We can artificially induce parthenogenesis by exposing cells to radiation. We have even induced parthenogenesis in species not naturally capable of it, such as rabbits. Also, ova start dividing when sperm enter them. If you can simulate this they'll start dividing anyway and you end up with a haploid individual.
4. Self-reproduction
If an organism produces both gametes that are motile and gametes that are not motile, it is hermaphrodite. What if an individual produces a gamete that is motile and gamete that is not motile which then merge to form a new individual. Conception is the trade mark of sexual reproduction. However, if the gametes are from one organism, the DNA within the offspring is derived from one single individual organism, which is asexual reproduction. This is most familiar as self-pollination in plants. There are species of coral, which are coelenterates, which are capable of but do not actively engage in self-reproduction while other species actively engage in it.
This is called self-reproduction, self fertilization, or selfing, and could be either autogamy or non-parthenogenic automixis. I mentioned this in the section on parthenogenesis yet if a fully developed sperm and ovum merge to form an individual, it's not considered parthenogenesis.
This is also an ancient form of cloning. People induce self-pollination in plants in order to obtain pure strains. You can induce self-reproduction in hermaphroditic species that aren't normally capable of it, such as snails. There exist humans that are hermaphrodites and in that case you could simply fertilize one of their ova with one of their own sperm, but obviously this is of limited practical value.
There are disadvantages to asexual reproduction which I described in the beginning. Therefore some hermaphroditic species evolved to decrease the likelihood of self-reproduction. Most hermaphroditic animals are incapable of self-reproduction. Some hermaphroditic plants are incapable of this. (Uyenoyama, 1995) Uyenoyama discusses various ways self-incompatibility could have arisen in plants.
An example of a situation where self-incompatibility would be advantageous in plants is when they are in a close relationship with a herbivore. (Strauss and Karban, 1995) If herbivores are highly adapted to a plant, then sexual reproduction could generate rare genotypes that the herbivores aren't adapted to. The higher the level of self- incompatibility, the more sexual reproduction takes place. this is one way self- incompatibility could be selected for. People have said that the environment can't change fast enough for sexual reproduction to be advantageous. A parasite-host relationship could be an example of a situation where the environment is changing fast enough. The plant-herbivore relationship of this example is similar. Sexual reproduction is advantageous for the plants so they can evolve away from another species.
Lastly, I want to address the question of what determines the proportion between the number of males and the number of females produced. I said before that if your purpose is to get two gametes together, you maximize the likelihood of that by having one travel to the other.
The gamete that is traveling will not have a 100% chance of reaching the other one. if the likelihood of the sperm reaching the ovum is less than 100% then there's the possibility that the genes of the male will not be passed on to future generations. That possibility would be disadvantageous and the absence of it would be advantageous. What form would the absence take? If say, the likelihood of a sperm reaching an ovum was 50% then if two sperm were created probably at least one of them would reach the ovum. It's still possible neither would but it's safe to assume at least one will. You don't want to waste energy creating more sperm than necessary. Of course in reality, the likelihood of a specific sperm reaching an ovum is far less than 50% and so a very large number of sperm are created. If you create a very large number of sperm it's safe to assume that at least one will reach the ovum.
If you create one sperm that will probably reach an ovum then you'll probably have one offspring. If you create a large number of sperm where on average only one will reach an ovum, again you'll probably have one offspring. However, if you were to create even more sperm than that you could have a situation where on average two or more sperm would reach an ovum. Then you would have even more offspring. The more offspring you have the greater the likelihood your genes would be passed on to future generations. If you are already creating a large number of sperm anyway then after that it's relatively little effort to increase the number of sperm by a few percent, if by doing so you would increase the number of offspring you have. Therefore, males can have more offspring than females. Among chordates, the number of offspring a female has per year is limited by the litter size for that species. However, a male can impregnate dozens of females, and have a number of offspring that is several times the litter size for that species. If this is true, you would need only a small number of males and a large number of females to ensure the propagation of the species. The species would be most likely to survive by maximizing its population, which it would do by maximizing the number of females since they limit the number of offspring produced. All you need is a few males and a large number of females. This is why traditionally ranchers kept one bull and a large number of cows. Also, hunters can decimate almost all of the male deers without threatening the population since very few male deers are necessary.
There is something else that happens simultaneously. The number of offspring that a female can have per year is limited by the litter size while since a male can impregnate several females, the number of offspring that a male can have per year is several times the litter size. If your purpose is to maximize the number of offspring you have then being male would be advantageous. If you have more sons than daughters, you would end up having more grandchildren. There exist genes where if you possess those genes, you're more likely to have more sons than daughters. Individuals with those genes would have more grandchildren, which will possess those genes also, and so the fraction of the population possessing those genes would increase. More males would be produced than females. These two opposing forces balance exactly so the same number of males and females are produced. If you were to suddenly increase the number of females, the average number of offspring that a single male would have would increase, and maleness would become advantageous. For that reason, more males would be produced and the balance would be restored.
There exists species of insects in which all reproduction is by sibling mating. Notice in that case, the number of grandchildren you have is maximized by having as few sons and as many daughters as possible. In these species, in each litter, there is usually one male, whose sole purpose is to impregnate his sisters, and many females. I point out that the example of reproduction solely through sibling mating is similar to asexual reproduction. The advantage is that reproduction is more likely since there is no question as to whether or not males and females can find each other, since they are born simultaneously. The disadvantage is that you don't increase the genetic variation of the population.
With social insects, because of haploid determinism, a daughter of a singly mated female is 75% similar to her sisters, 50% similar to her offspring , and 25% similar to her brothers. (Hasegawa, 1995) He said that since the workers are more closely related to sisters than brothers, they somehow cause the queen to have daughters.
In studies of the Australian bee Exaneura bicolor, larger populations have smaller female to male ratios than small populations. (Schwarz, 1995) He says that the mean reproductive value of daughters increases with the number of daughters. Such increases probably come from social interactions such as cooperative defense, increased task efficiency, and lower per capita costs in nest construction.
Literature Cited
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