The plant that bears both male and female reproductive organs is called monoecious. Conversely, the plant that bears only one type of reproductive organ is called dioecious. In flowering plants, the female reproductive organ is the pistil whereas the male reproductive organ is the anther.
The pistil contains the ovary, which in turn, contains ovules. Inside the ovules are the egg cells. The anther bears the pollen grains. Inside the pollen grains are the sperm cells. The sperm cells in the pollen have to reach the ovule and this is facilitated by pollination. There are two types of pollination: self-pollination and cross-pollination. Self-pollination occurs especially in monoecious flowers since the male and female organs are present in a single flower.
In cross-pollination, the pollen is transferred from the male flower to the female flower. The transfer may be facilitated by wind or by insects.
Based on the mode of pollination, the types of sexual reproduction in plants are autogamy for self-fertilization and allogamy for cross-fertilization. Allogamy is the more common type of reproduction among higher plants. For pollination to occur, the pollen sticks to the stigma of the pistil and grows a tube through the style of the pistil to reach the carpel containing the ovule.
Fertilization occurs when the sperm cell fertilizes the egg cell whereas another sperm cell fertilizes the endosperm nuclei. Thus, the zygote will be diploid whereas the endosperm will be triploid from the union of a sperm cell and two female cells.
The zygote develops into an embryo whereas the endosperm develops into nutritive tissue surrounding the embryo within the seed. Most animals reproduce sexually. Because finding a mate is essential in reproducing by sexual means most animals display sexual dimorphism, sexual selection, and courtship rituals.
Sexual dimorphism refers to the occurrence of two sexually distinct forms such that the male differs morphologically from the female of the same species. For example, male birds have colorful plumage compared with the plumage of female birds. Female birds choose a mate based on desirable qualities. Mate selection and courtship rituals are ostensible in other animals as well, including humans.
Sexual reproduction in humans naturally is by sexual means only. The process entails courtship and mate selection, copulation, pregnancy, childbirth, and prenatal care.
The partner chooses a potential mate essentially based on the qualities that ensure siring an offspring. The couple engages in sexual intercourse for internal fertilization to take place. Only a single sperm would be able to fertilize a viable ovum produced immediately by meiosis. The haploid sex cells form the diploid zygote that will next undergo mitosis to become an embryo. The embryo then develops organs and becomes a fetus inside the female womb. After the gestation period usually, about days , the female gives birth by pushing the fetus out of the birthing canal.
The newly born child, then, receives nutrition by lactation. Postnatal care continues until the child becomes independent. Do you have a question you want to ask about sexual reproduction? Join us! Participate in our Forum: Asexual and sexual reproduction differences. One of the major sexual reproduction advantages is to ensure that the chromosome number of a species remains across generations. Humans, for instance, have 46 chromosomes.
Half of it comes from the father and the other half from the mother. Prior to the union of the sperm cell and the egg cell, the gametes undergo meiosis to produce haploid gametes. Because of haploidy, the union of the gametes keeps up the number of chromosomes the same for all somatic cells. The sperm cell contains 23 chromosomes and the ovum has also 23 chromosomes. When they combine at fertilization, the zygote that develops into a new individual will have the same total number of chromosomes, Another advantage of sexual reproduction is greater genetic variation.
During meiosis, genetic recombination and the interchange of genes between homologous chromosomes occur. This ensures that the newly formed zygote — while containing the original number of chromosomes — will possess a genome that is genetically distinct from, and not a clone of, either parent. Chicken eggs are an example of a hard shell. The eggs of the egg-laying mammals such as the platypus and echidna are leathery. This process helps protect the eggs until hatching.
This occurs in some bony fish like the platyfish Xiphophorus maculatus, Figure In viviparity the young are born alive. They obtain their nourishment from the female and are born in varying states of maturity. This occurs in most mammals Figure Reproduction may be asexual when one individual produces genetically identical offspring, or sexual when the genetic material from two individuals is combined to produce genetically diverse offspring.
Asexual reproduction in animals occurs through fission, budding, fragmentation, and parthenogenesis. Sexual reproduction may involve fertilization inside the body or in the external environment. A species may have separate sexes or combined sexes; when the sexes are combined they may be expressed at different times in the life cycle.
The sex of an individual may be determined by various chromosomal systems or environmental factors such as temperature. Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization.
This can occur either outside the bodies or inside the female. The method of fertilization varies among animals. Some species release the egg and sperm into the environment, some species retain the egg and receive the sperm into the female body and then expel the developing embryo covered with shell, while still other species retain the developing offspring throughout the gestation period. Learning Objectives By the end of this section, you will be able to: Describe advantages and disadvantages of asexual and sexual reproduction Discuss asexual reproduction methods Discuss sexual reproduction methods Discuss internal and external methods of fertilization.
Concept in Action View this video to see a hydra budding. Exercises In which group is parthenogenesis a normal event? Compared to separate sexes and assuming self-fertilizing is not possible, what might be one advantage and one disadvantage to hermaphroditism? Answers B A A Temperatures can vary from year to year and an unusually cold or hot year might produce offspring all of one sex, making it hard for individuals to find mates.
A possible advantage of hermaphroditism might be that anytime an individual of the same species is encountered a mating is possible, unlike separate sexes that must find an individual of the right sex to mate. Also, every individual in a hermaphrodite population is able to produce offspring, which is not the case in populations with separate sexes. A disadvantage might be that hermaphrodite populations are less efficient because they do not specialize in one sex or another, which means a hermaphrodite does not produce as many offspring through eggs or sperm as do species with separate sexes.
Other answers are possible. Glossary asexual reproduction: a mechanism that produces offspring that are genetically identical to the parent. The known methods of reproduction are broadly grouped into two main types: sexual and asexual. In asexual reproduction, an individual can reproduce without involvement with another individual of that species. The division of a bacterial cell into two daughter cells is an example of asexual reproduction.
This type of reproduction produces genetically-identical organisms clones , whereas in sexual reproduction, the genetic material of two individuals combines to produce offspring that are genetically different from their parents. Humans provide an example of the former, while seahorses provide an example of the latter. The eggs hatch and the offspring develop in the pouch for several weeks. Sexual reproduction in seahorses : Female seahorses produce eggs for reproduction that are then fertilized by the male.
Unlike almost all other animals, the male seahorse then gestates the young until birth. Organisms that reproduce through asexual reproduction tend to grow in number exponentially. However, because they rely on mutation for variations in their DNA, all members of the species have similar vulnerabilities.
Organisms that reproduce sexually yield a smaller number of offspring, but the large amount of variation in their genes makes them less susceptible to disease. Many organisms can reproduce sexually as well as asexually. Aphids, slime molds, sea anemones, and some species of starfish are examples of animal species with this ability.
When environmental factors are favorable, asexual reproduction is employed to exploit suitable conditions for survival, such as an abundant food supply, adequate shelter, favorable climate, disease, optimum pH, or a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supply resources. When food sources have been depleted, the climate becomes hostile, or individual survival is jeopardized by some other adverse change in living conditions, these organisms switch to sexual forms of reproduction.
Sexual reproduction ensures a mixing of the gene pool of the species. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur. In addition, sexual reproduction usually results in the formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent.
Asexual and sexual reproduction, two methods of reproduction among animals, produce offspring that are clones or genetically unique. Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. This type of reproduction occurs in prokaryotic microorganisms bacteria and in some eukaryotic single-celled and multi-celled organisms. Animals may reproduce asexually through fission, budding, fragmentation, or parthenogenesis.
Fission, also called binary fission, occurs in prokaryotic microorganisms and in some invertebrate, multi-celled organisms. After a period of growth, an organism splits into two separate organisms. Some unicellular eukaryotic organisms undergo binary fission by mitosis. Unfortunately, empirical data have not indicated that fitness surfaces curve in just the right way for these models to work in real-life situations.
To make matters worse, sexual reproduction often entails costs beyond the recombination load described earlier. To reproduce sexually, an individual must take the time and energy to switch from mitosis to meiosis this step is especially relevant in single-celled organisms ; it must find a willing mate; and it must risk contracting sexually transmitted diseases. This last cost is often called the "twofold cost of sex. These are substantial costs—so substantial that many species have evolved mechanisms to ensure that sex occurs only when it is least costly.
For instance, organisms including aphids and daphnia reproduce asexually when resources are abundant and switch to sex only at the end of the season, when the potential for asexual reproduction is limited and when potential mates are more available. Similarly, many single-celled organisms have sex only when starved, which minimizes the time cost of switching to meiosis because mitotic growth has already ceased.
Although various mechanisms might reduce the costs of sex, it is still commonly assumed that sex is more costly than asexual reproduction, raising yet another obstacle for the evolution of sex. The aforementioned points might lead one to conclude that sex is a losing enterprise. However, sex is incredibly common. Furthermore, even though asexual lineages do arise, they rarely persist for long periods of evolutionary time.
Among flowering plants, for example, predominantly asexual lineages have arisen over times, yet none of these lineages is very old. Furthermore, many species can reproduce both sexually and asexually, without the frequency of asexuality increasing and eliminating sexual reproduction altogether.
What, then, prevents the spread of asexual reproduction? The first generation of mathematical models examining the evolution of sex made several simplifying assumptions—namely, that selection is constant over time and space, that all individuals engage in sex at the same rate, and that populations are infinitely large.
With such simplifying assumptions, selection remains the main evolutionary force at work, and sex and recombination serve mainly to break down the genetic associations built up by selection. So, it is perhaps no wonder that this early generation of models concluded that sex would evolve only under very restrictive conditions.
Subsequent models have relaxed these assumptions in a number of ways, attempting to better capture many of the complexities involved in real-world evolution. The results of these second-generation models are briefly summarized in the following sections. Current models indicate that sex evolves more readily when a species' environment changes rapidly.
When the genetic associations built up by past selection are no longer favorable, sex and recombination can improve the fitness of offspring, thereby turning the recombination load into an advantage. One important source of environmental change is a shift in the community of interacting species, especially host and parasite species.
This is the so-called "Red Queen" hypothesis for the evolution of sex, which refers to the need for a species to evolve as fast as it can just to keep apace of coevolving species. The name of this hypothesis comes from Lewis Carroll's Through the Looking Glass , in which Alice must run as fast as she can "just to stay in place.
Sex can also be favored when selection varies over space, as long as the genetic associations created by migration are locally disadvantageous. Whether this requirement is common in nature remains an open question. Organisms that reproduce both sexually and asexually tend to switch to sex under stressful conditions. Mathematical models have revealed that it is much easier for sex to evolve if individuals that are adapted to their environment reproduce asexually and less fit individuals reproduce sexually.
In this way, well-adapted genotypes are not broken apart by recombination, but poorly adapted genotypes can be recombined to create new combinations in offspring. Models that account for the fact that population sizes are finite have found that sex and recombination evolve much more readily.
With a limited number of individuals in a population, selection erodes easily accessible variation, leaving only hidden variation Figure 2. Recombination can then reveal this hidden variation, improving the response to selection. By improving the response to selection, genes that increase the frequency of sex become associated with fitter genotypes, which rise in frequency alongside them.
Interestingly, the requirement that fitness surfaces exhibit weak and negative curvature is relaxed in populations of finite size; here, fitness surfaces may be uncurved or positively curved and still favor sex. Figure 2: Selection in finite populations leaves hidden variation. This diagram depicts a population consisting of 14 haploid individuals who carry plus or minus alleles at each of four sites in their genome left panel.
In a new environment favoring the plus alleles, selection will, over time, increase the frequency of the plus alleles throughout the genome right panel.
For example, in a hotter climate, alleles conferring tolerance to higher temperatures would rise in frequency. Selection favors the good gene combinations here, the ones containing two plus alleles and eliminates the bad gene combinations. In the absence of sex, the only variation that remains after several rounds of selection is hidden in the sense that plus alleles at the first site are found with minus alleles at the second site or vice versa. This problem is irrelevant in an infinitely large population, because mutation will immediately create beneficial combinations e.
Two populations are represented as black circles with fourteen line segments, each composed of four black plusses or minuses. The population at left, representing the Initial population, contains two line segments with two plus signs, seven line segments with one plus sign, and five line segments with zero plus signs.
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