Evidence of Evolution: Study Guide (Biology 7.6)

Many sciences, one conclusion

In Topic 7.5 you used allele frequencies to detect evolution within a population over a few generations. Over millions of years, evolution produces far larger changes, and you cannot watch them directly. The evidence for them comes from several separate sciences. Each uses different methods and was developed independently, yet they agree: living things have changed over time and are related by common descent.

Kind of data	From which science	Example of what it shows
Geological	study of rocks and fossils	fossils appear in a consistent order through rock layers; extinct forms and transitional forms exist
Physical	physics of radioactive decay	the absolute ages of rocks and of once-living remains
Biochemical	molecular biology	related species share DNA and protein sequences; the more closely related, the fewer the differences
Geographical	distribution of species and fossils	species on islands resemble those on the nearest mainland; matching fossils on continents that were once joined
Mathematical	statistics and models	allele-frequency models (Topic 7.5) and calculated rates of sequence change

Fossils and the rock record

A fossil is the preserved remains or trace of an organism from the past. Fossils show species that no longer exist and, in some lineages, a series of forms that change step by step. For example, fossils of early whales include four-legged animals and later forms with tiny hind limbs, linking modern whales to land-living ancestors.

To use fossils as evidence of change, you need to know how old they are. There are three main approaches.

1. The age of the rock layers

Sedimentary rock forms as layers of sediment settle and harden, one on top of the other. In undisturbed rock, lower layers are older than the layers above them. This gives a relative age: you can say one fossil is older than another, but not by how many years.

2. Radioactive decay of isotopes

Some isotopes are unstable and decay into other atoms at a constant rate, unaffected by temperature or chemistry. The half-life is the time taken for half of the radioactive atoms in a sample to decay. After one half-life, 1/2 is left; after two, 1/4; after three, 1/8; after n half-lives, (1/2)ⁿ.

Carbon-14 forms in the atmosphere and enters living things through photosynthesis and food. While an organism is alive, its proportion of carbon-14 matches the air. When it dies, it stops taking in carbon, and its carbon-14 decays with a half-life of about 5,700 years. Comparing the carbon-14 left with the starting amount gives the time since death. After about 50,000 years so little remains that it cannot be measured reliably, so carbon-14 only dates fairly recent remains of once-living things.

Older fossils are dated indirectly. Volcanic ash contains minerals with isotopes that have much longer half-lives (potassium-40 is one). Dating ash layers above and below a fossil gives an age range for the fossil.

3. Geographical data

Rock layers and the fossils in them can be matched between different places. If a layer at one site contains the same distinctive fossils as a dated layer at another site, the two layers are likely to be about the same age. Geography also tracks where land masses were. Fossils of the reptile Mesosaurus, about 275 million years old, are found in both southern Africa and eastern South America. These coasts are now an ocean apart but were joined when the reptile lived.

Morphological evidence: homologous and vestigial structures

Homologous structures have the same basic arrangement of parts because they were inherited from a common ancestor, even if they now do different jobs. The forelimbs of mammals are the classic case:

Animal	Use of forelimb	Bones present
human	grasping	one upper-arm bone, two forearm bones, wrist bones, finger bones
bat	flying	the same set, with very long finger bones supporting the wing
whale	swimming	the same set, short and flattened inside a flipper
cat	walking	the same set, with the wrist and finger bones forming the foot

If each limb had been built separately for its job, there is no reason for a wing, a flipper and a hand to share the same bones. Shared structure despite different function points to shared ancestry.

Vestigial structures are reduced remnants of structures that were fully developed and functional in ancestors. Examples include the small pelvic bones of whales, the tiny pelvic spurs of pythons, the small wings of ostriches and other flightless birds, and the non-seeing eyes of some cave-dwelling fish and salamanders. A vestigial structure makes sense only if the organism descended from ancestors that used it.

Background, for contrast: analogous structures, such as the wings of a bird and of a butterfly, do the same job but are built differently. They arose separately because similar environments favoured similar solutions. They are not evidence of a recent common ancestor.

Molecular evidence: comparing sequences

All living things use DNA, and related species carry related versions of the same genes. When two populations split, each lineage gains its own mutations. The longer ago they split, the more differences build up. So:

fewer differences in DNA or amino acid sequence → more recent common ancestor.

DNA comparisons are usually more sensitive than protein comparisons. Because the genetic code has more than one codon for most amino acids, some DNA changes do not alter the protein (silent mutations, Topic 6.7). Molecular data are especially strong evidence because they are independent of anatomy: if a family tree built from bones matches one built from genes, two separate lines of evidence agree.

Biogeography: where species live

Species on oceanic islands tend to resemble species on the nearest mainland, not species living in similar climates elsewhere. The plants and animals of the Galápagos Islands, for example, most resemble those of South America. This is what you expect if the islands were colonised from the nearest mainland and the colonists then changed.

Worked example 1: dating fossils in a rock column

Question. Use Figure 1. (a) Put fossils X, Y and Z in order from oldest to youngest. (b) Give the possible age range of fossil Y. (c) Charcoal from the base of the soil layer contains 12.5% of the carbon-14 found in living wood. Taking the half-life as 5,700 years, calculate its age. (d) A second charcoal sample has 20% left. Estimate its age.

(a) In undisturbed rock, lower layers are older. Z is lowest, then Y, then X. Order: Z (oldest), Y, X (youngest).

(b) Y lies above ash B (2.4 million years) and below ash A (1.8 million years). So Y is between 1.8 and 2.4 million years old. Z is older than 2.4 million years; X is younger than 1.8 million years.

(c)

1. 100% → 50% → 25% → 12.5%: that is 3 half-lives. Check: (1/2)³ = 1/8 = 0.125.
2. Age = 3 × 5,700 years = 17,100 years.

(d) 20% is not a whole number of half-lives: it lies between 2 half-lives (25%) and 3 half-lives (12.5%). Read it from a decay curve (Figure 2): about 2.3 half-lives, so about 2.3 × 5,700 ≈ 13,000 years.

Interpretation. Z is marine clam shells, but X is freshwater snails. Where there is now land, there was once sea. The column records change in the environment and in the organisms living there over more than 2 million years. Note the assumptions: the layers have not been overturned or disturbed, and no carbon-14 was added to or lost from the charcoal after death.

Worked example 2: comparing DNA sequences

Question. The table shows the same 24-nucleotide stretch from inside a gene in four fictional beetle species, P, Q, R and S. (a) Count the differences between each pair. (b) Describe what the data show about relatedness. (c) P and Q differ in DNA but code for the same amino acids here. Explain how.

Species	Sequence (coding strand, in codons)
P	ATG GCT AAC GTT CCA GAT TGC AAA
Q	ATG GCT AAC GTC CCA GAT TGT AAA
R	ATG GCA AAC GTC CCT GAG TGC AAG
S	ACA GCA AAT GTC TCT GAT TGC CAA

(a) Compare position by position:

	P	Q	R	S
P	–	2	5	8
Q	2	–	5	8
R	5	5	–	7
S	8	8	7	–

(b) Describe with numbers (skill 4.B):

1. Specific data points: P and Q differ at only 2 of 24 positions (92% identical). S differs from P and from Q at 8 positions (67% identical).
2. Pattern: R is equally different from P and Q (5 each). S is the most different from every other species (7 or 8).
3. Relationship: P and Q share the most recent common ancestor. R shares a more distant common ancestor with them. S shares the most distant common ancestor with the other three.

(c) The two differences are in the third base of a codon: GTT and GTC both code for valine, and TGC and TGT both code for cysteine. These are silent differences, so the protein sequence is the same. This is why DNA comparisons can separate very closely related species that protein comparisons cannot.

Limits. This is only 24 nucleotides from one gene. Real studies compare thousands of positions across many genes so that chance similarities do not mislead.

Describing data well

• Quote specific values with units, not just “higher” or “lower”.
• State the trend: increases, decreases, levels off, or no clear pattern.
• State the relationship between variables, such as “the older the island, the greater the DNA difference”.
• Keep description separate from explanation: first say what the data show, then why.

Common misconceptions

• “Older fossils are always deeper.” Only in undisturbed layers. Folding, faulting or erosion can disturb the order.
• “Carbon-14 can date dinosaur bones.” After tens of thousands of years almost no carbon-14 is left. Fossils millions of years old are dated using longer-lived isotopes in volcanic layers.
• “A half-life is half the lifetime of the isotope.” It is the time for half of the remaining atoms to decay. After two half-lives, a quarter is left, not none.
• “Vestigial means useless.” It means reduced and no longer serving its original main function.
• “Similar-looking structures always show common ancestry.” Analogous structures look alike because of similar functions, not shared descent.
• “Humans evolved from chimpanzees.” Both share a common ancestor. Sequence similarity shows how recently that ancestor lived, not that one living species turned into another.
• “One line of evidence is enough.” The case for evolution is strong because independent lines of evidence agree.

Where this leads

This topic follows Topic 7.5, Hardy–Weinberg Equilibrium, which detects evolution in living populations. Next, Topic 7.7, Common Ancestry, looks at the cell-level and molecular features that all eukaryotes share. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.