Problem 10 - Biology: The Essentials

Question

Some researchers suggest that a giraffe's long neck results from competition for foliage with other types of animals; others say it is the product of sexual selection. How might each mechanism explain how a long-necked species evolved from an ancestral population with short necks? How does each explanation compare to how Lamarck might have explained it?

Step-by-Step Solution

Answer

Natural selection suggests giraffes evolved long necks to reach higher foliage, while sexual selection suggests long necks are attractive to mates. Lamarck would argue giraffes' necks stretched over their lifetimes and this trait was inherited.

Step 1 - Understanding the Evolutionary Mechanisms

Identify the two proposed mechanisms: competition for foliage (natural selection) and sexual selection. Natural selection suggests that giraffes with longer necks could reach higher foliage and survive better, whereas sexual selection implies that longer necks might be more attractive to mates.

Step 2 - Natural Selection Explanation

Explain how natural selection works: Giraffes with longer necks had a survival advantage because they could reach higher leaves in trees, obtaining more food. Over generations, these giraffes reproduced more, passing the long-neck trait to their offspring, leading to a population of long-necked giraffes.

Step 3 - Sexual Selection Explanation

Describe how sexual selection functions: Female giraffes might prefer males with longer necks, as they could be seen as stronger or more dominant. This preference leads to long-necked males reproducing more often, passing this trait to their offspring, resulting in the evolution of long-necked giraffes.

Step 4 - Comparing to Lamarck's Theory

Explain Lamarck's theory: Jean-Baptiste Lamarck would argue that giraffes' necks became longer because they stretched their necks to reach higher leaves during their lifetime. These acquired characteristics were then passed on to their offspring. Contrast this with natural and sexual selection, which rely on genetic traits being passed down without the need for the trait to be acquired during the organism's lifetime.

Key Concepts

Natural Selection

Natural selection is a fundamental concept in evolutionary biology. It explains how traits that are beneficial for survival and reproduction become more common in a population over generations. For giraffes, imagine a scenario where some individuals have slightly longer necks. These giraffes could reach higher leaves on trees, especially during times when food is scarce at lower levels.

Here's how it works:

• Giraffes with longer necks have better access to food.
• These giraffes are more likely to survive and reproduce.
• Their offspring inherit the longer neck trait.
• Over many generations, the population evolves to have longer necks.

This process doesn't happen overnight. It's a gradual change that occurs as those advantageous traits help individuals survive and pass on their genes.

Sexual Selection

Sexual selection is another mechanism that can drive evolution. It focuses on traits that improve an individual's chances of attracting a mate and reproducing. For giraffes, imagine females showing a preference for males with longer necks. This choice might be because longer necks are perceived as a sign of strength or health.

Here's the process:

• Females prefer males with longer necks.
• These males are more successful at finding mates and reproducing.
• Their offspring inherit the longer neck trait.
• Over generations, long necks become more common in the population.

This selective mating leads to an increased frequency of the longer neck trait, demonstrating how sexual selection can shape the physical characteristics of a species.

Lamarckism

Lamarckism is an early evolutionary theory proposed by Jean-Baptiste Lamarck. According to Lamarck, traits acquired during an organism's lifetime could be passed on to its offspring. For giraffes, Lamarck suggested that their ancestors had short necks but stretched them to reach higher leaves. Over time, this stretching would elongate the neck, and the long neck trait would be inherited by their offspring.

Let's break it down:

• Giraffes stretch their necks to reach high foliage.
• These stretched necks are then passed on to their offspring.
• Over generations, necks become progressively longer.

While Lamarck's idea was innovative for its time, we now understand that traits are passed through genetic information, not through acquired characteristics. Natural and sexual selection depend on genetic traits being passed down, not on physical changes happening during an individual's life.