Evidence for Evolution
This lesson covers:
- How the fossil record can be used to support evolution
- How comparative anatomy can be used to identify homologous structures
- How comparative biochemistry can be used to determine evolutionary relationships
The theory of evolution by natural selection
Both Darwin and Wallace independently proposed the theory of evolution by natural selection.
They suggested that organisms best suited to their environment are more likely to survive, reproduce, and pass on their advantageous characteristics to their offspring.
Evidence for evolution from the fossil record
Palaeontology, the study of life's history as recorded in fossils, involves examining organisms preserved in rock layers. These fossils show how organisms have gradually changed over time.
Key evidence from the fossil record supporting evolution includes:
- Simple bacteria and algae fossils are found in the oldest rocks, progressing to more complex vertebrates in newer rocks.
- Plant fossils appear before those of animals that feed on these plants, indicating a natural order of evolution.
Anatomical similarities between fossils demonstrate shared evolutionary ancestry. Extinct species can be compared with living species to understand evolutionary relationships.
Why is the fossil record incomplete?
There are a few reasons why we don't have a full record of fossils for every organism that has lived on Earth.
Why the fossil record is incomplete:
- Many organisms decompose before they can fossilise.
- Fossilisation is uncommon, and requires specific conditions for an organism to be preserved.
- Over time, many fossils have been lost due to erosion or geological processes.
- Many organisms have not yet been discovered.
- Certain organisms, especially those with soft bodies, are less likely to fossilise, leading to gaps in the record.
Evidence for evolution from comparative anatomy
Comparative anatomy examines the anatomical structures of different living species to find similarities and differences. It provides evidence for evolution, as organisms with similar structures likely evolved from a common ancestor.
How comparative anatomy provides evidence for evolution:
- Homologous structures are physical features in different species that have a similar underlying structure but may serve different functions.
- Organisms who share homologous structures likely evolved from a common ancestor, and have adapted these structures for different functions.
- Homologous structures are evidence for divergent evolution, where organisms evolve different adaptive traits as they occupy new ecological niches.
An example of a homologous structure is the pentadactyl limb found in vertebrates like mammals, birds, reptiles, and amphibians. Despite their varied uses (running, flying, and swimming), they have a common bone structure, suggesting a shared ancestral origin.
Evidence for evolution from comparative biochemistry
Comparative biochemistry involves studying the molecular aspects of organisms to uncover evolutionary relationships.
Useful molecules to study evolutionary links:
- Cytochrome c - This is a highly conserved protein involved in cellular respiration, so slight changes can help identify evolutionary links.
- Ribosomal RNA - This molecule is integral to protein synthesis so it changes slowly, making it useful for showing connections between species that diverged long ago.
- Nuclear, mitochondrial, or chloroplast DNA - Species that are more closely related will have more similar DNA sequences.
- Messenger RNA - Base sequences of mRNA are complementary to DNA so can assess DNA diversity.
- Amino acids - If they are closely related evolutionarily, two species have more similar amino acid sequences because they are determined by mRNA and DNA.
The hypothesis of neutral evolution
The hypothesis of neutral evolution states that most variability in a molecule's structure does not affect its function.
Why this is useful in the study of evolution:
- 'Neutral' changes that don't affect function accumulate at a fairly regular rate as they are not affected by natural selection.
- Comparing the rates of neutral substitutions in the molecular sequences of different species lets scientists estimate the time since two species diverged from a common ancestor.
- Generally, a greater number of differences indicates a more ancient divergence.