Question

read this passage about a variable inherited trait in brown trout.

a brown trout with its body length shown
brown trout are fish that may migrate, or repeatedly travel between habitats. brown trout that live in oceans often migrate to lakes or rivers to reproduce. to reach the place where they reproduce, the trout may need to pass through many different areas.

some areas are harder for trout with certain body lengths to pass through. a longer trout may struggle to swim through a shallow stream. a shorter trout may struggle to jump over a large dam or waterfall. if the trout cannot pass through these areas, they may be unable to finish migrating. trout that do not finish migrating are less likely to reproduce.

the graph below describes an isolated population of brown trout over a time period long enough for natural selection to occur. the graph shows body length measurements for a representative sample of the population at the start of this period and for another representative sample at the end. at the start of this time period, the brown trouts environment changed.

body length (cm)
sample at start
sample at end
34 38 42 46 50 54 58 62 66 70 74 78 82 86

describe how the brown trout population changed over this time period. use the theory of natural selection to construct a possible explanation for this change.

the average body length ________ in the population. this suggests that trout with shorter bodies were ______ likely to reproduce after the populations environment changed. this may have occurred because the environment changed to have significantly ______ dams. so trout with shorter bodies were more likely to ________ migrating.

Explanation:

1. Analyzing the box plot: The sample at the start has a higher median and range of body lengths shifted to larger values, while the sample at the end has a lower median and range shifted to smaller values, so average body length decreased.
2. Natural selection logic: Traits that improve reproductive success become more common. Since shorter trout became more prevalent, they must have had higher reproductive success.
3. Linking to the passage: Shorter trout struggle less with jumping over dams; if the new environment had more dams, shorter trout would be more likely to complete migration and reproduce.

Answer:

The average body length decreased in the population. This suggests that trout with shorter bodies were more likely to reproduce after the population's environment changed. This may have occurred because the environment changed to have significantly more dams. So trout with shorter bodies were more likely to complete migrating.