What is the null hypothesis for the pelvic spine frequency in stickleback populations?

• A. A random sample would show no difference in spine presence between populations.
• B. A random sample would contain roughly the same number with and without pelvic spines.
• C. All individuals have pelvic spines.
• D. No stickleback population has pelvic spines.

Answer: (B)

In testing a trait’s frequency, the default assumption is that there’s no bias in how that trait appears. For pelvic spine presence in sticklebacks, this means a random sample would show about equal numbers with and without pelvic spines. That sets a clear baseline: the proportion with spines is roughly 0.5. If the data deviate from this (significantly more or fewer than half), it suggests a real difference in frequency that might be due to population differences or environmental factors. The other options describe extreme states or focus on whether populations differ, but they don’t state the basic expected distribution in a random sample the way the null about equal presence and absence does.