Why does fertilization and meiosis alternate

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.

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 individual organisms? Variation is the outcome of sexual reproduction, but why are ongoing variations necessary? All species coevolve with other organisms.

For example, predators coevolve with their prey, and parasites coevolve with their hosts. A remarkable example of coevolution between predators and their prey is the unique coadaptation of night flying bats and their moth prey.

Bats find their prey by emitting high-pitched clicks, but moths have evolved simple ears to hear these clicks so they can avoid the bats. The moths have also adapted behaviors, such as flying away from the bat when they first hear it, or dropping suddenly to the ground when the bat is upon them. Each tiny advantage gained by favorable variation gives a species an edge over close competitors, predators, parasites, or even prey. The only method that will allow a coevolving species to keep its own share of the resources is also to continually improve its ability to survive and produce offspring.

As one species gains an advantage, other species must also develop an advantage or they will be 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 up become extinct. In animals, sexually reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring.

The germ cells are generated early in the development of the zygote. Art Connection Fungi, such as black bread mold Rhizopus nigricans , have a haploid multicellular stage that produces specialized haploid cells by mitosis that fuse to form a diploid zygote. The haploid multicellular stage produces specialized haploid cells by mitosis that fuse to form a diploid zygote.

The zygote undergoes meiosis to produce haploid spores. Each spore gives rise to a multicellular haploid organism by mitosis. Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism. In some plants, such as ferns, both the haploid and diploid plant stages are free-living. The diploid plant is called a sporophyte because it produces haploid spores by meiosis.

The spores develop into multicellular, haploid plants that are called gametophytes because they produce gametes. The gametes of two individuals will fuse to form a diploid zygote that becomes the sporophyte.

Show Hint Hint: Figure Yes, it will be able to reproduce asexually. What is a likely evolutionary advantage of sexual reproduction over asexual reproduction? Sexual reproduction involves fewer steps. There is a lower chance of using up the resources in a given environment. Sexual reproduction results in variation in the offspring. Sexual reproduction is more cost-effective. Show Hint Hint: C. Which type of life cycle has both a haploid and diploid multicellular stage?

Show Hint Hint: D. What is the ploidy of the most conspicuous form of most fungi? Show Hint Hint: B. Show Hint Hint: A. Both populations will die.

Mutations during asexual reproduction decrease plant fitness. Consumers do not trust identical-appearing produce. Larger portions of the plant populations are susceptible to the same diseases. Spores are not viable in an agricultural setting. Show Hint Hint: a. Show Hint Hint: Sexual reproduction increases the genetic variation within the population, because new individuals are made by randomly combining genetic material from two parents.

Show Hint Hint: Organisms with a diploid-dominant life cycle make haploid gametes by meiosis, while all their somatic cells are diploid. Show Hint Hint: Haploid-dominant organisms undergo sexual reproduction by making a diploid zygote. On the surface, creating offspring that are genetic clones of the parent appears to be a better system. 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 whenever circumstances are favorable by asexual budding, fragmentation, or asexual eggs. These methods of reproduction do not require another organism of the opposite sex. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually.

In addition, in asexual populations, every individual is capable of reproduction. In sexual populations, the males are not producing the offspring themselves, so in theory an asexual population could grow twice as fast.

However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexuality and meiosis so common? This is one of the important unanswered questions in biology and has been the focus of much research beginning in the latter half of the twentieth century. There are several possible explanations, one of which is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of the population.

Thus, on average, a sexually reproducing population will leave more descendants than an otherwise similar asexually reproducing population. The only source of variation in asexual organisms is mutation. This is the ultimate source of variation in sexual organisms, but 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 crossovers during prophase I and random assortment at metaphase I.

It is not in dispute that sexual reproduction provides evolutionary advantages to organisms that employ this mechanism to produce offspring.

But why, even in the face of fairly stable conditions, does sexual reproduction persist when it is more difficult and costly for individual organisms? Variation is the outcome of sexual reproduction, but why are ongoing variations necessary? All species co-evolve with other organisms; for example predators evolve with their prey, and parasites evolve with their hosts. Each tiny advantage gained by favorable variation gives a species an edge over close competitors, predators, parasites, or even prey.

The only method that will allow a co-evolving species to maintain its own share of the resources is to also continually improve its fitness. As one species gains an advantage, this increases selection on the other species; they must also develop an advantage or they will be outcompeted.

No single species progresses too far ahead because genetic variation among the progeny of sexual reproduction provides all species with a mechanism to improve rapidly. Species that cannot keep up become extinct. Fertilization and meiosis alternate in sexual life cycles.

What happens between these two events depends on the organism. The process of meiosis reduces the chromosome number by half. Thus, on average, a sexually-reproducing population will leave more descendants than an otherwise similar asexually-reproducing population. The only source of variation in asexual organisms is mutation. This is the ultimate source of variation in sexual organisms, but, 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, dividing the chromosomes among gametes. The process of meiosis produces 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 three main categories of sexual life cycles: diploid-dominant, demonstrated by most animals; haploid-dominant, demonstrated by all fungi and some algae; and the alternation of generations, demonstrated by plants and some algae.

The Sexual Life Cycle : In animals, sexually-reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring. It is not in dispute that sexual reproduction provides evolutionary advantages to organisms that employ this mechanism to produce offspring. But why, even in the face of fairly stable conditions, does sexual reproduction persist when it is more difficult and costly for individual organisms?

Variation is the outcome of sexual reproduction, but why are ongoing variations necessary? Possible answers to these questions are explained in the Red Queen hypothesis, first proposed by Leigh Van Valen in All species co-evolve with other organisms; for example, predators evolve with their prey and parasites evolve with their hosts. Each tiny advantage gained by favorable variation gives a species an edge over close competitors, predators, parasites, or even prey.

The only method that will allow a co-evolving species to maintain its own share of the resources is to also continually improve its fitness. As one species gains an advantage, this increases selection on the other species; they must also develop an advantage or they will be out-competed. No single species progresses too far ahead because genetic variation among the progeny of sexual reproduction provides all species with a mechanism to improve rapidly. Species that cannot keep up become extinct.

The main categories of sexual life cycles in eukaryotic organisms are: diploid-dominant, haploid-dominant, and alternation of generations. In sexual reproduction, the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents.

Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends upon the organism.