Question
If asexual reproduction survives at all, conditions must sometimes favor it, even in competition for a niche with sexually reproducing species. Some insects, like aphids,
If asexual reproduction survives at all, conditions must sometimes favor it, even in competition for a niche with sexually reproducing species. Some insects, like aphids, are both asexual and sexual reproducers. Though rare, those species that reproduce asexually tend to persist.
Efficiency, for one thing, is on the side of asexual organisms: Without the need for males, every clonal individual passes a full set of identical genes onto the next generation. In addition, a single female can establish a new population. This gives asexual creatures on the fringes of a habitat an edge in colonizing a new one.
Clonal organisms with tolerance for a broad range of conditions also exist. Even if they do not vary much genetically, they possess what scientists have dubbed "general-purpose genotypes." One example is a type of minnow found in Minnesota that tolerates both highly oxygenated and poorly oxygenated waters. Able to survive in an unpredictable environment, this minnow outlasts more specialized strains.
Asexual animals arose from diversified sexually reproducing ancestors. Often they are hybrids between two different species, and may possess two complete sets of chromosomes.
One such asexual organism is the whiptail lizard in the U.S. Southwest, Mexico, and South America, which consists only of females who reproduce by parthenogenesis. They appear to be the only known unisexual reptile.
Clones generally hit a dead end, going extinct when their limited variation collides with changing environmental conditions or with better-surviving sexual reproducers. With their unusual way of reproducing, they continue to fascinate scientists because they are natural experiments in interactions between fixed genotypes and varying environments.
Can sex earn its keep?
It would seem best to reproduce asexually or be a hybrid of some sort. However, some scientists have come up with a theory about why sexual reproduction is still needed from an evolutionary standpoint. Rather than correcting mutations or diversity that occurs during sexual reproduction, it may be better to allow those mistakes to occur, then "remove" them later - like through natural selection.
However, the problem is the speed at which asexual individuals reproduce. A sexually reproducing population could inevitably be driven extinct by the asexual organism's productivity in producing offspring. A new hypothesis has come out - the Red Queen Hypothesis - coined from Alice In Wonderland: "It takes all the running you can do, to keep in the same place."
This describes the never-ending interaction between hosts and disease, or predator and prey. As species that live at each other's expense continue to evolve, they are engaged in a constant struggle for survival advantage as their environment is constantly changing. So, this means sexual reproduction is needed. Those mutations or the genetic diversity from the mixing of the gametes allows new traits to appear that could help fight disease or evade predators.
Think about our example of sickle cell in humans - this is a detrimental disease that, according to evolution by natural selection, should have died out. But, it persists, as this is the method by which humans have to fight off malarial infections in certain regions of the world. If organisms only reproduce asexually and did not have the sickle cell allele, they would all die from malarial infection.
So, each type of strategy is useful from the context of evolution and natural selection. As we've seen repeatedly, this is mostly dependent on the environmental conditions of the organism and how their traits allow them to either survive in that environment, or die.
From:http://www.pbs.org/wgbh/evolution/library/01/5/l_015_01.html
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