GEOL 104 Dinosaurs: A Natural History
Fall Semester 2006
Evolution III: Patterns and Processes
- 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
- 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
- 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,
- 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.
- 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
- For example, the wings of birds and bats were initially hands; the mouthparts of
various arthropods were legs; etc.
- 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
- Peramorphosis: descendant populations will develop structures beyond the adult
form of original population
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Last modified: 12 January 2007