* Direct benefits -the male's trait provides a benefit in terms of female's fecundity (offspring produced) or her survival
* Direct Benefits include:
- Nupitial Gifts
- Nest sites/provisioning
- Protection from predators
* Choosy individuals receive direct benefits from their mates and this results in immediate increased fecundity, or the number of offspring produced. If the competitive sex displays an ornamental trait that reliably indicates some direct benefit then strong selection will favor mating bias. Having a mating preference is advantageous in this situation because it directly affects reproductive fitness. Direct benefits are widespread and the evidence for this mechanism of evolution is well supported in empirical studies.
* One example of a sexually selected trait with direct benefits is the bright plumage of the northern cardinal, a common backyard bird in the eastern United States. Male northern cardinals have conspicuous red feathers while the females are more cryptic in coloration. In this example, the females are the choosy sex and will use male plumage brightness as a signal when picking a mate because males with brighter plumage have been shown to feed their young more frequently than males with duller plumage. This increased help caring for the young lifts some of the burden from the mother so that she can raise more offspring than she could without help.
* In the great reed warbler, females tend to be attracted to males with longer song repertoires since they tend to sire offspring with improved viability. In doing so, they gain indirect benefits for their own young. In the Utetheisa ornatrix, females select males based on body size, systemic content of pyrrolizidine alkaloid, and glandular content of hydroxydanaidal. As a result, these females demonstrate direct and indirect phenotypic benefits: they have offspring that are less vulnerable to predation because of their increased size and higher alkaloid content, increasing viability and fitness.
•Sensory bias (or pre-existing bias or sensory exploitation)
-Females have a pre-existing preference for some stimulus
•For example, suppose females are attracted to orange because their food is orange
-Male traits that are similar to the stimulus attract females
•Males with orange markings will attract females' attention
* The sensory bias hypothesis states that the preference for a trait evolves in a non-mating context and is then exploited by one sex in order to obtain more mating opportunities. The competitive sex evolves traits that exploit a pre-existing bias that the choosy sex already possesses. This mechanism is thought to explain remarkable trait differences in closely related species because it produces a divergence in signaling systems which leads to reproductive isolation.
Sensory bias has been demonstrated in guppies, freshwater fish from Trinidad and Tobago. In this mating system, female guppies prefer to mate with males with more orange body coloration. However, outside of a mating context, both sexes prefer animate orange objects which suggests that preference originally evolved in another context, like foraging. Orange fruits are a rare treat that fall into streams where the guppies live. The ability to find these fruits quickly is an adaptive quality that has evolved outside of a mating context. Sometime after the affinity for orange objects arose, male guppies exploited this preference by incorporating large orange spots to attract females.
Another example of sensory exploitation is in the water mite Neumania papillator, an ambush predator which hunts copepods (small crustaceans) passing by in the water column. When hunting, N. papillator adopts a characteristic stance termed the 'net stance' - their first four legs are held out into the water column, with their four hind legs resting on aquatic vegetation; this allows them to detect vibrational stimuli produced by swimming prey and use this to orient towards and clutch at prey. During courtship, males actively search for females - if a male finds a female, he slowly circles around the female whilst trembling his first and second leg near her. Male leg trembling causes females (who were in the 'net stance') to orient towards often clutch the male. This did not damage the male or deter further courtship; the male then deposited spermatophores and began to vigorously fan and jerk his fourth pair of legs over the spermatophore, generating a current of water that passed over the spermatophores and towards the female. Sperm packet uptake by the female would sometimes follow. Heather Proctor hypothesised that the vibrations trembling male legs made were done to mimic the vibrations that females detect from swimming prey - this would trigger the female prey-detection responses causing females to orient and then clutch at males, mediating courtship. If this was true and males were exploiting female predation responses, then hungry females should be more receptive to male trembling - Proctor found that unfed captive females did orient and clutch at males significantly more than fed captive females did, consistent with the sensory exploitation hypothesis.
Other examples for the sensory bias mechanism include traits in auklets, wolf spiders, and manakins. Further experimental work is required to reach a fuller understanding of the prevalence and mechanisms of sensory bias.
* Another example is plant self-incompatibility alleles. When two plants share the same incompatibility allele, they are unable to mate. Thus, a plant with a new (and therefore, rare) allele has more success at mating, and its allele spreads quickly through the population.
* In human pathogens, such as the flu virus, once a particular strain has become common, most individuals have developed an immune response to that strain. But a rare, novel strain of the flu virus is able to spread quickly to almost any individual, causing continual evolution of viral strains.
*A classic example is the Hawk-Dove model of interactions among individuals in a population. In a population with two traits A and B, being one form is better when most members are the other form. As another example, male common side-blotched lizards have three morphs, which either defend large territories and maintain large harems of females, defend smaller territories and keep one female, or mimic females in order to sneak matings from the other two morphs. These three morphs participate in a rock paper scissors sort of interaction such that no one morph completely outcompetes the other two. Another example occurs in the scaly-breasted munia, where certain individuals become scroungers and others become producers.
* Positive selection can be seen in the evolution of warning coloration (aposematism) in toxic or distasteful organisms. Signalling theory proposes that the advantage of such coloration is that predators can learn to avoid potential prey with that coloration. For example, in the Batesian mimicry complex between a harmless mimic, the scarlet kingsnake (Lampropeltis elapsoides), and the model, the eastern coral snake (Micrurus fulvius), in locations where the model and mimic were in deep sympatry, the phenotype of the scarlet kingsnake was quite variable due to relaxed selection. But where the pattern was rare, the predator population was not 'educated', so the pattern brought no benefit. The scarlet kingsnake was much less variable on the allopatry/sympatry border of the model and mimic, most probably due to increased selection since the eastern coral snake is rare, but present, on this border. Therefore, the coloration is only advantageous once it has become common. 6th EditionDavid L Nelson, Michael M. Cox 2nd EditionDavid E. Sadava, David M. Hillis, H. Craig Heller Joseph S. Levine, Kenneth R. Miller 11th EditionLisa A. Urry, Michael L. Cain, Peter V Minorsky, Steven A. Wasserman