# How do harsh conditions cause an increase in the male birth ratio?

I've heard that in harsh conditions more human baby boys are born than girls because a bigger ratio of them will die before reproductive age. But in which aspects of the random gene combination while conception can harsh conditions (and which of their characteristics) actually can make a difference?

• edited - gender is a social construct
– AliceD
May 12 '16 at 10:55
• @Christiaan Yeah but now it looks like I'm talking about sexual selection. May 12 '16 at 11:00
• Your last sentence doesn't make sense. Maybe improve the question somewhat.
– AliceD
May 12 '16 at 11:06
• Don't understand the question and video at all. In this video host doesn't give us any proofs that's difference (where is the source) is significant assuming we have $p_{xy} = \frac{1}{2}$. May 12 '16 at 11:31
• "Why is there a male-biased sex-ratio at birth in harsh conditions?" would be a better way to express this... As someone studying the effects of environmental conditions on the evolution of lifespan in humans, I'm going to say the video wasn't great. May 12 '16 at 11:37

Issues with the post

Sexual selection is a mode of natural selection where members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (intrasexual selection). This question has nothing to do with sexual selection.

What do you mean by random gene combination? More globally I don't understand the last sentence.

As a consequence, I don't understand the question. IMO, the post should be closed as unclear. However, I can imagine a question that I would consider interesting. I am answering to the question Is it possible that selection yield to a sex-ratio that deviates from 1:1 at equilibrium? How does it relate to environmental conditions?.

Fisherian scenario

Evolution of sex-ratio under a standard Fisherian scneario is often refered to as Fisher's Principle.

Basics

In the vast majority of strictly sexual species, every individual has exactly one mother and one father. It means that at equal sex-ratio the average relatedness of all offspring and all males in the previous generation is equal to the average relatedness of all offsprings and all females int he previous generation. When the sex-ratio is biased toward more females (for example), males have on average a higher reproductive success than females. As a consequence individuals that produce more males will increase their fitness and will increase in fecundity. As a consequence, the adult sex-ratio will always go back to 1:1 (fifty-fifty).

Now, it is important to understand that the adult sex-ratio is not necessarily the same as the babies sex-ratio. If typically, there is higher mortality in males than in females (could happen depending on the species and the environment), then selection would favour a bias sex-ratio toward males in babies. The adult sex-ratio will in any case be 1:1

Kin/group selection (non-fisherian scenario)

See for example the case of social spider Aviles (1993).

What is good for the population?

Thinking about the same problem but thinking about what is good for the population (rather than for the individual). For the population, the limiting factor in the number of babies produced are often the females. As a consequence, it would be good for the population if the sex-ratio were biased toward females.

When can alleles that are beneficial for the population overweight what is beneficial for the individuals

Under a standard fisherian scenario this cannot happen though selection will increase the frequency of alleles that spread the best and not the ones that are good for the population. However, typically when the relatedness within the population is high enough (you should read about group selection and kin selection) such trait that are good for the population may evolve.

Recommendation

I recommend that you take some time an introductory class to evolutionary biology such as Understanding Evolution for example. It is fast and you will learn more and faster than what you can learn here with your question. I think I already gave you this suggestion a number of times.

• Oh, sorry, I let myself the second part on later and now I can see there's not much of an example of an actual non-fisherian scenario. Is there any actually found in nature? From the video I got that there is because otherwise it wouldn't be about the ratio of born males - which is the same for the fisherian scenario. May 16 '16 at 6:57
• Yes, some social spider for example have a biased sex-ratio toward females Aviles (1993). May 16 '16 at 14:17
• I'm grateful for that the answer contains the reason why these traits should evolve, but I didn't catch mutch about the true mechanism an organism can affect which sex will the fetus be when the gamets fuse. May 19 '16 at 8:45
• @Probably The mechanism is totally different discussion and is out-of-the-scope of this post. The above answer assumes that mutations can change the offspring sex-ratio of an individual to any value between [0-1]. Before understanding this mechanism you will need to understand the sex-determination systems. May 19 '16 at 14:51
• Typically for an ESD (Env. Sex Setermination), the mother can affect the sex ratio of the, say eggs by modifying the temperature of incubation (which can be modified by the characteristic of the nest or how deep they are burrowed). But manipulation of sex-ratio in pure GSD (Genetic Sex Determination) exist too such as killing the offpring very early depending on the sex or by making the sex determination under control of several loci you can reach all kind of sex-ratio in offspring. May 19 '16 at 14:52