Natural Selection:

• has withstood huge number of tests.
• Has been refined since Darwin & Wallace's time, but basic observations and mechanisms have been observed.

Evolution produces several important patterns:

• Divergence from common ancestors
  • Two (or more) distinct variations in an ancestral population convey their own advantage against the rest of the population
  • Over time, these two (or more) variations will become more distinct from each other
  • If they diverge enough, they will no longer be able to mate with each other: will be different species
  • Divergence can also occur (perhaps more commonly!) if an ancestral population is divided into two or more by changes in geography: because natural selection works by chance survivals, it is unlikely that exactly the same variations of the ancestral population will survival in the two or more separated populations. Over time, if the populations meet again, the accumulation of variations may be significant enough that they are distinct species.
• Adaptive Trends
  • Ancestor and descendants form a lineage (historical line). If the same basic adaptations are selected for and elaborated over time, this is called a trend. (e.g., longer and longer legs for fast running; longer and longer necks for browsing in trees, etc.)
• Adaptive radiations
  • If a new adaptation (or loss of competitor group) occurs, many different variations from a common ancestral population might survive (new or unoccupied "niches" in environment). Over a geologically short period time, a common ancestor can radiate into many different descendant lineages.
• Convergence from different ancestors
  • Some adaptations are mechanically advantageous and easy to produce developmentally
  • Different lineages of organisms can independently develop some of the same features, even though ancestors were quite different (i.e., streamlining in sharks, tunas, ichthyosaurs & dolphins).
• Living fossils
  • Populations little changed over long periods of geologic time.
• Exaptation
  • Formerly "preadaptation": the co-option of a structure that previously had some entirely different use for a new use
  • Seems to be the more common pattern of evolution than the appearance of entirely novel structures
  • For example, the wings of birds and bats were initially hands; the mouthparts of various arthropods were legs; etc.
• Heterochrony
  • Evolution by changes in rate of development from embryo to adulthood
  • Two major forms of heterochrony:
    • Pedomorphosis: descendant populations will retain some juvenile features into adulthood
    • Peramorphosis: descendant populations will develop structures beyond the adult form of original population

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Last modified: 12 January 2007