Encyclopedia of Evolution.pdf - Online Reading Center

ased, to eat small plankton near the surface of the lakes;
others preferred eating material that had settled to the bottoms
of the lakes. Males and females mated preferentially in
the same locations where they ate, which set them on the road
to becoming two separate populations. Now, the surface- and
the bottom-dwelling populations have begun to evolve unique
adaptations to their new modes of life: The mouths of the
bottom dwellers are larger and turned downward; those of
the surface dwellers are smaller and not downturned.
In each of these cases, natural selection has probably
favored the specialization on different food resources. If the
populations interbred, the resulting offspring might be inefficient
at exploiting either of the parental food resources.
Specialization on different symbionts. symbiogenesis is
the origin of new species as a result of the symbiotic coevolution
of two or more species. For example, a population of
bacteria may take up residence in a new kind of host cell, for
example an amoeba, and reproduce inside that host cell. Whenever
the amoeba reproduces, the bacteria are passed on to the
new amoebae. The amoeba represents a new environment for
this population of bacteria. This population is isolated from the
bacteria from which it originally came and adapts to a new set
of conditions. The amoebae, which have evolved a tolerance
for the bacteria, may also be isolated from other amoebae if
the other amoebae cannot tolerate the bacteria. By depending
on each other, the amoebae and the bacteria are isolated from
other amoebae and bacteria. The result can be a new species of
amoeba and a new species of bacteria. In time, the bacteria may
degenerate to such an extent that they are no longer recognizable
as a distinct species. The first stages of this process were
actually observed to occur in a laboratory setting.
Differential success at fertilization. Even if individuals
from two populations succeed in mating, their reproduction
may still not be successful, if the male reproductive cells (sperm
of animals, or pollen nuclei of plants) are inferior at fertilizing
the female reproductive cells (eggs of animals, or ovules of
plants). The sperm cells from one population may swim faster
and suffer fewer deaths on their way to the egg than the sperm
cells from another population; this will tend to isolate the two
populations. The female reproductive tract of many vertebrates
represents a hostile environment through which only the hardiest
sperm can travel—that is, the sperm that are best adapted
to those particular conditions. In flowering plants, pollen grains
land on the stigma of the flower, germinate, and grow a pollen
tube down into the ovary of the flower. Some pollen grains
are better at doing this than others. Frequently, the stigma of a
flower will not even permit the wrong kind of pollen—either
pollen that is genetically identical to it, or pollen that comes
from the wrong species—to germinate in the first place.
Postzygotic isolating mechanisms usually result because
the offspring of matings between species (hybrids) or between
populations have inferior reproduction. For example, hybrids
between oak species can occur but are relatively rare; there
are even reports of hybrids between the great cat species of
Africa, although this is more common in zoos than in the
wild. The rarity of such hybrids—they do not establish their
own populations that persist and spread—suggests that they
have inferior growth and reproduction.
isolating mechanisms
On some of the uninhabited Galápagos Islands, species
of Darwin’s finches have postzygotic isolation, as well as
the prezygotic isolation described above. The frequent shifts
between rainy weather and drought keep the large-beaked
finches and the small-beaked finches from successfully interbreeding.
During times of rain, small seeds are abundant and
small-beaked finches prosper; during droughts, large seeds are
abundant and large-beaked finches prosper. Natural selection
seldom favors beaks of intermediate size in these populations.
However, on the Galápagos Islands that are inhabited, where
human activity causes a continuous availability of seeds,
the finches with intermediate beaks are successful. Natural
selection seems to be favoring isolating mechanisms on the
uninhabited islands, but not favoring them on the islands
inhabited by humans.
Among plants, hybrids between species frequently occur
but may result in offspring with incompatible chromosomes.
If the chromosomes within the cells of the offspring
are unable to match up with one another during meiosis, the
offspring are sterile (see Mendelian genetics). However,
among plants, sterile individuals may have options that are
not available to animals.
• Many plants reproduce asexually, a process called apomixis.
The embryo produced inside the seed is an exact genetic
copy of the parent. The apomictic plants may constitute a
separate species; they are reproductively isolated from both
of the species that produced them, because they do not
have sexual reproduction at all.
• Plants frequently undergo spontaneous doubling in the
number of chromosomes. When the number of chromosomes
doubles in an animal egg cell, the egg will probably
die. Most animals tolerate only the most minor of chromosome
changes; even then, as in Down syndrome or trisomy
21 in humans, a single extra chromosome results in sterility
and retardation. However, a plant egg cell with doubled
chromosomes may survive. A sexually sterile plant, if it
undergoes chromosome doubling, may produce offspring
that are fertile because the doubled chromosomes now
form pairs. This allows meiosis to occur. The population
of plants with doubled chromosomes, however, may not be
able to interbreed with either of the ancestral populations.
The hybrid is then instantly isolated from both parents
and has characteristics different from both; it becomes an
“instant species.” This has been observed in the goatsbeard
Tragopogon and in the sunflower Helianthus. This form of
speciation is common in the plant kingdom. The study of
chromosomes suggests that almost half of all plant species,
including 95 percent of fern species, originated by chromosome
doubling following hybridization.
The inferiority of offspring produced by interbreeding
between two populations (a postzygotic isolating mechanism)
may promote the evolution of prezygotic isolating mechanisms.
Conversely, the benefit conferred by mating with conspecific
individuals may promote the evolution of isolating
mechanisms. Anatomical differences that prevent interbreeding
may be favored by natural selection because they encourage
individuals to mate only with other individuals that are simi