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Through sexual reproduction, genetic material is exchanged. This induces genetic recombination. The significance of this is obvious: the exchange of material increases the variation. This holds particular advantages to populations and is considered by evolutionists to be an innovation that greatly enhances the evolutionary process.
We know what sexual reproduction achieves. It increases the variation. However, increased variation in the genotype is of no value until it is expressed in the phenotype. The new varieties must be expressed in the offspring before natural selection can feast on this increased variation. The process that brings about the variation (sexual reproduction) is not subject to selection, only the result thereof (the increased variation in the offspring) is subject to selection.
Again, we are faced with the awesome question, how did sexual reproduction arise? If the process was not subject to selection, then only two options remain: chance or design. It requires a great deal of faith to believe in the chance development of sexual reproduction. At the genetic level, sexual reproduction is extremely complex and scientists have investigated these processes with a sense of wonder.
The exchange of gametes requires a modified form of cell division which is the process of meiosis. During meiosis, the number of chromosomes is halved, resulting in the gametes having half the chromosomes. Sexual fusion of two gametes then restores the number of chromosomes. Variation in the genome is greatly increased by two processes occurring during meiosis: independent assortment and crossing over. Both these processes are extremely complex, but in themselves are not subject to selection. They rearrange the genetic material, resulting in new combinations of the material. As this reshuffling occurs at the level of the genotype, it is not subject to natural selection until the new combinations have been expressed in the phenotype.
i) Independent Assortment
Independent assortment is achieved when chromosomes line up in homologous pairs and move independently to the one pole or the other. The process is governed by complex enzyme systems which in turn must also have come about by chance. The possible variation that can be achieved by independent assortment depends on the number of chromosomes present. In humans, there are 46 chromosomes, which would arrange themselves in 23 homologous pairs. There are thus 80 trillion possible variations.
ii) Crossing Over
Crossing Over is an awe-inspiring process. When homologous chromosomes are lined up during meiosis, they can, in a very precise way, exchange genetic material. There are five steps in achieving this:
a) Enzymes open the double helix of DNA in the aligned chromosomes to permit intermolecular base pairing.
b) One strand of each helix is cut at equivalent positions.
c) The enzyme ligase joins them to form a half-chromatid chiasma (because only one strand of each chromatid cross over), resulting in a cross-shaped molecule.
d) The cross-shaped molecule is cut in half by an enzyme, leaving a break in one strand of each recombinant.
e) The break is sealed by ligase.
The process has to be extremely precise. If even one nucleotide is transferred incorrectly, the genetic message becomes useless. A typical textbook description for the process will illustrate this complexity:
A normal crossover is really a miraculous process. Somehow the genetic material from one parental chromosome and the genetic material from the other parental chromosome are cut up and pasted together during each meiosis, and this is done with complete reciprocity. In other words, neither chromosome gains or loses any genes in the process. In fact, it is probably correct to say that neither chromosome gains or loses even one nucleotide in the exchange. How is this remarkable precision attained?i
It might be safely said that the crossing over process is more complex than anything man has ever designed. However, it would have had to come into existence by chance if the evolutionary paradigm is accepted. Chance or design are the options at this level, and chance requires more faith than most could muster.
Read about the next mechanism: transposable elements
i. D. Suzuki et al., An Introduction to Genetic Analysis (San Francisco: W.H. Freeman and Company).
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