Download 3-4 種とは何か 種が生物の基本的な単位であることをほとんどの生物

1
3-4
種とは何か
種が生物の基本的な単位であることをほとんどの生物学者が認め、リンネの二分法によって種の命名指示がなされるが、
種とは何か、種はどのように定義されるべきかに関しては未だに一致がない。これは理論的な問題 で、実際上は種は表
現型レベルの性質によって識別、定義される。この表現型レベルの性質は変異、変化が不可避的なため、できるだけ不
変の性質を抽出して分類を行うのが分類学の仕事になる。そこには多くの困難が伴う。そして、その困難が理論的な問
いを生み出す。それが種の理論的な定義である。定義は２種類に大別で きる。一つは共時的な定義で、時間とは無関係
に種のメンバーを特定しようとする。他は通時的ですべての時間に渡って種を定義しようというものである。
a. The phenetic species concept
数量表現的な種概念は数量分類を種カテゴリーに適応するもので、直観的には「互いに類似していて、他の集合とは
異なるような有機体の集合」として種を定義する。進化論はこの種概念には何ら貢献しない。種は表現レベルで特定化
され、それは実践的には役立つがそれ以上ではない。その背後にある種の理論的な概念は表現クラスター以外のもので
ある。（固有名の定義に関する Frege, Russell と Kripke を思い出してみよう。）表現的な種概念の問題はそれが進化論と何
も関係がない点である。それは強いていえば形態学と関係しているに過ぎない。種が進化にとって極めて重要な概念で
あることを考えると、その定義が進化に関係ないというのはどこかがおかしいのである。
b. The biological species concept
Mayr’s definition: species are groups of interbreeding natural populations that are reproductively isolated from other such groups.
生物学的な種概念では、遺伝子プールがおおよそ種と同じものとなる。遺伝子プールによって種の同一性と他種との区
別・相違が説明される。種を実際に定義する上での形態的な基準は交配による結果を反映するものとみなされる。交配
が一定の形態的な一様性を与えるのである。
c. The isolation species concept
(isolating mechanisms) ある種を近隣の別の種と区別するのは何か。それは二つの種の交配を禁止する隔離メカニズムであ
る。幾つかの隔離メカニズムには次のようなものがある。
(Premating mechanisms) Ecological or habitat isolation, Seasonal or temporal isolation, Sexual or ethological isolation, Mechanical
isolation, Gametic isolation,
(Postmating mechanisms) Hybrid inviability, Hybrid sterility, Hybrid breakdown
d. The recognition species concept
種はそのメンバー相互を認識するシステムをもっている(SMRS; specific mate recognition system)。これを共有するメンバ
ーの集合が種として定義される。
# Isolating mechanisms and specific mate recognition systems can be considered as two ways of defining species as reproductive
communities; the biological and recognition concepts are two parts of a more general reproductive species concept. (cf. Otte, D. and
J. A. Endler (eds.), Speciation and Its Consequences, Sinauer, 1989, Part One, 3-84)
e. The ecological species concept
生態学的な種概念は、種をある特定の生態学的ニッチに適応した有機体の集合として定義する。雑種は自然の適応帯に
適応しない故に、種間の交配はそれが起こらないように選択される。これによって、なぜ自然では交配が制限されてい
るか説明できる。
# Selection and gene flow can both explain the integrity of species. The biological species concept explains the integrity of species
by gene flow, the ecological concept by selection. The two processes are usually correlated, but can be tested between in special
cases. Selection can be strong enough to overcome gene flow (selection can produce divergence despite gene flow), and selection
can maintain a species’ integrity in the absence of gene flow. （遺伝と選択のバランス）
f. The cladistic species concept
分岐的な種概念は、種を二つの分岐点の間の進化的な系統群のメンバーとして定義する。生物学的、生態学的な種概念
が時間的に適用されれば、それは系統群を区切ることになる。
g. A pluralistic species concept
多元論的な種理解
種が実在するか、それとも種は概念に過ぎないのか。この問いに対して種の定義はそれぞれ異なる解答を与える。表現
的な性質の集まりとしての種は何も答えられない。生物学的な種概念では種は実在することになる。しかし、種よりも
上位の分類レベルについては何も主張していない。生態学的な種概念では、すべての分類レベルが同じ程度の実在性を
もっている。
# Species in evolutionary theory are “individuals” rather than “classes.” (Hull, Ghiselin)
2
Species: The Concept and Meaning of Species
Species come about as the result of many gradual changes; sexual reproduction, mutation, gene flow, and genetic drift take place, and then
the new traits that result from these processes are selected by nature through a process of natural selection. Environments are continuously
changing in tim e, and they differ from place to place. Natural selection, therefore, favors different characteristics in different situations. The
accumulation of differences eventually yields different species.
External similarity is the common basis for identifying individuals as being members of the same species. Yet a bulldog, a terrier, and a
golden retriever are very different in appearance, but they are all dogs because they can interbreed. People can also interbreed with one
another, and so can cats, but people cannot interbreed with dogs or cats, nor can these with each other. It is clear, then, that although species
are usually identified by appearance, there is something basic, of great biological significance, behind similarity of appearance. Individuals
of a species are able to interbreed with one another but not with members of other species. This is expressed in the following definition:
Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.
The ability to interbreed is of great evolutionary importance, because it determines that species are independent evolutionary units. Genetic
changes can only originate in single individuals, so they can only spread to members of their own species, but not to individuals of other
species. Individuals of a species share a common gene pool that is not shared by individuals of other species. Different species have
independently evolving gene pools because they are reproductively isolated.
Although the criterion for deciding whether individuals belong to the same species is clear, there may be ambiguity in practice for several
reasons. For example, it may not be known for certain whether individuals living in different sites belong to the same species, because it is
not known whether they can naturally interbreed. Another reason for ambiguity is rooted in the nature of evolution as a gradual process. Two
geographically separate populations that at one time were members of the same species later may have diverged into two different species.
Since the process is gradual, there is not a particular point at which it is possible to say that the two populations have become two different
species.
A related situation pertains to organisms living at different times. There is no way to test whether or not today's humans could interbreed
with the humans that lived thousands of years ago. The ancestors of modern humans that lived five hundred thousand years ago are classified
in the species Homo erectus, whereas present-day humans are classified in a different species, Homo sapiens. There is not an exact time at
which Homo erectus became Homo sapiens, but it would not be appropriate to classify remote human ancestors and modern humans in the
same species. Biologists distinguish species in organisms that lived at different times by saying that if two organisms differ from each other
about as much as two living individuals belonging to two different species differ, they will be classified in separate species and given
different names.
(The definition of species given above applies only to sexual organisms. Asexual organisms, such as Bacteria and blue-green algae, are
classified into different species according to criteria such as external morphology, chemical and physiological properties, and genetic
constitution.)
Reproductive Isolation and Reproductive Isolating Mechanisms
A species is an interbreeding natural population that is reproductively isolated from other such groups. There are many different types
biological incompatibilities that can cause a population to be reproductively isolated from another population, thus dividing these populations
into two separate species. These biological properties of organisms that prevent interbreeding are called reproductive isolating mechanisms.
There are two general categories of reproductive isolating mechanisms. Prezygotic reproductive isolating mechanisms take effect before
fertilization and prevent the formation of hybrids between members of different populations through ecological, temporal, behavioral,
mechanical, morphological, and gametic isolation. Postzygotic reproductive isolating mechanisms take effect after fertilization and reduce
the viability or fertility of hybrids or their progeny. (Geographic isolation, also called physical isolation, such as elm trees on different islands,
salmon in different rivers, or squirrels in different mountain ranges, is not a reproductive isolating mechanism, since it is not related to a
biological property of organisms.)
Prezygotic Reproduc tive Isolating Mechanisms
Ecological isolation
Populations may occupy the same territory but live in different habitats and so not meet. This reproductive isolating mechanism is known as
ecological isolation, or habitat isolation. It is well illustrated by the pig frog, rana grylio , and the gopher frog, rana areolata, that live in the
area around New Orleans, Louisiana. The pig frog is extremely aquatic and lives in deep ponds, lakes, and marshes among lily pads and
emergent vegetation. The gopher frog occupies mammal and tortoise burrows during the day, but is active at night around the margins of
swamp areas. The pig frog breeds in deep water and has no ecologic contact with the gopher frog.
Temporal isolation
Populations may mate or flower at different seasons or different times of day. This reproductive isolating mechanism is known as temporal
isolation or seasonal isolation. Three tropical orchid species of the genus Dendrobium flower just for a single day, the flowers opening at
dawn and withering by nightfall. Flowering in these three species all occurs in response to certain meteorological stimuli, such as a sudden
storm on a hot day, but the lapse between the stimulus and flowering is eight days in one species, nine in another, and ten or eleven in the
third. Fertilization between the three different species becomes impossible because at the time when the flowers of one species open, those of
the other species have already withered or are not yet mature.
Ethological Isolation
Ethological isolation, or behavioral isolation, occurs when members of the opposite sex of different species don’t acknowledge each other
3
because neither presents the correct mating rituals. In most animal species, members of the two sexes must search for each other and come
together in a very species-specific manner. Complex courtship rituals then take place, often with the male taking the initiative and the female
responding. This can then generate additional actions by both the male female, and eventually there is copulation. These elaborate rituals
play a significant part in species recognition. If either of the two sexes decides that the sequence of events in the mating process is incorrect,
then the entire process will be interrupted. Courtship and mating rituals have been extensively analyzed in some mammals, birds, and fishes,
and in a number of insect species.
Ethological isolation is often the most potent reproductive isolating mechanism to keep animal species from interbreeding. It can be
strong even among closely related species. For example, the closely related gray tree frog, hyla versicolor, and the pine woods tree frog, hyla
femoralis, frequently breed in the same ponds. Female frogs locate mates by the calls given by the males after they reach the breeding site.
Although the gray tree frog and pine woods tree frogs are very similar in physical characteristics, their male breeding calls are very different.
In the gray tree frog the call is a short, loud trill that lasts no more than three seconds. The call of the pine woods tree frog consists of a series
of sonorous but very quick, abrupt calls. Female tree frogs can distinguish between the two calls and no mixed mating occurs.
Mechanical isolation
Copulation is often impossible between different animal species because of incompatible shape and size of the genitalia. In plants,
variations in flower structure may impede pollination. This is known as mechanical isolation. In two species of sage from California, the
two-lipped flowers of Salvia mellifera have stamens and style in the upper lip, whereas Salvia apiana has long stamens and style and a
specialized floral configuration. S. melliferais pollinated by small or medium -sized bees that carry pollen on their backs from flower to
flower, while S. apiana is pollin ated by large carpenter bees and bumblebees that carry the pollen on their wings and other body parts.
Morphological isolation
Morphological isolation occurs when differences in shape or size prevent mating between species. Differences in size prevent copulation
between the oak toad, bufo quercicus, and the Gulf Coast toad, bufo vallicpes. The oak toad is very small, attaining a maximum length of one
and one-quarter inches in females, while the smallest of male Gulf Coast toads are almost twice as long. In such a case, size alone prevents
interbreeding. The male oak toads are much too small to grasp female Gulf Coast toads, and male Gulf Coast toads are so large that they are
more likely to eat female oak toads than try to mate with them. Even if they were to engage in courtship, a female oak toad would drown
under the weight of a male Gulf Coast toad during copulation.
Gametic isolation
Gametic isolation occurs when fertilization cannot take place between species. In some animals with internal fertilization, sperm may fail to
survive in the sperm receptacles of females of other species. In plants, pollen grains of one species typically fail to germinate on the stigma
of another species so that the pollen never reaches the ovary and fertilization cannot occur. In many aquatic animals the ova and sperm are
shed into the water. In such animals, gametes of different species generally fail to attract one another. For example, the sea urchins
Strongylocentrotus purpuratus and Strongylocentrotus franciscanus can be induced to release their eggs and sperms simultaneously, but
fertilizations that result are between eggs and sperms of the same species.
Postzygotic Reproductive Isolating Mechanisms
It is sometimes possible to obtain hybrid zygotes between members of different species through artificial insemination or artificial mixtures
of ova and spermatozoa.
Hybrid inviability
Often, hybrid zygotes do not develop fully, and, thus, die before they are born. This postzygotic isolating mechanism is called hybrid
inviability. The hybrid embryos artificially created between of sheep and goats, for example, die in the early developmental stages before
birth. Hybrid inviability is common in plants, whose hybrid seeds often fail to germinate or die shortly after germinat ion.
Hybrid sterility
However, hybrid zygotes sometimes do develop into adults, but in many cases the adults fail to develop functional gametes and, thus, are
sterile. This is known as hybrid sterility or hybrid infertility. The most famous of all animal hybrids, the mule, (a cross between horses and
donkeys,) is sterile. Hybrid sterility is frustrating for geneticists and evolutionists because genetic analysis of the differences between species
requires a study of the second and later generations.
Hybrid breakdown
Even though viable and fertile hybrids between two species are obtained, some barrier to exchange of genetic material may still exist if
hybrids of the second generation are inviable or weak. This as hybrid breakdown is a very common phenomenon. Hybrids between the cotton
species Gossypium barbadense, Gossypium hirsutum , and Gossypium tomentosum appear vigorous and fertile, but their offspring die as
seeds or early in development, or they develop into sparse, weak plants unable to reproduce to create a third generation.
Speciation
Via the definition of species and the biological factors that differentiate between the species, it is not difficult to determine the cause of
speciation, or the formation of new species. Since species are groups of populations reproductively isolated from one another, asking about
the origin of new species is equivalent to asking how reproductive isolation arises between populations. Via the definition of species, we can
arrive at the conclusion that when one gene pool differs from the original gene pool from which it came to such an extent that the members
of one population can no longer successfully mate and reproduce with members of the other population, speciation has occurred. There are
basically two general modes of speciation, one being divergence and the other being transformation.
Divergence, or cladogenesis, is the appearance and establishment of more than one species from one original species. During this process,
4
the original species may actually go through change as it diversifies into two or more new species, or it may remain stagnant as one more or
species arises from it. In such a situation, breeding studies can be attempted to determine the degree of reproductive isolation existing among
the two communities.
Transformation, or anagenesis, refers to the cumulative changes over time whereby one gene pool changes into another that is presumably
reproductively isolated from the first; thus, a new species is very gradually created while the old species is gradually lost. Transformation is
very subjective because evolutionists cannot always agree on exactly when the new species has become reproductively isolated from the
original species. This, of course, cannot be tested because the first population no longer exists, it having been transformed into the later one.
Here's an alternate way of looking at divergence and transformation. Speciation generally involves at least one of two processes: either the
environment of a particular species changes, or the species itself changes in the way that it lives and survives in its environment. If only one
way of survival emerges, and thus only one species emerges from the original species, transformation has occurred. However, if more than
one way of survival emerges, and multiple species emerge from the original species, divergence has occurred.
As one might assume, the nature of speciation is such that it is taking place all the time and, at any particular period of time, there are large
numbers of animal populatio ns that are at different stages of speciation.
An example of populations that have diverged a very small amount from a common ancestor are those of a moth, hyponomeuta padella ,
which are still considered to be part of the same species. In one population or subspecies the eggs are laid on apple leaves and the adults are
usually dark gray, while in the other, the eggs are laid on hawthorn leaves and the adults tend to be lighter in color. Members of these
subspecies occasionally mate with one another, although breeding experiments show that about twice as many matings take place between
individuals of the same subspecies.
Such a circumstance may continue until the populations become completely differentiated into separate species. It happens quite commonly,
however, in both animals and plants, that opportunities for hybridization arise between two populations that are becoming genetically
differentiated. It is possible that that the hybrids manifest little or no reduction of Darwinian fitness, so that gene exchange between the two
populations again proceeds freely, eventually leading to their integration back into a single gene pool. (It is also possible that genetic
separation has occurred to such an extent that reduction of fitness in the hybrids is sufficiently large to cause one of the postzygotic
reproductive isolating mechanisms to kick in and prevent the re-integration of the two now separated species.)
Specific means by which divergent speciation may occur are generally categorized into allopatric and sympatric speciation and adaptive
radiation. This is based on the amount of geographic separation between species.
Allopatric speciation
Allopatric speciation, or geographic speciation, begins when a population is divided into two or more smaller populations by a geographic
barrier so that interbreeding between the resulting populations is prevented. This may occur when a few colonizers reach a geographically
separate habitat, perhaps an island, lake, river, isolated valley, or mountain range. In another process, a population may be split into two
geographically separate ones by topographic changes, such as the end of water flow between two lakes, a burned area devoid of food, or an
area covered with volcanic lava. Also, an invasion of competitors, parasites, or predators into an intermediate zone can create a barrier that
divides a population. Once reproductively isolated by the barrier, gene frequencies in the two populations can diverge due to natural selection,
mutation, genetic drift, or gene flow. If the gene pools sufficiently diverge, interbreeding between the populations will not occur if the barrier
is removed. As a result, new species have formed.
Sympatric speciation
Sympatric speciation refers to the formation of new species without the presence of a geographic barrier. There are several factors that can
trigger sympatric speciation. Introgressive hybridization, or sometimes simply hybridization, can occur when two distinctly different forms
of the same species, or two closely related species that are normally reproductively or geographically isolated, mate and produce progeny.
This can occur as a result of physical changes in the environment due to fires, floods, earthquakes, and other natural disasters, or changes in
climate in the environment, that allow the closely related species to come in contact for the first time. It can also happen along a common
geographic boundary shared by both species or subspecies called a hybrid zone. In cases of introgressive hybridization, it is possible that the
genetic variation of the hybrids is greater than that of either parent and permits population of hybrids to evolve and adapt to environmental
conditions in the hybrid zone beyond the range of either parent. As time passes, the hybrid population, because of its genetic differences, is
exposed to different selection pressures, and eventually diverges from both parent populations to form a new species.
Somewhat related to hybridization is parapatric specation. Parapatric speciation occurs between populations of a continuous cline, or series
of related populations that live close to one another but generally do not co-mingle. Generally what happens is species at the edge of one
population mate with species at the edge of a nearby population of the same cline, thus new species is created through hybridization, but so
that only a few members of the parent populations are affected. In effect, parapatric speciation can be thought of simply a result of limited
gene flow. Allo-parapatric specation occurs when populations are initially separated, as in allopatric speciation, but then are united, allowing
parapatric speciation to finish the speciation process. This generally occurs when natural barriers are destroyed through some sort of
catastrophe.