GED Science: Evolution and Natural Selection Concepts
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Overview
This chapter covers how populations change over many generations and the evidence that living things share common ancestors. GED questions often describe a population over time or show a cladogram.
Natural selection needs inherited variation: individuals differ in traits that can be passed on. When some traits help individuals survive and reproduce in their environment, those traits become more common over generations. Examples include bacteria becoming resistant to antibiotics and dark peppered moths becoming more common on soot-darkened trees. When people choose which plants or animals breed, it is artificial selection.
Evidence of common ancestry includes similar bone structures in different species, such as the front limbs of humans, whales, and bats. A cladogram is a branching diagram of relationships; groups that share a more recent branch point are more closely related. When a population is split by a barrier, the groups can change until they can no longer interbreed, forming new species. This is speciation.
Key concepts
Natural selection
Individuals with helpful inherited traits survive and reproduce more, so those traits become more common over generations.
Memory tipPopulations change, not individuals.
Artificial selection
People choose which organisms breed to get desired traits, such as cows that give more milk.
Memory tipPeople choose = artificial; the environment chooses = natural.
Adaptation
An inherited trait that helps an organism survive and reproduce in its environment.
Memory tipAdaptations are inherited, not learned during one lifetime.
Homologous structures
Body parts with the same basic bones but different jobs, such as human arms and bat wings. They suggest common ancestry.
Memory tipSame bones, different jobs.
Cladogram
A branching diagram. A more recent shared branch point means a closer relationship.
Memory tipRead from the branch points, not from left to right.
Speciation
Formation of a new species, often after groups are separated and can no longer interbreed.
Memory tipA barrier plus many generations can make new species.
Often tested
- Organisms do not change their traits on purpose; selection acts on variation that already exists.
- Antibiotic resistance is natural selection in action.
- A cladogram does not show one living species turning into another.
3 sample questions
Verified practice questions from this unit.
Question 1. For natural selection to happen, the individuals in a population must
- ① all be exactly the same
- ② differ in traits that can be passed to offspring
- ③ never compete for food or mates
- ④ choose which traits to pass on
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Answer: ② differ in traits that can be passed to offspring
Key point: Natural selection needs inherited variation
Natural selection needs variation (differences) in traits, and those traits must be inherited. Individuals with helpful traits survive and reproduce more, so those traits become more common over generations.
Wrong choices
- all be exactly the same: If every individual were identical, no trait could be favored over another.
- never compete for food or mates: Competition for limited resources is part of what drives selection.
- choose which traits to pass on: Organisms cannot choose which traits they pass on.
Question 2. When an antibiotic is used, most bacteria die, but a few survive because they carry a gene for resistance. If this happens again and again over many generations, what is the most likely result?
- ① All bacteria of this kind die out.
- ② Resistant bacteria become more common in the population.
- ③ Each bacterium changes its own DNA on purpose to resist the drug.
- ④ The resistance gene disappears from the population.
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Answer: ② Resistant bacteria become more common in the population.
Key point: Selection makes helpful inherited traits more common
The antibiotic acts as a selection pressure. Bacteria with the resistance gene survive and reproduce, passing the gene on. Over many generations, a larger share of the population is resistant.
Wrong choices
- All bacteria of this kind die out.: The resistant bacteria survive and multiply, so the population does not die out.
- Each bacterium changes its own DNA on purpose to resist the drug.: Bacteria do not change on purpose. The resistant ones were already there and survived.
- The resistance gene disappears from the population.: The drug kills the bacteria without the gene, so the gene becomes more common, not less.
Question 3. In one area of England, about 10% of peppered moths were dark before factories were built. After many years of factory soot darkened the tree bark, about 90% of the moths were dark. Which explanation is best supported?
- ① Birds preferred to eat dark moths.
- ② Dark moths were harder for birds to see on dark bark, so more of them survived and reproduced.
- ③ Soot on the trees turned light moths dark.
- ④ The moths chose to become dark to hide.
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Answer: ② Dark moths were harder for birds to see on dark bark, so more of them survived and reproduced.
Key point: Camouflage that helps survival becomes more common over generations
Moth color is inherited. On dark bark, dark moths were better hidden from birds, so they survived and reproduced more. Over generations, the dark form became more common. This is natural selection.
Wrong choices
- Birds preferred to eat dark moths.: If birds ate more dark moths, dark moths would have become rarer, not more common.
- Soot on the trees turned light moths dark.: Soot does not change a moth's genes. Moth color is an inherited trait.
- The moths chose to become dark to hide.: Organisms do not choose to change their inherited traits.
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