Sexual reproduction

Sexual reproduction is a biological process through which descendants are created by combining the parent's genetic material. Sexually reproduction involves two different adult sexes (male and female) that produce sex cells (sperm, ovum) through a special type of cell division called meiosis. These cells possess only half of the DNA of normal cells, and are genetically unique from all others in the body due to genetic recombination. Offspring are then produced following the fusion of the sex cells to form a zygote. After fertilization, the resulting zygote is in possession of DNA from both parents, and all the information required to build the adult organism.

There are 3 modes of gestation that occur in animals:
 * Oviparous: The female lays eggs that develop over a period of a few months..
 * Ovoviviparous: The eggs are hatched in the oviduct of the female. The embryos develop in the uterus until fully grown.
 * Viviparous: The embryo is nourished inside of the female by a placenta.

Gametogenesis

 * Main Article: Meiosis

Egg and sperm develop from primordial germ cells in the gonads. These primordial germ cells are diploid, meaning that they have all the normal chromosomes of the organism in pairs. In humans, this means that they have 22 pairs of autosomes, and one pair of sex chromosomes, or 46 total. Before the primordial cell is to become an ovum or sperm ready to combine with a gamete of the complementary type to produce a new organism (at first a zygote) containing the normal number of chromosomes, it must undergo a special type of cell division whereby each gamete acquires only half the diploid number.

In humans, each mature ovum or sperm must include only 23 single (not paired) chromosomes. Mature ova or sperm cells are haploid, indicating that their 23 chromosomes in their nuclei are unpaired (and after they combine, then the resulting single cell the zygote is again diploid). The process whereby the diploid primordial germ cells develop into haploid gametes is called meiosis. Mitosis is part of the life cycle of any cell, but meiosis or meiotic division occurs only in the development of haploid ova and sperm from diploid primordial germ cells. The process itself appears as though the cell nucleus is undergoing two rounds of mitosis, but omits the step of replicating DNA on the second cycle. In the “first round,” the differentiating primordial germ cell replicates its DNA, and then in the “second round” it divides again (without another replication). In the second division, the pairs of chromosomes separate, leaving each of the new cells with just one copy of each of the 22 (in humans) autosomes and just one sex chromosome.

Fertilization

 * Main Article: Fertilization



Fertilization is the union of an ovum and sperm cell. It comprises a sequence of events that begins with the contact of a sperm cell with an egg cell and ends with the fusion of their two pronuclei to form a new diploid cell, called a zygote.

Sperm are equipped with flagellum that allows them to travel to the ovum. In cases of external fertilization, both ovum and sperm and frequently released into water and fertilization is not possible in drought conditions for the specific reason that motility requires an aqueous condition. During internal fertilization in humans, sperm must travel through the cervix, uterus, and then up the uterine tube. Smooth muscle contractions in the uterine tubes as well as ciliary activity (waving of hair-like structures) of the tube’s lining both are important in the transport of sperm up, and of the ovum into and then down, the uterine tube.

Frequently tens or even hundreds of millions of sperm are ejaculated, but only one fertilizes an ovum. Those sperm that do come into the vicinity of the ovum must get through the material covering the ovum (the corona radiata and the zona pellucida) and finally contact and bind to the ovum’s membrane, by means of specialized structures in the head of the sperm cell. When a sperm does get into the ovum, then the ovum membrane changes so that other sperm cannot enter. Meanwhile, the sperm cell in the egg is also undergoing changes and its specialized structures fall away. Prior to nuclear union, the haploid nuclei of both the sperm and the egg are called male and female pronuclei. Both swell, as their densely packed DNA loosens up prior to replication, and they also migrate toward the center of the ovum. Then their nuclear membranes disintegrate and the paternally and maternally contributed chromosomes pair up, an event called syngamy. At this time, the chromosome number is restored, and a new complete genome comes into being. The result of syngamy is an entity with an individual genome. The fertilized egg is now called a zygote. It is at this point already entering the first stage of its first mitotic division, and beginning cleavage.

Embryogenesis

 * Main Article: Embryology

Cleavage
After fertilization, the zygote proceeds immediately to the first cleavage (cell division) and subsequent cell divisions follow rapidly. The zygote is a very large cell, but the first waves of rapid cell division occur without increase in cell volume. The result is a closely bound mass of cells each of more typical cell size. At this stage the cells are called blastomeres, and the organism as a whole is called a morula from the time it has 16 blastomeres to the next stage. As it is undergoing this very rapid cell division, the organism is also migrating down the uterine tube toward the uterus.

Vertebrates, like humans, have bilateral symmetry and therefore polarity in three dimensions (head-tail, or back-front, and left-right). Establishing polarity is one of the most basic manifestations of emerging specialization. But the egg is roughly spherical, and it is not readily apparent how polarity is established. Although it had been shown long ago that the point of sperm entry determines the plane of first cleavage (and thus subsequent ones) in amphibian eggs, mammals were believed until recently to remain spherically symmetrical until later in development. Recent data on mammalian zygotes, however, suggests that the point of sperm entry may similarly determine the cleavage plane. Even the first two cells resulting from the first cleavage may have different propensities, which persist through the next divisions as the progeny of one cell tend to become the body of the offspring and progeny of the other cell become the embryo’s contribution to the placenta and other supporting structures. The word “fate,” however, might be too strong, because the cells of such very early embryos are resilient to perturbations—if one cell is removed, the remaining ones can compensate.

Gastrulation
Gastrulation begins when fluid starts to accumulate between the blastomeres of the morula. The fluid-filled spaces run together, forming a relatively large fluid-filled cavity. At the point when the cavity becomes recognizable, the organism is called a blastocyst. The outer cells of the blastocyst, especially those around the blastocyst cavity, assume a flattened shape. The flattened cells of the exterior blastocyst are the trophoblast. They become the embryo’s contribution to the placenta and other supporting structures. On one side of the blastocyst is a group of cells that project inside into the blastocyst cavity; this is the inner cell mass, or embryoblast, and its progeny form the body of the new offspring.

The cells of the inner cell mass can give rise to progeny differentiating into all the types of cells in the adult body, so they are called pluripotent. They have not usually been described as totipotent because, the inner cell mass having already differentiated from trophoblast, the cells of the inner cell mass were believed to be no longer able to give rise to the cells of the trophoblast. Recent work, however, describes culture conditions under which human embryonic stem cells can differentiate to trophoblast cells. Although the new offspring itself develops only from the inner call mass, the trophoblast is not just passive padding. Its progeny are the essential and specialized connection between the embryonic and maternal systems. Embryonic stem cells can be isolated from the inner cell mass.

Genetic Inheritance

 * Main Article: Genetics

In sexually reproducing organisms, each gene in an individual is represented by two copies, called alleles—one on each chromosome pair. There may be more than two alleles, or variants, for a given gene in a population, but only two alleles can be found in an individual. Therefore, the probability that a particular allele will be inherited is 50:50, that is, alleles randomly and independently segregate into daughter cells, although there are some exceptions to this rule.

The term diploid describes a state in which a cell has two sets of homologous chromosomes, or two chromosomes that are the same. The maturation of germ line stem cells into gametes requires that the diploid number of each chromosome be reduced by half. Hence, gametes are said to be haploid—having only a single set of homologous chromosomes. This reduction is accomplished through a process called meiosis, where one chromosome in a diploid pair is sent to each daughter gamete. Human gametes, therefore, contain 23 chromosomes, half the number in somatic cells—all the other cells of the body.

Because the chromosome in one pair separates independently of all other chromosomes, each new gamete has the potential for a totally new combination of chromosomes. In humans, the independent segregation of the 23 chromosomes can lead to as many as 16 to 17 million different combinations in one individual's gametes. Only one of these gametes will combine with one of the nearly 17 million possible combinations from the other parent, generating a staggering potential for individual variation. But this is only the beginning. Even more variation is possible from the recombination between sections of chromosomes during meiosis as well as the random mutation that can occur during DNA replication. With such a range of possibilities, it is amazing that siblings look so much alike!

Evolutionary Problems
Evolutionists seeking to explain the various challenges to the evolution of sexual reproduction have suggested several solutions, but none of these are adequate. An article from the BBC published in an October 12th, 2000 article relates in saying; Evolutionary biologists have long puzzled over why sex evolved at all.

Origin of Sexual Reproduction
The evolutionary model clearly has enigmatic stops, so much so that a clear picture cannot be seen. There remains two presuppositions one holds when approaching the origin of sexual reproduction:
 * The organism had to develop and maintain two unique reproductive systems simultaneously until sexual reproduction became dominant within an asexual population through many improbable mutations and lack of natural selection.
 * Or, both or either biological abilities were designed into the organisms that required them.

Certainly any answer to either question dealing with this part of the past is not set in stone, so to know how sexual reproduction came into existence is a question of faith rather than scientific observation or fact. However given the lack of logic within the view of the evolutionist regarding survival of these systems and the scientific improbability dealing with the chance of such through mutation, design of these biological systems is clearly or equally logical and scientific.

Simultaneous Development
If sexual reproduction were to evolve in a step-wise fashion through mutational change within an established population of asexual organisms. This obviously would involve an enormous number of chance mutations, because sexual reproduction is a vastly complex biological mechanism. In order to do such, given the evolutionary framework it must bear some survival advantage in order to spread throughout the population. If each step of evolution from asexual reproduction to sexual reproduction did not have a survival advantage sufficient to spread the new trait, then it would be eliminated from the gene pool by genetic drift or natural selection.

So, one might ask, what survival advantage is the production of sperm without the production of an egg? Sperm are totally useless without an egg, just as an egg is totally useless without sperm. So neither sperm nor egg could have developed alone according to evolutionary theory. Until the capacity to produce both sperm and egg was fully developed, both were totally useless, thus having no survival advantage, and thus not being aided by the evolutionary mechanism of "millions of generations," or "deep time."

The benefit of asexual reproduction

 * Main Article: Asexual reproduction

Scientists in the above BBC article explain that asexual reproduction is exceedingly more efficient than sexual reproduction, and were puzzled as to why the ruthless efficiency of evolution chose a less efficient method. They theorized that the reduction in mutations may have been the reason sexual evolution evolved.

However, researchers suggested there was no significant difference in the rate of mutations, and scientists dismissed the harmful mutations theory. Other research has suggested that in human populations (sexual reproduction), detrimental mutations may be accumulating faster than they are being eliminated by selection. 

This would mean that sexual reproduction has yet another disadvantage over asexual reproduction. Yet taking classical evolutionary theory into account, negative or non beneficial mutations, those that were forming sexual reproduction over the more efficient asexual reproduction should not of been chosen to spread through the population.

Population genetic theory
Scientists initially proposed that population genetic theory explained the evolution of sexual reproduction, because linkage became an obstacle in asexual reproduction. However, research from the Proceedings of the National Academy of Sciences indicated "linkage limits the rate and degree of adaptation even in recombining genomes," meaning the problem exists in either form. Thus, the problem of why sexual reproduction evolved is still a mystery.

Meiotic recombination
Scientists have also suggested that meiotic recombination and sexual reproduction both cause shuffling of parental genomes, thus promoting genetic diversity, an asset to survival. However, researchers have already raised several objections to this claim.

The first is that sexual reproduction and recombination do not always increase genetic variation. When genetic variation is added, it is not clear why greater genetic variation should generally be adaptive. The same recombination is also a mechanism for destroying favorable combinations of mutations. 

From the journal Science:. Reproductive strategies such as sexual reproduction and recombination that involve the shuffling of parental genomes for the production of offspring are ubiquitous in nature. However, their evolutionary benefit remains unclear.

Additional Information

 * Evolutionary Theories on Gender and Sexual Reproduction by Bert Thompson, Ph.D. and Brad Harrub, Ph.D.
 * Argument: Evolution of sex by Jonathan Sarfati, with Michael Matthews
 * A Critic Takes On the Logic of Female Orgasm Researcher claims female orgasm has no evolutionary purpose whatsoever.
 * Sex, the Queen of Problems in Evolutionary Biology Evolution News and Views, July 13, 2011.