Sunday, 23 December 2007
Evolution of sex
The evolution of sex is a major puzzle in modern evolutionary biology. Many groups of organisms, notably the majority of animals and plants, reproduce sexually. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance. However, since the hypotheses for the origins of sex are difficult to test experimentally, most current work has been focused on the maintenance of sexual reproduction. Several explanations have been suggested by biologists including W. D. Hamilton, Alexey Kondrashov, George C. Williams, Harris Bernstein, Carol Bernstein, Michael M. Cox, Frederic A. Hopf and Richard E. Michod to explain how sexual reproduction is maintained in a vast array of different living organisms.
It seems that a sexual cycle is maintained because it improves the quality of progeny (fitness), despite reducing the overall number of offspring (twofold cost of sex). In order for sex to be evolutionarily advantageous, it must be associated with a significant increase in the fitness of offspring. One of the most widely accepted explanations for the advantage of sex lies in the creation of genetic variation. Another explanation is based on two molecular advantages. First is the advantage of recombinational DNA repair (promoted during meiosis because homologous chromosomes pair at that time), and second is the advantage of complementation (also known as hybrid vigor, heterosis or masking of mutations).
For the advantage due to creation of genetic variation, there are three possible reasons this might happen. First, sexual reproduction can bring together mutations that are beneficial into the same individual (sex aids in the spread of advantageous traits). Second, sex acts to bring together currently deleterious mutations to create severely unfit individuals that are then eliminated from the population (sex aids in the removal of deleterious genes). Last, sex creates new gene combinations that may be more fit than previously existing ones, or may simply lead to reduced competition among relatives.
For the advantage due to DNA repair, there is an immediate large benefit to removal of DNA damage by recombinational DNA repair during meiosis, since this removal allows greater survival of progeny with undamaged DNA. The advantage of complementation to each sexual partner is avoidance of the bad effects of their deleterious recessive genes in progeny by the masking effect of normal dominant genes contributed by the other partner.
The classes of hypotheses based on the creation of variation are further broken down below. It is important to realise that any number of these hypotheses may be true in any given species (they are not mutually exclusive), and that different hypotheses may apply in different species. However, a research framework based on creation of variation has yet to be found that allows one to determine whether the reason for sex is universal for all sexual species, and, if not, which mechanism is acting in each species.
On the other hand, the maintenance of sex based on DNA repair and complementation applies widely to all sexual species. This explanation for the maintenance of sex is explored further as Section 6.2 Sex is maintained by DNA repair and complementation, under “Other explanations” below.
It seems that a sexual cycle is maintained because it improves the quality of progeny (fitness), despite reducing the overall number of offspring (twofold cost of sex). In order for sex to be evolutionarily advantageous, it must be associated with a significant increase in the fitness of offspring. One of the most widely accepted explanations for the advantage of sex lies in the creation of genetic variation. Another explanation is based on two molecular advantages. First is the advantage of recombinational DNA repair (promoted during meiosis because homologous chromosomes pair at that time), and second is the advantage of complementation (also known as hybrid vigor, heterosis or masking of mutations).
For the advantage due to creation of genetic variation, there are three possible reasons this might happen. First, sexual reproduction can bring together mutations that are beneficial into the same individual (sex aids in the spread of advantageous traits). Second, sex acts to bring together currently deleterious mutations to create severely unfit individuals that are then eliminated from the population (sex aids in the removal of deleterious genes). Last, sex creates new gene combinations that may be more fit than previously existing ones, or may simply lead to reduced competition among relatives.
For the advantage due to DNA repair, there is an immediate large benefit to removal of DNA damage by recombinational DNA repair during meiosis, since this removal allows greater survival of progeny with undamaged DNA. The advantage of complementation to each sexual partner is avoidance of the bad effects of their deleterious recessive genes in progeny by the masking effect of normal dominant genes contributed by the other partner.
The classes of hypotheses based on the creation of variation are further broken down below. It is important to realise that any number of these hypotheses may be true in any given species (they are not mutually exclusive), and that different hypotheses may apply in different species. However, a research framework based on creation of variation has yet to be found that allows one to determine whether the reason for sex is universal for all sexual species, and, if not, which mechanism is acting in each species.
On the other hand, the maintenance of sex based on DNA repair and complementation applies widely to all sexual species. This explanation for the maintenance of sex is explored further as Section 6.2 Sex is maintained by DNA repair and complementation, under “Other explanations” below.
What is SEX?
First topic must by this :-)
Sex refers to the male and female duality of biology and reproduction. Unlike organisms that only have the ability to reproduce asexually, many species have the ability to produce offspring through meiosis and fertilization. Often, individuals of the two sexes attract one another and communicate their readiness to procreate through biological changes, or, in social species, through courtship behaviours.
An organism's sex is defined by its biological role in reproduction, not according to its sexual or other behavior. The female sex is defined as the one which produces the larger gamete and which typically bears the offspring. In contrast, the male sex has a smaller gamete and rarely bears offspring. In some animals and many plants sex may be assigned to specific structures rather than the entire organism. Earthworms, for example, are normally hermaphrodites. In humans, "sex" is often perceived as a dichotomous state or identity for most biological and social purposes - such that a person can only be female or male. But many factors, including one's biology, environment, psychology and social context, have a role in determining how a particular person, and those around them, view their sex. Although the table below shows common differences between males and females, many people do not correspond to "male" or "female" with regard to every criterion. Additionally, about 1 to 1.7 percent of human beings exhibit biological sexual ambiguity to the degree that they cannot be physically classified as exclusively male or female. This is known as intersex. A person with intersex may have biological characteristics of both the male and female sexes.
Sex refers to the male and female duality of biology and reproduction. Unlike organisms that only have the ability to reproduce asexually, many species have the ability to produce offspring through meiosis and fertilization. Often, individuals of the two sexes attract one another and communicate their readiness to procreate through biological changes, or, in social species, through courtship behaviours.
An organism's sex is defined by its biological role in reproduction, not according to its sexual or other behavior. The female sex is defined as the one which produces the larger gamete and which typically bears the offspring. In contrast, the male sex has a smaller gamete and rarely bears offspring. In some animals and many plants sex may be assigned to specific structures rather than the entire organism. Earthworms, for example, are normally hermaphrodites. In humans, "sex" is often perceived as a dichotomous state or identity for most biological and social purposes - such that a person can only be female or male. But many factors, including one's biology, environment, psychology and social context, have a role in determining how a particular person, and those around them, view their sex. Although the table below shows common differences between males and females, many people do not correspond to "male" or "female" with regard to every criterion. Additionally, about 1 to 1.7 percent of human beings exhibit biological sexual ambiguity to the degree that they cannot be physically classified as exclusively male or female. This is known as intersex. A person with intersex may have biological characteristics of both the male and female sexes.
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