Comparative Methods Flashcards
What is the comparative method
A way to explain variation in traits (like behaviour) across species by comparing patterns and drawing broader inferences
Why is the comparative method useful
It allows us to go beyond single-species studies and explore how ecological and evolutionary factors shape traits across species
What was Darwin’s (1871) contribution to comparative methods
He studied sexual selection and compared polygamous vs monogamous species to understand trait differences
What did John Crook study in the 1960s
Social systems in weaver birds and their ecological correlates, especially diet and mating systems
Describe the red-headed weaver’s social and mating system
Monogamous, lives in dispersed territories, both parents raise the young
Describe the southern masked weaver’s social and mating system
polygynous, lives in colonies, males often compete for multiple females
What correlation did Crook find in weaver birds
Monogamy associated with insect diets; polygamy associated with seed diets
Why do insect-based diets lead to monogamy
Insects are dispersed and hard to gather alone, requires both parents to feed offspring efficiently
Why do seed-based diets allow polygamy
Seeds are found in large patches, can support group foraging, freeing males to compete for mates
What are the three problems with Crook’s data
- Species aren’t independent data points
- Correlation ≠ causation
- Direction of causality unclear
What are two key limitations of comparative methods
- Ignoring alternative hypotheses
- Failing to quantify ecological variables properly
What is sexual size dimorphism
Differences in body size between males and females of a species
What patterns were found in primate sexual dimorphism
Species with more females per male (higher sexual selection) show greater male size
What alternative hypothesis could explain size dimorphism
Niche differentiation - reducing competition between sexes
Why aren’t species always independent data points
Closely related species may share traits due to common ancestry, not independent adaptation
What is pseudoreplication in comparative studies
Mistaking related species as independent samples, inflating sample size and misleading results
How do independent contrasts solve non-independence
By comparing trait differences between closely related species pairs and treating those differences as data points
What is the benefit of using independent contrasts
They control for shared ancestry, making comparisons statistically valid
What’s a key limitation of comparative methods overall
Observational - can’t prove causation, only correlation
What is a strength of comparative methods
Identify broad evolutionary trends and test hypotheses across many species
What are the strengths of experimental studies
Provide detailed, causal understanding of specific behaviours
What are the limitations of experimental studies
May lack generality; not all questions are testable experimentally
How do comparative and experimental methods complement each other
Comparative methods show big-picture patterns, experiments test mechanisms behind them
