Chapter 18: Introduction to Darwinian Evolution

What is Evolution?

• Evolution is the accumulation of inherited changes within a population over time.
  • Evolution is the unifying concept of biology because it links all fields of the life sciences into a coherent body of knowledge.

Pre-Darwinian Ideas about Evolution

• Jean Baptiste de Lamarck was the first scientist to propose that organisms undergo change over time as a result of some natural phenomenon rather than divine intervention.
  • Lamarck thought that organisms were endowed with a vital force that drove them to change toward greater complexity over time.
  • He thought that organisms could pass traits acquired during their lifetimes to their offspring.
• Charles Darwin's observations while voyaging on the HMS Beagle were the basis for his evolutionary theory.
  • Darwin tried to explain the similarities between animals and plants of the arid Galápagos Islands and the humid South American mainland
• Darwin was influenced by artificial selection, in which breeders develop many varieties of domesticated plants and animals in just a few generations.
  • Darwin applied Thomas Malthus's ideas on the natural increase in human populations to natural populations.
  • Darwin was influenced by the idea that Earth was extremely old, an idea promoted by Charles Lyell and other geologists.

Darwin and Evolution

• Charles Darwin and Alfred Russel Wallace independently proposed evolution by natural selection, which is based on four observations.
  • First, genetic variation exists among the individuals in a population.
  • Second, the reproductive ability of each species causes its populations to have the potential to geometrically increase in number over time.
  • Third, organisms compete with one another for the resources needed for life, such as food, living space, water, and light.
  • Fourth, offspring with the most favorable combination of inherited characteristics are most likely to survive and reproduce, passing those genetic characteristics to the next generation.
• Natural selection results in adaptations, evolutionary modifications that improve the chances of survival and reproductive success in a particular environment.
  • Over time, enough changes may accumulate in geographically separated populations to produce new species.
• The modern synthesis combines Darwin’s evolutionary theory by natural selection with modern genetics to explain why individuals in a population vary and how species adapt to their environment.
• Mutation provides the genetic variability that natural selection acts on during evolution.

Evidence for Evolution

• Direct evidence of evolution comes from fossils, the remains or traces of ancient organisms.
  • Layers of sedimentary rock normally occur in their sequence of deposition, with the more recent layers on top of the older, earlier ones.
  • Index fossils characterize a specific layer over large geographic areas.
  • Radioisotopes present in a rock provide a way to accurately measure the rock’s age.
• Biogeography, the geographic distribution of organisms, affects their evolution.
  • Areas that have been separated from the rest of the world for a long time contain organisms that have evolved in isolation and are therefore unique to those areas.
• At one time the continents were joined to form a supercontinent.
  • Continental drift, which caused the various landmasses to break apart and separate, has played a major role in evolution.
• Homologous features have basic structural similarities even though the structures may be used in different ways because homologous features derive from the same structure in a common ancestor.
  • Evolutionary affinities exist among the organisms that have homologous features.
• Homoplastic features evolved independently to have similar functions in distantly related organisms.
  • Homoplastic features demonstrate convergent evolution, in which organisms with separate ancestries adapt in similar ways to comparable environmental demands.
• Vestigial structures are nonfunctional or degenerate remnants of structures that were present and functional in ancestral organisms.
  • Structures occasionally become vestigial as species adapt to different modes of life.
• Molecular evidence for evolution includes the universal genetic code and the conserved sequences of amino acids in proteins and of nucleotides in DNA.
• Evolutionary changes are often the result of mutations in genes that affect the orderly sequence of events during development.
  • Development in different animals is controlled by the same kinds of genes, which indicates that these animals have a shared evolutionary history.
• The accumulation of genetic changes since organisms diverged, or took separate evolutionary pathways, has modified the pattern of development in more complex vertebrate embryos.
• David Reznick and John Endler have studied the effects of predation intensity on the evolution of guppy populations in the laboratory and in nature.
  • Such experiments are a powerful way for investigators to test the underlying processes of natural selection.