Exam I Review Sheet handed out

With the discovery of evolution by natural selection, biologists from Darwin and Wallace's time onward have documented many different patterns and processes in evolution. Sometimes they refer to "microevolution" (changes within an species) and "macroevolution" (patterns on the larger scale; changes from one species to another, or between different lineages of ancestors and descendants). It is important to remember that "micro-" vs "macro-" is just a matter of scale and perception: at the level of individuals and populations, there is just variability, heritability, and superfecundity.

The most important pattern: the Tree of Life. Darwin and Wallace demonstrated the reality of Divergence through Time and Common Ancestry:

• 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.
• Common Ancestry
  • Closely related species are close because their common ancestor diverged relatively recently in Earth history
  • Other species are more distantly related because of divergences of THEIR common ancestors even farther back in time
  • No separate origins for differnt groups; instead, patterns of common ancestry and diverging descendants

Other important patterns and processes:

• Sexual Selection, a variation of Natural Selection recognized by Darwin, where the variation is "being more sexy" (and thus have better than average chance of breeding, and thus passing on "sexiness", compared to other members of the population [increased reproductive success]). Explains many extravagent display structures and behaviors (such as peacock tails, bird song, lion manes, etc.)
• Correlated Progression: Ancestor and descendants form a lineage (historical line). Sometimes a particular life habit favors the slight increase in multiple different traits (e.g., longer and longer legs, more compact body, more effcient heart and respiration for fast running; longer and longer necks, longer legs, better cropping teeth and/or grasping tongue for browsing in trees; more and more streamlined body profile, more paddle like legs, more dorsal nostris, etc. in swimmers; etc.) Traits that go against the general trend will be selected against; traits that go with the general trend will be selected for. (For many people, this series of trends in adaptations represents the totality of evolution)
• Adaptive Radiation: If a population evolves some significant new adaptation, or colonizes a region without competitors, or is present when competitors die off, many different variations from that common ancestral population might survive (fill new or unoccupied "niches" (ways of life) in environment). Over a geologically short period time, a common ancestor can radiate into many very different descendant lineages.
  • Niche Partitioning: during an adaptive radiation, the early members of the divergence will (naturally) still be relatively similar to each other (and to their common ancestor) in terms of size, shape, behavior, etc. Over time, those variations in each lineage that are least like their relatives will more likely survive, because they will have less competition. Consequently, the different species will "partition" (divide up) the niches and the resources.
• Convergence: 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: Species do not have a fixed duration, but will persist until the evolve into something else and/or go extinct. In some cases, species (or genera) may persist for extremely long periods of time with no major changes.
• Exaptation: Formerly called "preadaptation", the co-option of a structure that previously had some entirely different function 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 arms and 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:
  • Paedomorphosis: descendant populations will retain some juvenile features into adulthood
  • Peramorphosis: descendant populations will develop structures beyond the adult form of ancestor
• Extinction:
  The termination of a lineage. (If a species "dies out" by evolving into another species, this is more properly called a pseudoextinction). Extinctions occur throughout Earth History. What is more remarkable is Mass Extinction: the geologically-sudden disappearance of many diverse groups of organisms, which are not immediately replaced by ecological equivalents. Some mass extinction events seem to correlate with asteroid impacts; others with major volcanic episodes; others with glaciation.

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Last modified: 10 January 2007