Variation: Features that can be any value within a range (e.g., height, mass). These are usually controlled by many genes and the environment. If you graph this, it looks like a "bell curve."
2. Discontinuous Variation: Features that fall into distinct categories (e.g., blood group, eye colour). These are usually controlled by one or a few genes and are not affected by the environment.

Quick Review: Height is continuous (genetic + environment). Blood type is discontinuous (genetic only).

6. Adaptations: Built for Survival

An adaptation is a feature that increases an organism's chance of survival and reproduction. There are three types you need to know:

1. Anatomical Adaptations: Physical features (internal or external).
Example: A cactus has spines to stop animals from eating it.
2. Behavioural Adaptations: The way an organism acts.
Example: Birds migrating south for the winter to find food.
3. Physiological Adaptations: Processes that happen inside the body.
Example: A snake producing venom or a desert animal producing very concentrated urine to save water.

Did you know? Organisms from different groups can show similar anatomical features if they live in similar environments. This is called convergent evolution (like the wings of a bird and a bat).

7. The Mechanism of Natural Selection

This is the "engine" that drives evolution. If you are asked to explain this in an exam, follow these steps:

1. Mutation: A random change in DNA creates new alleles.
2. Variation: This leads to differences in characteristics within a population.
3. Selection Pressure: A struggle for survival occurs (e.g., predators, disease, competition for food).
4. Survival of the Fittest: Individuals with advantageous characteristics are more likely to survive.
5. Reproduction: These survivors breed and pass on their advantageous alleles to their offspring.
6. Inheritance: Over many generations, the proportion of the population with the "good" trait increases.

Evolution in Modern Times

Evolution isn't just something that happened millions of years ago. It’s happening now and has big implications for humans:

• Pesticide Resistance: Insects evolve to survive the chemicals farmers use to kill them.
• Drug Resistance: Bacteria (like MRSA) evolve to survive antibiotics. This is a major medical crisis!

Key Takeaway: Natural selection requires variation, a selection pressure, and inheritance of alleles. It is why we need to be careful with how we use antibiotics!