Encyclopedia of Evolution.pdf - Online Reading Center

432 appendix
freshwater habitats separated by vast expanses of saltwater.
Most botanists are unaware of how many seeds a little
bit of pond mud can contain: From three tablespoonfuls
of pond mud, I grew 534 plants! My own experiments
have also shown that seeds can even be carried inside of
predatory birds that have eaten prey that had eaten fruits
and seeds! Incredibly enough, even locusts can carry seeds,
when they are blown great distances in the wind. Some
plant parts have even been carried on icebergs.
The freshwater species are remarkably similar in widely
separated regions. How did this happen? Since lakes do not
last forever, and rivers shift course, freshwater species have
to disperse from one location to another, in order to persist.
Because freshwater species are unusually good at dispersing
from one freshwater location to another, it is not surprising
that they have dispersed widely in the world. Although freshwater
fishes usually cannot survive in saltwater, nor saltwater
fishes in fresh, there are many groups of fishes that contain
both fresh and saltwater species; evolutionary adaptation to
differences in salinity must not, therefore, be very difficult.
Indeed, some individual fish (such as salmon) can acclimatize
to changes in saltwater vs. freshwater. Freshwater fishes,
then, could have dispersed even through saltwater. Ducks
fly over great distances and could have dispersed freshwater
snails from one place to another. As every child knows, salt
kills terrestrial snails; but freshwater snails can even survive
floating in saltwater, so long as they have sealed themselves
into their shells with a membrane.
Natural selection therefore explains not just the origin of
species but of groups of species throughout the world.
chapter 14. Mutual Affinities of Organic Beings
“From the most remote period in the history of the world
organic beings have been found to resemble each other in
descending degrees, so that they can be classed in groups
under groups. This classification is not arbitrary like the
grouping of stars in constellations.” Rather than a confusion
of types, like a sky full of stars, organisms occur in recognizable
groups, such as the many species of lizards, of oak trees,
of carnivores. The species in each of these groups are adapted
to live in a great variety of environmental conditions and
parts of the world. This fact is so familiar that we hardly stop
to ask ourselves why it should be so. This taxonomic pattern,
I will explain, results from the operation of natural selection
over the entire history of the Earth.
As explained in earlier chapters, most species eventually
become extinct. The few survivors are the ones that produce
the species that come later. The groups that contain the
largest number of species, and the species that contain the
most individuals, are the ones that are least likely to become
extinct. As a result, to him who has will more be given: The
largest groups become even greater in importance. For example,
there are millions of species of animals, but they fall into
just a couple of dozen groups [phyla], and the many species
of flowering plants fall into just two groups, the monocots
and the dicots. This is because all of the animals and flowering
plants are the evolutionary descendants of just the
few ancient populations that did not become extinct. A further
example of this is the discovery of Australia, which has
resulted in the discovery of an enormous number of new and
unique species, and in the discovery of no new insect classes
and very few new plant families.
Biologists classify organisms not just to keep track of all
the information but also in an attempt to understand their
relationships to one another, based on similarities and differences.
Biologists could use an artificial system of classification—for
instance, to classify all blue organisms together,
as in blue whales, bluebirds, and bluebonnet flowers. But
biologists find such arbitrary classifications exceedingly
unsatisfying and strive instead to produce a natural system
of classification which closely resembles lineages of descent.
These natural systems are used by virtually all taxonomists,
not just those who would agree with me about evolution. I
have shown that these lineages of descent are not metaphorical,
that they do not reflect some unknowable “plan of the
Creator,” but have really resulted from a community of
descent. Biologists recognize, rather than invent, the patterns
of relationship that define the genera of organisms: As Linnaeus
said, the characteristics do not define the genus, but the
genus defines the characteristics [see Linnaean system].
It might seem obvious that the most important characteristics
for the survival of an organism would also be the
most important characteristics for its classification, but this
is not the case. Biologists have long recognized that fishes
and dolphins are very different on the inside, however similar
their external adaptations to swimming may appear. Among
plants, the leaves are extremely important as organs of survival,
but their shapes are virtually useless in classification—
no two leaves even on the same tree have exactly the same
shape.
Sometimes we classify organisms on the basis of characteristics
that appear not to be important to the function and
survival of these organisms. This actually makes sense, from
an evolutionary viewpoint, because natural selection will have
acted upon, and wrought great diversity of form in, those
characteristics that are important; the unimportant characteristics,
such as the rudimentary and now useless remnants of
petals in the flowers of grasses, are those that will have survived
without modification from ancient times and represent
evidence of ancestry. Often, these rudimentary characteristics
are found in the embryonic stage of the organism’s development.
The flowering plants are divided into monocots and
dicots on the basis of the number of leaves possessed by the
embryo, inside the seed. If we base our classification upon a
single characteristic, we may err; but when we base it on sets
of characteristics, shared by all the species in a group, we can
have confidence in the resulting pattern.
Evolutionary ancestry not only explains the taxonomic
arrangement of groups but also the amount of difference
among the various groups. Families, or genera, of organisms
that differ more greatly from one another are simply those
that have diverged, through evolution, for a longer period of
time since their common ancestor.
The evolutionary classification of species is a process
almost exactly parallel to the evolutionary classification of