Evolution: Origin of species
Before we can seriously ponder the origin of species (how new species come into being),
we must first make a serious attempt to get a handle on exactly what is meant by
"species." Here's a definition that works most of the time for sexually reproducing,
multicellular organisms:
Species: A group of like individuals capable of interbreeding and producing fertile
offspring under natural conditions. "Fertile offspring" takes care of cases such as
donkeys and horses, because mules are sterile. "Under natural conditions" takes care of
lions/tigers and wolves/dogs. There are many more examples, but this ought to clarify
the intended point.
As I've pointed out on numerous occasions, this definition of a species does not apply to
bacteria and archaea because they do not reproduce sexually. However, these organisms
are given species names out of habit and necessity. Microbiologists concede that
"species" of bacteria are not the same as species of higher organisms. Bacterial strains
(E. coli 0157:H7) identifies the strain of E. coli that produces the nasty toxin…quite
different from the normal strains found in the colon of all warm blooded animals.
Species have changed over time. The fossil record clearly shows us that new species
have come into existence (speciation), and the vast majority of species that have ever
existed have gone extinct. Extinction and speciation "events" occur at what appears to be
a very slow rate, but dramatic events in Earth's history have resulted in mass extinctions.
Mass extinctions leave huge ecological voids…niches to be filled. This appears to result
in accelerated rates of speciation/divergence. The fits and starts in the rate of
evolutionary change is referred to as punctuated equilibrium (which is a self-defining
concept if you pause for a moment to consider it).
Populations are the smallest units than can evolve. Individuals do not evolve (forget
about evolving your Pikachu into a Raichu…that's only a fantasy). It is critical that you
get a handle on the concept of a population. By definition, a population is a sub-set of
a species reproductively isolated from other populations of the same species. For
example, the migratory orcas that travel from Alaska to Tierra del Fuego (and back) each
year are a separate population from the orcas that are year-round residents of the San
Juan and Canadian Channel Islands. The two populations have different behaviors
(migration, diet, language), and they appear to be disinclined to interbreed when the
migratory population passes through.
Microevolution - A change in allele frequencies: The criteria for Hardy-Weinberg
equilibrium provide us with a starting point for understanding how a population might
undergo microevolution, for if any of the criteria are violated, allele frequencies change.
Mutation WILL happen, albeit very slowly, but it is the ultimate source for new alleles some of which are bad, but some of which may be advantageous. A population in H-W
Equilibrium is one that is NOT evolving.
Natural selection acts on phenotypes. Of course, underlying the phenotype is a
genotype, and alleles can (and do) change in frequency within a population over the
generations as selection pressure is exerted on the population. (Recall that a population
in Hardy-Weinberg equilibrium is one that is not evolving, but the criteria for such
equilibrium can only last so long on a planet that is continuously changing in so many
ways.) Microevolution is most simply defined as the change in allele frequencies within
a population over generations. The ultimate climax of microevolution would be
speciation. Two reproductively isolated populations diverge so significantly that they are
no longer the same species (see definition above).
Many traits (phenotypes) are controlled by multiple genes…polygenic traits. For a nice
overview of how natural selection works on polygenic traits, please check out Kimball's
overview. Pay special attention to 1. Directional selection, 2. Stabilizing selection, and 3.
Disruptive selection. This is not a hot link (we don't all have .docx capability), so you'll
have to copy and paste:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Evolution.html#Evolution_and
_Adaptation
One of the criteria for a population in H-W Equilibrium, "sufficiently large" is often
violated. In such cases, the very small group of individuals starting anew is not
necessarily representative of the very large population from which they originated. This
can result in what is called the Founder Effect. Imagine a few dozen birds catching a
bodacious tailwind and ending up on an island hundreds of miles from home. This would
be the "founder" population.
Populations in H-W equilibrium are not allowed to have small groups of individuals
leaving (emigrating), and they can't allow small groups from other populations within
their species to join them (immigration). Either case would disrupt the equilibrium of
allele frequencies. Gene Flow disrupts the equilibrium of allele frequencies, and results
in microevolution.
A natural disaster could reduce the size of a population in a way that is simply bad luck
for those individuals that perish. The survivors are not necessarily more fit, they just got
lucky. The lucky survivors' collective allele frequencies do not represent those of the
parent population. Such random change in allele frequencies is referred to as Genetic
Drift. A very similar scenario exists when conditions allow for only a small portion of a
population to survive and reproduce, and the survivors may or may not be more fit unlike the complete randomness of Genetic Drift. Such an event is referred to as the
Bottleneck Effect. It has been hypothesized that our species (Homo sapiens) was "forced
through a bottleneck" about 70,000 years ago, and this might partially explain why there
is so little genetic variation in our species.
Patterns of Evolution: Evolutionists of the 19th and early 20th century (including
Darwin Almighty) considered that evolutionary change was slow and
steady…Gradualism. We now have accumulated evidence that Punctuated Equilibrium
is a more accurate representation of the pace of change.
When unrelated species evolve in response to one another, we refer to this as coevolution. Such is the case with a plant and its pollinator, and so many other cases of
symbiosis that we must save this for another time. Co-evolution is extremely common as
all species are ultimately interdependent. Cooperation between species (symbiosis) has
been a driving force in evolution from the earliest cells to the complex interactions we
see today. Darwin's "struggle for survival" is not all about competition. Species - like
humans - benefit greatly from cooperation. "United we stand, divided we fall." It's been
that way for three and a half billion years.
In some cases, species that have diverged in their distant pasts may end up sharing the
same environment and over time each will evolve to meet the demands imposed by their
shared environment. Convergent Evolution (ex. Shark and dolphin) results when
unrelated species become increasingly, albeit superficially similar in appearance and
behavior despite having quite different genealogies.
Macroevolution: The evolution of entirely new groups of organisms at the higher
taxonomic levels is referred to as macroevolution. We also use macroevolution in
discussing the evolution of the heart, eyes, the brain, etc… For the present, however, we
will focus on microevolution, its causes, and its experimentally reproducible evidence.
Macroevolution will be woven into our exploration of the diversity of life, which is like
taking a mental journey to the ultimate zoo, botanical garden, aquarium, etc… and also
traveling backward in time. Kind of like the magic school bus.