Evolution Natural Selection Exercise 1 Answer Key

Understanding Evolution Through Natural Selection: A Detailed Exercise Answer Key and Explanation

The concept of natural selection stands as the cornerstone of modern evolutionary biology, providing a powerful, evidence-based mechanism for understanding the diversity of life on Earth. For students, working through targeted exercises is crucial for moving beyond memorizing definitions to truly internalizing how this process operates. This article provides a comprehensive answer key and in-depth explanation for a foundational "Evolution and Natural Selection" exercise, designed to clarify common points of confusion and solidify your grasp of the principles Charles Darwin first elucidated. The goal is not merely to supply answers, but to illuminate the reasoning behind them, transforming a simple answer key into a robust learning tool that builds lasting comprehension.

Deconstructing the Exercise: What Are We Really Testing?

Before diving into specific questions, it’s essential to understand the core tenets an exercise on this topic typically assesses. Natural selection is not a random process of change, but a non-random filter acting on existing variation. It can be broken down into four fundamental, sequential principles:

1. Variation: Individuals within a population exhibit heritable differences in traits (e.g., beak size, fur color, metabolic rate).
2. Inheritance: These traits are passed from parents to offspring through genetic transmission.
3. Selection: Environmental pressures (predation, climate, resource availability) cause some variants to survive and reproduce more successfully than others. This is often summarized as "differential reproductive success."
4. Time: Over many generations, the frequency of advantageous traits increases in the population, leading to evolutionary change.

A well-designed exercise will test your ability to identify these components in scenarios, distinguish natural selection from other processes like genetic drift or Lamarckian inheritance, and apply the logic to novel examples.

Exercise 1: Scenario Analysis and Principle Identification

Question 1: In a population of beetles, a mutation causes some individuals to have a green shell instead of the usual brown. Birds preferentially eat the brown beetles, as they are easier to spot against the green foliage. Over several generations, the proportion of green beetles increases.

• A) This is an example of natural selection because the environment (birds) selected for the green trait.
• B) This is genetic drift because the change is due to a random mutation.
• C) This is an example of Lamarckian inheritance because the beetles changed color to hide.
• D) This is not evolution because the beetles were always green.

Answer & Detailed Explanation: The correct answer is A. This scenario perfectly illustrates all four principles of natural selection.

• Variation: The population has two shell color variants (brown and green), originating from a random mutation.
• Inheritance: The green shell color is a heritable genetic trait.
• Selection: The selective pressure is predation by birds. The green beetles have a survival advantage (fitness advantage) in the green foliage environment, meaning they are more likely to survive to reproduce.
• Time: The change in the population's trait frequency (more green beetles) occurs "over several generations." Option B is incorrect because while the mutation is random, the increase in its frequency is non-random—it is directly caused by the selective pressure of bird predation. Genetic drift refers to random changes in allele frequency, especially in small populations, without regard to fitness. Option C describes the discredited idea of "inheritance of acquired characteristics," where an organism changes during its lifetime and passes that change on (e.g., a beetle straining to become green). The beetles did not change color; the proportion of already-green individuals changed. Option D is false because a change in the frequency of heritable traits in a population over generations is the definition of evolution.

Question 2: A population of bacteria is exposed to an antibiotic. Most bacteria die, but a few survive because they possess a random genetic mutation that confers resistance. These resistant bacteria reproduce, and soon the entire population is antibiotic-resistant.

• A) The bacteria developed resistance in response to the antibiotic.
• B) The antibiotic caused the mutation for resistance.
• C) The resistant bacteria were already present; the antibiotic simply selected for them.
• D) This is an example of artificial selection.

Answer & Detailed Explanation: The correct answer is C. This is the classic, real-world example of natural selection in action, often seen in the evolution of antibiotic resistance.

• The key insight is that the genetic variation (resistance) existed before the selective pressure (the antibiotic) was applied. The antibiotic did not create the resistance; it selected for the pre-existing resistant individuals by killing off the non-resistant majority. The survivors, now with no competition, reproduce and pass on the resistance gene. Option A implies a Lamarckian "need-based" change, which is incorrect. Option B confuses correlation with causation; the antibiotic is the selective agent, not the mutagen. While antibiotics can sometimes increase mutation rates, the specific resistance mutation is random and pre-existing. Option D is wrong because artificial selection involves directed breeding by humans (e.g., for crops or pets). Here, the selective pressure (the antibiotic) is environmental and non-directed.

Question 3: Which of the following is NOT a necessary condition for natural selection to occur?

• A) Heritable variation in traits.
• B) Differential reproductive success linked to those traits.
• C) A large population size.
• D) Environmental pressure that favors some variants over others.

Answer & Detailed Explanation: The correct answer is C) A large population size. While large populations can provide more raw material (variation) and make the effects of selection more statistically stable, natural selection can and does occur in small populations. The four core principles listed in the first section do not include population size. In fact, in very small populations, genetic drift can overpower selection, but selection is still a possible force. The absolute requirements are variation, inheritance, and differential success driven by the environment. A population of 10 beetles with a strong selective pressure (like a new predator) can still undergo natural selection if the heritable trait affects survival.

Deep Dive: Applying the Logic to Classic Case Studies

To master this concept