1.5.2Additional lessons
Evidence of Evolution — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 5 of 142
Part 1 of 5What Evolution Means
Evolution is a population-level process driven by inherited variation.
Definition
- Evolution is the change in the heritable characteristics of a population over successive generations, not change within one lifetime.
Heritable traits
- Only characteristics controlled by alleles can be passed on, so a plant losing leaves to grazing does not count as evolution.
Variation
- Random mutation and sexual reproduction produce new alleles and combinations, some of which make survival or reproduction more likely.
Natural selection
- Individuals with advantageous alleles leave more offspring, so those alleles become more common in the population over time.
Common ancestry
- Repeated splitting of lineages explains why all living species share features such as DNA, ribosomes and the genetic code.
Part 2 of 5Lamarck and Darwin Compared
Lamarck
- Proposed that organisms change during life through use and disuse, then pass these acquired characteristics to their offspring.
Darwin and Wallace
- Proposed that variation already exists, and that the environment selects which individuals survive to reproduce.
Giraffe example
- Lamarck
- stretching made necks longer.
- Darwin: longer-necked individuals reached more food and left more offspring.
Why Darwin is accepted
- Acquired changes such as muscle gained through exercise are not encoded in gametes,
- whereas selection acting on alleles is observed.
| Feature | Lamarckian view | Darwinian view |
|---|---|---|
| Source of change | Effort during life | Pre-existing heritable variation |
| Passed on? | Acquired traits inherited | Only allele-based traits inherited |
| Mechanism | Use and disuse | Differential survival and reproduction |
Part 3 of 5Evidence for Common Descent
Several independent lines of evidence point to the same conclusion.
Evidence for Common Descent
Sequence data
- Closely related species have more similar DNA and protein sequences,
- because fewer mutations have accumulated since their last common ancestor.
Molecular clock
- Mutations build up at a roughly steady rate, so counting sequence differences estimates when two lineages diverged.
Homologous structures
- The pentadactyl limb of humans, bats, whales and frogs has the same bone layout adapted to different uses.
Selective breeding
- Humans choose parents with desired traits;
- cabbage, kale and broccoli all descend from one wild species, showing selection can reshape a species.
Part 4 of 5Using Sequence Data
1
Extract DNA from the cells of each species being compared, or obtain the amino acid sequence of a shared protein such as cytochrome c.
2
Align the sequences so that corresponding positions can be compared one by one between the species.
3
Count the number of positions at which the bases or amino acids differ for each pair of species.
4
Fewer differences indicate a more recent common ancestor, and more differences indicate an older split.
5
Use the pattern of differences to build a phylogenetic tree showing how the species are related.
Part 5 of 5Exam Checklist & Tips
1.5.2 – Key Points for the Exam
- Evolution Change in the heritable characteristics of a population over generations.
- Homologous structure A structure with the same underlying anatomy in different species because of common ancestry, even if its function differs.
- Acquired characteristic A change gained during the life of an organism, which is not inherited unless it alters the alleles in gametes.
- Selective breeding Artificial selection in which humans choose which individuals reproduce, to increase desired traits.
- Molecular clock Use of the steady rate of mutation to estimate the time since two species diverged.
Common Mistakes
- Describing evolution as an individual changing during its lifetime rather than a population changing over generations.
- Claiming organisms evolve because they "need" a trait;
- variation arises by chance and selection acts afterwards.
- Treating homologous structures as having the same function; they share anatomy, not necessarily use.
- Saying DNA differences prove one species evolved from another living species, when they only show shared ancestry.
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