Sexual reproduction
Sexual reproduction is a type of
reproduction that results in increasing
genetic diversity of the
offspring. It is characterized by two processes. The first,
meiosis, involves the halving of the number of
chromosomes. The second process,
fertilization, leads the fusion of two
gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually
cross over to achieve
genetic recombination.
The
evolution of sex is a major puzzle in modern
evolutionary biology. The first
fossilized evidence of sexually reproducing
organisms is from
eukaryotes of the
Stenian period, about 1.2 to 1 billion years before the present time. Sexual reproduction is the primary method of reproduction for the vast majority of visible organisms, including almost all
animals and
plants.
Bacterial conjugation, the transfer of
DNA between two
bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar.
Contemporary evolutionary thought posits a few explanations for why sexual reproduction persists when
parthenogenesis appears in some ways to be a superior form of
reproduction. Sexual reproduction may persist because of selection pressure on the
clade itself- the ability for a population to radiate more rapidly due to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the 'ratcheting' of evolutionary speed as one clade competes with another for a limited resource.
|
In the first stage of sexual reproduction, 'meiosis', the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During 'fertilization', haploid gametes come together to form a diploid zygote and the original number of chromosomes (2n) is restored. |
Many
protists and
fungi reproduce sexually. Although they are unicellular, at times of reproduction the "father" cell and the "mother" cell combines together. Next, their genetic information combines together into a new formation, and by
cell division the offspring is born.
In
flowering plants, a
stamen produces gametes called
pollen grains, which attach to a
pistil, in which the female gametes (ovules) are located. Here, the female gamete is fertilized and develops into a seed. The ovary, which produced the gamete then grows into a
fruit, which surrounds the seed(s). Plants may either
self-pollinate or
cross-pollinate.
Male and female birds and reptiles both have
cloacae, an opening through which eggs, sperm, and wastes pass. Intercourse is performed by pressing the lips of the cloacae together, during which time the male transfers his sperm to the female. The female lays
amniotic eggs in which the young gestate. Nevertheless, a few species, including most waterfowl and ostriches, have a phallus shaped organ analogous to the mammals' penis.
In
placental mammals,
offspring are born as juveniles: complete animals with the
sex organs present although non-functional. After several months or years, the sex organs develop further to maturity and the animal becomes
sexually mature. Most female mammals are only
fertile during certain periods and during those times, they are said to be "in heat". At this point, the animal is ready to mate. Individual male and female mammals meet and carry out
copulation. For most mammals, males and females exchange sexual partners throughout their adult lives.
The mammalian male
The male reproductive system contains two main divisions: the
penis, which is inserted into the female and carries the sperm inside it, and the
testes, which produce the sperm. In humans, both of these organs are outside the
abdominal cavity, but they can be primarily housed within the abdomen in other animals (for instance, in
dogs, the penis is internal except when mating). Having the testes outside the abdomen best facilitates
temperature regulation of the sperm, which require specific temperatures to survive.
Sperm are the smaller of the two gametes and are generally very short-lived, requiring males to produce them continuously from the time of
sexual maturity until death. They are motile and swim by
chemotaxis.
The mammalian female
The female reproductive system likewise contains two main divisions: the
vagina and
uterus, which act as the receptacle for the sperm ejaculated by the male during
copulation, and the
ovaries, which produce the female's
ova. All of these parts are always internal. The vagina is attached to the uterus through the
cervix, while the uterus is attached to the ovaries via the
Fallopian tubes. At certain intervals, the ovaries release an ovum (the singular of ova), which passes through the fallopian tube into the uterus.
If, in this transit, it meets with sperm, the sperm penetrate and merge with the egg,
fertilizing it. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then implants itself in the wall of the uterus, where it begins the processes of
embryogenesis and
morphogenesis. When developed enough to survive outside the womb, the cervix dilates and contractions of the uterus propel the fetus through the birth canal, which is the vagina.
The ova are larger than sperm and are generally all created by birth. They are for the most part stationary, aside from their transit to the uterus, and contain nutrients for the later
zygote and
embryo. Over a regular interval, a process of
oogenesis matures one ovum to be sent down the Fallopian tube attached to its ovary in anticipation of fertilization. If not fertilized, this egg is flushed out of the system through
menstruation in humans and
great apes and reabsorbed in all other mammals in the
estrus cycle.
Gestation
Gestation, called
pregnancy in humans, is the period of time during which the fetus develops, dividing via
mitosis inside the female. During this time, the fetus receives all of its
nutrition and oxygenated blood from the female, filtered through the
placenta, which is attached to the fetus'
abdomen via an
umbilical cord. This drain of
nutrients can be quite taxing on the female, who is required to ingest significantly higher levels of
calories. In addition, certain
vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. The length of gestation, called the
gestation period, varies greatly from species to species; it is 38 weeks in humans, 56-60 in
giraffes and 16 days in
hamsters.
Birth
Once the fetus is sufficiently developed, chemical signals start the process of birth, which begins with contractions of the uterus and the dilation of the cervix. The fetus then descends to the cervix, where it is pushed out into the vagina, and eventually out of the female. The newborn, which is called an
infant in humans, should typically begin
respiration on its own shortly after birth. Not long after, the placenta is passed as well. Most mammals eat this, as it is a good source of protein and other vital nutrients needed for caring for the young. The end of the umbilical cord attached to the young's abdomen eventually falls off on its own.
Monotremes
Monotremes, only five species of which exist, all from
Australia and
New Guinea, lay
eggs. They have one opening for excretion and reproduction called the
cloaca. They hold the eggs internally for several weeks, providing nutrients, and then lay them and cover them like
birds. After less than two weeks the young hatches and crawls into its mother's pouch, much like marsupials, where it nurses for several weeks as it grows.
Marsupials
Marsupials reproduce in essentially the same manner, though their young are born at a far earlier stage of development than other mammals. After birth, marsupial
joeys crawl into their mother's pouch and attach to a
teat, where they receive nourishment and finish developing into self-sufficient animals.
*
Biological reproduction*
Bird*
Flowering plant*
Mammal*
Reptile*
Sex organ*
Sexual intercourse, sometimes called
making love# Pang, K. "Certificate Biology: New Mastering Basic Concepts", Hong Kong, 2004#
Journal of Biology of Reproduction, accessed in August 2005.
