read this passage about a variable inherited trait in alewives.
alewives are small fish found in oceans and lakes. alewives eat prey such as zooplankton, which come in different sizes.
alewives with a bigger gape width, which is the distance between the corners of the fish’s mouth, are better at catching bigger zooplankton. alewives with a smaller gape width are better at catching smaller zooplankton.
the graphs below describe an isolated population of alewives over a time period long enough for natural selection to occur. the graphs show gape width 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 alewives’ environment changed.
graphs: sample at start (purple dots) and sample at end (blue dots) with gape width (mm) on x - axis from 3 to 6
describe how the alewife population changed over this time period. use the theory of natural selection to construct a possible explanation for this change.
the average gape width in the population. this suggests that alewives with larger gape widths were likely to survive and reproduce than alewives with smaller gape widths. this may have occurred because the average size of zooplankton in the alewives’ environment . after the environment changed, alewives with larger gape widths were likely to be at catching food.

Question

read this passage about a variable inherited trait in alewives.
alewives are small fish found in oceans and lakes. alewives eat prey such as zooplankton, which come in different sizes.
alewives with a bigger gape width, which is the distance between the corners of the fish’s mouth, are better at catching bigger zooplankton. alewives with a smaller gape width are better at catching smaller zooplankton.
the graphs below describe an isolated population of alewives over a time period long enough for natural selection to occur. the graphs show gape width 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 alewives’ environment changed.
graphs: sample at start (purple dots) and sample at end (blue dots) with gape width (mm) on x - axis from 3 to 6
describe how the alewife population changed over this time period. use the theory of natural selection to construct a possible explanation for this change.
the average gape width  in the population. this suggests that alewives with larger gape widths were  likely to survive and reproduce than alewives with smaller gape widths. this may have occurred because the average size of zooplankton in the alewives’ environment . after the environment changed, alewives with larger gape widths were likely to be  at catching food.

Accepted Answer

# Brief Explanations:
To solve this, we analyze the graphs and natural selection principles:
1. **Average gape width change**: At start, purple dots (sample) have more at 4, some at 5, one at 6. At end, blue dots have more at 3, 4, 5. Wait, no—wait, count the dots. Wait, start: 4 has 5, 5 has 4, 6 has 1. End: 3 has 3, 4 has 5, 5 has 2. Wait, no, maybe I miscounted. Wait, the first graph (start) has 5 at 4, 4 at 5, 1 at 6. Second (end) has 3 at 3, 5 at 4, 2 at 5. Wait, no, maybe the x - axis is gape width. Wait, the key is to see the shift. Wait, maybe the average gape width decreased? No, wait, maybe I made a mistake. Wait, the problem is about alewives and natural selection. Let's think about the blanks:

First blank: The average gape width (increased/decreased/ stayed same). Looking at the graphs: Start sample: 4 has 5, 5 has 4, 6 has 1. End sample: 3 has 3, 4 has 5, 5 has 2. Wait, no, maybe the number of dots at lower gape widths (3) increased, and at higher (6) disappeared. Wait, maybe the average gape width **decreased**? No, wait, maybe I got it wrong. Wait, the second blank: alewives with larger gape widths were (more/less) likely to survive. If the average gape width decreased, then larger gape widths were less likely. Third blank: average size of zooplankton (increased/decreased). If larger gape widths are less successful, maybe zooplankton size decreased, so smaller gape widths are better. Fourth blank: alewives with larger gape widths were likely to be (better/worse) at catching food. If zooplankton is smaller, larger gape widths are worse.

Wait, let's re - examine the graphs. Sample at start: gape widths 4 (5 dots), 5 (4 dots), 6 (1 dot). Sample at end: 3 (3 dots), 4 (5 dots), 5 (2 dots). So the number of alewives with gape width 3 (smaller) increased, and 6 (larger) disappeared. So average gape width **decreased**. Then, alewives with larger gape widths were **less** likely to survive. This may be because zooplankton size **decreased** (so smaller gape widths are better for catching). Then, alewives with larger gape widths were likely to be **worse** at catching food.

But let's check the logic of natural selection. If the environment changed, and the population's gape width shifted towards smaller, then the selective pressure favored smaller gape widths. So:

1. The average gape width **decreased** in the population.
2. Alewives with larger gape widths were **less** likely to survive and reproduce.
3. This may have occurred because the average size of zooplankton **decreased**.
4. After the environment changed, alewives with larger gape widths were likely to be **worse** at catching food.

But maybe I made a mistake. Alternatively, if the average gape width increased, then larger gape widths were more likely, zooplankton size increased, and larger gape widths were better. But the graphs show more dots at 3 (smaller) and less at 6 (larger). So probably the average gape width decreased.

So filling the blanks:

1. decreased
2. less
3. decreased
4. worse

But let's confirm with the theory of natural selection. Natural selection acts on heritable traits. If the environment changes (e.g., zooplankton size decreases), then individuals with traits (smaller gape width) that are better adapted to the new environment (catching smaller zooplankton) will survive and reproduce more, leading to a change in the population's average trait (decreased gape width). So alewives with larger gape widths (less adapted to smaller zooplankton) are less likely to survive and reproduce. Zooplankton size decreased, so larger gape widths are worse at catching smaller zooplankton.

# Answer:
The average gape width $\boldsymbol{\text{decreased}}$ in the population. This suggests that alewives with larger gape widths were $\boldsymbol{\text{less}}$ likely to survive and reproduce than alewives with smaller gape widths. This may have occurred because the average size of zooplankton in the alewives’ environment $\boldsymbol{\text{decreased}}$. After the environment changed, alewives with larger gape widths were likely to be $\boldsymbol{\text{worse}}$ at catching food.

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