The selective pressure of contraceptives

Question

Is it possible that the use of contraceptives will produce evolution humans? Contraceptives can reduce reproductive success, so shouldn't selection favour resistance to them?

In this case it might be useful to think of evolution as a "black box", an information processing mechanism where only input and output is relevant. From this point of view, we would expect the box to produce an output comparable to the resistance that evolution will produce in a population experiencing a poisoning effect which seriously damages its reproduction mechanism.

The evolutionary "output" from such poisoning has been studied in great detail. Are there any lessons relevant to future human evolution in these studies?

Answer 1

Interesting question, but the answer is simpler than you might think.

Antibiotics put selective pressure on bacteria. Bacteria resistant to that antibiotic evolve as only the resistant ones survive. But this is the key, there has to be at least some already resistant bacteria from which the new (resistant) population will emerge.

With contraception, this is not the case. There are no spermatozoids capable of penetrating condoms, for example, and you should also bear in mind that in most cases mild hormonal disbalance is enough to prevent conception. It's a very delicate process, and contraception is basically an overkill.

Even if some spermatozoids could penetrate condoms, that trait would have to be hereditary in order for evolution to occur.

If you'd want to prove that contraception constitutes selective pressure on humans you'd first have to prove that there are spermatozoids and egg cells that can beat contraception, and that it's hereditary.

Evolution doesn't work the way you think it does. If you kill a random person, even if you kill a million people, it's not necessarily selective pressure. There are rules to evolution, and one of them is that it has to be hereditary.

This is not.

It wouldn't even come close to being artificial selection, let alone natural selection.

Besides, there's 7.3 billion of us. More than ever. We can hardly say contraception is reducing the world's population.

You also fail to see one thing, that a person who uses contraception in teenage years still has kids as a young adult. Contraception just delays pregnancy until you're ready. In evolution, there's a concept of "surviving long enough to reproduce". You can both use contraception and be an organism with 10 kids - though not at the same time.

Answer 2

Interesting question. I did not fully understand the entirety of your post so I will answer the question posed in the first sentence, namely:

Does it seem probable that the existence of contraceptives (which has reduced human reproduction to below survival-level in many populations) will be a completely new selective pressure on human evolution?

Yes, the use of contraceptives is a selective pressure. The couples that don't reproduce simply don't pass on their genes so evolution "is favouring" those couples that don't use contraception. This is a bit tricky because if a couple uses contraception for life and doesn't reproduce at all, they are evolutionary dead ends and selective pressure can't act on them because there isn't anything passed to the next generation. For the couples that use contraception for longer or more often, leading to delayed childbirth or less children born, then the parents would have contributed less genes to the next-generation's gene pool so they may be wiped out more easily. Thus contraception reduces the fitness of humans using it if used often enough to keep the populations using it below replacement levels.

Answer 3

If contraception helps us to achieve the proximal rewards of sex without reaching one of the ultimate goals (reproductive success), you could say that the availability of contraceptives inhibits reproductive output. But, I don't think that it's as simple as that. The availability of contraception merely means that the desire to have sex no longer automatically leads to the conception of children. Instead, a wish to have children often first needs to get into play. Thus, you would expect a strong fitness reward for individuals who are capable of effectively planning for children and not postponing it indefinitely into the future until they are no longer fertile.

As mentioned in previous answers, you won't get too see condom-tearing supersperm any time soon, but I dare predict that we should expect to see an increase in condom-tearing behavior over the next century or so. Currently, I think there's definitely a shift in selective pressures on behavioral genes related to reproduction.

A caveat is of course that changing behavioral output could also be induces by cultural evolution. Cultures may instill taboos on contraceptives (think of the Catholic Church's attitude towards condoms), or cultures may facilitate child care to stimulate child-bearing at a reproductively healthy age.

An altogether other possibility is that the average age at which the decline of fertility sets in will significantly increase.

Answer 4

In principle, contraceptives are a barrier to reproduction, and selection will approximately operate by maximising reproductive. Therefore, one would expect that selection would favour "resistance" to contraceptive methods. However, there is a big difference between selection and adaptation (evolution by selection).

An adaptive response can only evolve if there is heritable variation for the trait, in this case, resistance to contraceptives. For example, imagine we had a pill taken by men which disrupts sperm production as our only contraceptive method. This pill is not 100% successful, as some men still produce viable sperm because they have a gene/allele which prevents the pill from damaging sperm. Given enough time (and assuming no other contraceptive/abortive measures can be taken), then this gene should spread through the population, because carriers will produce more offspring (who in turn are likely to carry the gene).

This hypothetical scenario is similar to that of the evolution of antibiotic resistance in bacteria. We use antibiotics to kill populations of bacteria, but some members of the population have genes that protect them from the effects. The carriers of the resistance genes will then produce the next generation, all of whom carry the resistance gene.

Similarly, you can think of populations where toxic chemicals are more present. In some populations, arsenic is present at high levels. Arsenic is toxic and will decrease reproductive success (because there is a higher tendency die before proper opportunity to reproduce), so consider the environmental arsenic as our "contraceptive" in this scenario. Genetic variance in the ability to metabolise arsenic arose (e.g. by mutation) and spread through a population because of the advantages it gave.

In reality, there is little-to-no genetic resistance to plethora of contraceptive methods we can deploy - the failure of contraceptives is random not genetic. Therefore, we are not going to evolve resistance to contraceptives any time soon.

Furthermore, contraceptives may reduce the rate at which sex leads to reproduction, but it may allow parents to choose when to produce offspring and how many to produce more easily. As such, the children might be provided with better developmental environments or have a lower rate of infant mortality etc. so fitness could be higher in this group compared to those who are resistant to contraception. If so, this could select against contraceptive resistance. It's not reproductive success that matters, but the tricky concept of fitness.

Answer 5

Do you mean that (A) the choice of using contraception is part of the human phenotype and we can study the selection pressures that apply to it or that (B) you consider the absorption of a contraceptive as an external selection pressure and are wondering what evolutionary answer this selection pressure will cause?

If (A): One can only study the evolutionary pressures applying on the use of contraception under the following conditions:

• The choice of using contraceptive is inheritable
• People who do use contraceptive have significantly less (or more) kids over their entire life than people who do not

I am not sure any of these conditions is fullfilled.

If (B): If it was as simple as a "poisoning effect which seriously damages its reproduction mechanism", you could expect the emergence of a resistance allele (for example that would make females insensitive to oestrogens in oral contraceptives).

However I guess that persons who use contraception are likely to choose abortion if pregnant, decreasing the fitness effect of such a potential resistance allele.