Mechanisms of Evolution

• What does natural selection do?

  facilitates adaptive evolution
• Directional selection

  favors individuals at one extreme end of the phenotypic range; common when environment changes or members migrate
• Disruptive selection

  favors individuals at both extremes of the phenotypic range
• Stabilizing selection

  favors intermediate phenotypes; reduces variation
• Genetic drift

  changes in gene pool of small population due to chance events
• Bottleneck effect

  population reduced by disaster; gene pool of survivors not representative of original population
• Founder effect

  few individuals colonize isolated habitat; smaller gene pool not representative of original large gene pool
• Effects of genetic drift

  Significant in small populations, random, loss of genetic variation, harmful alleles become fixed
• Gene flow

  migration of alleles into or out of a population
• What does gene flow do to populations?

  Reduces genetic differences between them
• Is all evolution adaptive?

  no
• Evolution equation

  random variation + natural selection = evolution
• Speciation

  the formation of one or more new species from an existing species
• Anagenesis

  changes within a lineage over time
• Cladogenesis

  branching; basis for biological diversity
• Biological species concept

  population or group of populations whose members have the potential to interbreed to produce fertile offspring
• Reproductive isolation

  existence of biological barriers that impeded two species from producing viable offspring
• Prezygotic barriers

  block fertilization from occurring
• Habitat isolation

  populations live in different habitats and do not meet
• Temporal isolation

  When two species breed at different times
• Behavioral isolation

  differences in courtship or mating behavior
• Mechanical isolation

  anatomical incompatabilities
• Gametic isolation

  sperm can't fertilize egg
• Postzygotic barriers

  prevent hybrid zygotes produced by two different species from developing into viable adults
• Reduced hybrid viability

  zygotes fails to develop
• Reduced hybrid fertility

  offspring are sterile
• Hybrid breakdown

  second generation offspring are feeble or sterile
• Biological species concept does not apply to

  asexual organisms, fossils
• Morphological species concept

  defines a species by body shape and unique set of features
• What organisms can morphological species concept be applied to?

  sexual or asexual
• What is a disadvantage of the morphological species concept?

  relies on subjective criteria
• Ecological species concept

  defines a species in terms of ecological niche
• What organisms can the ecological species concept be applied to?

  sexual and asexual
• What does the ecological species concept emphasize?

  role of disruptive natural selection
• Phylogenic species concept

  defines a species as a set of organisms with a unique genetic history
• Allopatric speciation

  formation of new species in populations that are geographically isolated
• Adaptive radiation

  common ancestor spreads to several new environments and gives rise to new species
• Sympatric speciation

  The formation of new species without geographic separation
• Autoploidy

  diploid gametes fuse to form tetraploid zygote
• Allopolyploid

  two species combine chromosomes to form polyploids