Which Statement Is True For All Sexually Reproducing Plants And Animals?
Chapter 7: Introduction to the Cellular Basis of Inheritance
7.i Sexual Reproduction
Learning Objectives
By the end of this section, you will be able to:
- Explain that variation amongst offspring is a potential evolutionary advantage resulting from sexual reproduction
- Describe the iii different life-cycle strategies among sexual multicellular organisms and their commonalities
- Understand why you lot could never create a gamete that would be identical to either of the gametes that made yo
Sexual reproduction was an early evolutionary innovation afterwards the appearance of eukaryotic cells. The fact that virtually eukaryotes reproduce sexually is prove of its evolutionary success. In many animals, it is the only mode of reproduction. And withal, scientists recognize some real disadvantages to sexual reproduction. On the surface, offspring that are genetically identical to the parent may appear to exist 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 do good to an organism that tin can produce offspring past asexual budding, fragmentation, or asexual eggs. These methods of reproduction practise not require another organism of the opposite sex activity. There is no need to expend energy finding or attracting a mate. That free energy tin can exist spent on producing more offspring. Indeed, some organisms that pb a solitary lifestyle have retained the power to reproduce asexually. In improver, asexual populations only have female individuals, so every private is capable of reproduction. In contrast, the males in sexual populations (half the population) are not producing offspring themselves. Considering of this, an asexual population tin abound twice as fast as a sexual population in theory. This means that in contest, the asexual population would have the advantage. All of these advantages to asexual reproduction, which are as well disadvantages to sexual reproduction, should mean that the number of species with asexual reproduction should be more common.
Nonetheless, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexual reproduction so mutual? 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 amid offspring is very of import 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 adjacent when unlike parents combine their unique genomes, and the genes are mixed into different combinations past 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 every bit when the gametes combine in fertilization.
The Cerise Queen Hypothesis
There is no question that sexual reproduction provides evolutionary advantages to organisms that employ this mechanism to produce offspring. The problematic question is why, even in the face of fairly stable conditions, sexual reproduction persists when it is more difficult and produces fewer offspring for private organisms? Variation is the outcome of sexual reproduction, simply why are ongoing variations necessary? Enter the Reddish Queen hypothesis, offset proposed by Leigh Van Valen in 1973. 1 The concept was named in reference to the Ruby-red Queen'due south race in Lewis Carroll's volume, Through the Looking-Glass, in which the Red Queen says ane must run at full speed merely to stay where one is.
All species coevolve with other organisms. For example, predators coevolve with their prey, and parasites coevolve with their hosts. A remarkable instance of coevolution betwixt predators and their prey is the unique coadaptation of night flying bats and their moth prey. Bats detect their prey past emitting high-pitched clicks, merely moths have evolved uncomplicated ears to hear these clicks and then they tin can avert the bats. The moths have besides adjusted behaviors, such every bit flying away from the bat when they first hear it, or dropping suddenly to the ground when the bat is upon them. Bats have evolved "tranquility" clicks in an endeavor to evade the moth's hearing. Some moths take evolved the ability to reply to the bats' clicks with their own clicks as a strategy to confuse the bats echolocation abilities.
Each tiny advantage gained by favorable variation gives a species an border over shut competitors, predators, parasites, or even prey. The only method that will allow a coevolving species to keep its own share of the resources is besides to continually improve its power to survive and produce offspring. Equally 1 species gains an advantage, other species must also develop an reward or they will exist outcompeted. No single species progresses too far ahead because genetic variation among progeny of sexual reproduction provides all species with a mechanism to produce adapted individuals. Species whose individuals cannot keep upward become extinct. The Red Queen's catchphrase was, "It takes all the running you can do to stay in the same place." This is an apt description of coevolution between competing species.
Life Cycles of Sexually Reproducing Organisms
Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends on the organism. The process of meiosis reduces the resulting gamete's chromosome number by one-half. Fertilization, the joining of two haploid gametes, restores the diploid condition. At that place are three main categories of life cycles in multicellular organisms: diploid-dominant, in which the multicellular diploid phase is the most obvious life stage (and there is no multicellular haploid stage), every bit with most animals including humans; haploid-dominant, in which the multicellular haploid phase is the most obvious life stage (and there is no multicellular diploid phase), as with all fungi and some algae; and alternation of generations, in which the two stages, haploid and diploid, are apparent to one degree or another depending on the group, as with plants and some algae.
Nearly all animals employ a diploid-dominant life-cycle strategy in which the but haploid cells produced past the organism are the gametes. The gametes are produced from diploid germ cells, a special cell line that only produces gametes. In one case the haploid gametes are formed, they lose the ability to dissever again. In that location is no multicellular haploid life phase. Fertilization occurs with the fusion of two gametes, commonly from dissimilar individuals, restoring the diploid state (Figure 7.2 a).
If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?
Most fungi and algae employ a life-cycle strategy in which the multicellular "torso" of the organism is haploid. During sexual reproduction, specialized haploid cells from two individuals join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores (Figure 7.two b).
The third life-cycle type, employed by some algae and all plants, is called alternation of generations. These species accept both haploid and diploid multicellular organisms as office of their life cycle. The haploid multicellular plants are called gametophytes because they produce gametes. Meiosis is not involved in the product of gametes in this example, equally the organism that produces gametes is already haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular institute called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will develop into the gametophytes (Effigy 7. 2 c).
Department Summary
Well-nigh all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis appears to be i of the advantages of sexual reproduction that has made it so successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces genetically unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. Fertilization, the fusion of haploid gametes from two individuals, restores the diploid condition. Thus, sexually reproducing organisms alternate between haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly. There are 3 main categories of life cycles: diploid-dominant, demonstrated by virtually animals; haploid-dominant, demonstrated past all fungi and some algae; and alternation of generations, demonstrated by plants and some algae.
Glossary
alternation of generations: a life-bike type in which the diploid and haploid stages alternating
diploid-dominant: a life-cycle type in which the multicellular diploid stage is prevalent
haploid-dominant: a life-cycle type in which the multicellular haploid phase is prevalent
gametophyte: a multicellular haploid life-cycle stage that produces gametes
germ cell: a specialized prison cell that produces gametes, such as eggs or sperm
life cycle: the sequence of events in the development of an organism and the product of cells that produce offspring
meiosis: a nuclear sectionalization process that results in four haploid cells
sporophyte: a multicellular diploid life-cycle stage that produces spores
Footnotes
i Leigh Van Valen, "A new evolutionary law," Evolutionary Theory 1 (1973): one–30.
Source: https://opentextbc.ca/biology/chapter/7-1-sexual-reproduction/
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