chapter 24

• Darwin visited the Galápagos Islands and found them filled with plants and animals that lived nowhere else in the world.
  • He realized that he was observing newly emerged species on these young islands.
• Speciation—the origin of new species—is at the focal point of evolutionary theory because the appearance of new species is the source of biological diversity.
• Microevolution is the study of adaptive change in a population.
• Macroevolution addresses evolutionary changes above the species level.
  • It deals with questions such as the appearance of evolutionary novelties (e.g., feathers and flight in birds) that can be used to define higher taxa.
• Speciation addresses the question of how new species originate and develop through the subdivision and subsequent divergence of gene pools.
• The fossil record chronicles two patterns of speciation: anagenesis and cladogenesis.
• Anagenesis, phyletic evolution, is the accumulation of changes associated with the gradual transformation of one species into another.
• Cladogenesis, branching evolution, is the budding of one or more new species from a parent species.
  • Only cladogenesis promotes biological diversity by increasing the number of species.

In the ocean, on either side of the Isthmus of Panama, are 30 species of snapping shrimp; some are shallow-water species, others are adapted to deep water. There are 15 species on the Pacific side and 15 different species on the Atlantic side. The Isthmus of Panama started rising about 10 million years ago. The oceans were completely separated by the isthmus about 3 million years ago.

Why should deepwater shrimp on different sides of the isthmus have diverged from each other earlier than shallow-water shrimp?

A) They have been geographically isolated from each other for a longer time.
B) Cold temperatures, associated with deep water, have accelerated the mutation rate, resulting in faster divergence in deepwater shrimp.
C) The rise of the land bridge was accompanied by much volcanic activity. Volcanic ash contains heavy metals, which are known mutagens. Ash fall caused high levels of heavy metals in the ocean sediments underlying the deep water, resulting in accelerated mutation rates and faster divergence in deepwater shrimp.
D) Fresh water entering the ocean from the canal is both less dense and cloudier than seawater. The cloudy fresh water interferes with the ability of shallow-water shrimp to locate mating partners, which reduces the frequency of mating, thereby slowing the introduction of genetic variation.

In the following figure, the isthmus separates the Pacific Ocean on the left (side A) from the Atlantic Ocean on the right (side B). The seawater on either side of the isthmus is separated into five depth habitats (1—5), with 1 being the shallowest.

In which habitat should one find snapping shrimp most closely related to shrimp that live in habitat A4?

A) A3
B) A5
C) B4
D) either A3 or A5
E) any species from any one of the side A habitats