Sexual reproduction

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Revision as of 22:44, 24 June 2009

A sperm attempts to penetrate the ovum coat to fertilize it.

Sexual reproduction is a biological process by which organisms create descendants by combining their genetic material. These organisms have two different adult sexes (male and female).

Gametes (sperm, ovum, pollen) are first formed during a unique type of cell division called meiosis. These cells possess only half of the DNA of normal (somatic) cells, and are therefore known as haploids. During meiosis, genetic recombination also takes place, and as a result, each gamete is genetically unique from all others.

New individuals are produced by the fusion of the haploid cells to form a zygote. This process is known as fertilization. After fertilization, the resulting zygote is a single diploid cell in possession of DNA from both parents, and all the information required to build the adult organism. The growth from zygote to adult is achieved through somatic cell division called mitosis.

Meiosis

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

Fertilization is a procees that begins after the union of an egg and sperm cell. Fertilization itself 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.

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Modes of Gestation

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.

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;

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 effecient 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.[2] [3]

However, researchers suggested there was no significant difference in the rate of mutations, and scientists dismissed the harmful mutations theory. [4] Other research has suggested that in human populations (sexual reproduction), detrimental mutations may be accumulating faster than they are being eliminated by selection. [5]

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 effeciant 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.[6] 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. [7]

From the journal Science:.

Related References

• 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.

See Also

• Sex can't have evolved Response to Talk.Origins
• Reproduction
• Meiosis
• Embryo
• Fetus

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