Other Types Of Natural Selection
Natural selection can come in an infinite variety of forms. Every organism will be more or less successful depending on what genes it carries and how those genes interact with the environment. Genes can cause novel ways of processing nutrients, allow for different structures to be formed, and allow for old structures to be repurposed. Completely different organisms occupying the same niche are often found to have similar structures. These structures were not obtained from a common ancestor but from the forces of natural selection alone. Natural selection is the primary driving force behind all the different forms and functions of life on Earth.
What Is The Relationship Between Sexual And Natural Selection
Thus, it seems that the diverse coloration in this species is promoted by both natural and sexual selection.
I had always thought of sexual selection as a subset of natural selection. What’s the right way to think about the relationship between the two?
- 2$\begingroup$sexual selection is a set of special cases within natural selection. clearly any living thing which does not have gender will not undergo sexual selection.$\endgroup$
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 kinds in the one it is between individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners.
The whole argument is moot when dealing with individuals that aren’t reproducing sexually, of course.
Some quotes from the abstract of the article “The Logic of Analog Signaling and the Theory of Signal Selection” by Amotz Zahavi & Avishag Zahavi:
Which Sex Is Under Stronger Selection
Sex roles are defined by differences in gametes: females produce relatively few, highly nutritious gametes, whereas males produce comparatively abundant, smaller, motile gametes. Because only a single gamete of each type is required to produce an offspring, there will be an excess of male gametes that will not fertilize any eggs. This asymmetry leads to Bateman’s principle, whereby female reproduction is primarily limited by their access to resources to nourish and produce these large gametes, whereas male reproduction is mainly limited by access to females . Therefore males typically compete among themselves for access to females, whereas females tend to be choosy and mate only with preferred males.In sexually reproducing species, every offspring has one father and one mother, so the average reproductive success is equal for both males and females. A successful male can potentially sire many offspring. If a male gains a disproportionate share of reproduction, he will take away reproductive opportunities from other males, leading to a high reproductive variance among males. A successful female, on the other hand, will not take away reproductive opportunities from other females, leading to a smaller variance in reproductive success. The higher the reproductive variance, the stronger the effects of sexual selection . Strong sexual selection typically results in sexually dimorphic traits that are exaggerated, or more elaborate, in the sex with highest reproductive variance .
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Mate Choice: The Chosen Sex
Mate choice means, by definition, that there is nonrandom fertilization success with respect to one or more phenotypes amongst members of the chosen sex, and so there is sexual selection acting on that sex. All members of the choosy sex may prefer the same few individuals, or indeed the same single individual, which will dictate how strong sexual selection on the chosen sex is. Alternatively, the choosy sex may differ amongst themselves in their preferences. This could of course lead to complex patterns of sexually selected fitness amongst the chosen sex. Nonetheless, this situation still seems relatively straightforward.
Crucially, what this means is that resources can have both primary and secondary sexual function, as with genitalia. An animal needs resources to survive and produce offspring in the first place, as they also need genitalia, but this is natural selection on primary sexual function, the selection that means an animal is able to engage in competition for gametes. If resources or genitalia then differentially influence access to gametes, they gain secondary sexual function and come under sexual selection. And there is now abundant evidence that sexual selection has driven divergence in animal genitalia, including in terms of rapid evolutionary change, in both males and females .
Next, we will consider the choosy sex, and how choice itself arises.
Strategic Signalling By Females
Theory predicts that females adjust the quantity of pheromone used each day by balancing the risk of not attracting a male against age-specific mortality, with females commencing signalling at a low rate, and unmated females increasing that effort as they age . The results of the field experiments conducted by Johnson et al. suggest that female signalling effort is directed more to attracting particular kinds rather than numbers of males. Thus, females can exercise mate choice by signalling not through some form of sex-role reversal, but rather through the action of sexual selection on the morphology of male organs of sense.
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Darwin’s Model Of Sexual Selection
Darwin’s model, which later came to be known as the DarwinFisher model , has been invoked as a potential explanation for sexual selection in monogamous taxa. In Darwin’s version of the model , he assumes a population with 2 types of females: the more vigorous and better-nourished individuals and the less vigorous and healthy . Males also are variable in quality, and they arrive before females on the breeding grounds . The higher-quality females are ready to breed before the lower-quality females, and they choose to pair with the higher-quality males . Thus, the model predicts assortative mating by quality , and Darwin argues that the higher-quality pairs will produce more offspring than the lower-quality pairs. In Darwin’s discussion of this model, we find the only occurrence of a clear explanation for the evolution of mate choice in his entire book :
Such pairs would have an advantage in rearing offspring, more especially if the male had the power to defend the female during the pairing-season, as occurs with some of the higher animals, or aided in providing for the young.
The Male Accessory Reproductive Glands And Ducts
The accessory reproductive glands, and principally the prostate gland and seminal vesicles, secrete the major fluid component of the semen, in which spermatozoa are suspended at ejaculation. The primates, in common with all eutherian and metatherian mammals, possess a prostate gland, but seminal vesicles are absent in marsupials, whilst among eutherians they vary tremendously in size or are lacking in some cases. There is now considerable evidence that these variations result, in part, from postcopulatory sexual selection . Thus, in primates that have polygynandrous mating systems, such as macaques, most baboons, mandrills and chimpanzees, the seminal vesicles are very large, whereas in monogamous and polygynous taxa they are significantly smaller .
Seminal vesicle sizes and seminal coagulation ratings for primate genera in which females mate primarily with single partners, or with multiple partners, during the fertile period. Data are ratings on a 4-point scale for seminal vesicle size and seminal coagulation . *** p< 0.002 .
Vas deferens length and thickness in mammals where females have single-partner versus multiple-partner mating systems. * p< 0.05, ** p< 0.01 .
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Models Of Indirect Benefits
In some species, the males appear to provide nothing to the females but sperm, yet they have elaborate ornaments for which females show preferences . These systems are especially perplexing from a sexual selection standpoint because the benefits of choice are not at all obvious. Consequently, a tremendous amount of effort has been devoted to creating explanatory models of female-choice evolution in such systems . These types of models have commonly been described as indirect-benefits models because the female’s choice of males provides her with no immediate, measurable benefits. Rather, the female’s fitness increases as a consequence of her offspring having higher fitness if she pairs with a preferred male. Several excellent reviews have addressed the dizzying array of such models , so rather than review them, we attempt here to organize them into a few major categories. We would like to suggest that there are 3 main categories of indirect-benefits models. We have no doubt that such a categorization will be controversial. Indeed, we are at odds with at least one other perspective that suggests that all indirect-benefits models should be lumped into a single category of FisherZahavi models . We believe there is much heuristic value in keeping them separate, so to stimulate discussion we propose the following 3 classes of models.
A Definition Of Sexual Selection
After what we see as a continuity from the words and works of Darwin, as captured by and
, it was this second component that was elusive to Darwin. However, the traditional focus on access to mates as the keystone to sexual selection is perhaps because humans are not broadcast spawners. We will return to the case when competition for mates may not yield sexual selection below.
Importantly, gametes as a resource can vary in two ways, and thus be limiting in two ways. They can vary in quantity, and they can vary in quality. Competition can therefore arise for access to gametes in terms of getting access to the most gametes, but also in terms of access to gametes of the highest quality. We will revisit the importance of competition in terms of quality below .
Second, we are agnostic as to the sexual identity of the competitors . This means that we are also agnostic as to whether or not sexual selection happens in both sexes/mating types at the same time all options are possible. However, implicit in our definition is that individuals of the same sex, same mating type, or tissues with the same sexual function , compete for the gametes of the opposite sex or complementary mating type. In other words, sexual selection arises from competition within a sex or mating type.
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The Second Major Triumph
In our view, another major triumph in sexual selection research came from advances in quantitative genetics and formal selection theory, which resulted in quantitative techniques for the measurement of selection in natural populations . These approaches lead to the stark realization that what we are really talking about are selection coefficients on sexually selected phenotypic traits, which leads to the seemingly simple question of what determines the magnitude of a selection coefficient in a natural population.
Effect On Female Fitness
Male-male competition can both positively and negatively affect female fitness. When there is a high density of males in a population and a large number of males attempting to mate with the female, she is more likely to resist mating attempts, resulting in lower fertilization rates. High levels of male-male competition can also result in a reduction in female investment in mating. Many forms of competition can also cause significant distress for the female negatively impacting her ability to reproduce. An increase in male-male competition can affect a female’s ability to select the best mates, and therefore decrease the likelihood of successful reproduction.
However, group mating in Japanese medaka has been shown to positively affect the fitness of females due to an increase in genetic variation, a higher likelihood of paternal care and a higher likelihood of successful fertilization.Exposure to environmental estrogens, such as some herbicides, can confuse female choice of males.
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How Does Sexual Selection Work
Sexual selection, the process through which individuals compete for mates, primarily takes two forms: intersexual selection and intrasexual selection. Intersexual selection, often referred to as mate choice, involves individuals of one sex choosing among members of the opposite sex based on the attractiveness of certain traits that those individuals possess. Intrasexual selection, also called mate competition, involves one sex competing with members of the same sex for access to mates.
Typically, the sex that is choosing mates is the one that invests more in gamete production prior to mating. Conversely, the sex that is chosen is also the sex that fights with members of the same sex for access to mates, and is traditionally the one that invests less in gamete production. In many species females produce just a few large and costly eggs, while males produce many, small and less expensive sperm. Because of this difference in gamete production and investment, known as anisogamy, females are typically the choosy sex and males typically compete with other males for access to females.
When Does Sexual Selection Act
Sexual selection can affect reproductive success at multiple reproductive stages. First, it acts during all the processes that lead to acquiring mating opportunities . Darwin referred exclusively to pre-copulatory sexual selection in his discussions, erroneously assuming that mating would inevitably result in reproductive success. In recent years, evidence that copulatory and post-copulatory events play an important role in determining the outcome of fertilization and reproduction has been increasing. Post-copulatory selection refers to the events that occur during and after mating. Post-copulatory male-male competition is known as sperm competition a term coined by Parker who recognized that when females mate with multiple males, their ejaculates compete inside the female reproductive tract for access to eggs. Sperm competition has resulted in the evolution of morphologically modified sperm that increase the likelihood of fertilization in many taxa . Post-copulatory female choice refers to the ability of females to affect the likelihood that sperm from a particular male fertilizes their eggs, and their decision to invest in offspring based on the identity of the male with whom they mate. Females exert this choice via morphological, chemical and behavioral adaptations. This type of selection is called cryptic choice because it occurs inside the female reproductive tract and cannot be detected from behavioral studies alone .
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Examples Of Sexually Selected Traits
There are the following examples of sexually selected traits, it is important to understand that these traits evolve due to intrasexual selection, the evolution of male armaments that provide individuals with an advantage when fighting off potential competitors are the best-suited examples of such trait. Some of the examples of such ornamental traits are as follows-
Antlers in deers
High body weight of male
Fighting capability among males.
Fun Fact: Why Do Humans Have Two Nipples
A good rule of thumb for mammals is to have twice as many nipples as offspring that you produce at one time. A female cow typically has two offspring at a time, and has four nipples a small dog has eight nipples and a large dog has ten nipples because typical litter size is four to five pups at once. Thus, human females, who typically gestate one fetus at a time, have two nipples.
Large penises and breasts
Human penises are quite unique! Despite common slang terms that imply otherwise , the human penis contains no bones. Unlike most of our closest evolutionary relatives, like chimpanzees and bonobos, human males do not have a penis bone, or baculum. Instead, human males must maintain an erection by pumping blood into the penis. Evolutionary biologists have speculated that the loss of the baculum in humans may be due to sexual selection by human females. Because the human erection relies on a type of hydraulic pumping system, erection failure can be an early warning of certain health conditions. Thus, human females are able to use the male erection as a clear sign of good health in potential partners.
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Intraspecific Sexual Selection And Interspecific Isolation
Fisherian sexual selection may amplify initially small differences in genetic variation between populations . Speciation by divergent sexual selection on colouration would imply that colouration not only mediates species-assortative mating, but is also subject to directional sexual selection within species. Cichlid mating systems are diverse and sexual selection is expected to be stronger in some lineages than others. In particular, polygynous mating systems and female-only parental care set the stage for potentially strong sexual selection by female choice. But does such intraspecific choosiness target the same colouration traits that also determine assortative matingtd:quest In the Lake Victoria haplochromine Pundamilia nyererei it does: females prefer redder males, and red colouration is also the main cue determining species-assortative preferences . In the Malawi haplochromine Labeotropheus fuelleborni, male colouration is also subject to intraspecific female choice, but the colour traits that females find most attractive within populations do not mediate preferences between populations . Together, these studies suggest that intraspecific sexual selection on haplochromine colouration might be common, but more species need to be investigated in order to draw general conclusions.
Christine M. Drea, in, 2015
Difference Between Natural Selection And Sexual Selection
Natural Selection vs Sexual Selection
There are several types of selections such as natural selection, sexual selection, artificial selection etc. Selection of organisms is defined as some sort of functional relationship between fitness and phenotype. Selection is the basic concept which helped Charles Darwin to introduce his theory of evolution. Some people describe that the sexual selection is a special form of natural selection. Darwin mainly used the concept of sexual selection to introduce and understand certain aspects of the reproductive biology of animals that he was unable to ascribe to natural selection. However, there are some differences between these two concepts. Darwin noted that many sexual characteristics are caused due to the process of natural selection, but certain changes are made due to both forms of selection.
What is Natural Selection?
Any coherent difference in fitness among phenotypically different organism is known as natural selection. The ability of survival and reproducibility of an organism is used to measure fitness of that particular organism.
What is Sexual Selection?
Sexual selection is another type of selection which involves the selection of traits based on their role in courtship and mating processes. In other words, it is the mating success among individuals in a particular population. Those who mate successfully may pass their traits to the next generation and that would enhance the mating success.
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