The main idea here is that sticklebacks are a powerful model for rapid adaptation because they show repeated, predictable evolutionary changes in many independent populations within a short time frame. This parallel evolution occurs when freshwater sticklebacks repeatedly evolve similar traits—like reduced armor plates and pelvic girdles—despite starting from related marine populations. The consistency of these changes across different populations reveals natural selection acting under similar ecological pressures, making it possible to study both the phenotypic shifts and their genetic underpinnings, often in just a few generations. In this system, researchers can link quick changes in visible traits to specific genetic changes, explore how different populations converge on the same solutions, and compare how similar environments drive similar outcomes. The fact that these evolutions happen repeatedly and quickly, across many independent populations, makes sticklebacks an especially informative model for understanding evolution in action. The other statements don’t fit because the genome is not simply trivial to analyze, sticklebacks do respond to selection (they are classic examples of rapid adaptive change), and they are not restricted to laboratories—they are found in wild freshwater and marine environments worldwide.

This question is about showing evolution in action through repeated natural experiments. Sticklebacks are powerful model organisms because they undergo rapid, repeated evolutionary changes after shifting from marine to freshwater environments in postglacial lakes. In many independent lake populations, they converge on similar traits—like reduced armor plating and changes in pelvic structures—despite being geographically separate. This parallel evolution provides a clean, natural setup to compare how selection acts and how genetic variation is used to produce adaptive traits, all within short timescales that we can study directly. The repeated, predictable patterns across many populations make them ideal for uncovering how evolution proceeds in response to ecological change. Regarding the other statements: sticklebacks do have genetic variation and do not rely on ancient fixed traits; their ongoing adaptation is well documented. Fossil records exist for the lineage, but the strongest evidence for their use as a model comes from contemporary populations and genetic data showing repeated, parallel evolution.

This question is about proving causality by directly manipulating the genetic variant and watching the effect on the trait. The strongest way to test if a variation in Eda causes changes in plate number is to swap the Eda allele between individuals using genome editing and then compare plate numbers in a shared genetic background. By editing only the Eda allele and keeping all other genes and the environment the same, any difference in plate number can be attributed to the specific Eda variant. If the swapped allele leads to a different plate count, that provides direct evidence that Eda variation drives the trait. Observing natural populations or comparing knockouts alone introduces confounds. A cross between species or transferring Eda across backgrounds brings many uncontrolled genetic differences and environmental factors, muddying the link between Eda variant and plate number. A knockout shows whether Eda is necessary for the trait, but it doesn’t test how natural allelic differences influence plate number in a controlled background.

The main idea is that historical selection can leave lasting traces in the genome that stick around even after the selective pressure has ended. When a beneficial allele sweeps through a population, nearby variants can hitchhike with it, creating a sweep that reduces genetic diversity and creates extended regions of linkage. After selection ends, recombination gradually erodes these patterns, but remnants—like unusually high frequencies of certain alleles and lower diversity in the surrounding region—can persist as a “ghost” of the past selection. Those genomic signatures let us infer that adaptation occurred in the past, even though the current conditions no longer favor that allele. So the best description is that past selection leaves detectable signatures in the genome, such as elevated allele frequencies or reduced diversity, even after selection ends. The other options don’t capture this idea: phenotypic changes disappearing isn’t about lingering genomic traces, the notion that ghosts cause spurious signals misstates the concept, and it’s false to say selection leaves no genomic signature at all.