Unit 7 · Natural Selection

The decision that organizes this unit

Mechanism route and repair branches

1. Main route: Describe the variation among individuals and identify evidence that at least part of the variation can be inherited.
2. Main route: Connect the environmental condition to survival or reproduction through a specific trait mechanism rather than a statement of need.

• Diagnostic cue: Organisms are said to develop a needed trait and pass it on. Wrong branch: Environmental demand directly creates an adaptive inherited change. Repair: Begin with existing or newly generated heritable variation, then connect the trait to differential reproductive contribution.

1. Main route: Predict the direction of allele or trait-frequency change across generations while preserving migration, drift, and mating alternatives.

• Diagnostic cue: A high-fitness individual is described as strongest or longest-lived. Wrong branch: Fitness is detached from reproduction and environment. Repair: Define fitness as relative contribution of heritable variants to later generations under the specified conditions.

1. Main route: Use Hardy-Weinberg or another null model as a comparison and avoid naming a force from model departure alone.
2. Main route: For phylogeny, identify the most recent shared node, define the clade, and treat rotation around a node as topology-preserving.

• Diagnostic cue: Two adjacent tips are called closest relatives. Wrong branch: Page position replaces node history. Repair: Trace each lineage backward to its most recent common ancestor and ignore rotations that leave connections unchanged.

Load-bearing representation lab

Trait means rise in chronological order from generation 0 to generation 5 to generation 10, and the displayed 95-percent intervals do not overlap. The directional shift is therefore larger than the represented uncertainty across these sampled generations. If the trait variation is heritable and the comparison conditions are appropriate, such a repeated generational change can support directional selection. The figure alone does not establish heritability, fitness differences, or the selective agent, so those conditions must be supplied by the study rather than inferred from rising means. Compute each interval from its printed mean and half-width before describing separation; do not replace the stated 95-percent interval with standard deviation language. Another common error calls the increase adaptive solely because the mean rises. Drift, environmental change, and sampling can also shift a phenotype, and adaptation specifically requires a heritable fitness-related change. A defensible response cites the ordered means and nonoverlapping intervals, then states which additional evidence would connect that pattern to natural selection. Generation labels are time points, not independent treatment names.

Translate the drawing into nested groups rather than measuring visual distance between tips. A and B share a recent internal node, C and D share another, and all four ingroup taxa belong to the larger clade beyond the outgroup split. Those ancestor-descendant sets define the represented topology. The note that branch lengths are not to scale means horizontal or vertical spacing cannot be used as elapsed time or amount of evolutionary change. Rotating descendants around an internal node can change left-to-right order while leaving every clade and most-recent-common-ancestor answer unchanged. A common misread calls adjacent labels closest relatives even when they do not share the relevant node, or treats the outgroup as an ancestor of the other tips. For any pair, trace both branches backward until they meet and compare that node with the meeting node for an alternative pair. The tree supports relative common ancestry, not a claim that living taxa evolved from one another. It also carries no character-state evidence by itself; that separate evidence must be mapped onto the topology explicitly.