There is an interesting paper by Alphey, Fu et al in the PNAS describing how genetic engineering could be used to fight the dengue virus
Here are some basic facts:
* Over 50 million people each year are infected by the dengue virus
* Only female mosquitoes bite ( suck blood ) and so they are the ones that can carry the infection to humans.
* There is only one species of mosquito that is responsible for dengue infections, it's called aedes aegypti.
* Fu, Alphey et al seem to have found a way of genetically engineer the males which cause their female offspring to be flightless, but their male offspring will be fine.
* Flightless females can't infect humans with dengue
* Flightless females won't be able to reproduce.
So let's see what would happen if we release some affected males into the wild. They will compete with the unaffected males for mates. There will be a reduction in the number of healthy females in the next generation who will be able to reproduce.
The male offspring of the affected males will be able to carry the genetic modification to the next generation and generations of males after that. At each generation there will be reduction in the number of healthy females.
It sounds like this could lead to the extinction of the (aedes aegypti) mosquito.
Alas, there is a little flaw in the logic...
We can look at this a few different ways:
1: Suppose in nature a genetic mutation can take place in the male of a species such that he'll produce healthy males with the same mutation and females that won't be able to reproduce. With the logic above that would suggest that that could lead to a reduction in healthy females in all future generations with potentially devastating consequences for the species. If just 3% are carriers and then we would have a reduction of approximately 3% in the population in every future generation. Sounds like extinction is on the way.
Well, I posit that such mutations are not uncommon in all species but yet we still have life on the planet. So they must be some flaw...
2: Let's look at it another way. Without going into the mathematics yet, if a creature has a gene which is severely detrimental to the procreation of its offspring. Then having that gene is a significant evolutionary handicap. And so you'd expect natural selection to mercilessly wipe out that gene. It is just survival of the fittest.
3: Let A be the affected gene, which is dominant over the unaffected gene U.
* Males with AA, AU and UU can reproduce
* Females with UU can reproduce
* Females with AA or AU can't
If we release some number of mosquitoes with AA genes into the wild. We'll call them generation 1. They'll mate with the wild UU females and produce AU male and female offspring. There will be no AA males in generation 2. Since to have AA male in generation 2 we'd need a female carrier of A to reproduce, but they can't. Only the pure UU females can reproduce.
The really key thing is that as we go from generation 2 to 3, the AU males will reproduce with the UU females, but only half of their male offspring will carry the A gene and similarly at each subsequent generation, the proportion of males with the A gene will be halved. In a few quick generation the A gene will have died out and we'll be left with a pure UU population of healthy flying male and female mosquitoes.
If you'd like to see a really simple mathematical model showing this in action,
have a look at the following spread-sheet:
* To see the calculation details, click here, requires (free) Google docs account.
* To view the results click here, no Google docs account required.
So what should we do: if we release some AA males into the wild, the effect on the native population will be rather small since the proportion of male carriers is going to halve in each generation subsequent to the second generation. So even if there is an enormous release of the AA males and they far outnumber the unaffacted males (UU), then within a few short generations we will still find that the A gene will be (almost) completely wiped out.
It would seem that a huge release of AA males every second generation would have some effect on the mosquito population, but it is rather impractical.
Bearing in mind that only half of the male offspring of AU males will carry the A gene, for the A gene to thrive into future generations, we would need the AU males to be far superior to the UU males when competing for mates. However if that could be achieved a future problem that we would face is that if there were any (UU) females that had a prefernce for UU males over AU males, then they would have an enormous evolutionary advantage and we would then find that in a few generations they would dominate thus wiping out the A gene once again.
_____________________________________________________________________________
I got the following response from Luke Alphey:
You are both completely right and completely wrong at the same time!
Yes, the RIDL gene (A gene in your terminology) will be rapidly eliminated from the target population after a single initial introduction of the gene (e.g. in the form of released homozygous males). This puts this strategy, with other sterile-insect methods, in the class of 'self-limiting' genetic control strategies (in contrast to 'self-sustaining' or invasive strategies where the introduced gene has some method of spreading itself through the wild population, typically a selfish-DNA-based gene drive system).
However, the premise of a single release is incorrect. The key to sterile-male (or sterile-insect) methods is to establish a sufficient population of sterile males in the field to ensure that enough wild females mate a sterile male and then maintain this for at least several insect generations while the target population declines. For 'conventional' irradiated insects, it is even more obvious that the sterile insects will have no viable offspring at all, so the effect will only be for (approx) the lifespan of the released steriles. Therefore, periodic release is required, with a frequency less than the lifespan of the sterile males. This is perfectly practical, and there is plenty of precedent from the conventional Sterile Insect Technique, which uses radiation-sterilised insects to control a range of agricultural pests (e.g. New World screwworm and Mediterranean fruit fly).
The fact that the RIDL gene will disappear rapidly from the target population and is maintained only by periodic (effectively constant) release is often considered a desirable 'safety' feature. Though it is very hard to see how this approach could cause any undesirable side-effects, it is nonetheless reassuring to know that simply stopping releases would lead to rapid elimination of the gene from the population. This is in contrast to some other proposed methods using modified mosquitoes, such as refractory insect/gene drive systems or Wolbachia.
I'd be happy to discuss this further, but there are also a few papers you might like to look at.
Open access (some may ask your name, but they are free):
http://jbiol.com/content/8/4/40 short review which discusses self-limiting and self-sustaining methods
http://www.biomedcentral.com/content/pdf/1741-7007-5-11.pdf research article which models different genetic-sterility strategies
http://www.eurekah.com/chapter/3233 Book chapter discussing RIDL and SIT approaches, particularly in the context of mosquitoes
..(there is ) an earlier PNAS paper (2007) which models versions of this approach for dengue control and a review on sterile-male methods for mosquito control (from Vector-borne and Zoonotic Diseases, epub ahead of print).
Tuesday, February 23, 2010
Monday, September 28, 2009
Contraception, Abortion and Evolution: Population growth round two
Men who have lots of sex and indeed lots of different partners tend to have more offspring. For all of history and pre-history that has been the case. There have always been some second order effects such as, those with lots of different partners would have greater risk of contracting a venereal disease which can prevent further offspring being produced due to infertility or even death. On top of that, those who sleep around probably make more enemies. It is conceivable that they have a higher murder rate than their monogamous counterparts. Though I don't have anything other than anecdotal evidence to support this particular assertion.
Another issue is that the promiscuous males could loose their regular partners, who prefer to find a more faithful man.
However, in spite of all of that, more sex generally produces more offspring.
But in the last few decades the use of reliable contraception has become wide-spread, also in many countries abortion is readily available. In the past people generally had the choice to have sex or not. That led to pregnancies. Now there are two different choices, whether or not to have sex and it is a separate issue to choose to have kids. So the people who are having the most children are the ones who want to have them the most and not the ones who want sex the most. This is an enormous change as a driver of evolution. Also lesbians, or indeed any woman who can't find or doesn't want a male sex partner, can just go along the a sperm bank. So sex is not even a prerequisite to having kids.
New positive drivers that will lead to more children:
1: Following a religious organisation that discourages contraception and abortion. Perhaps the success of science in developing contraceptives will lead to a less scientific and more religious species.
2: Strong desire to have children. In the past, from an evolutionary point of view, it was very important to be able to attract a mate, have a desire to have sex and then the desire to look after the child when he arrived. Now there is a new issue, there needs to be a desire to have children, which clearly related to the desire to look after them once they have arrived, but they are distinct.
New negative drivers that will lead to fewer children:
1: Ambition in women to succeed in the world at the cost of having fewer children. A woman who doesn't want a career but just wants to start having lots of babies will clearly be more successful from an evolutionary point of view. In the past when there was more of a risk of starvation, the more worldly mothers could help provide better for her kids. Now with very high survival rates, having the kids in the first place is the key issue.
2: The alpha male types who never want to settle down, at least partially motivated by the fact that they have a constant supply of available females. In the past such men would have many offspring, even if they didn't stick round long enough to look after many of them. However, now with reliable contraception, the man who wants to settle down young have have lots of kids has an evolutionary advantage.
All of this is not just going to affect humanity in the future. To a certain extent, the effects have been seen already. For example in Northern Ireland, we saw a growth of the catholic population relative to the protestants at least in part due to the catholic prohibition on contraception.
Whether religiousness is down to nurture or nature, we can indeed witness that people who observe a rule that forbids the use of contraception have an evolutionary advantage.
We may well see population growth taking off once again as over the next few generations there will be a growth in the desire of people to have children because those who want to have children have a new evolutionary advantage.
On the other hand, the desire to look after children once they arrive is possibly now less important than it was previously, since in wealthy countries at least, social services often step in to look after children that have been neglected by their parents.
Another issue is that the promiscuous males could loose their regular partners, who prefer to find a more faithful man.
However, in spite of all of that, more sex generally produces more offspring.
But in the last few decades the use of reliable contraception has become wide-spread, also in many countries abortion is readily available. In the past people generally had the choice to have sex or not. That led to pregnancies. Now there are two different choices, whether or not to have sex and it is a separate issue to choose to have kids. So the people who are having the most children are the ones who want to have them the most and not the ones who want sex the most. This is an enormous change as a driver of evolution. Also lesbians, or indeed any woman who can't find or doesn't want a male sex partner, can just go along the a sperm bank. So sex is not even a prerequisite to having kids.New positive drivers that will lead to more children:
1: Following a religious organisation that discourages contraception and abortion. Perhaps the success of science in developing contraceptives will lead to a less scientific and more religious species.
2: Strong desire to have children. In the past, from an evolutionary point of view, it was very important to be able to attract a mate, have a desire to have sex and then the desire to look after the child when he arrived. Now there is a new issue, there needs to be a desire to have children, which clearly related to the desire to look after them once they have arrived, but they are distinct.
New negative drivers that will lead to fewer children:
1: Ambition in women to succeed in the world at the cost of having fewer children. A woman who doesn't want a career but just wants to start having lots of babies will clearly be more successful from an evolutionary point of view. In the past when there was more of a risk of starvation, the more worldly mothers could help provide better for her kids. Now with very high survival rates, having the kids in the first place is the key issue.
2: The alpha male types who never want to settle down, at least partially motivated by the fact that they have a constant supply of available females. In the past such men would have many offspring, even if they didn't stick round long enough to look after many of them. However, now with reliable contraception, the man who wants to settle down young have have lots of kids has an evolutionary advantage.
All of this is not just going to affect humanity in the future. To a certain extent, the effects have been seen already. For example in Northern Ireland, we saw a growth of the catholic population relative to the protestants at least in part due to the catholic prohibition on contraception.
Whether religiousness is down to nurture or nature, we can indeed witness that people who observe a rule that forbids the use of contraception have an evolutionary advantage.
We may well see population growth taking off once again as over the next few generations there will be a growth in the desire of people to have children because those who want to have children have a new evolutionary advantage.
On the other hand, the desire to look after children once they arrive is possibly now less important than it was previously, since in wealthy countries at least, social services often step in to look after children that have been neglected by their parents.
Tuesday, September 22, 2009
Cause of homosexuality
Evolution has to answer some tough questions, such as how could the human eye evolve (see Appendix 2 below) or why did peacocks evolve to have such a ridiculous (but beautiful) tail (see Appendix 3 below). In many respects evolution is a ruthless driver of efficiencies. But there are some anomalies. For example, why are there so many homosexuals? If there were a gene for gayness, then surely evolution would wipe it out. I'd like to proposes an explanation.
But first let me discuss tennis! ( It will be relevant, I promise)
When a tennis player is about to take a shot, naively we might think that the best strategy would be to hit the ball in such a way as to maximise the probability of it going in, that would be to hit it back gently into the middle of the court. However competitive players almost never do that.
If the player hits the ball harder (faster) then it will increase the probability of the ball going out, but if he were to gently hit the ball, then the opponent will almost certainly take advantage of the weak shot and hit a winner. So the optimal shot involves taking a calculated risk that the ball will go out. ( See Appendix 1 for a plot of some probabilities and expected score)
Now returning to evolutionary psychology: why are there so many gay people? Well, consider the following hypothesis: There are some genes which give people an advantage, such as say ability to recognise emotions in others. But rather than working just as a black and white (i.e. binary) scale, it is much more gray than that and that the genes can also lead to a small possibility (probability) of homosexuality.
Someone who carries these genes can still have an advantage over others who don't even though they expect some percentage of their offspring will be gay. Over all the effect of having the genes is positive.
Well, what could such a set of genes be? Or what quality do they give the people who have the genes. It could be a 'feminine-side'. A man who can express emotions and empathise with others who display their emotions can have an advantage over someone with very limited social skills. However having such genes could also lead to a probability of homosexuality. And so a genetic cause of homosexuality gets passed from one generation to the next in a bundle of genes with net beneficial effect.
The next question is how do we test this hypothesis? Well, in some families all the kids are tall. There is a high correlation in height of siblings. If the hypotheis is true, we would expect a lower correlation of homosexuality between siblings, since the genes are causing a probability of homosexuality and not actually homosexuality, unlike genes which cause height. But the correlation would still be positive.
Appendix 1
Let's look at the strategy of the tennis player in a mathematical way.
Let v be the speed of the ball that player A hits.
Ignoring drop shots for now, we'll consider speeds from the easy shot into the middle of the court up to the very high speeds which are very difficult to keep in.
Let p(v) be the probability that a shot hit with speed v by player A will go in.
Let q(v) be the probability that player B, on the other side of the net, will be able to return the ball again conditional on A's shot, which was hit with speed v, being in.
For both p(v) and q(v), low speeds will give high probabilities, high speeds will see the probabilities dropping towards 0.
For a given shot, we'll award A a score of
-1 when A's shot is out, that happens with probability (1-p(v))
0 when A's shot is in and B returns it successfully, that happens with prob: p(v) * q(v)
+1 when A shot is in and B is unsuccessful in returning it, that happens with prob p(v) * ( 1 - q(v))
We can now work out the expected score when A hits the ball with speed v:
E(S(v)) = -1 * (1 - p(v)) + 0 + 1 * p(v) * ( 1 - q(v))
E(S(v)) = 2 * p(v) - 1 - p(v) * q(v)
Below is a plot of what E(S(v)) might look like against v.
So we can see from the plot above, that to maximise his expected score, player A needs to hit the ball with a speed that will result in there being a reasonably high probability of the ball going out.
In exactly the same way, genes can be advantageous to the carrier, even if it means there is a significant probability of having homosexual offspring as long as they confer some other advantage.
Appendix 2
The evolution of the eye has been well explained. I won't go into the details here except to say that a useful introductory article can be found in wikipedia
Appendix 3
The best explanation I've heard for why peacock's beautiful tails have survived generations of ruthless evolution is that if a peahen were to have a preference for fit males with smaller tails, then her male offspring who have inherited the short tail from their father, would have trouble finding a mate. The cause of their problem would be that their mother had a opinion on which was the best mate that differed from the consensus. So, when choosing a mate, your children will thank you for going with the consensus.
But first let me discuss tennis! ( It will be relevant, I promise)
When a tennis player is about to take a shot, naively we might think that the best strategy would be to hit the ball in such a way as to maximise the probability of it going in, that would be to hit it back gently into the middle of the court. However competitive players almost never do that.
If the player hits the ball harder (faster) then it will increase the probability of the ball going out, but if he were to gently hit the ball, then the opponent will almost certainly take advantage of the weak shot and hit a winner. So the optimal shot involves taking a calculated risk that the ball will go out. ( See Appendix 1 for a plot of some probabilities and expected score)
Now returning to evolutionary psychology: why are there so many gay people? Well, consider the following hypothesis: There are some genes which give people an advantage, such as say ability to recognise emotions in others. But rather than working just as a black and white (i.e. binary) scale, it is much more gray than that and that the genes can also lead to a small possibility (probability) of homosexuality.
Someone who carries these genes can still have an advantage over others who don't even though they expect some percentage of their offspring will be gay. Over all the effect of having the genes is positive.
Well, what could such a set of genes be? Or what quality do they give the people who have the genes. It could be a 'feminine-side'. A man who can express emotions and empathise with others who display their emotions can have an advantage over someone with very limited social skills. However having such genes could also lead to a probability of homosexuality. And so a genetic cause of homosexuality gets passed from one generation to the next in a bundle of genes with net beneficial effect.
The next question is how do we test this hypothesis? Well, in some families all the kids are tall. There is a high correlation in height of siblings. If the hypotheis is true, we would expect a lower correlation of homosexuality between siblings, since the genes are causing a probability of homosexuality and not actually homosexuality, unlike genes which cause height. But the correlation would still be positive.
Appendix 1
Let's look at the strategy of the tennis player in a mathematical way.
Let v be the speed of the ball that player A hits.
Ignoring drop shots for now, we'll consider speeds from the easy shot into the middle of the court up to the very high speeds which are very difficult to keep in.
Let p(v) be the probability that a shot hit with speed v by player A will go in.
Let q(v) be the probability that player B, on the other side of the net, will be able to return the ball again conditional on A's shot, which was hit with speed v, being in.
For both p(v) and q(v), low speeds will give high probabilities, high speeds will see the probabilities dropping towards 0.
For a given shot, we'll award A a score of
-1 when A's shot is out, that happens with probability (1-p(v))
0 when A's shot is in and B returns it successfully, that happens with prob: p(v) * q(v)
+1 when A shot is in and B is unsuccessful in returning it, that happens with prob p(v) * ( 1 - q(v))
We can now work out the expected score when A hits the ball with speed v:
E(S(v)) = -1 * (1 - p(v)) + 0 + 1 * p(v) * ( 1 - q(v))
E(S(v)) = 2 * p(v) - 1 - p(v) * q(v)
Below is a plot of what E(S(v)) might look like against v.
So we can see from the plot above, that to maximise his expected score, player A needs to hit the ball with a speed that will result in there being a reasonably high probability of the ball going out.
In exactly the same way, genes can be advantageous to the carrier, even if it means there is a significant probability of having homosexual offspring as long as they confer some other advantage.
Appendix 2
The evolution of the eye has been well explained. I won't go into the details here except to say that a useful introductory article can be found in wikipedia
Appendix 3
The best explanation I've heard for why peacock's beautiful tails have survived generations of ruthless evolution is that if a peahen were to have a preference for fit males with smaller tails, then her male offspring who have inherited the short tail from their father, would have trouble finding a mate. The cause of their problem would be that their mother had a opinion on which was the best mate that differed from the consensus. So, when choosing a mate, your children will thank you for going with the consensus.
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