Evolution Evidence and Mechanisms

Grade 9 · Science · Study Guide

Examine the multiple lines of evidence supporting evolution and the key mechanisms — natural selection, mutation, genetic drift, and gene flow — that drive evolutionary change.

Evolution is the change in heritable characteristics of biological populations over successive generations. Charles Darwin and Alfred Russel Wallace independently proposed natural selection as the primary mechanism of evolution in 1858. The theory is supported by multiple independent lines of evidence.

Fossil evidence shows a progression of life forms over geological time, including transitional fossils like Tiktaalik (fish to tetrapod) and Archaeopteryx (dinosaur to bird). Comparative anatomy reveals homologous structures — such as the forelimbs of humans, whales, bats, and cats — which share a common ancestral skeleton despite different functions. Vestigial structures (like the human coccyx or whale pelvis remnants) are reduced organs that had functions in ancestors. Molecular evidence — similarities in DNA and protein sequences among related species — provides the most precise measure of evolutionary relationships.

Natural selection occurs when individuals with favorable heritable traits survive and reproduce more successfully than others, leading those traits to increase in frequency over generations. For natural selection to occur, there must be variation in the population, the variation must be heritable, and differential reproductive success must result. Other mechanisms include mutation (the ultimate source of new genetic variation), genetic drift (random changes in allele frequencies, most impactful in small populations), and gene flow (movement of alleles between populations). Speciation occurs when populations become reproductively isolated and diverge into separate species.

Key terms

Natural Selection: The process by which organisms with favorable traits survive and reproduce more successfully, increasing those traits in the population.
Homologous Structures: Anatomical structures in different species that share a common ancestral origin, suggesting evolutionary relatedness.
Vestigial Structure: A reduced or non-functional organ that was functional in an ancestor, serving as evidence of evolution (e.g., human appendix).
Genetic Drift: Random changes in allele frequencies in a population, particularly impactful in small populations; not driven by selection.
Speciation: The evolutionary process by which reproductively isolated populations diverge and form new, distinct species.
Transitional Fossil: A fossil that shows intermediate characteristics between an ancestral group and its descendant group, documenting evolutionary change.

Flashcards

Evolution: Change in the heritable characteristics of a population over successive generations through mechanisms including natural selection.
Natural Selection: Differential reproductive success based on heritable variation; drives adaptation over generations.
Adaptation: A heritable trait that increases an organism's fitness (survival and reproductive success) in its environment.
Gene Flow: The movement of alleles between populations through migration; tends to reduce genetic differences between populations.
Analogous Structures: Structures in different species that have similar functions but different evolutionary origins (e.g., bird wing and insect wing).
Fossil Record: The preserved remains and traces of past organisms in rock layers; provides a timeline of life's history.
Artificial Selection: Selective breeding by humans to enhance desirable traits in plants and animals; demonstrates how selection changes populations.
Reproductive Isolation: Any barrier that prevents gene flow between populations, a prerequisite for speciation.

Questions

Which type of evidence compares DNA and protein sequences to determine evolutionary relationships?
1. Fossil evidence
2. Comparative anatomy
3. Molecular evidence
4. Biogeography
Show answer
Answer: Molecular evidence
Molecular evidence compares DNA sequences, RNA, and protein sequences across species. More similar sequences indicate more recent common ancestry, providing a quantitative measure of evolutionary distance.
Natural selection can act on traits that are not heritable and therefore cannot be passed to offspring.
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Answer: False
Natural selection can only act on heritable traits — those encoded in DNA that can be passed from parents to offspring. Acquired traits (like a scar) are not heritable and cannot be selected for.
The random change in allele frequencies in a small population, not due to natural selection, is called genetic ___.
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Answer: drift
Genetic drift is the random fluctuation of allele frequencies due to chance events, particularly powerful in small populations. It can cause alleles to be lost or fixed regardless of their adaptive value.
The wing of a bat and the arm of a human have the same underlying bone structure. These are examples of:
1. Analogous structures
2. Vestigial structures
3. Homologous structures
4. Convergent structures
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Answer: Homologous structures
Homologous structures share the same underlying anatomy (same bones: humerus, radius, ulna, carpals) because they were inherited from a common ancestor, even if they serve different functions today.
Mutation is the ultimate source of new genetic variation in a population.
Show answer
Answer: True
Mutations are random changes in DNA sequence that create new alleles. Without mutation, there would be no new genetic variation for natural selection, genetic drift, or gene flow to act upon.

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