Question

unit 7: natural selection
biology by rachel taylor
directions: for each example, identify and explain the trait variation with the initial advantage, describe the environmental change that took place, and identify and explain the trait variation with the advantage after the environmental change. then, interpret each graph by describing the data

example | variation with initial advantage | environmental change (selective pressure) | variation with advantage after environmental change | graph interpretation
--- | --- | --- | --- | ---
peppered moths | light moths were able to camouflage with light trees and hide from predators | air pollution soiled light bark and turned it dark | dark moths were now able to blend in better with the environment | after the trees became dark, the # of light moths decreased in frequency and the dark moths increased in frequency
elephants | tusks were useful for defense, digging for water and food, and competing for mates | 20th century poaching for ivory became a selective pressure | |
cod fish | larger size allowed for more offspring and better competition for food | intense overfishing targeting larger individuals | cod populations evolve to mature at smaller sizes | the average size of cod decreased over time due to the removal of larger individuals
stickleback fish | heavily armored sticklebacks were protected from large marine predators | move to freshwater from marine: no large predators, nutrients poor, small predators favoring faster - moving fish | lightly armored sticklebacks grow faster, mature sooner, and escape small predators better | marine: high % heavily armored, low % lightly armored; freshwater: low % heavily armored, high % lightly armored
cane toads | | | |

1. cane toad leg length evolution

in 1935, cane toads were introduced to australia to control crop pests. they became invasive and spread rapidly. toads that had longer legs were able to travel farther and colonize new areas faster. however, this rapid movement came at a cost—longer - legged toads suffered joint damage and arthritis, reducing their survival. over time, natural selection favored shorter - legged toads, which were still able to spread but without suffering injuries.

Explanation:

Step1: Analyze Cane Toad Trait Variation

The initial advantage for cane toads was longer legs, which helped them travel farther and colonize new areas faster (supporting rapid spread).

Step2: Identify Environmental Change

The environmental change (selective pressure) is the cost of rapid movement: longer - legged toads suffered joint damage and arthritis, reducing their survival.

Step3: Determine New Advantageous Trait

After this environmental change, shorter - legged toads had the advantage as they could still spread but without the severe joint issues, so natural selection favored them.

Step4: Interpret the Graph

The graph (Cane Toad Leg Length and Joint Damage) likely shows a positive correlation between leg length and joint damage. As leg length increases, joint damage (e.g., measured as severity or prevalence) also increases. So, over time, the frequency of shorter - legged toads would increase because they have less joint damage and better survival, while longer - legged toads decrease due to lower survival from joint problems.

Answer:

• Variation with initial advantage: Longer - legged cane toads could travel farther and colonize new areas faster.
• Environmental change (selective pressure): Longer - legged toads suffered joint damage and arthritis, reducing their survival.
• Variation with advantage after environmental change: Shorter - legged cane toads could still spread but had less joint damage, so they had better survival.
• Graph interpretation: The graph likely shows a positive relationship between leg length and joint damage. Over time, the population of shorter - legged toads increases (as they have higher survival) and longer - legged toads decreases (due to lower survival from joint issues).