Lecture 10 - 3444
Harvesting-induced evolution in bighorn sheep
Lecture by Dave Coltman for Biology 3444F on November 26, 2024
Learning Objectives
Distinguish between quantitative and discrete trait variation
Describe partitioning of quantitative trait variance into components
Relate heritability, selection, and response to selection, especially in selective harvesting
Compare QTL mapping and GWAS for studying genetic architecture of quantitative traits
Discuss molecular markers in studying genetic basis of quantitative traits
Threats to Biodiversity
Discussion on various factors threatening biodiversity, including overharvesting.
Overharvesting & Exploitation
Highlights how selective management of species for sport and trophy hunting affects populations.
Revenue generated from hunting is reinvested for wildlife management and habitat protection.
Careful management can prevent overharvesting; however, harvesting remains selective.
Questions raised about the evolution of species in response to selective harvesting.
Quantitative Trait Variation
Discrete traits are often governed by a single gene and show Mendelian inheritance.
Many traits exhibit continuous variation, influenced by multiple genes, each contributing a small effect.
Quantitative traits are measurable numerically, complicating the study of genetic variations.
Causes of Quantitative Traits
Traits are influenced by many genes and by gene interactions such as dominance and epistasis.
Environmental variation also plays a significant role.
Quantitative Genetic Variation
Quantitative genetics uses trait means, variances, and co-variances to describe populations.
Variance reflects dispersion around the mean; co-variance measures relationships between different traits.
Genetic Variation and Quantitative Traits
Phenotype (P) = Genetics (G) + Environment (E)
Variance in phenotype can be divided into genetic (VG) and environmental (VE) sources.
Additive genetic variation (VA) is crucial for understanding heritability and adaptation.
Estimating Additive Genetic Variation (VA)
VA measurement allows for estimating resemblance between parents and offspring and informs natural selection processes.
Heritability can be expressed as a fraction of phenotypic variance: [ h^2 = \frac{VA}{VP} ]
The Meaning of Heritability
Represents genetic resemblance between parents and offspring.
Measures potential for evolutionary change and ability to predict a trait based on genetics.
Heritability is not a fixed trait; it varies by population, environment, and time.
Estimating Heritability
Can be computed through evaluating offspring against parental values in natural populations with sufficient sampling.
Example: Human height has a heritability estimate of 0.57.
Heritability and Evolution
Breeders' equation: [ R = h^2 S ]
Heritability predicts inherited variance available for selection; selection differential (S) is the difference between population mean and selected parents.
Bighorn Sheep: Traits and Evolutionary Response
Bighorn sheep exhibit sexual dimorphism in body mass and horn size, influencing social rank and reproductive fitness.
Horn size is critical for trophy status and is associated with fitness.
Selective Hunting and Evolution
Trophy hunting leads to artificial selection, potentially causing evolutionary changes in traits with heritable basis.
Minimum horn size regulations create artificial selection pressures on horn growth.
Ram Mountain Study
Examined selective pressures of hunting on bighorn sheep at Ram Mountain.
Investigated heritability and evidence of evolutionary changes due to selective hunting.
Trophy Selection Outcomes
57 trophy rams harvested at Ram Mountain since 1972 with a 4/5 curl horn length restriction until 1996.
Mean harvesting rate was 40% of legal rams annually before regulations were tightened.
Impacts on Horn Size
Trophy hunting emphasizes long-horned rams, affecting reproductive success and social structures in populations.
Age and horn length correlate with mating success, showcasing selective pressure.
Hunting and Reproductive Success
Hunted rams face reduced reproductive lifespans and mean age at harvest is 6.3 years.
Reproductive success is affected by age-related horn growth.
Genetic Variation in Horn Size
Hunting suppresses horn size due to phenotypic selectivity, leading to expected evolutionary responses if heritable.
Animal Model and Pedigree Analysis
Continuous monitoring since 1972 aids in understanding genetic variance through established pedigrees.
Microsatellites used to confirm familial relationships and model heritability across generations.
Changes in Breeding Values
Study of horn length and body weight showed significant breeding value differences and changes over decades.
Horn length has a heritable value of 0.69, weight at 0.41, indicating selective evolutionary responses.
Longitudinal Studies of Trait Values
Evaluated changes in breeding values and trait values over time, indicating selection pressures and evolutionary trends.
Modern Studies and Methodologies
Recent models incorporate Bayesian analytics for better estimation of heritable traits post-hunting.
Investigation into molecular markers and their association with horn growth and other traits continues.
Genetic Markers and QTL Studies
Discussion of QTL mapping and GWAS approaches to correlate genetic markers with quantitative traits.
Challenges in identifying genes of significant effect remain, indicating complexity of trait inheritance.
Conclusion: Future Directions
Ongoing research exploring selective pressures, genetic markers, and conservation implications unveils complexities in wildlife management.
Importance of developing high-density SNP panels to enhance detection capabilities for trait variation.
Where else might genetic responses occur?
Considerations for evolutionary responses in other harvesting scenarios like logging and habitat manipulation.
Questions and References
Detailed references for further reading on topics discussed in the lecture.