Organisms exhibit a variety of patterns relative to reproduction, and different points of view may be taken with
regard to classification of these patters. For our purposes, we shall consider that there are two basic types of
reproduction, sexual and asexual.
Sexual reproduction is always associated with a type of nuclear division called meiosis which occurs at some
point in the life cycle of the organism involved.
Furthermore, except for a few atypical cases, sexual reproduction is characterized by the union of gametes, or
specialized reproductive cells, in the formation of a new individual. Such gametic union is followed by the
fusion of the game tic nuclei and the association of their chromosomes: this entire sequence of events is known
The two most significant features of sexual reproduction at least in terms of our emphasis in this chapter, are
meiosis and chromosomal association, mostly because of their genetic implications. It is sufficient for our
purposes to regard any process in which there is the production of organisms without the formation so new
chromosomal associations as asexual reproduction.
The vast majority of organisms exhibit sexuality, a phenomenon which, in its most obvious form, becomes
apparent in the most obvious form, becomes apparent in the existence of two sexually distinct kinds" of
individuals within a species.
Typically, the two cells which meet and unite in sexual reproduction are morphologically dissimilar, one being
relatively large and nonmotile, the other being relatively small and motile. When this is the case, the larger
gamete is termed an egg or ovum, and the smaller one is called a sperm. Whenever an individual is capable of
producing sperm it is designated a male; if it produces eggs, it is a female.
The major exceptions to this typical manifestation of sexuality are extremely interesting. Among several of the
algae and fungi, there are two sexually distinct strains within a species that are morphologically
indistinguishable in every detail. Sexual reproduction is carried on through the union of gametes, but there is
no discernible structural difference between them. Because there is no basis for designating one of the strains
"male" and the other "female", it is common to refer to one as the "plus" strain and the other as the "minus"
strain in a completely arbitrary manner.
Certain protozoa exhibit a phenomenon called multiple sexuality, in which there are various levels of sex rather
than two contrasting forms or strains. In multiple sexuality, the concepts of maleness and femaleness are, of
course, entirely without meaning.
Although most species of animals are dioeciously, some are monoecious. In general, the most complex animals
are dioeciously, whereas the monoecious conditions are limited to the less complex forms. An individual
member of a monoecious species is called a hermaphrodite the common earthworm is such an animal. It is
interesting to note that dioecious species an individual may occasionally be seen which possesses certain
characteristics of both sexes. Such an individual is called a pseudohermaphrodite.
Such an individual is not functional both as a male and a female as is a true hermaphrodite, and its appearance
in the species is considered abnormal. In plants, sexuality is generally obscured to such a degree as to render it
virtually unknown to the casual observer. Nevertheless, most plant species feature sexual reproduction. In
contrast to the situation among animals, the more complex plant species tend to be monoecious rather than
dioecious, although these terms have a slightly different meaning in botany. In dioeciously organisms, cross fertilization occurs of necessity. However, in monoecious organisms, a
possibility exists for selffertilization. This is seldom realized in monoecious animals, because most
hermaphrodites produce eggs and sperm at different times. In monoecious plants, selffertilization is common,
but even so, cross fertilization is the rule.
Few species fail to exhibit sexuality in some form, but in spite of this, asexual reproduction is very widespread
in the world of life. Many organisms reproduce most of the time in this manner, with sexual reproduction
occurring rarely or occasionally.
In general, asexual reproduction is limited in the animal kingdom to certain members of the lowest phyla in the
scale of complexity, notably the Protozoa, Porifera, Cnidaria, and Platyhelminthes, of the phyla we have
considered as major ones. In the plant kingdom, however some of the most advanced plants reproduce in this
fashion with regularity.
Fundamentally, there are two ways in which asexual reproduction may occur. The first method might be
termed somatic reproduction, the essence of which is the production of a new individual from a part of the
This form of reproduction involves more t