Meiosis

Meiosis, which evolved 1.4 billion years ago in eukaryotes, both reduces the number of chromosomes needed for sexual reproduction, and contributes to genetic variation, leading to the process of evolution. In 1876, Oscar Herwtig was the first to recognize the role of the cell nucleus and chromosomal reduction in sea urchin eggs during meiosis, and noted that the fusion of the egg and sperm and their nuclei contributed to the inherited traits of the offspring. August Weismann extended these findings in 1890 when he found that meiosis required two cycles of cell division if the number of chromosomes was to remain stable. The word meiosis (Greek = “lessen”) refers to the reduction in the number of chromosomes by one half in the daughter cells after sexual reproduction.

During the process of meiosis, there is a recombination of genes contributed by each parent. The result is an offspring that has a unique combination of genes, providing the opportunity for evolution to occur.

Hereditary information is passed from parents to their offspring in genes written in the language of DNA. In asexual reproduction, which occurs in prokaryotes (bacteria) and a few eukaryotes, the organism simply divides, with the resulting offspring an exact genetic replica of its single parent, inheriting both its strengths and weaknesses; in the absence of a mutation, no evolution is possible. By contrast, in sexual reproduction, which occurs in most eukaryotes, each parent contributes genes. The genome of one diploid germ cell, composed of DNA wrapped in a chromosome, undergoes DNA replication, followed by two rounds of division (process of reduction division—called meiosis), resulting in haploid cells called gametes. Each gamete, containing a complete set of chromosomes, fuses with a gamete of the opposite sex during fertilization to form a new diploid cell or zygote.

Enabling Evolution to Occur

As a direct result of the process of crossing over during meiosis, there is a recombination of genes, with a scrambling of alleles (alternate forms of each gene). The resulting offspring has a unique combination of genes contributed by each parent that is genetically different than either parent. This genetic diversity creates an opportunity for changes to occur in the offspring through natural selection. Natural selection is the basis for evolution and the opportunity for organisms to successfully meet the demands associated with a changing and often challenging environment.