The development of species: evolution and classification

Welcome to the Story of Life: Evolution and Classification

In this chapter, we are going to explore how biologists make sense of the incredible variety of life on Earth. We’ll look at how we group organisms together (classification) and the amazing ways creatures change over time to survive (evolution). This is part of your Cell Division and Development module because evolution is essentially the long-term result of genetic changes being passed down through generations!

Don’t worry if this seems like a lot of names and numbers at first—we’ll break it down into simple steps and use some handy tricks to remember the details.

1. Biological Classification: Sorting the Natural World

Biologists use a hierarchy to classify organisms. Think of this like a filing system or a home address. You start with a big category (like a country) and get more specific (like a house number).

The Taxonomic Ranks

You need to know these eight levels in order, from the biggest (most inclusive) to the smallest (most specific):

1. Domain (The biggest "bucket")
2. Kingdom
3. Phylum
4. Class
5. Order
6. Family
7. Genus
8. Species (The most specific level)

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

What is a "Species"?

It sounds simple, but scientists actually use two main ways to define a species:

1. Biological Species Concept: A group of organisms that can breed together to produce fertile offspring. (Example: A horse and a donkey can breed, but their offspring—a mule—is sterile, so horses and donkeys are different species).
2. Phylogenetic Species Concept: A group of organisms that share the same evolutionary ancestry and have similar physical or genetic traits. They are the smallest group at the "end" of an evolutionary branch.

Quick Review: Classification goes from broad (Domain) to specific (Species). A species is defined either by who they can mate with or their shared history.

2. Evidence for Evolution: Fossils vs. Molecules

How do we know who is related to whom? We use two main types of evidence.

Observable Evidence (Fossils)

By looking at fossils, scientists can see how body shapes (like those of ancient humans, or hominids) have changed over millions of years. This is "old school" evidence based on what we can see.

Molecular Evidence (DNA)

This is the modern way! By comparing DNA sequences, we can see exactly how many "typos" or differences exist between two species. The fewer the differences, the more closely related they are.

DNA Barcoding

Imagine scanning a cereal box at the supermarket. DNA barcoding works the same way. Scientists pick a specific gene that stays relatively similar within a species but is different between species.

- In Animals: We often use a gene in the mitochondria called cytochrome c oxidase 1.
- In Plants: We use genes found in the chloroplasts.

Did you know? We use mitochondrial or chloroplast DNA because these organelles have their own DNA, and there are hundreds of them in a single cell, making the DNA much easier to find and study than the DNA in the nucleus!

3. Phylogenetic Trees: Drawing the Family Tree

A phylogenetic tree is a diagram that shows how species are related.
- Extant species: Species that are alive today (found at the very tips of the branches).
- Extinct species: Species that have died out (their branches stop before the present day).

Important Point: Sometimes evidence conflicts! For example, looking at bones might suggest one relationship, but looking at DNA might suggest another. This is common when studying hominids (humans and our ancestors) and hylobatids (gibbons).

Key Takeaway: Evolutionary trees are "works in progress." As we get better DNA technology, we often have to redraw them!

4. Adaptations: The Tools for Survival

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

1. Anatomical Adaptations: Physical features of the body.
Example: Humans having a large brain size and the ability to walk on two legs (bipedalism).
2. Physiological Adaptations: Internal processes or chemical changes.
Example: Humans developing "lactose tolerance" (the ability to digest milk as adults) or changes in skin pigmentation to protect from UV rays.
3. Behavioural Adaptations: The way an organism acts.
Example: Humans using tools or developing cultural rituals for social bonding.

Plant Adaptations

Plants also adapt to their surroundings:
- Water: Cacti have spines instead of leaves to reduce water loss.
- Temperature: Some plants produce "antifreeze" chemicals to survive extreme cold.

5. Natural Selection: How Evolution Happens

Evolution doesn't happen by magic; it happens through natural selection. Think of it as a four-step process:

1. Genetic Variation: In any population, individuals have slightly different DNA (mutations).
2. Selection Pressure: Something makes life hard (e.g., a new predator, a change in climate, or disease).
3. Survival of the Fittest: Individuals with "better" adaptations are more likely to survive and reproduce.
4. Inheritance: They pass those winning genes to their children. Over many generations, the whole population changes.

The Mystery of Language

Scientists still debate how humans learned to speak. This is a great example of a scientific question with competing theories because we don't have fossils of "words"!
- The "Mother Tongues" hypothesis suggests language started as a way for mothers to soothe babies.
- The "Gossip" hypothesis suggests it evolved to help large groups of humans bond socially.

6. Measuring Biodiversity

Biodiversity is the variety of life. It can be measured at three levels:
- Genetic: Variation of genes within a species.
- Species: The number of different species in an area.
- Ecosystem: The variety of habitats (like forests, oceans, and deserts).

Simpson's Index of Diversity (\( D \))

We use this formula to see how healthy an ecosystem is. A high value (closer to 1) means the habitat is diverse and stable. A low value (closer to 0) means it is dominated by just one or two species and is easily damaged.

\( D = 1 - (\sum (\frac{n}{N})^2) \)

Note: You don't need to memorize the formula, but you should know that \( n \) is the number of individuals of one species, and \( N \) is the total number of all individuals.

Calculating Genetic Diversity

To see how much genetic variety is in a population, we look for polymorphic gene loci (genes that have more than one version/allele).

The Formula:
\( \text{proportion of polymorphic gene loci} = \frac{\text{number of polymorphic gene loci}}{\text{total number of loci}} \)

Common Mistake to Avoid: Don't confuse "species richness" (how many species there are) with "species evenness" (how many of each species there are). Simpson's Index looks at both!

Summary Checklist

- Can you list the ranks from Domain to Species? (Remember Good Soup!) [ ]
- Do you know the difference between anatomical, physiological, and behavioural adaptations? [ ]
- Can you explain the four steps of natural selection? [ ]
- Do you understand why DNA barcoding uses mitochondrial genes in animals? [ ]