lesson 7

• Species


  Defined by appearance (Linnaeus) or potential to interbreed and produce fertile offspring (biological species concept)
• Speciation


  • Formation of new species when individuals can no longer interbreed with the rest of the group
• Reproductive barriers cause speciation

  1. Prezygotic barriers prevent fertilization

  2. Postzygotic barriers reduce fitness or prevent development of fertile offspring
• Allopatric speciation


  Reproductive barriers arise due to physical separation of a population into two groups that cannot interbreed
• Sympatric speciation


  Populations diverge genetically while sharing a habitat, leading to reproductive isolation and speciation
• Allopatric speciation


  • Galápagos tortoise subspecies on different islands
• Sympatric speciation


  • Cichlid fish diversifying into several species in a small African lake
• Polyploid speciation


  Gametes from two species unite, producing the first cell of a new species with more chromosomes than either parent
• Gradualism


  Evolution proceeds in small, incremental changes over many generations
• Punctuated equilibrium


  Long periods of little change are interrupted by bouts of rapid change
• Evolution might occur quickly when a key adaptation arises, like the diversification of flowering plants
• Evolution might also occur quickly following a mass extinction, as surviving organisms exploit new resources in the changed environment
• Evolution might occur quickly when a key adaptation arises, such as the diversification of the first flowering plants
• Extinction


  A species is extinct when all of its members have died
• Background extinction rate


  The pace at which species go extinct due to gradually changing environments
• Earth has witnessed five mass extinctions in the last 600 million years, where many species went extinct in a short time
• Impact theory


  Meteorites or comets caused some mass extinctions, with the debris suspended in the atmosphere dramatically changing the environment
• Plate tectonics (Earth's shifting land masses) might also explain mass extinctions
• Humans are changing so much of the biosphere that we may be experiencing a 6th mass extinction, such as the crash of bluebird and woodpecker populations due to the introduction of the European starling
• Taxonomy


  The science of describing, naming, and classifying species
• Taxonomic hierarchy


  Organizes species into progressively larger groups
• Phylogenetics


  The study of evolutionary relationships among species
• Phylogenetic tree


  Depicts evolutionary relationships based on descent from common ancestors
• Cladogram


  A type of phylogenetic tree
• Clade


  A group of organisms consisting of a common ancestor and all of its descendants
• According to the cladogram, a bird is more closely related to a dinosaur than a crocodile
• The most recent common ancestor of species 7, 8, and 9 is the node labeled C
• "Protista" is not a clade because not all descendants from a common ancestor are included in the group
• Researchers hypothesized that a mutation changed the flower color of some Mimulus plants, starting the speciation event that separated two species
• Selective breeding to create varieties mimicking the color of the other species showed different frequencies of pollinator visits, shedding light on how the species might have diverged
• A mutation in a gene affecting flower color may have caused a shift in pollinator preferences, reproductively isolating some individuals in the population and leading to the divergence into two species