Sexual selection is a mode of selection that operates on an organism's ability to obtain a mate. Sexual selection allows some individuals to out compete others because they are better at securing mates. Unlike natural selection, it does not take into account fitness. This functions as an interesting form of selection where changes in a population over time can both benefit and harm a species. For example, a male peacock's bright feathers might make it more attractive to a female, but may also be more attention-grabbing to potential predators. Here is a good article explaining sexual selection.
1) Intersexual Selection: Selection between the sexes.
Causes of Sexual Selection:
2) Intrasexual Selection: Selection within the sexes.
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Here is a link to a video of how Deer would use their antlers to fight. (even if they are in your yard)
Human Sexual Selection:This is an article that describes how sexual selection is present in humans and goes into depth about how some of our choices are due to psychological evolution over the years.
Traditional social sexual norms may have arisen from the biology of sexual selection. Because sperm production requires significantly less energy than egg production, females are the limiting factor in reproduction. Therefore, females tend to be more choosy with their partners, whereas males are more likely to try to have as many sexual partners as possible. This could be the basis for the common (but arguably outdated) sentiment that men will tend to sleep around more than women will, and that women have an easier time finding sexual partners.
This is an article explaining a recent study on human sexual selection with twins and they compared male and female desires in fraternal and identical twins .
This is an article about sexual selection in humans in a twin study of human mating preferences in men and women.
This article describes the effect of MHC's on sexual selection like we discussed in class.
In this article, the role of the parent-child relationship is examined in human sexual selection, as seen in the "bad boyfriend" struggle that often happens with a parent and child; interesting connection to evolution and how it relates to selecting a mate.
This is an interesting article about the evolution of the human Mating Practices and how it is very similar to our evolutionary ancestors.
Interesting article that says recently women have been programmed to prefer the Beta males vs. the Alpha male norm.
Great article about parental investment and how that relates to sexual selection.
There are also psychological issues in human sexual selection that conflict with the "norms" seen in other species for sexual selection. Here is a chart between the male and female psychological long-term mating strategies from a psychology class I took here at UGA.
Female long-term mating strategies include selecting a mate who is:
Here's a video of sexual conflict in water striders.
Asexual reproduction refers to a single parent that can produce offspring independently via parthenogenesis. There is no mating or mixing of genes. Essentially, offspring of asexual reproduction are clones of their parents. Another benefit to asexual reproduction is that there is less energy invested in finding males and mating.
What are the disadvantages to asexual reproduction?
Organisms of the genus Hydra are asexual animals. They reproduce by budding, which occurs when an offspring grows out of the body.
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Other examples of asexual reproduction include Gemmules or Internal Buds. This occurs when a parent releases a specialized mass of cells that develops into offspring. For example, Sponges exhibit gemmules reproduction. Organisms can also exhibit fragmentation, regeneration, and parthenogenesis. Parthenogenesis occurs when there is a development of an egg, but that egg has not been fertilized into an individual. This is seen in most kinds of wasps, bees, and ants that lack sex chromosomes.
Sexual Reproduction refers to how a female and male gamete fuse to mix genes and create an organism genetically unique from its parents. Independent assortment of the organism's chromosomes allow for more possible combinations as well as more adaptations to be passed on. The benefits of an organism using sexual reproduction allows for adaptability, variation, and loss of deleterious genes. There are two ways for fertilization to occur. The first is external fertilization where the egg is fertilized outside the body, and the second is internal fertilization where the eggs are fertilized within the female reproductive tract.
Sexual selection of the mate normally lies with the gender who provides more resources for the survival of the offspring.
Monogamy: describes the formation of a lasting male and female pair. Monogamy is more common in organisms that live in uniform environments, where resources such as territory and food are uniformly distributed. Because there is little difference between the quality of territories held by individual males, selection would favor monogamy because female fitness in that habitat would be the same. Most birds are classified as monogamous organisms. The book describes two kinds of monogamy - sexual monogamy and social monogamy. In sexual monogamy, the animal only reproduces with its bonded mate. In social monogamy, a pair bond is formed, but one or both of the animals involved sneak in extra matings. The book gives the superior fairy wren as an example of social monogamy.
Polygamy: describes the acquisition of two or more mates. If the habitat is diverse, with some parts more productive than others, competition may be intense and some males will settle for territories poorer in resources. A female may find it more advantageous to join another female in a territory of a male defending richer resources. Examples of polygamous animals include lions, hippos, and some species of monkey.
Polygyny: when one male mates with multiple females.
Polyandry: when one female mates with multiple males.
Mating Systems - Nature.com
Here's an interesting article discussing how sperm competition contributes to the formation of new species. It covers how sexual selection can contribute to the formation of sexual organs and processes.
When discussing sexual selection as a tenant of non-random mating and a subsequent force that facilitates the evolution of sex (mode and frequency) within a species, it is important to consider the role sexual conflict plays in the development of reproductive behavior, preference, and success. Sexual conflict occurs when males and females of a species have varying/conflicting morphological and/or behavioral modes of reaching their optimal reproductive fitness. These conflicts can be interlocal or intralocal depending on the location of selection. This means that conflicting alleles can occur at the same loci (intralocus) or at different loci (interlocus) between the sexes. These differences present major conflicts by limiting the reproductive fitness of one sex in favor of the other or by causing an antagonistic co-evolutionary “tug-of-war” where the behavioral or morphological mechanisms of one sex shifts in response to those of the other. In either case, evolution can and does create costly sexual conflicts between species driving the species to “ move, acclimate, adapt, or die.” Put simply, sexual conflict is driving several evolutionary mechanisms that can have effects on population size, allele frequency, and reproductive success—all of which further drive the evolution of species. This all means that conflict, on the reproductive level, plays a major role in how evolution works within species and how it can lead to several interesting changes to the mode and frequency of sexual reproduction. For example, I learned in entomology that dragonflies have a special penis shape that allows them to scoop out the semen of previous males from a female in order for their sperm to be the one to fertilize the female's egg. This is a very interesting evolutionary quirk.
What is also important to speciation is the ability for males to have reproductive success with a female. Research studies have shown the in flies, females take an active role in deciding who fathers the offspring. Female flies continuously mate with separate partners, but are capable of pushing the sperm from their reproductive track. This is considered to be "non-random mating" on a postcopulatory stand. This can lead to speciation by evolution. Understanding this sexual selection is difficult and still an ongoing study. Read more here!
Here is a journal explaining the sexual conflict theory.
Essentially the probability of a successful reproducing species is dependent on the amount of sexual conflict existing in a population. The lower the sexual conflict, the higher chances for reproduction and genetic recombination.
Types of sexual conflict:
1. Senescence (Aging) - process of the accumulation of biological and cellular changes that disrupts the metabolism. An example is Male Senescence in Darkling Beetles.
2. Traumatic Insemination (ex. Bedbugs) - male pierces the female's abdomen and injects sperm through the wound into her abdominal cavity.
3. Forced copulations (Sexual Coercion link) - forcing species of another sex to mate with one another.
Here is a link to a video of the Praying Mantis.
7. Nuptial gifts: males give females gifts of nutrients at mating to keep her strong after reproduction. This also factors into the praying mantis cannibalism. This has links to increasing female fitness.
8. Ducks in which males are reproductively over active and female uses ducts to select on certain sperm.
Rensch's Rule: allometric (the study of the relationship of body size to shape, anatomy, physiology and finally behavior) law that relates sexual size dimorphism to the bigger sex. It states that size dimorphism increases when the male is bigger, and vice versa when the female is bigger. This topic is well covered in this review you can find here.
Lek Mating and Paradox
Leks are aggregations of reproductive males competing for females and are defined by males displaying and females choosing, most commonly in bird species. Congregating about dominant males are sub-dominant males that may steal mating opportunities.
The premise behind Leks is that the female chooses traits of their preference. Over time, it is expected that males would exhibit less genetic variance, therefore removing the female's choice, yet the choice still exists. This genetic variation mystery is known as the "lek paradox". Female choosing should lead to directional selection -- "runaway selection" -- however, that does not appear to be the case.
Scientists think that there is still genetic variation in humans due to beneficial mutations. Beneficial mutations can make some males more resistant to disease, and males with a greater genetic diversity are rated as more attractive by women (remember the MHC study in class).
Male Senescence in Darkling Beetles:
Attached below is a handout I made for a sexual conflict course here at UGA. The handout outlines the effects of sperm senescence on reproduction rates and mate choice. The handout corresponds to the article "Male Reproductive Senescence As A Potential Source Of Sexual Conflict In A Beetle" written by Carazo et. al. The article looks specifically at the interesting conflicts that arise between the aggressive mating of older males and the declining quality of their sperm. The main argument suggests that age plays a major role in reproductive success/behavior and mate choice, creating a dynamic form of conflict between males and females. --Simply, male aging can and does affect female fitness.
Penis, Presentation, Predation, and Progeny: Sexual Conflict in Trinidadian Guppies
Here is another handout I made for the sexual conflict course I took here at UGA. The handout looks at the effect of sexual harassment on the penis size and color of male guppies and the increased predation risks female guppies encounter in order to avoid sexual harassment. The combined effects of these pre and post copulatory mechanisms have drastic effects on the overall health of the female's brood. It was shown, in the articles listed below, that females that experience high levels of sexual harassment have smaller and less attractive male offspring and that females will enter waters with more predators in order to escape these forced copulations.
Handout overview: Looking at both the pre and post-copulatory effects of sexual conflict in the Trinidadian Guppy (Poecilia Reticulata), sexual harassment of the female guppy has been show to have multiple co-evolutionary and behavioral affects. For simplicity, the affects of sexual harassment have been divided into four main categories-Penis, Presentation, Predation, and Progeny. When examining these four areas in tandem, conclusion can be drawn on the effects that sexual harassment is having on the mating patterns, brood size, allele distribution, and overall fitness of both male and female guppies. Simply, understanding the codependence of these categories begins to shed light on the process of sexual conflict from its origins to its cross-generational effects and begins to predict further morphological and social changes that result from this battle for mating dominance.
Disadvantages of Sexual Conflict
When it comes to sexual selection, it is a great way for females to see who is "best fit" to mate with. However, sexual selection--as shown in this article--can also prove to be dangerous for certain species. Yes, whatever it is that certain creatures do to attract the females, it also puts them at a greater risk for being predated. This is an example of how natural and sexual selection do not always select for the same traits and therefore the overall result is a balance of the two.
Videos, Diagrams, and Articles
Sexual conflict hast a vast arrray of displays. This is a page describing how male water striders use blackmailing techniques to intimidate the female into having sex if his advances are not wanted.