Why does sexual selection evolve beautiful features?

Question

I have a very raw understanding of sexual selection:

Say a group of females of a certain species "like" some feature of a certain groups of males; by "like" I mean some behaviour that makes this group of females tend to mate with males having this feature, then the females are selecting their genes which make them feel appealed to males with the feature, and then this process becomes exponent since the group of females becomes bigger and forces the population of males to have this feauture, until an equilibrium is reached. I know this also happens with the roles interchanged.

My first question is, why do females tend to like males that have high amounts of these features, more than the average in the population?

My second question is, why do the end products of these selections are such beautiful features? Perhaps this may sound a little arrogant but I don't understand how an animal can appreciate this beauty, I suppose this is to make some sort of selection of healthy males.

Thanks

Answer 1

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 female. This might be the provision of food by the preferred male, access to a good territory or shelter that is under the control of the preferred male, protection by the preferred male, that sort of thing. Females will accordingly favour male traits that advertise potential direct benefits. These might include male size/condition/colouration, or even built structures like those of the bowerbirds. Basically, these traits show that the male knows how to look after himself, and by extension, he knows how to look after her (as males in good condition generally inhabit the best habitats and will offer the best protection and parenting).

Indirect benefits These don't do much for the female, but they increase the genetic quality (and thus potential fitness) of her offspring. This is a much more complicated area of mate choice, and can be split into a bunch of (non-exclusive) hypotheses (below). And of course, attractive males may well offer both direct and indirect benefits.

Sensory bias: Males may, by chance, possess a trait that holds intrinsic appeal for a female. The best explanation might be that it mimics the colour or shape of a food source (e.g. fruit) that the animal eats, and thus the animal has pre-evolved preference for that colour or shape (e.g. red patches). From that point, males with more of that trait, whatever it is, will tend to be more attractive, for no rational reason. This hypothesis is sound, but is currently lacking good empirical evidence.

Fisherian sexy sons: This can in fact lead on from an initial sensory bias. Females may choose males that they find 'sexy', because their offspring will also be sexy, and thus be successfull in reproduction. This can lead to 'runaway' selection for sexier and sexier sons, at considerable survival cost for males. In this case, natural selection and sexual selection work in opposing directions.

Indicator mechanisms: This is the handicap principle or 'good genes' principle. If a structure is costly to produce or maintain, or increases the risk of predation, and yet the male possessing it is still alive, he must have good genes to have withstood such a handicap. It may also advertise that the male is not infested with parasites. The extravagant plumes of birds may be largely explained by this one, but also sexy sons and sensory bias.

Genetic compatibility: Individuals seek partners that are genetically compatible. A certain proportion of the fitness of the offspring depends on the specific combination of genes (i.e. it's not additive), and individuals that can signal or detect signals in such a way that they can choose compatible partners will have an evolutionary advantage. The best example is scent (e.g. preference for dissimilar MHC immune genes - see the human t-shirt experiment).

These mechanisms are well known in theory, but not always well-supported by evidence (yet). This is probably the best reference.

Answer 2

I'm not completely certain about the vocabulary used in this context so let me know if you wish clarification

There are a few hypothesis why females care about the sexual features of males, the two most prominent are:

• The sexy son hypothesis states that a sexually attractive male will have sexually attractive offspring which, in turn, is also more likely to mate. This way a female choses to maximize the chance to pass on it's genes to next generations.
• The good genes hypothesis says that males who can afford to develop features that do not benefit their survival but are only for the purpose of sexual selection are healthy, strong and fit. If not, they would not spend resources on sexual attractiveness.

But why are all these features beautiful? Well, the purpose here is that they are beautiful, in terms of attractive, to the recipient of the message: the potential partner. We as humans may find these features also beautiful, as is most likely the case with the colourful feathers of the peafowl. But what about the antlers of deer? Beautiful, dangerous? This is subjective for us ..