Encyclopedia of Evolution.pdf - Online Reading Center

solar system
———. On Human Nature. Cambridge, Mass.: Harvard University
Press, 1978.
solar system See universe, origin of.
speciation Speciation occurs when two or more species
diverge from a single ancestral population. natural selection
and sexual selection can produce evolutionary
change within lineages of organisms over time. However,
as many writers have pointed out, Darwin’s book (see Darwin,
Charles; origin of species [book]) did not actually
explain how two or more species could evolve from one species,
but only how one species could change into another.
Something more is needed, besides natural selection, to
explain speciation. A certain amount of reproductive isolation
is necessary. If one population of organisms is separated
into two populations, the two groups cannot interbreed. This
could result from geographical isolation, in which the two
populations are isolated from one another by impassible barriers
such as deserts or oceans. Or it could result from characteristics
of the organisms themselves: isolating mechanisms
are negative processes that prevent interbreeding between
groups, and specific mate recognition systems are positive
processes that encourage breeding within groups.
Once even partial reproductive isolation occurs, the
genes of the two populations no longer mix completely
together. The two populations may eventually become two
species because new genetic combinations, and new mutations,
that occur in one population will be different from
those in the other. The two populations could then diverge
along different evolutionary lines:
• Natural selection may favor different characteristics in each
group strongly enough to compensate for interchange of
genes (gene flow) between the populations. After the populations
have begun to diverge, natural selection may favor
isolating mechanisms and specific mate recognition systems
that complete the process of speciation.
• Different and random genetic changes (genetic drift) may
occur in each group (see founder effect). Even if natural
selection favors the same characteristics in both populations,
they will probably diverge because they will not have the
same genes upon which natural selection can act.
Evolutionary biologists distinguish species on the basis
of either a biological or a phylogenetic species concept. The
biological species concept (see Mayr, Ernst) recognizes the
potential to interbreed under natural conditions as the definition
of species membership. Even though this concept has
some difficulties (hybridization between recognized species
occurs frequently, and investigators cannot know the interbreeding
potential of geographically isolated populations or
fossil species), it is the concept that most closely reflects the
evolutionary process itself. The phylogenetic species concept
recognizes that species are distinct if experts can distinguish
them.
Natural or sexual selection may occur rapidly after
reproductive isolation, as the incipient species adjusts by
directional selection to the new environmental conditions or
the new set of species with which it is in contact. This may
be followed by a long period of stabilizing selection. This has
occurred frequently enough, according to many paleontologists,
to produce a pattern of punctuated equilibria in the
fossil record.
Speciation can occur in different ways, as described in
the following sections.
Allopatric speciation. Allopatric speciation occurs when
populations are geographically or ecologically isolated. This
has occurred frequently on islands, where dispersal of plants
and animals from the mainland is rarely successful, and once
it occurs, the island population is unlikely to disperse back
to the mainland. This is the process that has produced the
vast number of unique endemic species found only on certain
islands (see biogeography). The presence of many endemic
species on islands was one of Darwin’s chief evidences that
evolution has, in fact, occurred. Most species of nettles are
small, stinging herbs; but in Hawaii, a stingless nettle bush
has evolved, in isolation from mainland nettles. Hawaii has
or had many species of birds, snails, insects, plants, and
many other species found nowhere else in the world. Not
surprisingly, Hawaii has the longest list of threatened and
endangered species of any state in the United States (see biodiversity).
Given long enough time, the geographic separation
of populations almost inevitably produces new species
through allopatric speciation.
Peripatric speciation. Peripatric speciation is similar
to allopatric speciation but involves the geographical isolation
of small populations from the main population. Small
populations can be important in speciation, since the genetic
makeup of small populations can change more rapidly than
that of large populations (see population genetics). A classic
experimental demonstration of rapid evolution in small
populations is the study of evolutionary change, over the
course of a year and a half, in fruit flies (see Dobzhansky,
Theodosius). Dobzhansky established 20 large, and 20
small, populations of fruit flies, each with a certain chromosome
at 50 percent frequency. In most populations, this chromosome
became less frequent. In all of the large populations,
the chromosome became less frequent to about the same
extent; they all evolved similarly. In the small populations,
some hardly evolved at all, and some evolved a great deal;
the small populations had a much greater range of evolutionary
outcomes than the large populations.
Parapatric speciation. A large population may evolve
into two or more species by specialization on different habitats
within the geographical range of the ancestral species.
Sympatric speciation. Reproductive isolation can also
occur between two populations that live in the same location.
Examples include:
• If two species of plants open their flowers in different seasons
(for example, the autumn-flowering vs. the springflowering
species of alder trees of the genus Alnus), they
cannot interbreed even if they live in the same habitat.
• If two species of plants depend upon different pollinators
(such as the hummingbird-pollinated vs. the bee-pollinated