Spring Semester 2014 Evolution II: Tempo & Mode, Patterns & Process
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.
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
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
"We may look at Megatherium, armadillos & sloths as all offsprings of some still older type
some of the branches dying out." -- Notebook B, mid 1837-February 1838
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.
Thus, the basic pattern of the history of living things is a Tree of Life,
where the trunk and stems are lineages of ancestors, the branching points representing divergences between lineages, and
the tips of the branches living species (or extinct species that died without 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; many with major volcanic episodes; others with glaciation.
Co-evolution: Selection of one species due to activity of an interactor leads to counter-selection in response of the first
species
For example, plant species develop traits (shapes, colors, tastes of nectar) that favor a select few number of pollinators, thereby promoting
greater chance of getting their own pollen rather than some other plant's
Or, in the Galápagos: drier islands have fewer small plants, so tortoises preferentially feed on Opuntia cacti. Cacti on these
islands have evolved taller woody trunks, and in response the tortoises have evolved a "saddle-backed" shell that allows them to
reach higher than dome-backed ancestors.
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.
Here is Carl Sagan's summary of Darwinian evolution and the Tree of Life, from the TV series Cosmos:
And here is another summary of evolution and how it works (and how it ISN'T like the parody-version of evolution which Creationists claim scientists believe):