Meiosis
Another important purpose of Meiosis is to increase the genetic variability of the gametes produced. This increase comes in the forms of crossing over (during prophase of Meiosis I) and independent assortment (during metaphase of Meiosis I). As you study the phases of Meiosis, appreciate that crossing over and independent assortment produce new genetic combinations, and separating homologs reduces the ploidy. Read this section to learn about the events that occur during Meiosis. Try to conceptualize the similarities and differences that exist between Mitosis and Meiosis.
Figure 11.1 As the organisms shown above, each of us begins life as a fertilized egg (zygote). After trillions of cell divisions, we develop into a complex, multicellular organism.
The ability to reproduce is a basic characteristic of all organisms: Hippopotamuses give birth to hippopotamus calves; Joshua trees produce seeds from which Joshua tree seedlings emerge; and adult flamingos lay eggs that hatch into flamingo chicks. However, unlike the organisms shown above, offspring may or may not resemble their parents. For example, in the case of most insects, such as butterflies (with complete metamorphosis), the larval forms rarely resemble the adult forms.
Although many unicellular organisms and a few multicellular organisms can produce genetically identical clones of themselves through asexual reproduction, many single-celled organisms and most multicellular organisms reproduce regularly using another method - sexual reproduction. This highly evolved method involves the production by parents of two haploid cells and the fusion of two haploid cells to form a single, genetically recombined diploid cell - a genetically unique organism. In almost all sexually reproducing species, these two haploid cells differ in size, with the smaller cell called "male" and the larger one called "female." These haploid cells are produced by a type of cell division called meiosis. Sexual reproduction, involving both meiosis and fertilization, introduces variation in offspring that may account for the evolutionary success of sexual reproduction. Most eukaryotic organisms, both multicellular and unicellular, can or must employ some form of meiosis and fertilization to reproduce.
In most plants and animals, the zygote formed by fertilization, through thousands of rounds of mitotic cell division, will develop into an adult organism.
Source: OpenStax, https://openstax.org/books/biology-2e/pages/11-introduction
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