A lifeform or organism is defined as a chemical complexity capable of making a copy of itself. This is called reproduction. Asexual reproduction is a form of reproduction in which the DNA within the resulting offspring is derived from one single individual organism. Sexual reproduction is a form of reproduction in which the DNA within the resulting offspring is derived from more than one individual organism. All multicellular species except sponge, and some sponges also, are capable of sexual reproduction although there are many multicellular species that reproduce almost entirely by asexual reproduction. All organisms to have ever existed are capable of asexual reproduction although there are many species that reproduce almost entirely by sexual reproduction.
In order for evolution to take place, all members of a population can't be genetically identical. In order for genetic traits to spread through a population, one organism would have to be more likely to have offspring than another, meaning they would have to be genetically different. Genetic difference can come about through errors in the duplication of DNA. This includes recombination, point mutation, loss or duplication of genes, alterations in the arrangement of genes, and numerical changes in the number of chromosomes. Mutations can also come about by a mutagen, such as a cosmic ray, striking and altering of the base pairs of the DNA, where the bases of DNA are adenine, thymine, cytosine, and guanine. Many species rely entirely on these things to provide all of the genetic variation that makes evolution possible. However, it would be preferable if you could have more genetic variation than these things could provide. The more genetic variation that exists, the higher the likelihood that one individual would happen to possess a trait or combination of traits that is highly advantageous, or greatly increases the likelihood of them having offspring. If the environment changes, the higher the likelihood that an individual would possess traits that enable it to survive in the new environment. The more genetic variation within a population, the higher the likelihood that it can change along with a changing environment. Thus it is advantageous for a population to have as much genetic variation as possible.
Some bacteria and protozoa are capable of a process called "horizontal gene transfer" by which two individuals merge, exchange a few chromosomes, and then separate. Each individual afterward is genetically different than they were beforehand. Assuming that there exists individuals genetically identical to what the two individuals had been originally, there is afterward more genetic variation within the population. The genetic variation within the population has increased. The purpose of horizontal gene transfer is to increase the genetic variation within a population. The purpose of reproduction is to increase the number of individuals within a population. If you could combine these two processes into a single process, you could simultaneously increase the amount of genetic variation within a population, which is advantageous for the reasons stated, and increase the number of individuals within a population, which you have to do anyway if the population is to continue to exist and individuals die. This duel-purpose process is called "sexual reproduction", which you could think of as a combination of horizontal gene transfer, where you increase genetic variation, and asexual reproduction, where you increase the number of individuals.
There are advantages and disadvantages to both sexual and asexual reproduction. There are costs to sexual reproduction that don't exist for asexual reproduction. You have to expend energy in the synthesis of sex organs, finding a mate, courtship, and mating. These are costs unique to sexual reproduction. The advantage of sexual reproduction must outweigh these costs The advantage of sexual reproduction is that you increase the genetic variation of your offspring. If your offspring are all the same as yourself, they will all have the same fitness as yourself. If they are all different from yourself and each other, probably one of them will be better adapted to the environment than you are. If you can get your genes into an individual that's better adapted to the environment than you are, you maximize the likelihood of your genes being passed on to future generations. Also, it has to do with a changing environment. If you know what the environment is, you would just make offspring best suited to that environment. If you don't know what the environment is, meaning the environment's changing and you don't know what the future is, it's in your best interest to make offspring best adapted to several different environments. Probably one of them would end up being better adapted to what the environment ends up being than any one of them if you had made them all the same. Yet as I said there are disadvantages to sexual reproduction. One of the disadvantages is that reproduction itself is less likely, since you have to happen to have two individuals at the same place at the same time. The advantage of sexual reproduction as opposed to asexual reproduction is that you increase genetic variation. The advantage of asexual reproduction as opposed to sexual reproduction is that you only need one individual for reproduction to take place so it is therefore easier or more likely. Therefore, all multicellular life is capable of both sexual and sexual reproduction. For most multicellular species, such as almost all plants, almost all of their reproduction is asexual, although they are also capable sexual reproduction, which is why plants have flowers. For some species, such as humans, almost all of their reproduction is sexual, although they are also capable of asexual reproduction, although only when we're embryos, which is how identical twins are formed.
There exists forms of reproduction in which the DNA of the offspring is derived from either one or two individual organisms, but not any other number. Why is this? If there was a form of reproduction in which the DNA of the offspring was derived from three individual organisms, the likelihood of reproduction taking place would be even less than with two because you would have to happen to have three individuals at the same place at the same time. One of the disadvantages of sexual reproduction would be increased. You might think that the advantage of sexual reproduction would be increased also, but that's not true. The amount that genetic variation is increased from two generations of two individual reproduction is greater than the amount that genetic variation would be increased from one generation of three individual reproduction. If there were to exist a circumstance where it would be advantageous to increase genetic variation by a given amount in the shortest time possible, you could achieve that same effect by making the making the generations, or the time between when organisms reproduce and when their offspring reproduce, as short as possible. Therefore reproduction where the DNA of the offspring is derived from three individual organisms would have higher disadvantage, but not higher advantage, than reproduction where the DNA of the offspring is derived from two individual organisms. Thus it doesn't exist.
Obviously it would be advantageous to minimize all disadvantages. One of the disadvantages of sexual reproduction is the unlikelihood of happening to have two cells at the same place at the same time. Therefore you want to make this as likely as possible.
You want to maximize the likelihood of two cells coming together, which is the likelihood that both of the cells will reach the center location. There exists a given likelihood of one of the two cells reaching a specific location, and the likelihood that both of them will happen to is half that. If there's a likelihood of one of the two cells reaching a specific location, why not just let that location be the other cell, so there will be a 100% likelihood of the other cell reaching that location, since it's there anyway.
The likelihood of one of the cells reaching the other cell is the same as the likelihood of each of the cells reaching a third location, and thus twice the likelihood of both cells reaching a third location. Thus you greatly reduce one of the major disadvantages of sexual reproduction. These cells are called gametes. An organism that produces gametes that are motile is called male. An organism that produces gametes that are not motile is called female. An organism that produces both gametes that are motile and gametes that are not motile is called hermaphrodite. An organism that does not produce gametes has no gender.
Cells with half the normal number of chromosomes are called gametes. Motile gametes are called sperm. Non-motile gametes are called ova. What gametes an organism produces during its life determines its gender. All organisms fall into one of the following five categories.
1. Male - at some point in its life produces sperm, but never produces ova
2. Female - at some point in its life produces ova, but never produces sperm
3. Concurrent Hermaphrodite - produces both sperm and ova at the same time
4. Consecutive Hermaphrodite - produces both sperm and ova, but not at the same time
5. Asexual - does not produce either sperm or ova
The motility of the gametes, or what gametes are produced, is the only thing that defines gender. All other differences between male and female, and there are species where the differences are far more pronounced than in humans, are indirect repercussions of this one singular definition. It's similar to the fact that if a multicellular organism is photosynthetic, meaning at some point in its existence contains chlorophyll, it's a plant, and if it's not photosynthetic it's an animal, and all other differences between plants and animals are indirect repercussions of this singular definition.
If two cells, with the same number of chromosomes, come together and form one cell, the resulting cell has twice the number of chromosomes as in the original cell. The next generation would have four times the number of chromosomes as in the original cell, and so on. Obviously, this is not acceptable. In order for the cells that are formed when two cells come together to have the same number of chromosomes as in all the cells of the original organism, the cells that merge with other cells must have half the number of chromosomes as in all other cells of the original organism. The cells with half the usual number of chromosomes are formed by meiosis.
Let's say a cell goes through mitosis and each of the daughter cells undergo mitosis. The four resulting cells have the same number of chromosomes as the original. Let's say however, that during the first mitosis, instead of chromosomes lining up along the axis of the cell, entire pairs of reduplicated chromosomes line up. Then when they duplicate, instead of one from each pair of chromosomes going to each side of the cell, entire pairs of reduplicated chromosomes go to each side of the cell. The resulting cells each contain not one of each type of chromosomes, but two copies of half of the types. These cells each have the same number of chromosomes as the original. For the second "mitosis", the chromosomes don't have to duplicate since you already have pairs of reduplicated chromosomes. Thus the cells resulting from this division have half the number of chromosomes as their parents. Therefore, meiosis is essentially where a cell undergoes mitosis and the resulting cells undergo mitosis, where in the first division , not single chromosomes but pairs of chromosomes line up along the axis of the cell.
Here you can read about mitosis and meiosis.
Today, you occasionally have a situation where one or more chromosomes are copied more than once. Polyploidy is where you have extra sets of the entire genome. Perhaps some time in the past, there was a mutation that altered mitosis so that instead of simply the chromosomes being duplicated, the duplicates were duplicated again. You would then have homologous quadruplets or pairs of homologous pairs. Then perhaps each daughter cell would get an entire pair of each chromosome. Perhaps in each daughter cell, simply the presence of pairs of chromosomes in the first place would prevent the triggering of another round of duplication. You could imagine something like this giving rise to meiosis.
This doesn't address the question of how it became common place for the resulting gametes to then merge to produce new individuals. The earliest multicellular organisms had a juvenile stage that was unicellular, as we do of course. They would put off single cells that would drift through the ocean until they land somewhere and then they would develop into an adult. Very rarely, two such cells might just happen to run into each other and merge into one cell. That cell would then become an adult. There could have been genes where individuals that possess those genes would produce offspring cells that are more likely to combine with other such cells and survive combination. Individuals with those genes would have the advantages of sexual reproduction and would be more likely to pass their genes on to future generations than individuals without those genes. The fraction of the population with those genes would increase. You have the problem of offspring having twice the number of chromosomes as their parents. Perhaps then the mutation that gave rise to meiosis came about or became advantageous. Those with a tendency for meiosis to take place would be more likely to pass their genes on to future generations than those without this tendency. The fraction of the population possessing this tendency would increase. There exist species today that produce cells that could either combine with another cell and give rise to an individual or could develop into an individual without combining with another cell. Fungi are frequently capable of this.
Recently we've learned more about how meiosis is controlled genetically. We've identified genes, within which certain mutations can cause failure of meiosis. (Francis, 1995) Francis' team characterized a mutation in the gld-1 gene (germ line development) in caenorhabditis elangans. In gld-1 (null) hermaphrodites, germ cells enter the meiotic pathway normally but then exit pachytene and return to the mitotic cycle. This implies that meiosis is an elaborate variation of mitosis and that the gld-1 gene is required to not turn off the variations and to prevent the process from reverting to its earlier primitive form.
Another form of meiotic failure involves polyploidy. (Loidl, 1995) At the beginning of meiosis, homologous chromosomes pair up. Loidl studied triploid and tetraploid examples of the yeast saccharomyces cereisiae. For tetraploid, four examples of a given chromosome usually form two pairs, while for triploids, three examples of a chromosome usually form a triplet. Therefore tetraploids are more successful at forming homologous pairs during meiosis than triploids. For this reason, spore viability of triploids is about 40% of normal, while for tetraploids, it's only slightly less than normal.
All sexual reproduction to have ever taken place is by the process I've described. However, there is a great deal of variation among species as to the exact method by which the two cells are brought together. Many plants employ insects to achieve this and you could write a book on that subject alone. Almost all asexual reproduction is by mitosis, but not absolutely all of it. Now I shall describe the different types of asexual reproduction.
1. Mitosis
In order for an organism to be capable of mitosis it must be unicellular. However, all organisms, except viruses and those formed by budding, begin as a single cell. Thus if nearly all organisms are unicellular at some point in their existence than nearly all organisms are capable of mitosis at some point in their existence. A human zygote is no less a unicellular organism than a paramecium. Simultaneously, a human zygote is no less a member of the species homo sapien than you are. Thus humans are capable of reproduction by mitosis, albeit only for a few seconds. This, of course, is how identical twins are formed. Also, individual cells of multicellular organisms replicate by mitosis.
Mitosis probably did not exist in the earliest proteinoids or the earliest life to have existed, but it was derived shortly thereafter. Logically, if you wanted to make a copy of a bacterium, what you would have to do is first make a copy of the DNA, and second get a complete copy of the DNA at either side of the cell and divide the cell in half. This is mitosis. Even eukaryotic mitosis is usually described as an integration of the processes that duplicate cellular material and processes that partition the duplicated material into two viable daughter cells. Probably originally, cells divided in half without duplicating DNA. Each daughter cell ended up with some fraction of the original DNA. Somehow there was a mutation that caused replication of DNA. Originally, perhaps not all of the genome was replicated and the daughter cells randomly ended up with different fractions of the resulting DNA. Yet a few individuals would have genes that caused all of the DNA to be replicated and specifically caused each daughter to usually get one and only one copy of each chromosome. Individuals with those genes would be more likely to pass their genes on to future generations. The fraction of the population with those genes would increase. Selection would favor those that performed mitosis more efficiently. You end up with a situation where all life except viruses are capable of what we think of as mitosis. There still exist mutations that cause mitosis to fail but since any such mutations are fatal, those individuals are taken out of the population.
Artificially induced asexual reproduction is called cloning. When a bacterium, protozoan, or unicellular algae is placed in an environment with an adequate food supply it'll automatically undergo mitosis. Placing it in such an environment could be considered artificially induced mitosis. More interesting is inducing mitosis as a means of asexual reproduction in multicellular species when they're in their unicellular stage. When a zygote undergoes mitosis and becomes a bicellular embryo it's not different than when a protozoan undergoes mitosis and becomes two individuals. If the two cells of the embryo become separated, each will become a separate individual. This results in identical twins. If we artificially separate the two cells this will occur also. This was first done in 1892 by Driesch who separated bicellular sea urchin embryos by violently shaking them. This was done with rabbits by Adams and Rowson in 1968. This was done with humans by Still and Hall in 1993. Today we occasionally replicate sheep and cattle by this means. This type of cloning is called blastomere separation.
2. Budding
All organisms, except viruses and those formed by budding, begin as a single cell. For multicellular organisms, that cell divides by mitosis and the resulting cells divide by mitosis, as do all subsequent cells until the number of cells within the individual increases from one to the maximum number of cells to exist in the individual or whatever number of cells are present within the adult. Except for gametes which are formed by meiosis, all of the cells are genetically identical. However as the number of cells within the individual increases, within some cells different segments of DNA are activated or "read" than in other cells. Different cells then behave differently. This is differentiation. As time goes on the cells become more and more different. When humans are composed of, say, eight cells, they're all identical. In an adult human, cells are very different from each other. The complexity of a lifeform is determined by where they stop in the process. In some species, very little differentiation takes place so the cells never become very different from each other. In some species, a very large amount of differentiation takes place so the cells of the adult are very different from each other.
In a low-complexity organism, where very little differentiation has occurred, the individual cells of the adult are not that different from the original single cell that the organism began as. In a high-complexity organism, where a large amount of differentiation has occurred, the cells of the adult are very different from the original single cell that the organism began as. The more differentiation that takes place, meaning the more complex the organism, the greater the difference between the individual cells of the adult and the single cell that the organism began as.
In a sponge there is almost no differentiation. The individual cells are very similar to each other and the original cell it began as. Let's say you remove a single cell from a sponge. If that cell is identical to the original cell that gave rise to the adult then why would that cell not also give rise to an adult? There is no reason why it would not and indeed that is what would happen. If you took a sponge and forced it through a mesh, dividing it into individual cells, however many there are, each cell would become an adult identical to the one you started with. Here we're describing something involving human intervention. However, naturally sponges put off small pieces of themselves that become individuals. This is called budding, selfing, or vegetative reproduction.
The greater the complexity of the organism the less they are capable of this. Coelenterates also put off small pieces of themselves that become new individuals. If you take planarian and slice it lengthwise each half will become a new individual. They do this naturally. Beyond this point you're talking more about regeneration than about budding. if you take a starfish and slice it exactly in half, most likely both halves will die, if not then one half will live and the other will die. It is very unlikely, but it is possible, that both halves would live and you would have two starfish. If you cut the arm off a starfish it will grow a new arm but the arm won't grow a new starfish. If you take a certain salamander and cut off a leg it'll grow a new leg but obviously the leg won't grow a new salamander. It's physically impossible to cut a salamander in half and get two individuals. With humans, if you're cut, the cut will heal but that's the extent of our regenerative capability.
Plants are of a level of complexity by which they can easily reproduce by budding. I mentioned that there are multicellular species that reproduce almost entirely by asexual reproduction. A good example is bamboo, some species of which reproduce sexually (meaning flower) once every 70 years and otherwise reproduce asexually (meaning send up new shoots).
The origin of budding isn't difficult to imagine. Sometime if an organism was damaged or broke apart, if it was simple enough the pieces would become new individuals. Those more likely to have this happen to them or "do this" would be more likely to have offspring and pass their genes on to future generations.
Artificially induced budding is the oldest form of cloning and dates from at least the Fourth Millennium B. C. In Babylonia they would take cuttings from plants and bury them and the cuttings would grow into new plants. It could have existed much earlier since it was also done by some primitive tribes. This is, of course, artificially induced budding, and thus cloning. Most animal phyla are at a level of complexity by which they would be capable of budding in the early embryonic stages. Sometimes early embryos can divide in half and become two individuals. Some identical twins are formed in this way. Humans can reproduce by budding until 11 weeks gestation. Cells artificially taken from early embryos can also become individuals. The simpler the animal the later in its embryonic development would cells taken from it develop into an individual. We are capable of artificially induced budding of adult humans in the limited case of the separation of so-called "conjoined twins".
Cellular differentiation can begin even in the first division. (Cheng et al, 1995) Occasionally, one of the resulting cells is larger than the other. Cheng and his team used caenorhabditis elagans to illustrate asymmetric division.
3. Parthenogenesis
I described before how sexual reproduction probably evolved from a system where organisms released single cells which became individuals. Ironically, there's been a kind of reverse evolution where sexual reproduction evolved into a system where organisms reproduce asexually by releasing single cells which develop into individuals without combining with other cells, or something similar. These types of asexual reproduction are called parthenogenesis. Parthenogenesis that gives rise to males is called arrhenotoky, that gives rise to females is called thelytoky, and that gives rise to both is called amphitoky or deuterotoky.
First I'll describe the main type of parthenogenesis and then I'll mention all of variations of this, which are numerous. Parthenogenesis usually takes the form of meiosis in which the first division is abortive. Since the reduction division never takes place, the resulting cells are diploid. Since, they are diploid, they can develop into diploid individuals without combining with other cells. It's ironic that mitosis evolved into a more complex system called meiosis in order to get haploid cells, and meiosis evolved into an even more complex system of incomplete meiosis or parthenogenesis, simply to create diploid cells, which is what we had originally. In different examples of parthenogenesis, the second division of meiosis halts at different points. Sometimes it's completely nonexistent. Other times, the chromosomes are lined up along the axis with spindle fibers connecting them to the centromeres, but it finally ultimately doesn't actually take place. Presumably, the greater the completeness of the second division, the more primitive the form of parthenogenesis.
Occasionally, normally nonparthenogenic species undergo rudimentary parthenogenesis called tychoparthenogenesis. It is believed that parthenogenesis, meaning thelytoky, evolved from tychoparthenogenesis. There are some unanswered questions about how this occurred. Tychoparthenogenesis is a less efficient means of reproduction than either sexual reproduction or parthenogenesis. It squanders energy on near meiosis which serves no purpose. Why would individuals with a tendency for tychoparthenogenesis be more likely to pass their genes on to future generations? Perhaps in very stable environments sexual reproduction would be less advantageous and asexual reproduction would be more advantageous, and this would allow tychoparthenogenesis to get a toe-hold. Then those capable of more efficient parthenogenesis would be selected. There was some mutation that gave rise to tychoparthenogenesis and selection refined it to parthenogenesis. It's not difficult to imagine such a mutation since meiosis may frequently have errors. It's just these errors are usually lethal and you don't see them. Meiotic abnormalities may be common in sexual reproduction and such failures in meiosis provided the cellular basis for the restorational mechanisms of parthenogenesis. Parthenogenesis evolved by taking advantages of mistakes in sexual reproduction.
Motile gametes (sperm) are more likely to succeed in their journey if they are small-sized. the non-motile gametes (ova) have to be larger then they would be if the sperm were not small sized so the zygote would contain enough material. If a sperm ended up with diploid chromosomes it wouldn't possess enough cytoplasm to go through a series of divisions like a zygote. A diploid ovum would be almost the same size as a zygote and it would be difficult to say how it differed from a zygote. Therefore this form of reproduction exists only in females.
A good example of it is in aphids. at one point in their extremely intricate lifecycle, an individual starts developing an offspring within it before it itself is born. Therefore, the outermost individual has two generations telescoped within it.
I'll now list different types of parthenogenesis and related means of reproduction. If the cell that becomes an individual is not the result of two cells coming together, it's parthenogenesis and asexual reproduction. If it is the result of two cells coming together, it's sexual reproduction if they are from different individuals and asexual reproduction if they are from the same individual. If they are from the same individual, it's called autogamy if they are from different meiosis and automixis if they are from the same meiosis. Automixis that involves fusion of fully developed sperm and ova is called non- parthenogenic automixis. There are some plants, such as ferns, that produce two sperm and two ova at each meiosis. You could also have automixis where two of the ova produced by a female in one meiosis fuse and then develop. This is called parthenogenic automixis.
Let's go back to normal parthenogenesis where the cell is not the result of two cells fusing. The cell could be either the result of meiosis or not. If it is, you have the problem of the offspring being haploid. You could get around this with a process called endomitosis. Endomitosis is when during cleavage division nuclei fuse which doubles the number of chromosomes. If this occurs before meiosis, the form of parthenogenesis is called apomixis, not to be confused with another type of parthenogenesis which unfortunately has the same name. If it occurs after meiosis, it's called replicative haploid parthenogenesis. If it doesn't take place at all you have haploid parthenogenesis. The offspring are haploid and this is how arrhenotoky takes place. In addition to these three types of parthenogenesis that involve meiosis, there are also two types of parthenogenesis that do not involve meiosis. The original cell could be an oocyte which is a cell resulting of after the first division of meiosis. You are preventing meiosis from being complete instead of letting it be complete and then either restoring diploidy or not. This is the most famous type of parthenogenesis and what I discussed in the beginning of this section. This type of parthenogenesis is called apomixis, so it has the same name as the less famous type I mentioned earlier. Lastly, in flowering plants, the ovum that becomes an individual could be derived mitotically from a somatic cell in the ovule instead of a generative cell. This is called aposporous apomixis or apospory. These last two types of parthenogenesis produce diploid offspring.