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TL;DR:
There is a dearth of actual experimental evidence. However:
there is at least one study that confirmed the process ([STUDY #7] - Myxococcus xanthus; by Fiegna and Velicer, 2003).
Another study experimentally confirmed higher extinction risk as well ([STUDY #8] - Paul F. Doherty's study of dimorphic bird species an [STUDY #9] - Denson K. McLain).
...
39
Very intresting question. The problem is that animal intelligence is hard to measure not only for scientists, but probably also for the potential mate. Paradoxically, that is why selection for intelligence, if it occurred, may be very strong. One has to be smart in order to recognise smart behaviour, so preference and preferred feature are strongly connected....
29
Several years back, results of a twin study (Zietsch, et al., 2008; popular press in The Economist) suggested that genes associated with homosexuality make heterosexuals who carry the same genes more reproductively successful. This would explain the observation that, for example, sisters of homosexual males have more offspring.
We show that ...
23
Firstly I'll clarify that you are talking about simultaneous hermaphrodites rather than sequential hermaphrodites (1st one sex, then the other e.g. the limpet Patella vulgata).
It is perhaps easiest to address the question by countering it and asking why dioecy (2 sex systems/2 gonochoric types e.g. male and female) is better? As you have pointed out there ...
21
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 ...
18
Obviously selection would appear to not favour being homosexual, in an evolutionary sense it represents somewhat of a decrease in fitness: Homosexuals fail to reproduce successfully due to the requirement of both male and female gametes and reproductive organs, therefore significantly fewer than the average heterosexual couple. Certainly I don't think it ...
17
The male kakapo (Strigops habroptila) in that video is called Sirocco. Kakapo were (and still are) very close to extinction, so in the 1980s the Kakapo Recovery Programme was launched. As part of this programme, rangers monitor all known kakapo in the wild, visiting their nests and generally ensuring they are in good health. When Sirocco was a young chick, a ...
17
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 ...
10
First of all for the first 22 chromosomes (and the mitochondrial chromsome) are the same between women and men. The X and the Y together are something like 1.5% of the total DNA in the human cell. The X chromosome is also in common, so it has the same chromosomal (genetic) variations the same as women and men.
While I am not sure about the differences you ...
10
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
This is not a subject I know well, but I can point to several recent textbooks on homosexuality in animals, all with an evolutionary perspective. I remember that Bagemihl's book got good reviews when it was published. In general, I think it is important to acknowledge that homosexuality is common in many animal species and not just humans:
Bagemihl. 2000. ...
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
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 ...
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 ...
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, ...
6
At least for physical data (heigth, weight etc) you can have a look of the DINED Anthropometric database http://dined.io.tudelft.nl/dined/. Here you can find mean and standard deviation data for Dutch population studies on numerous anthropometric measures, and you can stratify the results by sex.
6
The biggest problem with this question, (not just here, but for everyone who asks it,) is drawing on our culturally constructed definition of maleness to look for mechanisms of genetic inheritance of what are considered "male" traits. We could measure frequency distributions of height, but we don't have a way to accurately probe any of the other qualities ...
5
I know nothing about biology however I did watch an amazing PBS documentary on cuttlefish that I think is fairly relevent.
From http://www.pbs.org/wgbh/nova/nature/kings-of-camouflage.html
NARRATOR: During mating, males outnumber the females, sometimes 10 to
one. And they're all looking for the chance to pass on their genes.
While a female lays eggs ...
5
Relating to your last comment on random fluctuations in survival, a recent theoretical paper by Lee et al. 2011 studies the effect of mating systems on demographic stochasticity in small population. No empirical data there though. Their main conclusion is that polygyny (in relation with sex ratio) can lead to high demographic variance, therefore lowering ...
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
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
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 "...
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