Chapter 11 of Raven Biology of Plants (Eighth Edition) explains how evolution operates, from Darwin’s theory of natural selection to the genetic mechanisms shaping populations and species. The chapter begins with Charles Darwin’s voyage on the HMS Beagle, where his observations of variation among Galápagos tortoises and finches, along with the writings of Thomas Malthus and Charles Lyell, inspired his theory that heritable variation combined with competition leads to differential survival and reproduction. Natural selection, akin to artificial selection by breeders, favors traits that increase reproductive success, gradually shifting allele frequencies. This insight led to modern population genetics, which defines a population by its gene pool—the sum of all alleles—and studies how gene frequencies change over time. The Hardy-Weinberg law provides a mathematical framework for nonevolving populations, showing that allele and genotype frequencies remain constant unless influenced by outside forces. Deviations from equilibrium reveal the agents of evolutionary change: mutation (the source of new variation), gene flow (allele movement between populations), genetic drift (random fluctuations especially strong in small populations via founder or bottleneck effects), and nonrandom mating (including inbreeding and self-pollination, which increase homozygosity).
The chapter then explores how natural selection acts on phenotypes rather than genotypes, shaping adaptations to both physical environments and interactions with other organisms (coevolution). Adaptation may take the form of clines, where traits vary gradually across geography, or ecotypes, genetically distinct populations suited to different habitats. Case studies such as lead-tolerant bent grass (Agrostis tenuis) and ecotypes of Potentilla glandulosa illustrate how selection molds local populations. Rapid adaptation is documented in examples of grazing pressure, heavy metal soils, and invasive species like purple loosestrife and kudzu.
Speciation, the formation of new species, is explained through the biological species concept (reproductive isolation), morphological species concept (structural distinctness), and phylogenetic species concepts (shared ancestry or characters). Speciation occurs allopatrically, when geographic barriers isolate populations, or sympatrically, often through polyploidy in plants. Autopolyploidy doubles chromosome sets within one species, while allopolyploidy combines genomes of different species, as seen in Tragopogon (goat’s beard), Spartina grasses, and the origin of bread wheat (Triticum aestivum). Hybridization, apomixis, and recombination speciation in plants like the anomalous sunflower also generate new species. Adaptive radiation, particularly in isolated environments like Hawaiian lobeliads, demonstrates how a single ancestor diversifies into many species occupying distinct ecological niches.
Finally, the chapter addresses macroevolution, the origin of major groups above the species level. Gradualism views evolution as the accumulation of small changes over long periods, while punctuated equilibrium, proposed by Eldredge and Gould, interprets the fossil record as reflecting long periods of stasis interrupted by bursts of rapid speciation. Together, these models underscore that evolution is dynamic, driven by both genetic processes and ecological pressures, and remains the unifying principle of biology.
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