Natural Selection and Speciation

Natural selection is the mechanism that drives evolution. Individuals with advantageous traits are more likely to survive, reproduce, and pass those traits to offspring. Over many generations, the frequency of beneficial alleles increases in a population.

1

Variation

Individuals in a population differ in heritable traits due to genetic variation (mutations, recombination).

2

Selection pressure

Environmental challenges (predators, disease, food availability, climate) mean not all individuals survive and reproduce equally.

3

Differential reproduction

Individuals with favourable traits are more likely to reproduce and pass those traits to the next generation.

4

Speciation

When populations become reproductively isolated (e.g., by geography), they evolve independently and may diverge enough to become separate species.

Australian example: The geographic isolation of Australia allowed marsupials to evolve independently. The thylacine (Tasmanian tiger) and placental wolf evolved similar body forms through convergent evolution despite being unrelated — both faced similar selection pressures as apex predators.

Types of Speciation

Allopatric Speciation

Speciation caused by geographic isolation. A physical barrier (mountain range, ocean, river) separates a population, preventing gene flow. The isolated populations evolve independently until they can no longer interbreed.

Example: Darwin's finches on the Galapagos Islands diverged from an ancestral South American finch after colonising different islands.

Sympatric Speciation

Speciation within the same geographic area, without physical separation. Can occur through polyploidy (especially in plants), disruptive selection, or assortative mating where subgroups preferentially mate with individuals sharing their traits.

Example: Many Australian orchid species have speciated sympatrically through adaptations to specific pollinators.

Phylogenetic Trees

A phylogenetic tree (or cladogram) is a branching diagram showing the evolutionary relationships among species or groups. It is read like a family tree — the most recent common ancestor is where two branches join (a node). Species that share a more recent common ancestor are more closely related.

Reading a phylogenetic tree

    ┌── Species A
  ──┤
    │  ┌── Species B
    └──┤
       └── Species C

• B and C share a more recent common ancestor (inner node) → they are more closely related to each other than to A.
• A, B, and C all share a common ancestor at the root.
• Branch lengths may represent evolutionary time or genetic distance.
• Clade: A group that includes an ancestor and all its descendants.

Molecular Evidence for Evolution

Modern molecular biology provides powerful independent evidence for evolution, supporting and refining relationships established from anatomy and fossils.

DNA sequence comparison

Species that diverged more recently share more similar DNA sequences. Humans and chimpanzees share approximately 98.7% of their DNA, indicating a relatively recent common ancestor (~6 million years ago). Humans and mice share about 85% of their protein-coding genes.

Protein similarity

The amino acid sequences of proteins like cytochrome c (found in all aerobic organisms) are highly similar across related species. More similar sequences indicate closer evolutionary relationships. This is because the underlying DNA is conserved.

Molecular clocks

Certain genes accumulate mutations at a roughly constant rate. By comparing gene sequences and counting differences, scientists can estimate when two species diverged — a molecular clock. This has helped refine the timing of major evolutionary events.

Vestigial structures and pseudogenes

Vestigial structures (e.g., the human tailbone, whale leg bones) and non-functional pseudogenes (broken copies of once-useful genes) provide evidence of descent from ancestors that used these structures. Humans retain a non-functional gene for egg yolk production, shared with other vertebrates.

Key Vocabulary

Term	Definition
Speciation	The evolutionary process by which one population diverges into two or more distinct species that can no longer interbreed.
Phylogenetic tree	A branching diagram showing inferred evolutionary relationships among species or groups based on shared characteristics.
Convergent evolution	When unrelated species independently evolve similar traits in response to similar environmental pressures.
Molecular clock	The use of mutation rates in DNA sequences to estimate the timing of evolutionary divergence events.

Worked Examples

1

Using a phylogenetic tree: Species A, B, C, and D. A and B share node 1; C shares node 2 with (A+B); D branches off at node 3 (root). Which two species are most closely related?

Step 1: Read the tree. A and B share the most recent common ancestor (node 1).

Step 2: C's most recent common ancestor with A or B is node 2 (less recent than node 1).

Step 3: D's most recent common ancestor with any other species is the root (node 3), the most ancient.

Answer: Species A and B are most closely related. D is the most distantly related to all others.

2

Two gene sequences differ by 10 mutations. If the mutation rate is 1 mutation per million years, when did the species diverge?

Step 1: Total mutations accumulated since divergence = 10.

Step 2: Since divergence, each lineage accumulates mutations independently. Total mutations = mutations in lineage 1 + lineage 2 = 10 total since divergence (5 per lineage if equal).

Step 3: Time = 10 mutations ÷ 1 mutation per million years = 10 million years since divergence.

Note: Molecular clocks assume a constant mutation rate, which must be calibrated using fossils or other evidence.

3

Explain why the flippers of a whale and the wings of a bat are considered homologous structures, and what this tells us about evolution.

Homologous structures share a common underlying structure (same bones: humerus, radius, ulna, carpals, digits) derived from a common ancestor, even though they are now used for different functions.

The whale flipper (swimming) and bat wing (flying) have the same bone arrangement as a human arm and a horse leg — all derived from the forelimb of a common tetrapod ancestor.

Evolutionary meaning: This is evidence of divergent evolution from a common ancestor. Natural selection has modified the same basic structure for different functions. It strongly supports the idea of common descent.

Knowledge Check

Select the correct answer for each question.

Question 1

What is the primary driver of natural selection?

Random mutations always improve fitness Heritable variation in traits that affect survival and reproduction All individuals in a population reproduce equally Evolution always moves toward greater complexity

Question 2

Allopatric speciation requires:

Two species living in exactly the same habitat Polyploidy in plant populations Geographic isolation that prevents gene flow between populations A mutation in a single individual

Question 3

In a phylogenetic tree, two species that share a more recent common ancestor are:

More distantly related Unrelated to each other Identical species More closely related

Question 4

The similar streamlined body shape of sharks (fish) and dolphins (mammals) is an example of:

Convergent evolution Homologous structures Allopatric speciation Genetic drift

Question 5

Which of the following best supports an evolutionary relationship between two species?

They live in the same geographic area They share highly similar DNA sequences and protein structures They have a similar diet They are both endangered

Evolutionary Biology