Evolution and Ecology

Biology 136
Spring 1998

Jonathan Brown, Assistant Professor, Ph.D.
Grinnell College

Table of Contents

Schedule and goals of lab exercises - see below for links to labs.

Lab 1 -- Homology and Analogy

Lab 2 -- Sources of Phenotypic Variation

Lab 3 -- Mendelian Genetics

Lab 4 -- Evolutionary Forces

Lab 5 -- Animal Behavior

Lab 6 -- Measuring Selection in Natural Populations

Lab 7 -- Field Community Ecology

Appendices

A -- Writing Scientific Papers in Biology

B -- Commonly Used Statistical Formulas

Lab 1 -- Homology and Analogy (Week 1)

Task: Discover and explain the evidence supporting hypotheses of homology or analogy for shared traits.

Conceptual goals:

• Study of structure and function is an integral part of the study of evolution.
• Evidence for common ancestry is found in a lack of logical structure/function fit (homology). This demonstrates that adaptation is constrained by organisms’ evolutionary history.
• Evidence establishing homology or analogy is garnered primarily from the study of the developmental origins and fine structure of traits.
• Distinction between homology and analogy is the cornerstone of recognition of patterns of descent with modification. Modern systematics relies on evidence from all areas of biology.

Knowledge development:

• Become familiar with characteristics that define major taxonomic groups of plants and animals.

Task: Estimate heritability of a number of phenotypic traits using a common garden experiment.

Conceptual goals:

• Phenotypic variation is a result of an interaction of genetic and environmental influences.
• The extent to which natural selection can produce evolutionary change depends on the degree of genetic determination of phenotypic variation, or heritability.
• Heritability is measured via experiments that measure genetic and environmental components of variation (not by sequencing DNA!).
• Different traits can exhibit different levels of heritability.

Skill development:

• Experimental design (common garden experiments)
• Calculation of mean and variance for data subsets
• Graphical presentation of means and confidence levels
• Presentation of results in the format of a scientific paper.

Task: Test Mendelian hypotheses of inheritance in corn and Drosophila for multiple phenotypes.

Conceptual goals:

• Mendel’s rules predict a certain distribution of phenotypic traits in offspring
• Gene and allelic interactions and physical linkage can alter expected distributions of phenotypes
• Analysis of expected distributions of phenotypes from experimental crosses can establish the relative positions of genes on chromosomes.
• Sample size can influence the ability to reject Mendelian hypotheses.

Skill development:

• Statistical test (C ²) used to test deviation from an expected distribution of categorical results.
• Generation of linkage maps.

Task: Explore via computer simulations the factors that influence evolutionary change within and among populations.

Conceptual goals:

• Evolution within populations occurs via the effects of natural selection, drift, mutation and gene flow. The effects of these forces can interact in unexpected ways, at times overwhelming the effects of natural selection.
• Characteristics of populations such as size and mating rules influence which of these forces prevails.
• Accumulation of the effects of drift, mutation and gene flow can result in population differentiation.

Skill development:

• Interpretation of results in graphical form.
• Statistical test of difference in means between 2 groups (overlapping 95% confidence intervals)

Task: Experimentally test a hypothesis about animal behavior and communicate the results in an oral presentation.

Conceptual goals:

• Animal behavior is studied by experiments that address hypotheses generated by careful observation.

• Proximate hypotheses establish the mechanistic explanations for an organism’s behavior. Ultimate explanations tie an animal’s behavior to its effects on survivorship or reproductive success.

Skill development:

• Experimental design, with particular emphasis on controls.
• Quantification of behavior.
• Statistical comparison of two means when samples are non-independent (paired).
• Oral presentation of results.

Task: Estimate the strength of natural selection on a single phenotype (gall size) of a gall-making fly. Compare the nature of selection between sites and/or years.

• Selection is generated by interactions between individuals and their environment. Predator-prey interactions can be a significant source of phenotypic selection.
• Phenotypic selection can be quantified in natural populations by comparing survivorship of varying individuals.
• Interactions between individual predators can produce directional, stabilizing or disruptive selection on a quantitative trait, depending on the characteristics and behavior or both species. When a species has interactions with many predators, the resulting selection on phenotypes is a sum of these individual effects.
• The strength of selection may vary from place to place and year to year.

Skill development:

• Unbiased sampling of a population.
• Calculation of the intensity of selection.
• Statistical tests of significance for differences in means (95% confidence intervals) and variance (F-test).
• Presentation of results in the format of a scientific paper.

Task: Perform an experiment that addresses the role of biotic or abiotic factors on populations or community structure in terrestrial or aquatic ecosystems and report the results in the format of a scientific poster.

Conceptual goals:

• Biotic interactions (e.g. predation) and abiotic factors (e.g. disturbance) can dramatically influence community composition.
• The roles of various influences on community structure are determined through replicated experiments.

Skill development:

• Sampling design
• Measurement of abiotic features of communities
• Calculation of species diversity indices
• Statistical test of significant differences in means.
• Poster presentation of experimental results.