1. the process by which organisms produce new organisms of the same type 2. a method of reproduction in which all the genes passed on to the offspring come from a single individual or parent

3. a method of reproduction that involves two parents producing offspring that
are genetically different from either parent
4. an organism that is genetically identical to the organism from which it was produced
5. the male sex cell
6. a sex cell produced by a female
7. an egg or sperm cell; a sex cell
8. the cell division process that divides the cell’s nucleus into two, each with a complete set of genetic material from one parent
cell
9. the cell division process that results in the production of sex cells (sperm and egg)

10. the process that combines a sperm cell and an egg cell
11. the cell that results from the joining of the egg and sperm

12. the process that transforms developing cells into specialized cells with
different structures and functions

13. the organ of the human female reproductive system that produces an egg cell, the female gamete


14. female reproductive hormone (either one)
15. female reproductive hormone (either one)

16. the organ, in female animals, where the embryo develops into a fetus
17. the organ that enables nutrients and oxygen to pass from the mother’s blood to the fetus, and waste products to pass from the fetus to the mother’s blood

18. the male reproductive organ, that produces sperm and the hormone testosterone

19. a hormone associated with male sexual development and reproduction

20. structure in female where fertilization occurs

Sexual reproduction was an early evolutionary innovation after the appearance of eukaryotic cells. The fact that most eukaryotes reproduce sexually is evidence of its evolutionary success. In many animals, it is the only mode of reproduction. And yet, scientists recognize some real disadvantages to sexual reproduction. On the surface, offspring that are genetically identical to the parent may appear to be more advantageous. If the parent organism is successfully occupying a habitat, offspring with the same traits would be similarly successful. There is also the obvious benefit to an organism that can produce offspring by asexual budding, fragmentation, or asexual eggs. These methods of reproduction do not require another organism of the opposite sex. There is no need to expend energy finding or attracting a mate. That energy can be spent on producing more offspring. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, asexual populations only have female individuals, so every individual is capable of reproduction. In contrast, the males in sexual populations (half the population) are not producing offspring themselves. Because of this, an asexual population can grow twice as fast as a sexual population in theory. This means that in competition, the asexual population would have the advantage. All of these advantages to asexual reproduction, which are also disadvantages to sexual reproduction, should mean that the number of species with asexual reproduction should be more common.

However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexual reproduction so common? This is one of the important questions in biology and has been the focus of much research from the latter half of the twentieth century until now. A likely explanation is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of those offspring. The only source of variation in asexual organisms is mutation. This is the ultimate source of variation in sexual organisms. In addition, those different mutations are continually reshuffled from one generation to the next when different parents combine their unique genomes, and the genes are mixed into different combinations by the process of meiosis. Meiosis is the division of the contents of the nucleus that divides the chromosomes among gametes. Variation is introduced during meiosis, as well as when the gametes combine in fertilization.

Explanation:

By the end of this section, you will be able to:

Explain that variation among offspring is a potential evolutionary advantage resulting from sexual reproduction

Describe the three different life-cycle strategies among sexual multicellular organisms and their commonalities

Understand why you could never create a gamete that would be identical to either of the gametes that made you

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