Fundamentals of Taxonomy, Phylogeny, and Population Evolution
Basic Taxonomy and Phylogeny
Basic taxonomy has its roots in Aristotle’s original 2 Kingdom system. Since that time, modern taxonomy has undergone countless revisions, including important contributions by Linnaeus, Whittaker, Woese (Table 24.2), and others. Be able to recognize and express important historical changes to our taxonomical schemes.
In biology today, modern taxonomy is synonymous with phylogenetic systematics. Understand the basic principles of grouping organisms based on evolutionary relationships and be able to interpret and generate phylogenetic trees (cladograms) that are derived based on evolutionary relationships. Be able to distinguish between homology (similar characteristics based on shared ancestry) and analogy or convergent evolution (similar characteristics due to similar environmental pressures and natural selection). Recognize the differences between valid taxonomical groupings (clades), monophyletic clades, and paraphyletic and polyphyletic groupings. Understand the difference and significance ancestral characteristics and derived characteristics play in grouping organisms based on evolutionary relationships. Be able to interpret phylogenetic trees with proportional branches (Figure 20.13). Understand the principle of maximum parsimony and the role it plays in helping to determine the most likely grouping of organisms based on phylogeny.
Molecular clocks use the rate of change in genes to estimate time related to evolutionary change. The number of nucleotide substitutions in related genes is assumed to be proportional to the time since they last shared a common ancestor. Molecular clocks are calibrated by plotting the number of genetic changes against the dates of branch points known from the fossil record.
Chapter 21: Evolution of Populations
Populations evolve, not individuals. Microevolution is a change in allele frequencies in a population over generations. Three mechanisms change allele frequency:
- Genetic Drift
- Gene Flow
- Natural Selection
Genetic variation results from mutations. Sexual reproduction generates genetic variation through the recombination of existing alleles. A population is a localized group of individuals capable of interbreeding and producing fertile offspring.
The Hardy-Weinberg principle can be used to measure and predict allele and genotype frequencies in a population that is not evolving. Two equations are important:
- p + q = 1 is used to determine allele frequencies
- p² + 2pq + q² = 1 is used to determine genotype frequencies
Given the frequency of an allele or genotype for a population, be able to determine the frequency of the other alleles and genotypes.
The five conditions for a population to be in Hardy-Weinberg equilibrium (not evolving) are rarely met in nature:
- No mutations
- Random mating
- No natural selection
- Large population size
- No gene flow
Hardy-Weinberg equilibrium is disrupted by:
- Genetic drift: Chance events cause fluctuations in allele frequencies; more pronounced in small populations, tends to reduce genetic variation.
- Gene flow: Transfer of alleles from one population to another.
- Natural selection: Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions.
There are three modes of natural selection:
- Directional Selection
- Disruptive Selection
- Stabilizing Selection