Concepts Summary

• Evolution is genetic change taking place within a group of organisms. It is a two-step process: (1) genetic variation arises, and (2) genetic variants change in frequency.
• Anagenesis refers to change within a single lineage; cladogenesis is the splitting of one lineage into two.
• Molecular methods offer a number of advantages for the study of evolution.
• The use of protein electrophoresis to study genetic variation in natural populations showed that most natural populations have large amounts of genetic variation in their proteins. The neutral-mutation hypothesis proposes that molecular variation is selectively neutral and is shaped largely by mutation and genetic drift. The balance hypothesis proposes that molecular variation is maintained largely by balancing selection.
• Variation in DNA sequences can be assessed by analyzing restriction fragment length polymorphisms, microsatellites, and data from direct sequencing.
• A species can be defined as a group of organisms that are capable of interbreeding with one another and are reproductively isolated from the members of other species.
• Species are prevented from exchanging genes by prezygotic or postzygotic reproductive isolation.
• Allopatric speciation arises when a geographic barrier prevents gene flow between two populations. With the passage of time, the two populations acquire genetic differences that may lead to reproductive isolating mechanisms.
• Sympatric speciation arises when reproductive isolation exists in the absence of any geographic barrier. It may arise under special circumstances.
• Some species arise only after populations have undergone considerable genetic differences; others arise after changes have taken place in only a few genes.
• Evolutionary relationships (a phylogeny) can be represented by a phylogenetic tree, consisting of nodes that represent organisms and branches that represent their evolutionary connections.
• Approaches to constructing phylogenetic trees include the distance approach, the maximum parsimony approach, and the maximum likelihood and Bayesian methods approach.
• Different parts of the genome show different amounts of genetic variation. In general, those parts that have the least effect on function evolve at the highest rates.
• The molecular-clock hypothesis proposes a constant rate of nucleotide substitution, providing a means of dating evolutionary events by looking at nucleotide differences between organisms.
• Genome evolution takes place through the duplication and shuffling of exons, the duplication of genes to form gene families, whole-genome duplication, and the horizontal transfer of genes between organisms.