22
Note: This is based on literature searches I've done a while ago out of general curiosity. I'm in no way an expert on human reproduction.
First, I'm not sure if you are asking about evolutionary reasons or the developmental causes for for a difference in sex ratio. Here, I will focus on the developmental causes.
There is much evidence for a male bias in ...
16
Here are a few examples. All are open to debate as to whether it answers your question.
Zoophilia in humans
When asking of example of a "species" that does something, it is easy to forget about the diversity of behaviours among individuals of that species. It raises the question of how frequent must the behaviour be in that species to accept that the ...
12
There are plenty of animals with brief procreative periods, as you suggest. In chimps, sex takes 10 to 15 seconds - much less time than humans. So the question is really why have humans evolved to be different?
I don't think any of the answers based on fitness are particularly compelling. It's not like sex feels like running a marathon or less physically ...
11
I think I can expand on the answer by @boo2060.
The evolution of female mate choice depends on females achieving higher fitness by choosing certain males over others. At the broadest scale, there are two mechanisms by which this can occur, direct benefits, and indirect benefits.
Direct benefits
These are material things that (surprise) directly benefit the ...
9
What are the evolutionary explanations for why women are weaker than men (on average), and is this difference adaptive?
All of the theories surrounding this fact are speculative; it would be difficult to prove "why" men are, on average, stronger than women.
One contributing theory is Female-Choice - basically that women had (or has) the ability to be more ...
9
There is an article in the Journal of Popular Science from 1885. However, I do know there is at least one more recent article floating around some where since I read it. At the moment, I can't find it but will update if or when I do.
The article goes on to state that during times of scarcity the number of male births outweighs the number of females whereas ...
8
I'm going to give a tentative answer full of speculation and guesswork, but it's too long to fit in a comment so here goes.
Sex duration is possibly a sort of human reproductive handicap. Sexual arousal can soften the cervix, increasing fertility during lengthy sex sessions. Lengthy sex exposes them couple to predators and takes time and energy, so if you ...
8
There are numerous examples of visual attraction in animals. An absolute classic of an experiment, taught to most/all evolutionary biology students, is the widowbird tail length experiment by Andersson. He experimentally manipulated the tail lengths of male widowbirds at random. Some tails were made longer and some shorter. From this experiment Andersson ...
7
Many organisms have multiple sexes. For instance, here is one which apparently has 7 (and each sex, or "mating type" can only mate with the others). The "gotcha" is that, mostly, they're unicellular organisms where mating in itself is weird to begin with. I haven't ever heard of a metazoan which truly has more than one sex (there may be variation within the ...
7
What does "evolutionary advantageous" mean?
First note that is sexual selection evolutionarily advantageous? is a question that makes no sense. An allele or a trait can be evolutionarily advantageous if it increases the fitness of its carrier. If an allele, an evolutionary process or an environmental change decreases the risk of extinction for a species, ...
7
Selection in one sex can produce a correlated response in the other when there is pleiotropy (or linkage disequilibrium) (see Lynch & Walsh 1998, and Lande 1979 & 1980).
Genes can be considered pleiotropic when they affect more than one trait, for example; the male and female forms of equivalent traits, and this can be seen as covariance between the ...
7
It does appear that Wikipedia has not provided the most appropriate reference for that statement, but it is repeated in the article it refers to and that you link to, both in the abstract:
Male sacrifice may be adaptive because cannibalized males increase their paternity relative to those that are not eaten
and in the body of the introduction:
Males ...
7
The process is self-reinforcing but the argument is not circular (no tautology implied).
As soon as some male traits are considered more sexy than others, then there is selection for females to like those traits even more, which causes those traits to raise in frequency, which increases the selection for liking these traits. In other words, in this model, ...
5
There is an interesting theory coming from a slightly different, yet related, field. It was developed by Paul Vasey and Doug VanderLaan of the University of Lethbridge, they are both evolutionary psychologists:
An Adaptive Cognitive Dissociation Between Willingness to Help Kin and Nonkin in Samoan Fa’afafine (or direct link)
They called their theory the "...
5
Absolutely. Quality by appearance is sometimes a big part of mate selection and sometimes it is not.
The size and cognitive capacity brain is probably important but not always. Primates are closest to us and have most similar tastes to us, have varying levels of interest in mate appearance. Most primates have a troupe dominance where a dominant male ...
5
The answer in extreme brief is yes. Only about 9% of whites and 16% of blacks engage in interracial marriage in the U.S.
But really this deserves a fuller discussion.
The predominant pattern of mate selection in human beings is to marry within their ethnic group. I say 'ethnic group' rather than 'race' because 'race' has no strong scientific ...
4
It is advantageous to have a skew towards more males. Males can be expendable as they are only required briefly for impregnation and can then focus on providing resources for the mother and young and protection. This breaks down when this is no longer the case, for example in humans we are closer to a 50/50 divide.
This article notes that in aphids, there ...
4
I'm not at a biologist, but I have a pet theory I'd like to throw into the ring: hermaphroditism tends to be unstable in a similar way to strong sex skews. Imagine species A where 90% of offspring are female; at moderate population density, the 10% male population will be more than sufficient to maintain the females, and having 90% of the population able to ...
4
You're not wrong, per se, but in practice they refer to two different concepts. I honestly think the Wikipedia article does a good job, in particular this sentence:
In summary, while natural selection results from the struggle to survive, sexual selection emerges from the struggle to reproduce.
It also cites Darwin:
The sexual struggle is of two ...
4
This is directly following the advice of Lande & Arnold (1983), saying:
Linear multiple
regression can be used first to estimate the forces of directional selection, $\beta$, and their standard errors. Then a quadratic multiple regression (16) or (Al), can be used to estimate the forces of stabilizing selection, $\gamma$, with their standard errors....
4
I have some insight into this from an engineering perspective. Years ago, I worked on some computational and search algorithms which used a growth factor to determine how far to widen a search space, or how much more memory to allocate. For some of these algorithms, I ran a series of random trials to search for an optimal growth factor and found that for ...
4
[C]an drift overcome sexual selection in the evolution of sexually dimorphic traits?
Short answer
Yes
Long answer
There are many ways to consider the effect of genetic drift. Here is one: One can approximate the probability of fixation of new mutations (using diffusion equations) with
$$P_{fix}≈\frac{1-e^{-4Ns} }{1-e^{-4s}}$$
, where $s$ is the ...
4
My favourite definition
Selection is a fitness differential associated to a genetic variance among individuals in a population.
You might want to have a look at What does fitness really mean?.
What I like about this definition
I personally went with this definition because it is short and clear cut and directly highlight the importance of mathematical ...
4
Probably not! There are certainly some physical reasons to doubt it. Looks like the mass difference between X and Y sperm is on the order of 0.1 picograms: X chromosome "weight"?
First, mass differences are not likely to affect swimming and motility of sperm, because sperm are at low Reynolds number -- the forces of fluid drag vastly overwhelm ...
4
Taken by itself I can agree that it might seem circular. However, in practice, all traits act and occur in a pre-existing background, i.e. in organisms with their genetic background and their living environment. All that is needed is a preexisting difference in fitness (for instance connected to the related good-genes hypothesis), that a new gene for female ...
4
Short answer, we don't know.
There are several competing hypotheses but they are all nearly impossible to test. Behavior often suffers the same problem as testing, we can come up with a hypothetical reason but since we are the only organism that does it, we have no good testing options. We can't put people in controlled conditions for generations and see if ...
4
Short answer
Survival does not get your genes into the gene pool, reproduction does.
thus reproduction based selective pressures can be stronger than survival based ones. there are plenty of organisms that die during or right after mating because of this.
Consider an example In species A finding a mate is fairly rare, it may only happen a few times in an ...
3
I'm not sure what you mean by "gender" in the context of non-human organisms. Gender is usually defined as the intersection between biologically determined sex, and the surrounding culture.
Maybe you could argue that in social insects like, say, ants, the fertile queen and the infertile female workers diverge so much in behavior and gene expression that ...
3
Yes, it is called Alternative Mating Strategies and is well known. As an example High-backed pygmy swordtail (a fish) are known for having two types of males, one that is large and aggressive and another that is small and looks like a female to avoid aggression from other males.
3
The closest thing you will find to what you're looking for, I think, are animals with multiple different mating strategies where one sex (usually the male) can have multiple distinct phenotypes. Some examples include:
Patas monkeys, where some males are large and have harems of females while others are dubbed "sneak maters" and instead of having their own ...
Only top voted, non community-wiki answers of a minimum length are eligible