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 as fertilization.
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 non-motile, 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 self-fertilization. This is seldom realized in monoecious animals, because most hermaphrodites produce eggs and sperm at different times. In monoecious plants, self-fertilization 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 parental body.
This form of reproduction involves more than a single cell whenever it occurs in multicellular organisms. The simplest form of somatic reproduction is seen in unicellular organisms such as protozoa, yeasts, bacteria, and certain algae whose cell division automatically results in their reproduction.
We have mentioned previously that some organisms such as Hydra produce buds which ultimately develop into independent organisms. Certain of the free-living flatworms undergo a longitudinal division of the body, which results in the formation of two organisms from one. Among the more complex plants, some species exhibit very elaborate modifications for somatic reproduction, such as the tubers of Irish potatoes, the runners of strawberries, and the bulbs of tulips.
As a second asexual method, some organisms exhibit the production of spores, which are single-celled units capable of growing into whole organisms. This method of reproduction is virtually limited to the plant kingdom, where it is demonstrated in some form by the vast majority of species.
Perhaps it will occur to the thoughtful student to wonder whether or not regeneration, which was discussed in the preceding chapter, constitutes a form of asexual reproduction.
In consideration of this question, it must be remembered that we limited the definition of this phenomenon to situations where parts of an organism are lost and replaced, or to those in which an entire organism develops from a part. Replacement of structures certainly does not constitute reproduction, nor can regeneration be considered a natural means of reproduction among organisms.
We can separate a planarian worm into several parts experimentally and induce the regeneration of new individuals, but the worm does not perform this task autonomously. Hence, it is best to regard the term regeneration as one describing a certain aspect of growth. To state the matter another way, growth and reproduction are two distinct phenomena.
It should not be thought that sexual and asexual reproduction is necessarily exclusive in a given species. Although sexuality has never been observed in some species and in others reproduction is entirely sexual a great many species exhibit both sexual and asexual reproduction. Hydra a common freshwater cnidarian is such an organism.
In addition to egg and sperm formation, which is a manifestation of sexuality, buds are frequently formed, and these eventually develop into adult individuals. Many flowering plants may be propagated from a root, stem, or leaf. This represents purely asexual reproduction, even though seeds may be formed as a result of sexual reproduction.
In comparing sexual and asexual reproduction in living organisms it might seem that asexual reproduction would have a number of advantages over sexual reproduction. An aquatic environment is frequently necessary for sexual reproduction to take place, particularly in such animals as hydra and most fishes where there is no direct contact between male and female individuals.
In addition, the fusion of gametes is usually dependent upon the random movement of one of the garftetic types. When sperm fail to make contact with eggs, the reproductive process is thwarted.
Although a number of hazards are inherent in sexual reproduction, it has become the dominant form of reproduction among organisms. Perhaps there is some significance in this.
Earlier, we introduced the idea that chromosomes with their genes play an important role as intrinsic developmental factors and in the control of metabolism at all growth levels. It should be apparent that whatever part genes play in the development and in the homeostasis of an organism, these effects must of necessity remain unchanged from parent to offspring in asexual reproduction.
This principle has been exploited by man in the asexual propagation of many plants such as fruit trees when it is desirable to ensure that all qualities of a parent plant are exactly reproduced in the offspring, and here, this reproductive feature is of distinct advantage.
On the other hand, it is the essence of typical sexual reproduction that gametes are formed in parent individuals by meiosis and are brought together infertiliazation with subsequent formation of a new individual. The process of meiosis, as we shall see, results in a random mixing of the genetic factors and their subsequent random distribution into gametes. Typically, the gametes are formed in separate parents; thus, fertilization results in a set of genetic factors from two different individuals.
By this means, the phenomenon of variability is made possible in the offspring, and apparently, it provides an advantage that outweighs the disadvantages mentioned above. We shall see later that sexual reproduction and variation have played important roles in the rise of new species, a process which is apparently responsible for the multiplicity of present-day forms.