"You still don't understand what you're dealing with, do you? The perfect organism. Its structural perfection is matched only by its hostility. I admire its purity. A survivor … unclouded by conscience, remorse, or delusions of morality." - Ash to Ripley (Alien, 1979)
Selective pressures are the driving mechanisms for change in populations, and at one point in time, selective pressures chose the mechanisms of sexual reproduction over those of asexual reproduction. To help understand what types of things can decide whether the selective pressures sexual to asexual reproduction, let's take a look at the advantages and disadvantages of sexual reproduction.
There are significant disadvantages to sexual reproduction, and while these are not all of them, here are some important ones. Most of these disadvantages are lessened, if not mitigated, by reproducing asexually.
Everything has an energy budget that it must balance by the end of the its life. Even on a smaller scale, if more energy than usual is consumed in one aspect of activity, then another such aspect suffers from a deficiency. Finding a mate is a very strenuous activity. For males, as can be seen in Superb Bird-of-Paradise (birds of paradise), elaborate mating rituals are very costly to the organism .This cost may be superficial as first, such as spending energy showing off to potential mates takes away energy from gathering food, but can also be less ostensible, such the price payed for such extravagant male characteristics - testosterone  - has its own set of adverse effects. Asexual mating removes the need to impress, and therefore the energy expenditure required to show off to the opposite sex. That very energy could be diverted towards reproducing more offspring, increasing the reproductive success of that organism.
The Video from class showing the intense mating rituals of birds of paradise:
Sex itself consumes time. As can be seen from these dolphins, mating can happen very quickly. And as can be seen in these octopi, staying in the mating encounters could potentially become dangerous (fellow on the right). The primary reason why this is true, is because both partners are vulnerable to attack during intercourse. Besides copulation taking up time, sexual reproduction is usually coupled with time spent investing in the next generation. This may be anything from sitting on eggs as they incubate, to gathering food, to having to work two jobs to make sure tuition is paid. This idea returns to the one mention above: energy budget. Time is as important a resource as energy when it comes to budgeting and managing it. Organisms that have to allocate time to make sure their future generation is successful, are taking that time away from other activities. Say in some species, such as the bonobo, males are extremely polygamous . If a male in that species had to expend extra energy and time to ensure that its progeny would survive, it would be directly impacting its ability to have access to other females and the potential increase in the total number of offspring it could have had. In fact, it is very difficult to find examples of creatures that have polygamous mating rituals and strong male parental care. This plays into the uncertainty males have about the father of the child.
An innate problem with sexual reproduction is that not all of an organism's genes are present in the next generation. Naturally, about a half of the genes in an individual comes from one parent, the other 50% coming from the other parent. The goal of reproduction is to make sure that the organism's own genes are successfully transmitted, yet sexual reproduction places a theoretical limit of only half said organisms' genes continuing. As successive generations reproduce, one's progeny becomes less and less representative of those original genes. Asexual reproduction guarantees 100% transmittance.
Whenever I hear the concept of only 50% of genes being transferred to the next generation, I remember a movie I watched when I was still in grade school. The movie is called Rabbit Proof Fence, and it told the story of three Aborigine girls who escaped from the "Moore River Native Settlement" to return to their families. The movie details the events of the Stolen Generation, a plan run by many agencies in Australia to remove Aborigine children and raise them in a western fashion, eventually having them bear children that were half Aborigine and half European in an attempt to breed out the Aborigine. Here is short clip film someone adapted and annotated for what seems to be a school project. I warn that some of the scenes are very intense.
Parasites play an interesting role in the ultimate selective pressure, opting for either sexual or asexual reproduction. Sexually Transmitted Diseases are commonly cited as disadvantages to sexual reproduction, but I hesitate to include them as something that was determinant of sexual or asexual reproductive strategies. STD's are opportunists that have evolved to become successful in their own right. By being passed from host to host through the exchange of fluids, STD's are just taking advantage of a easy bridge. Other forms of infection, such as maggots in Purple Martins, are similar in this regard (opportunists) to other notable STD's such as HIV . What can be considered influential in the determination of which strategy, sexual or asexual, is the extended concept of STD's. The passing of something deleterious. Harmful things can be anything from viruses and bacteria to simply destructive DNA. Say, in a hypothetical sense, a mutation occurs in a generally asexually reproducing bacteria that leads to a gene that would cause the cell to die. Suppose this gene is usually deactivated, but say in the presence of lactose, the mechanisms that would allow for the generation of lactase were defective and resulted in a protein that either did not digest the lactase, or otherwise led to harmful chemicals to persist in the cell. The cell, before the external pressure of lactose, would reproduce normally, and have many progeny. All of a sudden, there is a significant percentage of the population that would die because the harmful element was passed generation to generation. Asexual reproduction would not contact above the baseline, average intraspecies interactions (potentially even more reduced as contact and proximity required in mate selection would also be gone).
There wouldn't be boys in asexual reproduction. While some would call this an advantage to asexual reproduction, I chose to ambivalent. Regardless, there is an impact of boys being in a population. Say the distribution between males and females was equal, 50-50. Compared to population of a 100 asexually reproducing "females," there are half the amount of reproductive females. The reproductive success of females is restricted by the number of eggs they hold, and the reproductive success of males is, similarly, dependent on how many mates they can encounter . The theoretical maximum is therefore the total number of eggs females can consecutively get fertilized in their life times. The asexually reproducing population, would by default have twice as many females, and therefore a greater theoretical maximum.
There is snail native to New Zealand called the Mud Snail, Potamopyrgus antipodarum. The snail has the ability to reproduce both sexually and asexually . The asexually reproducing snail creates clones, while the sexually reproducing produces genetically varying offspring. Parasitic influences from shallow water trematodes select for sexually reproducing snails that would have a varying genetic resistance to the parasite. In deeper water, the asexually breeding snail dominates due to the lack of parasitic pressures .
Despite these substantial negatives against sexual reproduction, it was selected for, and still is. As I am sure you are aware, the purpose of all adaptations is to help a population survive. It is interesting that regardless of all the downsides, the positives make up for it.
"Cleaning up" is a loosely defined term in regards to DNA. The replication of genetic material has many steps and can be condensed into two main types of DNA replication: mitosis and meiosis . The focus of this post is not the differences between the two, but if you want to learn more, click here. The main relevant difference is the result of the two mechanisms. Mitosis results in two daughter cells, each genetic copies of their mother, while meiosis results in four haploid daughter cells, each a different genetic combination of the mother, and only half the genetic material. It is not to say that genetic information is lost in the process, but simply redistributed. Sexual reproduction requires the conjugation of a haploid egg and a haploid sperm, coming from the female and male respectively. What does this have to do with "cleaning up?" It all has to do with recombination . In sexual reproduction, genes have the chance to switch it up. In a mother cell of a sexually reproducing organism, half of the DNA comes from its mother and the other half from its father. So, when it divides into the four daughter cells, it could be assumed that it splits into two cells with the mother's genetic information, and two with the father's. This is not the case. During meiosis, pairs of chromosomes rearrange themselves, resulting in new combinations of maternal and paternal genetic material. Moreover, DNA can also partake in crossing over, which allows for individual chromosomes exchanging genetic material, increasing the genetic diversity of the resulting four daughter cells. These processes allow for new combinations of traits in progeny, causing the focus of selective pressures to fall upon individual traits and not just the successfulness of the entire genome. Overtime, mutations that could result in less favorable traits would be selected against, essentially cleaning the genetic material of a population. Evidence of this can be found in a study conducted by R. Stephen Howard & Curtis M. Lively, here.
One undeniable benefit to sexual reproduction is genetic variability. In a population that doesn't suffer from selective pressures doesn't necessarily feel the need for genetic variability. Certain selective pressures are time dependent. These could be environmental pressures, which are comparatively short-lived pressures. An example of a constant selective pressure is good old parasitism. As I was mentioning earlier, parasitism has a kind of two way street deal with sexual reproduction. Because of sexual reproduction, organisms, and males specifically, are more exposed to parasites. Increased energy expenditure on masculine displays of virility draw energy from other processes, such as immune response. This male immunity, which is already relatively weaker than a female's due to testosterone (necessary for secondary male characteristics), is further weakened by increased energy consumption elsewhere, increasing the compatibility filter for assailing parasites . But, because of genetic diversity, the likelihood that all individuals in the species are susceptible to a parasite decreases, thus selecting for a future generation to have a greater percentage of resistant individuals. This benefit has enormous benefits. An example where the lack of sexual diversity affected humans was the Irish Potato Famine. Almost all of the potatoes in Ireland were of the same lumper variety. These plants had be planted vegetatively, meaning that they were all clones . When the potato blight hit Ireland, nearly eradicated the entire potato population. The lack of genetic diversity led to the parasite infecting the entire population unrestricted. This is a major concern today, as farmers continue to plant the same variety of genetically modified grain in an attempt to increase their yield . There is a very real threat to these genetically modified crops, as they are already resistant to many generic parasites, and the hypothetical infection which could attack, would already be resistant plenty of preventative measures.
Are males worth it? To many species, the answer is yes. The benefits of sexual reproduction outweigh the costs for many species. And while there are still many species of organisms that continue to reproduce asexually, it is as nature intended. Parasitic pressures drove organisms to reproduce in such a way that resulted in increased genetic variability in the population as well as the steady removal of unsuccessful traits. Sexual reproduction doesn't speed up evolution, and doesn't benefit an individual, but the entire population. An interesting thing to consider as well, some of the very downsides of sexual reproduction have become positives in that they give a certain degree of satisfaction. Finding a mate, for example, isn't time consuming, as it is usually the goal.
 Zuk, M. Riddled with Life: Friendly Worms, Ladybug Sex, and the Parasites That Make Us Who We Are. Orlando: Harvest, 2008. Print.  Combes, Claude. The Art of Being a Parasite. Chicago: University of Chicago, 2005. Print.
Both males and females are affected by the same diseases and parasites. Even though this is true, on average women live about 5
years more than males . Males tend to have higher mortality rates than
females for almost all causes of death across the lifespan. The major difference peaks
in young adulthood when males reach reproductive maturity and begin to compete
for mates. There is not a parasite that is specific to just males or females,
but parasites kill twice as many males in developed countries and four times as
many males in underdeveloped countries as compared to females . It
seems that not only do males tend to get sicker than females, but
they also tend to be more susceptible to parasites. If parasites and diseases aren't specific to
gender, what is the cause for males' mortality rates?
Why Males are the Sicker Sex?
Physiologically: Testosterone Levels
Vertebrate males have different
levels of sex hormones than that of females. Testosterone is the major sex
hormone in males that improves their secondary sex characteristics and allows
them to produce sperm. Although this hormone is very important to males, it
seems to be a main reason of why males are the sicker sex . Testosterone depresses
immune cells, tissues, and organs. Not only does testosterone decrease the
immune system, it also increases other hormone levels such as cortisol, which
is a stress hormone that suppresses the immune system even further. The more
testosterone a male has, the weaker his immune system becomes . Not only does it
decrease the immune system but it can give males something called the
"Testosterone Storm". This is a surge in testosterone levels that
seems to make men act reckless .
Males have to search for females
to mate with which involves a good bit of roaming. Doing this expends a lot of
energy which has a negative impact on the immune system. Roaming will also increase
their exposure to parasties and pathogens. Sometimes mating rituals or
habits cause males to become more exposed to parasites because they must stay
in a certain area for a long period of time trying to seduce females .
Some males must fight or compete
with other males in order to mate with females. Male to male aggression is
highest when only a proportion of males are able to be considered a desirable
mate or marriage partner. Cultures of species with polygamous cultures
or species increase competition among males . Polygamous species favor males with higher testosterone levels because they will compete more, but this also decreases the immune system as mentioned earlier. Competition among males in a species fighting for a mate can receive fatal wounds which also contributes to a higher mortality rate than that of females. Males that must compete are usually much larger than the females in their species. Moore and Wilson found that the greater the difference in size between males and females, the higher the levels of parasitism in males. They also found the more intense the competition among the males, the sicker they were .
The Antechinus is a rodent-like
marsupial mouse that lives in Australia. Some antechinus species have short
term breeding periods. These animals are usually solitary except for during the
breeding periods. During a breeding period, both male and female antechinus
are ready to mate. This means that the male antechinus must fight for his
females. They spend the majority of rut competing with other males for females.
Competing with other males causes a lot of stress on the antechinus males. They are also constantly roaming as they try to find as many females as possible to mate with.
Stress produced by the environment or social interactions have been recognized
to increase the levels of corticosteroids which also affects the immune system. During this time, the male's digestive system will begin to break down due
to stress and lack of eating. The large amount of corticosteroids released
during this time cause immune system to failure. This makes the males unable to
defend themselves against disease and parasitism. After the breeding period, the females are
pregnant and all of the males die. The females can live up to two more years
after their first mating season .
The example of the Antechinus is
a dramatic example of how intensely males can be affected by competition,
constant roaming, and the release of a large amount of stress hormones during the mating season.
The Spadefoot toad is an
amphibian that lives in the desert. They spend the majority of their time
borrowed underground to keep cool. They come out during periods of rainfall to
find a mate. They will travel to where the rain has made a pool and sit in the
water calling for a mate. A female toad will visit a pond once a year and mate.
They then lay their eggs and go back to a life of being alone. The longer a
male toad is immersed in the water, the more susceptible he is to trematodes
that live in the water. They will get more trematodes than females simply
because they are exposed in the water pools for a longer period of time .
The Spadefoot is an example of roaming and exposure. They will roam searching for a pool of water
and stay in that location until a mate happens to hear his croaking.
have an advantage?
it is obvious that males will be sicker because of the previously stated
reasons, but do females have advantages that make them healthier? Indeed, they
do! Females have a sex hormone called Estrogen. This sex hormone is known to do
just the opposite of Testosterone. It increases the immune system by stimulating T cells . T cells function by recognizing self from non-self and help stimulate an immune response . Most females, with the exception of some
species, do not roam or search for their mates. They let their mates find them which decreases exposure to parasites.
Females can also pick and choose a mate from the males that they encounter. This means that they are not competing for their mates.They can
pick the healthier male to mate with. This will decrease the chance that the
male she chooses will have parasites that can be passed to her.
When estrogen is present, it increases the ability of a T cell to respond to an antigen and mediate an immune response .
What Is Coevolution? Coevolution is the process of two species putting
selective pressures on one another. The reciprocal selective pressures allow
both species to evolve, while not allowing one to get ahead of the other . Coevolution
is most likely to occur between species with a persistent relationship that are in close ecological interaction with each other. There are three types of symbiotic relationships that are formed
through coevolution. These relationships include: predator/prey, parasite/host,
and mutualistic .
The relationship between Cuculus canorus (cuckoo) and Acrocephalus
arundinaceus (reed warbler) is an excellent example of coevolution between a
parasite and its host. The cuckoo is a parasitic bird that tricks other bird
species into raising their own young . While the reed warblers are absent from
their nest, the female cuckoo swallows the egg of the host species and then
replaces the egg with her own young. The tricked reed warbler parents then proceed
to raise the egg as their own. Once the cuckoo egg hatches, the cuckoo tosses
the host eggs to their death . Once parasitized, the reed warbler’s
reproductive success is zero. Therefore, genetic variation that allows the reed
warblers to stop the cuckoos will be naturally selected for and passed on to
future generations . The cuckoos then reciprocate and evolve traits which
allow them to persist longer in the host nest. Both species are in a constant
arms race to outsmart the other species .
The female cuckoo will lay eggs that appear to mimic
the appearance of the eggs of their hosts, which hinders discrimination and
removal of their eggs by the reed warblers. In the first stage of coevolution
between C. canorus and A. arundinaceus, natural selection
favors the birds that better discriminate cuckoo eggs from their own . This rejection
behavior then puts selective pressure of the cuckoo to lay eggs that resemble
the host egg. This mimicry puts more pressure on the reed warbler to rejection
a foreign egg even if it greatly resembles its own . This cycle of better
detection and rejection continues as long as there is continual renewal of genetic
diversity in both populations.
Acacia Tree Ants
The relationship between the Pseudomyrmex ferruginea (acacia ant) and the Acacia cornigera (bullhorn acacia) is another example of coevolution.
Unlike the cuckoo and reed warbler, this is a mutualistic relationship. The
acacia ant no only depends on the plant for food and shelter but it also protects
the bullhorn acacia from preying insects and other plants . The acacia have evolved traits in order to
support this mutualistic relationship. The tree has swollen and hollow thorns that
serve as both the ants’ home as well as their protection. The tree also provides
the ants with food both as nectar and Beltian bodies . The ant has also evolved specific characters
to aid in maintaining this mutualism. The ants serve as a defense against
herbivores and they also remove fungal spores in order to prevent fungal
pathogens from entering the plant. The characters of both the ant and the
acacia are mutualistic traits that have evolved for the interaction in reciprocal
Coevolution plays a critical role in generating genetic
diversity. However, coevolution can come at a cost, especially between parasite
and host. In order for two species to both evolve, each much exert energy to
acquire new traits. Investment in certain traits can be costly, and can lead to
a decrease in fitness. For example, the male reed warbler increases surveillance
of the nest to decrease the chance of parasitism . This leaves the female reed
warbler susceptible to fertilization by another male. The cuckoo birds also
have costs to developing new traits. The smaller size egg more closely
resembles the host egg but it is also an easier prey . Both species have costs
to pay if they are going to be successful enough to pass their genes to the
next generation. References