Question

natural selection & fitness
helpful definitions
heritable variation - traits (like fur color, size, coat thickness) that can be passed from parent to offspring.
overproduction - organisms produce more offspring than can survive.
selected for - a trait that helps survival and reproduction.
selected against - a trait that reduces survival and reproduction.
selective pressure - environmental factor that “chooses” which traits survive (predators, weather, lack of food, etc.).
fitness - ability to survive and produce offspring that also survive.
part 1: darwin’s 5 points of natural selection practice
directions: read each scenario below and fill in the graphic organizer using evidence from each scenario.
scenario #1: there are three types of polar bears: ones with thick coats, one with medium coats, and one with thin coats. it is fall, soon to be winter time in the tundra. the temperatures are dropping rapidly and the bears must be kept warm or they will freeze to death. many of the bears have had 2 cubs each, but due to the extreme temperatures and harsh conditions, many mothers only have one cub left.
what is the heritable variation in this population?
what is the overproduction in this population?
what is the trait that is being selected for?
what is the selective pressure in this example?
selected against?
make a prediction: how will the population evolve overtime?

Explanation:

• Heritable variation: Polar bears have different coat - thicknesses (thick, medium, thin) which can be passed to offspring.
• Overproduction: Many polar bears have had 2 cubs each but due to harsh conditions, many mothers only have one cub left, indicating more offspring were initially produced than can survive.
• Trait selected for: Thick coats, as they help bears survive the dropping temperatures.
• Selective pressure: The dropping temperatures and harsh winter conditions in the tundra.
• Trait selected against: Thin coats, as they are less likely to keep bears warm in the cold.
• Prediction: Over time, the proportion of polar bears with thick coats will increase in the population as they are more likely to survive and reproduce, while the proportion of those with thin and medium coats will decrease.

Answer:

• What is the heritable variation in this population? Different coat - thicknesses (thick, medium, thin) in polar bears.
• What is the overproduction in this population? Many polar bears having 2 cubs each initially, with only one cub often surviving due to harsh conditions.
• What is the trait that is being selected for? Thick coats.
• What is the selective pressure in this example? Dropping temperatures and harsh winter conditions in the tundra.
• What is the trait selected against? Thin coats.
• Make a Prediction: How will the population evolve overtime? The proportion of polar bears with thick coats will increase, while the proportion of those with thin and medium coats will decrease.