Problem 2 - Campbell Biology

Question

Knowledge/Comprehension According to Hamilton's rule, (A) natural selection does not favor altruistic behavior that causes the death of the altruist. (B) natural selection favors altruistic acts when the resulting benefit to the recipient, corrected for relatedness, exceeds the cost to the altruist. (C) natural selection is more likely to favor altruistic behavior that benefits an offspring than altruistic behavior that benefits a sibling. (D) the effects of kin selection are larger than the effects of direct natural selection on individuals.

Step-by-Step Solution

Answer

Option (B) is correct; \( rB > C \).

Step 1 - Understand Hamilton's Rule

Hamilton's rule is a principle that predicts when an altruistic behavior will be favored by natural selection. The condition is: \( rB > C \), where \( r \) is the genetic relatedness between the altruist and the recipient, \( B \) is the benefit to the recipient, and \( C \) is the cost to the altruist.

Step 2 - Analyze Option (A)

Option (A) states that natural selection does not favor altruistic behavior that causes death of the altruist. This isn't necessarily true according to Hamilton's rule, as it depends on the relative values of \( r \), \( B \), and \( C \).

Step 3 - Analyze Option (B)

Option (B) correctly states Hamilton's rule: natural selection favors altruistic acts when the benefit to the recipient (\( B \)), weighted by relatedness (\( r \)), exceeds the cost to the altruist (\( C \)). In simplified terms, \( rB > C \).

Step 4 - Analyze Option (C)

Option (C) suggests that natural selection is more likely to favor altruism towards offspring over siblings. Hamilton’s rule doesn’t explicitly compare offspring versus siblings; it only provides a condition for when altruism is favored.

Step 5 - Analyze Option (D)

Option (D) implies that the effects of kin selection are larger than the effects of direct natural selection. Hamilton’s rule doesn’t make comparisons of magnitude between kin selection and direct selection; it simply states the condition for kin-selected altruism to occur.

Step 6 - Conclusion

Based on Hamilton's rule, option (B) is the correct answer because it properly describes the condition under which natural selection favors altruistic behavior.

Key Concepts

Natural Selection

Natural selection is a fundamental mechanism of evolution described by Charles Darwin. It explains how traits that improve an organism's chances of surviving and reproducing are passed down through generations.
Individuals with favorable traits are more likely to survive and reproduce, spreading those advantageous traits.
This process leads to the gradual adaptation of species to their environments over time.

For instance, imagine a population of beetles with varied coloring. If birds can easily spot and eat lighter beetles, darker beetles will be less visible and more likely to survive. Over generations, the beetle population will tend to become darker.

Hamilton's rule fits into this by explaining a special case of natural selection involving altruistic behavior, where an individual may help a relative at some cost to itself.

Altruistic Behavior

Altruistic behavior involves actions that benefit others at a cost to the individual performing the act. It's a bit puzzling from an evolutionary standpoint since it seems counterintuitive for an organism to reduce its own fitness to help another.
However, Hamilton's rule offers an explanation. It shows that altruistic behaviors can evolve if they benefit relatives who share the same genes.

A classic example is found in social insects like bees. Worker bees often sacrifice themselves to protect the hive, ensuring the survival of the queen and her offspring, who share a significant proportion of their genes.

Hamilton's rule quantifies this relationship using the formula: \(rB > C\), where:

• \(r\) is the genetic relatedness between the altruist and the recipient
• \(B\) is the benefit to the recipient
• \(C\) is the cost to the altruist

If the benefit to the recipient, adjusted for relatedness, exceeds the cost to the altruist, then the behavior is favored by natural selection.

Genetic Relatedness

Genetic relatedness is a measure of how many genes two individuals share due to common ancestry.
It's a critical factor in Hamilton’s rule and kin selection because it dictates how much an altruistic act benefits the altruist's own genes.
For example:

• Parent and child share 50% of their genes, so \(r = 0.5\)
• Siblings also share 50% of their genes, so \(r = 0.5\)
• Half-siblings share 25% of their genes, so \(r = 0.25\)
• First cousins share about 12.5% of their genes, so \(r = 0.125\)

These values help determine the likelihood that altruistic behaviors will evolve.
For instance, helping your sibling (\(r = 0.5\)) is twice as beneficial in genetic terms as helping your half-sibling (\(r = 0.25\)).
This framework helps biologists understand the dynamics of family cooperation and conflict in the animal kingdom.