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Lecture 6

BIOB51H3 Lecture 6: Lecture 6


Department
Biological Sciences
Course Code
BIOB51H3
Professor
Maydianne Andrade
Lecture
6

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Lecture 6
B. Comparative Method
Questions: Macroevolutionary questions (Pattern)
= origin of major differences among taxa
Factors leading to the evolution of some trait
Hypotheses about the origin of adaptations
Questions about evolutionary HISTORY
Technique: comparison of patterns of origin of traits across taxa (e.g., across species,
genera, etc.)
Assessment of relationship between evolution of X and evolution of Y
Compare to: microevolutionary questions examining change WITHIN species
Example 1
- Gorilla gorilla
- Pan troglodytes
Observations in primates:
Significant variation in testes size
Q: What explains the diversity of testes size in primates?
Hypothesis 1: Physiological requirement
Larger testes are present in larger species because larger endocrine glands are required to
maintain adequate testosterone levels in the body
Prediction:
Testes mass will increase with body mass across species.
Data: Testes mass increases with body mass across
primates
BUT there are some species that are quite off the line
BUT: Exceptions suggest physiology is not sufficient to explain all variation
Observations:
Gorilla 4x body size of chimp
Gorilla testes ¼ weight of chimp
Observation:
Different mating systems (relevant?)
o Gorillas are single male maters (from the perspective of a female)

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Females only mate with single male during one fertile period
o Chimpanzees have a multi-male mating system
Females often mate with multiple males in a single fertile
period
Why might different mating systems affect the evolution of a diversity of testes sizes?
Sperm competition: competition among the sperm of different males for fertilization of the
eggs of a single female
Q (pattern): Does mating system determine the evolution of testes size in primates?
Theory:
Sperm competition: competition among the sperm of different males for fertilization of the
eggs of a single female
Occurs if females mate with > 1 male during fertile period
One male adaptation to ‘win’ fertilization = increased ejaculate volume (more sperm)
Fact:
Larger ejaculates are produced by larger testes
Q: What explains the diversity of testes size in primates?
Hypothesis 2: Evolutionary response to sperm competition
Evolution of testes size depends on mating system
Prediction:
Larger testes (Y) evolve when females evolve to mate with more than one male (X)
TEST requires identification of ‘natural experiments’ in the PAST when a new mating system
evolved in a taxon = ‘X’ changed.
Does Y change as predicted?
Q (pattern): Does mating system determine the evolution of testes size in primates?
NOTE: To test hypothesis 2, must control for body size effect (already documented in test of
hypothesis 1)
Hypothesis 2: Evolution of testes size depends on mating system

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Done? NO
Problem: no consideration of evolutionary history (each species treated as an independent data
point)
Why is this a problem?
Prediction:
Larger testes (Y) evolve when females evolve to mate with more than one male (X)
TEST requires ‘natural experiments’ in the PAST when a new mating system evolved in a taxon =
‘X’ changed.
Does Y change as predicted?
Hypothetical evolutionary sequence
Additional speciation events, diversification independent of mating system
- No change in mating system
1. Change in mating system leads to evolution of difference in testes size (ONE
evolutionary event)
2. All other observed cases of association between mating system & testes size are due to
inheritance
NOT due to independent evolutionary events
Single-male systems, testes smaller than average
Multi-male systems, testes larger than average
P < 0.05 supporting our hypothesis
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