Biology (9610) | Species and taxonomy

Welcome to Species and Taxonomy!

Hello Biologists! This chapter is all about how we organize the incredible variety of life on Earth—from the smallest bacteria to the largest whales. Understanding classification (taxonomy) is like having a powerful filing system for living organisms. It helps us understand relationships, evolutionary history, and how life has diversified. Don't worry if the names seem complicated; we will break down the essential concepts step-by-step!

3.1.10.1 The Concept of a Species

The Basic Unit: Defining Life's Groups

The most fundamental unit in biological classification is the species. When scientists classify organisms, they start here.

The Biological Definition of a Species

For your syllabus, a species is defined using the concept of reproduction.

A species is the largest group of organisms capable of interbreeding to produce fertile offspring.

• Interbreeding: The organisms must be able to mate and produce offspring.
• Fertile Offspring: This is the crucial part. The resulting offspring must also be able to reproduce successfully. If they are sterile (unable to reproduce), the parents are considered different species.

Analogy: Think of different dog breeds (like a Chihuahua and a Great Dane). They look very different, but they can produce fertile puppies, so they are all the same species: Canis familiaris.

Universal Identification: The Binomial System

Imagine trying to talk about a common bird if every country called it a different name! To solve this problem, all species are identified globally using the binomial system, established by Carl Linnaeus.

A binomial consists of two parts: the name of its genus and the name of its species.

• The first part is the Genus (always capitalized).
• The second part is the species (never capitalized).
• The entire name must be written in italics (or underlined if handwritten).

For example:

Humans are Homo sapiens.
Lions are Panthera leo.
The common garden pea is Pisum sativum.

The Difficulty of Defining a Species (A Critical Look)

While the "interbreeding and fertile offspring" definition is excellent in theory, you must be able to recognise the difficulty of defining species and critically examine this definition in practice.

The definition works well for sexually reproducing animals, but it breaks down when considering real-world examples:

• Hybridisation (e.g., Mules): A horse (Equus caballus) and a donkey (Equus asinus) are clearly different species, yet they can interbreed. However, their offspring, the mule, is sterile (infertile). This example confirms the definition, but shows the boundary is often tested in nature.
• Asexual Reproduction: Bacteria or organisms that reproduce asexually (without mating) cannot be classified using the interbreeding test. We must rely solely on other factors, like genetics or morphology (physical structure).
• Geographical Separation: Two groups of organisms might be fully capable of interbreeding, but if they are separated by an ocean, they cannot physically do so. Are they still the same species? By the strict definition, yes, but they are genetically isolated.
• Fossil Records: When dealing with extinct organisms, reproduction cannot be observed. Classification must be based purely on skeletal morphology.

Common Mistake to Avoid: Don't assume that if two organisms look different, they are different species. Intraspecific variation (variation within a species, covered in 3.1.6) is huge! Think about a human (Homo sapiens) living in Alaska versus one living near the equator—they are the same species despite major differences in appearance and genetics.

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Key Takeaway for Species Concept: The definition is the best we have, but it's not perfect for every single organism, forcing scientists to look for additional evidence.

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3.1.10.2 Biological Classification (Taxonomy)

What is Biological Classification?

Biological Classification (or Taxonomy) is the process of arranging species into groups based on their similarities and differences.

The primary goals of classification are:

1. To arrange species in groups that reflect their relationships.
2. To arrange species in groups that may reflect their evolutionary origins (how closely related they are in terms of descent).

Each group within the classification system is called a taxon (plural: taxa).

The Taxonomic Hierarchy

Classification uses a hierarchy, which means it’s a system where smaller groups (taxa) are placed within larger groups, with no overlap between them. As you move down the hierarchy, the groups contain fewer organisms but they are more closely related.

The Seven Main Taxa (Groups)

You need to recall the standard taxonomic hierarchy in the correct order:

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

Memory Aid (Mnemonic): A classic way to remember the order is:
Dear King Philip Came Over For Great Soup.

Did you know? Organisms in the same Order are far more related than organisms in the same Kingdom, because the Order is a smaller, more specific grouping.

Clarifying Relationships: Modern Evidence for Classification

Historically, organisms were classified based solely on morphology (what they looked like). Today, classification is far more accurate because we use molecular evidence. You must know how two techniques help clarify taxonomic relationships:

1. Genome Sequencing (DNA/mRNA Base Sequences)

Genetic material is the ultimate blueprint. The more closely related two species are, the more recently they shared a common ancestor, and thus the more similar their DNA base sequences will be.

• By comparing the base sequence of DNA (or the mRNA sequence derived from it) between two species, scientists can quantify their genetic similarity.
• A small difference in base sequence suggests a very close evolutionary relationship (e.g., humans and chimps share ~98% of their DNA).
• A large difference in base sequence suggests that the species diverged much earlier in evolutionary history.

2. Immunology (Amino Acid Sequences)

Proteins are coded for by DNA, so analyzing protein structure is an indirect way to examine genetic similarity. Immunology specifically looks at the similarities in amino acid sequences of shared proteins (like albumin).

The process involves using antibodies to test how similar blood proteins are:

1. Extract a protein (e.g., blood serum albumin) from Species A (e.g., a human).
2. Inject the protein into Species B (e.g., a rabbit). The rabbit’s immune system recognizes the human protein as an antigen and produces specific antibodies against it (anti-human antibodies).
3. The anti-human antibodies are collected and mixed with blood samples from other organisms (Species C, D, E, etc.).
4. The more precipitate (clumped antigen-antibody complexes) that forms, the more similar the foreign protein is to the original human protein.

Example: If anti-human antibodies cause a lot of precipitation when mixed with chimp blood, but very little when mixed with lizard blood, it confirms that humans are more closely related to chimps than to lizards.