Sexual Selection & Mating Systems

Lecture 7: Sexual Selection & Mating Systems

Sexual Selection vs. Natural Selection

• Sexual selection favors traits increasing reproductive success.
• Sexual selection is linked to mating success, not necessarily survival.
• The two types of sexual selection are intrasexual and intersexual selection.
• Intrasexual selection involves competition within a sex.
• Intersexual selection involves one sex choosing a mate based on traits like bright plumage.

Why is Mate Choice Important?

• Females are usually choosier due to higher parental investment.
• Mate choice ensures better genes or resources for offspring.
• Mate choice leads to direct benefits like food, territory, and protection.
• Mate choice leads to indirect benefits like good genes for offspring.

Assessments - Proximate Causes of Mate Choice

• Assessments include nuptial gifts and territory.

Nuptial Gifts

• Material gifts include food, sticks, and nesting materials.
• Direct benefits include providing energy.
• Indirect benefits include signals of strong genes.

Territory

• Areas defended by a mate indicate resource availability.
• Territory is a direct benfit, indicating resource availability.
• Territory can be an indirect signal of good parental care.

Appearance

• Physical traits include color, tail length, and size.
• Attractive traits may be passed to offspring.
• Appearance can be correlated with health.

Parental Care

• Parental care includes behaviors that ensure offspring survival.
• Direct benefits ensures young are raised successfully.
• Indirect benefits signal strong genetic quality.
• Parental care can be assessed directly by observing care in previous offspring.
• It can be assessed indirectly by correlating traits like bright plumage in House Finches.

Sexual Conflict

• Sexual conflict occurs when one sex's strategy reduces the fitness of the other.
• Sexual conflict ensures one's genes are passed on at another's expense.
• Infanticide is an example of sexual conflict.
• Toxic semen reduces female's chance of mating again.
• Forced copulation is mating without consent.

Mating Systems

• Monogamy involves one male and one female.
• A pair bond is present in monogamy.
• Monogamy occurs when both parents are needed for care.
• Emperor Penguins are an example .
• Polygamy involves one individual mating with multiple others.
• Polygyny involves one male and many females. (ex. Bison)
• Polyandry involves one female, many males. (ex. Sandpipers)
• Polygynandry involves multiple males and females.
• Promiscuity involves mating with multiple partners and no pair bonds.
• Promiscuity can lead to extreme sexual selection and dimorphism. (ex. Bonobos, Dolphins)

Why Different Mating Systems Evolve

• Driven by the offspring's needs.
• Monogamy evolves if two parents are needed.
• Polygamy or Promiscuity evolves if only one or no parent is needed.

Key Takeaways

• Sexual selection favors traits that enhance mating success, not just survival.
• Mate choice stems from nuptial gifts, territory, appearance, and parental care.
• Sexual conflict arises when one sex benefits at the other's expense.
• Different mating systems evolve based on the needs of offspring and parental roles.

Lecture 8: Hardy-Weinberg Equilibrium & Evolution (Lecture 1/31/25)

Expectations if Evolution is Not Occurring

• All individuals have equal reproductive success if no microevolutionary mechanisms are acting.
• Predicted genotypic ratios remain stable across generations.
• Allele frequencies do not change.

Hardy-Weinberg (HW) Equilibrium Assumptions

• Infinite Population is violated by Genetic Drift.
• Isolated Population is violated by Gene Flow.
• No Net Mutations is violated by Mutations.
• Random Mating is violated by Sexual Selection.
• Equal Survival & Reproduction is violated by Natural Selection.
• Note: HW Equilibrium is robust to minor violations*

Hardy-Weinberg Equation

• Used to predict expected genotypic frequencies:
• p² + 2pq + q² = 1 (Genotype frequencies)
• p + q = 1 (Allele frequencies)
• Allele frequencies remain constant if a population is in HW Equilibrium.

Calculating Expected Genotype Frequencies

• Given actual allele frequencies (p & q):
• f(GG) = p²
• f(Gg) = 2pq
• f(gg) = q²

Using Statistics to Test for Evolution

• The Chi-Square (x²) Test is used to compare observed vs. expected frequencies.
• Null Hypothesis: no difference between actual and expected frequencies, so the Population is in HW Equilibrium.

P-Value Interpretation

• p ≥ 0.05 → Fail to reject the null meaning the Population is NOT evolving.
• p < 0.05 → Reject the null and Population IS evolving

Determining if Evolution is Occurring

• Calculate actual allele and genotype frequencies.
• Determine expected frequencies assuming HW Equilibrium.
• Compare observed vs. expected using statistical tests.
• If significantly different, evolution is occurring.

Solving Hardy-Weinberg Problems

• Identify given information such as allele, genotype, or phenotype frequencies.
• Check if the population is stated to be in HW Equilibrium and use HW equations to calculate missing values.

Lecture 9

Selection for Learning in Variable Environments

• Individuals benefit from learning in changing environments.

Characteristics of Learned Behaviors

• Behaviors exist on a continuum from fully learned to fully innate.
• Genotype (Alleles) influences behavior.

Types of Learned Behaviors

• Habituation
• Associative Learning
• Observational Learning

Habituation

• Definition: Loss of responsiveness to unimportant stimuli.
• Key Characteristic: Not associated with a response.

Associative Learning

• Definition: Learning to associate a stimulus with a response.

Types of Associative Learning

• Classical Conditioning
• Associating a neutral stimulus with a response (reflex)
• Example: Training an organism to associate a cue with a reward or punishment.

Operant Conditioning

• Reinforcement or punishment occurs after behavior.
• Learning through trial and error.

Observational Learning

• Definition: Learning by watching others and copying.
• Example: Young whooping cranes learn migratory routes by following older birds.

Characteristics of Innate Behaviors

• Behaviors exist on a continuum from learned to innate.

Key Characteristics of Innate Behaviors

• Genetically determined.
• Developmentally fixed (unchanging).
• Occurs without prior experience.
• Individuals exhibit similar behavior regardless of external/internal stimuli.
• Stimulus initiates innate behavior.

Evolution of Innate Behaviors

• Highly adaptive when:
• Opportunities for learning are low.
• Learning the wrong behavior is costly.
• Mistakes can be dangerous or fatal.
• However, not always adaptive in all contexts.

Fixed Action Pattern (FAP)

• Definition: A fixed sequence of behaviors that:
• are performed the same way every time, continues to completion and triggered by a simple sign stimulus.

Innate Behaviors' Advantages

• Passed directly to offspring.
• Consistent across populations.
• No time required to learn.

Innate Behaviors' Disadvantages

• Cannot be modified when circumstances change.

Learned Behaviors

• Adaptable to multiple variables.
• Can be modified for new conditions.

Learned Behaviors' Disadvantages

• Not passed genetically.
• Requires time to learn.
• Learning errors are possible.
• Often requires parental care.

Types of Animal Movements

Kinesis

• Random movement in response to stimulus intensity.

Taxis

• Defined as directed movement toward a stimulus (positive) or away from a stimulus (negative).

Orientation

• Using compass direction to determine movement.

Piloting

• Movement relative to landmarks.

Navigation

• Combines orientation + piloting

Lecture 10: Speciation and Reproductive Isolation (2/10/25)

Reduced Hybrid Fertility (Hybrid Sterility) (#7)

• Definition: Hybrids that are viable but sterile
• Postzygotic isolating mechanism.

Hybrid Breakdown (#8)

• Definition: First-generation hybrids (F1) are viable and fertile, but subsequent generations exhibit reduced fitness.
• Postzygotic isolating mechanism

Summary of Reproductive Isolating Mechanisms (RIMs)

Prezygotic Isolating Mechanisms

• Habitat isolation
• Temporal isolation
• Behavioral isolation
• Mechanical isolation
• Gametic isolation

Postzygotic Isolating Mechanisms

• Hybrid inviability
• Hybrid sterility
• Hybrid breakdown

Stages of Isolation

• Pre-mating attempt prevents mating.
• Post-mating attempt prevents fertilization.
• Post-production of offspring affects hybrid viability/fertility.

Concepts in Speciation

• Definition of a species, species concepts, and requirements for speciation
• Microevolution and its role in speciation
• Reproductive isolating mechanisms
• Mechanisms of speciation
• Allopatric Speciation (geographic isolation)
• Sympatric Speciation (without geographic isolation)

Speciation Process

• Key requirements: Microevolution + Reproductive Isolation lead to Speciation

Factors Contributing to Speciation

• Mutation, Natural Selection, Non-random Mating, Genetic Drift leads to Divergence
• Gene Flow prevents speciation unless reproductive isolation occurs

Allopatric vs. Sympatric Speciation

Allopatric Speciation ("Other Country")

• A population forms a new species due to geographic isolation.
• Initial RIM: Habitat isolation due to a physical barrier.

Sympatric Speciation ("Same Country")

• A subset of a population forms a new species without geographic isolation.
• Initial RIM: any isolating mechanism other than habitat isolation.

Outcomes of Secondary Contact After Allopatric Speciation

Reinforcement

• Isolation is maintained due to different RIMs

Fusion

• RIMs are insufficient; populations merge back together.

Stability

• Occasional hybrids form but are not enough to merge populations.

Sympatric Speciation in Plants and Animals

Polyploidy (Mutation)

• Goldfish, Gray Tree Frogs, Angiosperms, Ferns
• RIMs: Gametic isolation, hybrid sterility, hybrid inviability.

Habitat Differentiation (Natural Selection)

• Apple/Hawthorn Trees, Pea Aphids
• RIMs: Habitat isolation, temporal isolation.

Sexual Selection

• Cichlid Fish Mate Choice
• RIM: Behavioral isolation

Explore sexual selection, where traits boost reproductive success, differing from survival-focused natural selection. Discover intrasexual competition and intersexual mate choice, driven by varying parental investment. Learn how assessments like nuptial gifts and territory impact mate selection.