Patterns of Evolution and Speciation
Populations diverge (move away) from one another or from the "parent" population in
different ways. Selection pressure (natural selection) is the principle driving force. Note
that "natural selection" is considered a theory, and you ought to keep this in mind, but
there is a much, much broader theory of evolution where natural selection is only one
part.
Adaptive radiation is a very broad term that is used for both microevolutionary and
macroevolutionary trends. Species (micro) and groups (macro) diverge over vast periods
of time to fill ever-changing habitats and niches.
In consideration of multigene traits with continuous variation within the population
(microevolution):
Directional Selection: Traits such as height, coloration, speed, size, etc… are governed
by many genes, and the resulting distribution of phenotypes is represented by a bell
curve. Selection pressure may favor one extreme or the other and move the entire curve
in one direction. For example, if there is continuous variation in pigmentation and dark
pigmentation is favored by selective pressure, then the population will - over time become darker. This is an example of directional selection.
Stabilizing Selection: In another example, those individuals with the average phenotype
may be favored. This is the case with birth weight in human babies. We want the babies
to develop as much as they can and still make it through the birth canal. A 3-4 pound
baby can easily make through even a narrow birth canal, but is at a developmental
disadvantage. A baby weighing between 10-12 pounds is nicely developed, but might get
stuck. When selection pressure favors individuals with phenotypes in the middle of a
continuous range, we call it stabilizing selection.
Disruptive selection is the opposite of stabilizing selection in that both extremes are
favored, and individuals in the middle of the continuous phenotypic continuum are the
least fit. An example can be found in the case of "Darwin's Finches" (ask me to tell you
about this someday) from the Galapagos Islands. Having descended from a single parent
population, the species found today can be assessed based on the size and shape of their
beaks. Depending on the source of food available on an island, a standard beak may not
be long and slender enough to reach into crevices to pick out its little bits of food, and the
same beak may not be short and stout enough to crack open seeds hulls to get at the
yummy bits on the inside. So the "average" beak is at a disadvantage, while long and
slender (one extreme of the distribution) or short and stout (the other end) are favored.
Disruptive selection leads to divergence and ultimately a splitting of a parent species into
two new ones.
In consideration of macroevolutionary trends:
Coevolution: Competition is not the only force driving evolution. Since all life is
interdependent on other life forms, it is obvious that cooperation is also a significant
driving force. When species evolve in response to one another to the benefit of both, this
is called coevolution. Consider a flower and its specific pollinator. There are far too
many examples to list, but we will consider the results of hundreds of millions of years of
convergent evolution when our focus is on communities in ecosystems. In this context,
the result of coevolution is called mutualism or mutualistic symbiosis.
Convergent evolution: Another macroevolutionary trend occurs in diverse species that
find themselves competing within a similar ecological niche. Over time, the selection
pressures exerted by the habitat/niche result in organisms that are superficially similar,
yet have very different evolutionary genealogies. Such is the case with the shark and the
dolphin. The dolphin traces its ancestry to land dwelling species that are warm blooded,
placental mammals that get oxygen from lungs, not gills, and they nurse their young with
milk. While sharks and dolphins may share superficial characters, their ancestry and
phylogenetic history (phylogenetics is the term used to classify organisms on the
metaphorical "Tree of Life" based on genealogy/ancestry).
Speciation:
The species concept itself is difficult and not absolute. A common definition would be "a
group of closely related interbreeding individuals producing fertile offspring under
natural conditions." This excludes the majority of all life - the asexually-reproducing
bacteria, archaea, and most of the unicellular eukaryotic microbes.
Diverging populations - over time - interbreed with decreasing frequency. Since the
definition of "species" includes "interbreeding" then such diverging populations are
headed toward becoming new/different species. With rare exceptions (usually found in
plants) the complete reproductive isolation does not occur suddenly from one generation
to the next. This "separation phase" muddies the water of what constitutes a species.
Grizzly bears, Ursus arctos, and polar bears, Ursus maritimus, are
genetically/gametically capable of interbreeding, but they don't do this (with very rare
exceptions) so they are considered different species (this is where the "under natural
conditions" clause in the species definition comes into play). The same is true of lions,
Panthera leo, and tigers Panthera tigris. We see speciation even further along in the
sterile hybrids of horses, Equus caballus, and donkeys, Equus asinus, where the offspring
are sterile. (See also postzygotic barriers - "reduced hybrid fertility" below)
Reproductive isolating barriers:
Prezygotic (barriers that prevent egg and sperm from meeting to form a zygote) and
Postzygotic (barriers that do not allow the zygote to develop into a normal, healthy,
fertile individual).
Prezygotic:
Allopatric speciation ("different country") occurs over long periods of geographical
isolation. Archipelagos are great places to study allopatric speciation.
Sympatric ("same country") /Prezygotic speciation involves other mechanisms
that present barriers to interbreeding in the absence of geographical isolation. Examples
include:
1. Behavioral - one species might be diurnal, solitary, and sleep in trees, the other
nocturnal, social, and sleep together in dens. Such is the case with lions and
tigers. (Note: Not too far back in time, the geographical ranges lions and tigers
overlapped… not any more.)
2. Temporal - "Having to do with time" - ex. male and female flowers mature at
different times. Some animal species mate in different seasons in the same
geographic area.
3. Mechanical - The male and female "parts" don't fit. There are examples in both
the plant and animal kingdoms.
4. Habitat - You can find water-dwelling and land-dwelling species of garder snakes
within a few meters. Diverging populations have adapted to a preference for one
or the other habitat. Over time, you get yourself two different species in the
genus Thamnophis.
5. Gametic - A point is reached when the egg and sperm simply will not fuse their
respective nuclei. There are a number of specific types and examples.
Sympatric/Postzygotic:
1. Reduced hybrid viability - hybrids don't develop properly.
2. Reduced hybrid fertility - think about mules (male donkey x female horse) and
hinnies (male horse x female donkey). Typically completely sterile.
3. Hybrid breakdown - The zygote forms, but development is pretty much not going
to go anywhere.