Charles Darwin frequently used the phrases "survival of the fittest" and "the fight for existence" when explaining the theory of natural selection. His term choice was disastrous because it led many biologists to overemphasize differential mortality and ignore that natural selection ultimately only works via differential reproductive success.
Of course, differences in survivorship can influence the selection, but only if they cause a difference in reproduction. Self-replication or reproduction is a crucial aspect of the definition of life because successful offspring is the only thing that matters in natural selection.
Evolutionary biology has studied the link between parental care and offspring reproductive success. In many animal species, parental care, including feeding, grooming, and protecting offspring, is a crucial component of reproduction. The likelihood of offspring surviving and reproducing increases with parental care, which may result in the passing of advantageous features to subsequent generations.
Nevertheless, not every child who receives parental care does so equitably. Some children do better than others at turning parental care into reproductive success. The evolution of parental care may be significantly impacted by this ability, which is regulated by genetic and environmental variables.
Children with congenital diseases, including spina bifida, fibrocystic illness, cleft palate, or Down syndrome, are less likely to be fertile than healthy youngsters. Is there evidence that these youngsters are treated differently by their parents? One indicator is whether the children are entirely or partially abandoned
According to studies, a significant number of critically sick youngsters are institutionalised. According to the 1976 United States Census, more than 16,000 children (approximately 12 per cent of all institutionalised children) were never visited. Additionally, nearly 30,000 (almost 22 per cent) more patients were only seen once a year or fewer. Although these results are correlational and cannot be proven causative, they are consistent with the premise that parents spend less on children with anomalies.
What about children with disabilities who are neither institutionalised nor adopted? The prevalence of child physical abuse and neglect in the United States is estimated to be around 1.5 per cent. This gives a baseline against which other types of child abuse may be measured. Daly and Wilson (1981) collected several pieces of research indicating that children with anomalies are mistreated at significantly greater rates.
Throughout these studies, the percentage of children born with congenital physical defects which are mistreated ranged from 7.5 to 60 per cent, significantly more significant than the general population's base incidence of abuse.
Although the abuse may have produced a child's condition rather than being a result of the abuse in some of these situations, this is mainly ruled out in children who were born with the problem, such as those with spina bifida, cystic fibrosis, talipes, cleft palate, or Down syndrome.
The standard of parental care children receives is crucial in determining their capacity to reproduce successfully. Healthy adulthood and tremendous reproductive success are more likely to develop in offspring who receive excellent parental care.
For instance, in birds, young people whose parents feed more are more likely to mature, breed, and have more offspring than those fed less. The genetic makeup of the offspring is a further essential aspect. Children who inherit advantageous genes from their parents may be better able to translate parental attention into successful reproduction.
In a study of burying beetles, for instance, it was found that kids who acquired genes that made them more aggressive towards rivals were likelier to live and reproduce than those who did not.
The environment is also crucial in determining an offspring's capacity to reproduce successfully. For instance, regardless of how well their children are cared for by their parents, some species' environmental conditions, such as temperature or the chance of predation, can affect how they develop. Under these circumstances, even if the offspring receive less parental care than their siblings, the offspring who are better acclimated to their environment may be more successful at reproducing.
Finally, the relationship between parental care and offspring reproductive success can be influenced by social factors, such as competition between siblings for resources. Offspring better at competing for resources, such as food or nesting sites, may be more successful at converting parental care into reproductive success. In some species, siblings may even engage in aggressive behaviour towards each other to gain access to resources, further highlighting the importance of social factors in offspring reproductive success.
An ideal organism should balance its short-term chances of successful reproduction with its long-term chances of success to optimise its lifelong contribution to future generations. As a result, an individual with a high likelihood of significant future reproductive success should be less willing to risk its soma in current reproductive actions than an individual with a lower likelihood of such success.
An organism's residual reproductive value can be used to calculate the soma's current worth regarding its expectation of future progeny. Risk-taking is frequently required for successful reproduction, such as exposing oneself to predators, which decreases longevity and, consequently, future reproductive success.
Another factor influencing a kid's capacity to transform parental attention into reproductive success is whether the youngster is a man or a girl. Of course, assuming an equal sex ratio in the population, sons and daughters have equal reproductive success on average.
Nonetheless, the condition of the son or daughter may make one or both more likely to benefit from parental care. This is the significant discovery of the Trivers-Willard hypothesis. When parents are in good health, they will create more sons and spend more on boys, increasing their chances of generating a son who will be highly successful in the mating game.
According to the Trivers-Willard hypothesis, if the parents are in bad health or have fewer resources to spend on their daughters, they should invest more in them. In other words, being in "good" shape affects male reproductive success more than female reproductive success, as expected in a polygynous mating system.
Parents should bias investment towards sons if the parents are in good shape and daughters if the parents are in poor shape. Human tests of the Trivers-Willard theory have been inconclusive. A Trivers-Willard effect has been discovered in a few investigations. Female infants were more likely than male infants to be killed by their parents in one study, for example, as would be predicted by the hypothesis.
Similarly, among Kenya's Kipsigis, poorer households were more inclined to invest in their girls' educations than their boys', but wealthy ones were the opposite. Using years of schooling as a proxy for parental investment, researchers discovered that sons of high-status men received more education than girls. In contrast, daughters of low-status men received more education than boys.
She also discovered that higher-status males had more sons. According to research, tall and heavier parents had more males than daughters. Researchers found no indication that high-status parents spent more in boys than girls in a sample of 3,200 US children and no evidence that lower-status parents invested more in daughters than sons. In a rural sample from Dominica, researchers found no support for the Trivers-Willard hypothesis. Further research is needed to discover whether the postulated Trivers-Willard effects exist in diverse human populations.
A study of twins, one of whom was healthier in each pair, provides one direct test of the idea that parents are likely to invest in offspring based on their reproductive worth. Janet Mann, an evolutionary psychologist, studied fourteen infants: seven twin pairs, all of whom were born prematurely. Mann undertook thorough behavioural studies of the relationships between the mothers and their infants when the babies were four months old.
The interactions were observed when the dads were not there, and both twins were awake. Positive maternal behaviour was assessed using behavioural recordings, which included kissing, holding, calming, talking to, playing with, and staring at the newborn.
Each infant's health status was examined independently at delivery, discharge from the hospital, four months, and eight months. Medical, neurological, physical, cognitive, and developmental exams were all part of the health status examinations. Mann then examined the healthy baby hypothesis, which states that the child's health condition influences the degree of positive mother behaviour.
When the babies were four months old, nearly half of the moms displayed more positive maternal behaviour towards the healthier babies, while the other half showed no preference. However, when the infants were eight months old, all mothers had focused more positive maternal behaviour towards the healthier infant, with no reversals. In conclusion, the findings of this twin study corroborate the healthy baby theory, implying that mothers devote more maternal investment towards children with higher reproductive value.
A complex interplay of genetic, environmental, and social factors influences offspring's ability to convert parental care into reproductive success. Understanding this relationship is critical for understanding the evolution of parental care and the factors that shape the success of offspring in reproducing and passing on their genes to future generations.
Conflict is predicted to affect an offspring's ability to reproduce as an adult in certain species, including humans. Under certain circumstances, parents may try to turn their offspring permanently non-reproductive against their will.
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