Evidence for Evolution - Flashcards

Comparative Genomics

• Compares the DNA sequences / genomes of different species

• Scientists compare differences in DNA sequences and genes

• Species with more similar DNA sequences and genes are more closely related and thus share a more recent common ancestor

• Species with more differences in their DNA sequences and genes are less closely related and thus have a more distant common ancestor

• Differences in the DNA sequence or genes are causes by mutations which accumulate over generations

Protein Sequences

• Proteins are made of specific amino acid sequences determined by DNA

• Scientists can compare differences in the amino acid sequence of the same protein in different species

• Species with more similar amino acid sequence are more closely related and thus share a more recent common ancestor

• Species with more differences in their amino acid sequences are less closely related and thus have a more distant common ancestor

• Differences in the amino acid sequence are causes by mutations which accumulate over generations

Mitochondrial DNA

• Mitochondrial DNA (mtDNA) is small circular DNA found in mitochondria and inherited solely through the maternal line

• Scientists can compare differences in the mtDNA sequence between individuals, populations, or closely related species

• Species with more similar mtDNA sequences are more closely related and thus share a more recent common ancestor

• Species with more differences between their mtDNA sequences are less closely related and thus share a more distant common ancestor

• Differences in the mtDNA are causes by mutations which accumulate over generations

Bioinformatics

• Combines biology, computer science, mathematics, and statistics to analyse biological data (DNA sequences / genomes, protein sequences, or mtDNA)

• Computer programs compare large amounts of this biological data to identify similarities and differences between organisms.

• Species with more similar sequences are more closely related and thus share a more recent common ancestor

• Species with more differences in their sequences are less closely related and thus share a more distant common ancestor

Process of Fossil Formation

• Fossil: preserved remains, impressions or traces of organisms that lived in the past.

• Organism dies → soft tissues usually decompose or are eaten by scavengers/microorganisms.

• Hard structures such as bones, teeth and shells are more likely to remain.

• Remains are rapidly buried by sediment such as mud, sand or silt → reduces decomposition and damage.

• More sediment accumulates → becomes compacted into sedimentary rock.

• Mineral-rich groundwater can enter remains and deposit minerals → permineralisation preserves their structure.

• Fossils can also form as moulds, casts, impressions or traces such as footprints/burrows.

• Over time, erosion/geological processes expose the fossil → allowing discovery.

Conditions Required

• Fossilisation is rare and requires favourable conditions.

• Rapid burial → protects remains from scavengers, physical damage and decomposition.

• Low oxygen → reduces activity of decomposers → slows decomposition.

• Hard body parts (bones, teeth, shells) → resist decay → more likely to fossilise.

• Aquatic/sediment-rich environments are favourable → mud, sand or silt can rapidly cover remains.

• Continued sediment deposition + mineral-rich groundwater can assist preservation/permineralisation.

• Soft tissue can occasionally be preserved under exceptional conditions such as ice, amber or peat where decomposition is greatly reduced.

Issues with the Fossil Record

• The fossil record is incomplete → does not represent every organism that has existed.

• Incomplete fossilisation: most organisms decompose because fossilisation requires specific conditions.

• Preservation bias: hard-bodied organisms are much more likely to fossilise than soft-bodied organisms.

• Destroyed fossils: erosion, Earth movements, heat/pressure and human activity can damage or destroy fossils.

• Not discovered: many fossils remain buried and have not yet been found.

• Incomplete fossils: often only fragments are preserved → scientists must reconstruct/interpret organisms from limited evidence.

• Difficult to date: different dating techniques have particular age/material limitations.

• Despite these issues, fossils provide important evidence of evolutionary change over time.

Relative Dating - Definition

Relative dating determines whether a fossil or rock is older or younger than another, providing a chronological order.

Statiagraphy - How it Works, Pros, Cons

How it works

• Stratigraphy: studies layers of sedimentary rock called strata.

• Fossils are considered approximately the same age as the rock layer in which they occur.

• Principle of superposition: in undisturbed sedimentary strata, oldest layers are at the bottom and youngest at the top.

• Therefore: lower fossil = generally older; higher fossil = generally younger.

Advantages

• Simple way to place fossils into chronological order.

• Useful when fossils cannot be directly dated.

• Allows relative ages of fossils within strata to be compared.

Disadvantages

• Gives relative age only — not a numerical age.

• Folding, faulting and erosion can disturb strata.

• Missing rock layers can create gaps in the geological record

Index Fossils - How it Works, Pros, Cons

How it works

• Index fossils come from species that existed for a relatively short geological time.

• Finding the same index fossil in different locations suggests the rock layers are approximately the same age.

• Allows scientists to correlate strata between different geographical locations.

• Fossilised pollen can act as an index fossil and provide evidence of past vegetation and climate.

Advantages

• Allows rock strata from different locations to be compared.

• Useful even when corresponding strata occur at different depths.

• Some index fossils can also provide information about past environments/climate.

Disadvantages

• Requires an appropriate, identifiable index fossil to be present.

• Only provides a relative age, not an exact numerical age.

• Species must have existed for a limited geological period to be useful.

Absolute Dating

Absolute dating estimates the numerical age of a fossil

Carbon 14 Dating

How it works

• Living organisms continually obtain carbon, including radioactive carbon-14 (¹⁴C).

• When an organism dies, carbon intake stops.

• ¹⁴C then progressively decays into nitrogen-14.

• Carbon-14 half-life = approximately 5,730 years.

• Scientists measure remaining ¹⁴C relative to stable carbon to estimate time since death.

• Generally useful for organic material up to ~60,000 years old.

Advantages

• Provides an approximate numerical age.

• Useful for relatively recent organic remains.

• Can directly date suitable biological material.

Disadvantages

• Only useful for once-living/organic material.

• Not reliable for material much older than ~60,000 years → too little ¹⁴C remains.

• Contamination can affect accuracy.

Accelerator Mass Spectrometry (AMS)

How it works

• Specialised form of radiocarbon dating.

• Directly measures/counts carbon isotopes.

• Requires a much smaller sample than conventional radiocarbon dating.

• Can date very small quantities of organic material, including material associated with cave paintings.

Advantages

• Requires only a very small sample.

• Useful when available material is limited or valuable.

• Can analyse samples too small for conventional radiocarbon techniques.

Disadvantages

• Still uses carbon-14 → similar age/material limitations.

• Requires specialised and expensive equipment.

• Contamination can affect results.

Potassium - Argon Dating (K - Ar)

How it works

• Radioactive potassium-40 (⁴⁰K) decays to argon-40 (⁴⁰Ar).

• Potassium-40 half-life ≈ 1.3 billion years.

• Long half-life → useful for very old material.

• Particularly used to date volcanic rock.

• Fossils generally aren't directly dated → scientists date associated volcanic rock and infer the fossil's age.

Advantages

• Can date very old material beyond the range of C-14.

• Useful for ancient fossils when associated volcanic material exists.

• Provides an approximate numerical age.

Disadvantages

• Requires suitable volcanic rock/material.

• Fossil age is usually determined indirectly from surrounding/associated rock.

• Geological alteration or changes in argon can affect accuracy.

Dendrochronology

How it works

• Also called tree-ring dating.

• Trees generally produce one growth ring each year.

• Unknown wood samples are compared with overlapping ring patterns from known-age samples.

• Scientists can cross-date matching ring patterns.

• Environmental/climatic conditions affect ring width → distinctive rings can act as markers.

Advantages

• Can provide very precise dating, sometimes to an individual year.

• Does not rely on radioactive decay.

• Ring patterns also provide evidence about past climate/environmental conditions.

Disadvantages

• Restricted to suitable wood/tree material.

• Requires identifiable and overlapping tree-ring patterns.

• More limited time range than techniques such as K–Ar dating.