BrightPath Back to Course
Year 12 Science

Evidence for Evolution

Examine the multiple lines of evidence supporting biological evolution, including the fossil record, comparative anatomy, molecular biology (DNA), and biogeography.

The Fossil Record

Fossils are the preserved remains or traces of organisms that lived in the past. The fossil record provides a chronological account of life's history, showing how organisms have changed over time. Transitional fossils display features intermediate between ancestral and descendant groups, demonstrating evolutionary pathways.

Key Fossil Evidence

Oldest fossils

Stromatolites (~3.5 billion years) -- evidence of early prokaryotic life

↓

Transitional fossils

Tiktaalik (fish-to-tetrapod), Archaeopteryx (dinosaur-to-bird)

↓

Dating methods

Relative dating (stratigraphy) and absolute dating (radiometric, e.g., carbon-14)

Key point: The fossil record is incomplete because fossilisation requires specific conditions (rapid burial, hard body parts). Despite this, the fossils we have consistently show increasing complexity over time and provide strong evidence for common ancestry.

Comparative Anatomy

Comparing the anatomical structures of different organisms reveals evolutionary relationships. Homologous structures (same origin, different function) indicate common ancestry, while analogous structures (different origin, similar function) indicate convergent evolution.

Types of Anatomical Evidence

Homologous Structures

Same bone pattern in human arm, whale flipper, bat wing, horse leg -- common ancestor

Analogous Structures

Bird wing vs insect wing -- similar function but different embryological origin (convergent evolution)

Vestigial Structures

Human appendix, whale hip bones -- reduced structures that had function in ancestors

Embryology as Evidence

Comparative embryology shows that vertebrate embryos look remarkably similar in early stages, sharing features like pharyngeal arches and post-anal tails, suggesting common ancestry.

Developmental genes (e.g., Hox genes) are highly conserved across species, controlling body plan development in organisms as diverse as fruit flies and humans.

Molecular Biology and Biogeography

Molecular evidence compares DNA, RNA, and protein sequences across species. More closely related species have more similar sequences. Biogeography examines the geographic distribution of species and how it reflects evolutionary history and geological events.

DNA Sequence Comparison

Molecular Clocks

Mutations accumulate at roughly constant rates. The number of differences between DNA sequences estimates time since divergence.

Universal Genetic Code

All life uses DNA/RNA and the same genetic code, strongly suggesting a single common ancestor (LUCA).

Biogeographic Evidence

  1. 1. Island biogeography: Isolated islands have unique endemic species descended from mainland colonists (e.g., Darwin's finches on the Galapagos).
  2. 2. Continental drift: Similar fossils on now-separated continents (e.g., Glossopteris on all southern continents) show they once shared land connections.
  3. 3. Australian marsupials: Australia's long isolation led to adaptive radiation of marsupials filling niches occupied by placental mammals elsewhere.

Key Vocabulary

Homologous Structures

Anatomical structures in different species that share a common evolutionary origin but may have different functions (e.g., pentadactyl limb in vertebrates).

Transitional Fossil

A fossil showing intermediate characteristics between an ancestral group and its descendants, providing evidence of evolutionary change over time.

Biogeography

The study of the geographic distribution of species and how it relates to evolutionary history, geological events, and natural selection.

Molecular Clock

A technique that uses the rate of molecular change (DNA mutations) to estimate the time of divergence between species from a common ancestor.

Worked Examples

1

Explain how the pentadactyl limb provides evidence for common ancestry among vertebrates.

Step 1: The pentadactyl (five-digit) limb pattern is found in humans, whales, bats, horses, and other vertebrates.

Step 2: Despite different functions (grasping, swimming, flying, running), the same basic bone arrangement (humerus, radius, ulna, carpals, metacarpals, phalanges) is present.

Answer: These are homologous structures. The simplest explanation for the same underlying bone pattern in organisms with very different lifestyles is that they all inherited the plan from a common ancestor, which was then modified by natural selection.

2

Two species share 98% of their DNA sequence. Another pair shares 85%. Which pair is more closely related?

Step 1: Closely related species have had less time to accumulate mutations since diverging from their common ancestor.

Answer: The pair sharing 98% of their DNA is more closely related. Greater DNA similarity indicates more recent common ancestry and less evolutionary divergence.

3

Explain why similar marsupials and placental mammals exist on different continents (e.g., Tasmanian tiger vs wolf).

Step 1: Australia separated from other landmasses millions of years ago, isolating its marsupial populations.

Step 2: On both continents, similar ecological niches existed (e.g., large predator niche).

Answer: This is convergent evolution -- marsupials in Australia and placental mammals elsewhere independently evolved similar body forms and behaviours to fill equivalent ecological niches. Their similarities are analogous, not homologous.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Homologous structures provide evidence for evolution because they:

Question 2

Which of the following is a transitional fossil?

Question 3

The fact that all living organisms use DNA as their genetic material provides evidence for:

Question 4

The human appendix is considered a vestigial structure because it:

Question 5

Darwin's finches on the Galapagos Islands are an example of evidence from:

Key Concepts Summary

Year 12: Homeostasis Year 12: Speciation