All living things share one basic feature in life: reproduction. It is this motivation to reproduce that ensures the continuation of the species. This reproduction can occur in one of two ways:
- asexual reproduction;
- sexual reproduction.
Asexual reproduction involves just one parent and does not involve sex. Part of the parent breaks or splits away and grows and develops into a new individual living thing. The process is quick and simple but does however have the drawback of both the parent and offspring sharing the same genetic material and therefore any genetic defects are passed on and are not naturally bred out of the species. This means that the power to adapt is reduced and natural selection takes over. Most asexual reproduction takes place in plants although some animals such as the hydra (a tiny animal living in ponds) also reproduce asexually. The sea anemone is one animal that can reproduce asexually as well as sexually, so too is the aphid. Plants can reproduce or be reproduced asexually in a number of ways:
- by sending out runners: plants such as the strawberry send out horizontal stems which develop young plants at the end and send down roots into the soil. The stems then die back and the new plant becomes independent of its parent;
- by producing bulbs: as plants such as daffodil and garlic grow and develop new small bulbs are produced around its base. These themselves will grow and develop into independent plants. This explains why, if left in the ground, a few daffodil bulbs will develop over the years into a large number of plants;
- through their roots: some plants are able to reproduce from the remains of roots left in the ground. Gardeners know only too well how important it is to take out all the root system of a ‘weed’ to prevent it from reproducing new plants from all the pieces of root that are left.
We also make use of asexual reproduction as farmers and gardeners, and assist in the reproduction of plants by taking cuttings from the leaves and stems of certain plants and growing them on. It is possible to grow cuttings from one plant by grafting them onto another, related plant (e.g. fruit trees). This does not always produce a new species: in time, the graft usually returns to the original stock.
The process of sexual reproduction requires there to be two parents. Each parent is responsible for the production of sex cells or gametes. These cells, the male sperm and the female egg, are combined during fertilization and a new cell is formed from which the new plant or animal develops. Sexual reproduction takes longer than asexual reproduction but has the advantage that, rather than being genetically identical to the parent, the offspring inherit half of their genes from one of their parents and half from the other. This means that any genetic weaknesses may not always be carried on from one generation to another, although this does happen in some circumstances where the gene carrying a particular characteristic is dominant. Sexual reproduction can take place in one of two ways:
- external reproduction: some animals are able reproduce in a way that requires no sexual union. Eggs laid by the female are fertilized outside the body by the male’s sperm. Animals that reproduce in this way tend to lay large numbers of eggs to increase the chances of fertilization and survival of the offspring;
- internal reproduction: in some animal species, internal reproduction involves the sexual union or mating of the male and female. The male injects his sperm into the female so that fertilization can take place inside the female’s body. In mammals, the female then carries the young as it grows and develops from two cells to millions. This period is called the gestation period and its length varies from one species to another. Others species, like birds, lay fertilized eggs that are incubated until the young bird hatches. Animals that reproduce in this way tend to produce fewer sex cells as there is more likelihood of successful fertilization taking place.
Human reproduction takes place through internal sexual fertilization. During sexual intercourse sperm, produced in the male’s testes, in mixed with a liquid in the prostate gland to produce semen. This then travels through the penis and enters the vagina of the female. Here, sperm swim into the uterus and along the fallopian tubes. If the female has ovulated (produced a ripe egg cell), the sperm may fertilize the egg and a zygote is formed. Only one sperm is required for fertilization to occur out of the millions that are ejaculated from the male. Once fertilized, the zygote begins to change as it divides and develops. If this division leads to each part developing into a baby, then each will have the same genes, both will be either male or female and will be identical twins. If two eggs (or more) are released from the ovary and are fertilized, they will have different genes, may be either male or female and will be non-identical twins (or triplets etc). The fertilized egg then travels down the fallopian tube to the uterus where it implants in the lining of the womb and where the placenta also develops. The zygote continues to grow into an embryo (in humans the first eight weeks after conception), and then a foetus (an unborn offspring over eight weeks since conception). During the next nine months, the embryo is provided with food and oxygen through the umbilical cord, along which waste materials from the baby are also passed. As the embryo becomes fully developed, the female gives birth (usually after 40 weeks) with the baby being born through the female’s vagina.
Whereas asexual reproduction is widespread in plants and sexual reproduction widespread in animals, there are many plants (such as the strawberry) and some animals (such as the sea anemone) that are able reproduce both asexually and sexually.
