A male organism is the physiological sex that produces sperm. Each spermatozoon can fuse with ovum, in the process of fertilization. A male cannot reproduce sexually without access to at least one ovum from a female, but some organisms can reproduce both sexually and asexually. Most male mammals, including male humans, have a Y chromosome, which codes for the production of larger amounts of testosterone to develop male reproductive organs. Not all species share a common sex-determination system. In most animals, including humans, sex is determined genetically, but in some species it can be determined due to social, environmental, or other factors. For example, Cymothoa exigua changes sex depending on the number of females present in the vicinity; the existence of two sexes seems to have been selected independently across different evolutionary lineages. The repeated pattern is sexual reproduction in isogamous species with two or more mating types with gametes of identical form and behavior to anisogamous species with gametes of male and female types to oogamous species in which the female gamete is much larger than the male and has no ability to move.
There is a good argument that this pattern was driven by the physical constraints on the mechanisms by which two gametes get together as required for sexual reproduction. Accordingly, sex is defined operationally across species by the type of gametes produced and differences between males and females in one lineage are not always predictive of differences in another. Male/female dimorphism between organisms or reproductive organs of different sexes is not limited to animals. In land plants and male designate not only the female and male gamete-producing organisms and structures but the structures of the sporophytes that give rise to male and female plants. A common symbol used to represent the male sex is the Mars symbol, ♂ — a circle with an arrow pointing northeast; the symbol is identical to the planetary symbol of Mars. It was first used to denote sex by Carl Linnaeus in 1751; the symbol is called a stylized representation of the Roman god Mars' shield and spear. According to Stearn, all the historical evidence favours that it is derived from θρ, the contraction of the Greek name for the planet Mars, Thouros.
The sex of a particular organism may be determined by a number of factors. These may be genetic or environmental, or may change during the course of an organism's life. Although most species with male and female sexes have individuals that are either male or female, hermaphroditic animals, such as worms, have both male and female reproductive organs. Most mammals, including humans, are genetically determined as such by the XY sex-determination system where males have an XY sex chromosome, it is possible in a variety of species, including humans, to be XXY or have other intersex/hermaphroditic qualities, though one would still be considered genotypically male so long as one has a Y-chromosome. During reproduction, a male can give either an X sperm or a Y sperm, while a female can only give an X egg. A Y sperm and an X egg produce a male, while an X egg produce a female; the part of the Y-chromosome, responsible for maleness is the sex-determining region of the Y-chromosome, the SRY. The SRY activates Sox9, which forms feedforward loops with FGF9 and PGD2 in the gonads, allowing the levels of these genes to stay high enough in order to cause male development.
The ZW sex-determination system, where males have a ZZ sex chromosome may be found in birds and some insects and other organisms. Members of the insect order Hymenoptera, such as ants and bees, are determined by haplodiploidy, where most males are haploid and females and some sterile males are diploid. In some species of reptiles, such as alligators, sex is determined by the temperature at which the egg is incubated. Other species, such as some snails, practice sex change: adults start out male become female. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male. In some arthropods, sex is determined by infection. Bacteria of the genus Wolbachia alter their sexuality. In those species with two sexes, males may differ from females in ways other than the production of spermatozoa. In many insects and fish, the male is smaller than the female. In seed plants, which exhibit alternation of generations, the female and male parts are both included within the sporophyte sex organ of a single organism.
In mammals, including humans, males are larger than females. In birds, the male exhibits a colorful plumage that attracts females. Boy Female Gender Male plant Male pregnancy Man Masculinity Gentleman Wedgwood, Hensleigh. "On False Etymologies". Transactions of the Philological Society: 68
Antlers are extensions of an animal's skull found in members of the deer family. They are a single structure, they are found only on males, with the exception of the caribou. Antlers are shed and regrown each year and function as objects of sexual attraction and as weapons in fights between males for control of harems. In contrast, found on pronghorns and bovids such as sheep, goats and cattle, are two-part structures. An interior of bone is covered by an exterior sheath grown by specialized hair follicles, the same material as human fingernails and toenails. Horns continue to grow throughout the animal's life; the exception to this rule is the Pronghorn which regrows its horn sheath each year. They grow in symmetrical pairs. Antler comes from the Old French antoillier from some form of an unattested Latin word *anteocularis, "before the eye". Antlers are unique to cervids; the ancestors of deer had tusks. In most species, antlers appear to replace tusks. However, two modern species have tusks and no antlers and the muntjac has small antlers and tusks.
Antlers are found only on males. Only reindeer have antlers on the females, these are smaller than those of the males. Fertile does from other species of deer have the capacity to produce antlers on occasion due to increased testosterone levels; the "horns" of a pronghorn meet some of the criteria of antlers, but are not considered true antlers because they contain keratin. Each antler grows from an attachment point on the skull called a pedicle. While an antler is growing, it is covered with vascular skin called velvet, which supplies oxygen and nutrients to the growing bone. Antlers are considered one of the most exaggerated cases of male secondary sexual traits in the animal kingdom, grow faster than any other mammal bone. Growth occurs at the tip, is cartilage, replaced by bone tissue. Once the antler has achieved its full size, the velvet is lost and the antler's bone dies; this dead bone structure is the mature antler. In most cases, the bone at the base is destroyed by osteoclasts and the antlers fall off at some point.
As a result of their fast growth rate, antlers are considered a handicap since there is an immense nutritional demand on deer to re-grow antlers annually, thus can be honest signals of metabolic efficiency and food gathering capability. In most arctic and temperate-zone species, antler growth and shedding is annual, is controlled by the length of daylight. Although the antlers are regrown each year, their size varies with the age of the animal in many species, increasing annually over several years before reaching maximum size. In tropical species, antlers may be shed at any time of year, in some species such as the sambar, antlers are shed at different times in the year depending on multiple factors; some equatorial deer never shed their antlers. Antlers function as weapons in combats between males, which sometimes cause serious wounds, as dominance and sexual displays; the principal means of evolution of antlers is sexual selection, which operates via two mechanisms: male-to-male competition and female mate choice.
Male-male competition can take place in two forms. First, they can compete behaviorally where males use their antlers as weapons to compete for access to mates. Males with the largest antlers are more to obtain mates and achieve the highest fertilization success due to their competitiveness and high phenotypic quality. Whether this is a result of male-male fighting or display, or of female choosiness differs depending on the species as the shape and function of antlers vary between species. There is evidence to support that antler size influences mate selection in the red deer, has a heritable component. Despite this, a 30-year study showed no shift in the median size of antlers in a population of red deer; the lack of response could be explained by environmental covariance, meaning that lifetime breeding success is determined by an unmeasured trait, phenotypically correlated with antler size but for which there is no genetic correlation of antler growth. Alternatively, the lack of response could be explained by the relationship between heterozygosity and antler size, which states that males heterozygous at multiple loci, including MHC loci, have larger antlers.
The evolutionary response of traits that depend on heterozygosity is slower than traits that are dependent on additive genetic components and thus the evolutionary change is slower than expected. A third possibility is that the costs of having larger antlers exert enough selective pressure to offset the benefit of attracting mates. If antlers functioned only in male–male competition for mates, the best evolutionary strategy would be to shed them after the rutting season, both to free the male from a heavy encumbrance and to give him more time to regrow a larger new pair, yet antlers are retained through the winter and into the spring, suggesting that they have another use. Wolves in Yellowstone National Park are 3.6 times more to
Reproduction is the biological process by which new individual organisms – "offspring" – are produced from their "parents". Reproduction is a fundamental feature of all known life. There are two forms of reproduction: sexual. In asexual reproduction, an organism can reproduce without the involvement of another organism. Asexual reproduction is not limited to single-celled organisms; the cloning of an organism is a form of asexual reproduction. By asexual reproduction, an organism creates a genetically identical copy of itself; the evolution of sexual reproduction is a major puzzle for biologists. The two-fold cost of sexual reproduction is that only 50% of organisms reproduce and organisms only pass on 50% of their genes. Sexual reproduction requires the sexual interaction of two specialized organisms, called gametes, which contain half the number of chromosomes of normal cells and are created by meiosis, with a male fertilizing a female of the same species to create a fertilized zygote; this produces offspring organisms whose genetic characteristics are derived from those of the two parental organisms.
Asexual reproduction is a process by which organisms create genetically similar or identical copies of themselves without the contribution of genetic material from another organism. Bacteria divide asexually via binary fission; these organisms do not possess different sexes, they are capable of "splitting" themselves into two or more copies of themselves. Most plants have the ability to reproduce asexually and the ant species Mycocepurus smithii is thought to reproduce by asexual means; some species that are capable of reproducing asexually, like hydra and jellyfish, may reproduce sexually. For instance, most plants are capable of vegetative reproduction—reproduction without seeds or spores—but can reproduce sexually. Bacteria may exchange genetic information by conjugation. Other ways of asexual reproduction include parthenogenesis and spore formation that involves only mitosis. Parthenogenesis is the development of embryo or seed without fertilization by a male. Parthenogenesis occurs in some species, including lower plants and vertebrates.
It is sometimes used to describe reproduction modes in hermaphroditic species which can self-fertilize. Sexual reproduction is a biological process that creates a new organism by combining the genetic material of two organisms in a process that starts with meiosis, a specialized type of cell division; each of two parent organisms contributes half of the offspring's genetic makeup by creating haploid gametes. Most organisms form two different types of gametes. In these anisogamous species, the two sexes are referred to as female. In isogamous species, the gametes are similar or identical in form, but may have separable properties and may be given other different names. For example, in the green alga, Chlamydomonas reinhardtii, there are so-called "plus" and "minus" gametes. A few types of organisms, such as many fungi and the ciliate Paramecium aurelia, have more than two "sexes", called syngens. Most animals and plants reproduce sexually. Sexually reproducing organisms have different sets of genes for every trait.
Offspring inherit one allele for each trait from each parent. Thus, offspring have a combination of the parents' genes, it is believed that "the masking of deleterious alleles favors the evolution of a dominant diploid phase in organisms that alternate between haploid and diploid phases" where recombination occurs freely. Bryophytes reproduce sexually, but the larger and commonly-seen organisms are haploid and produce gametes; the gametes fuse to form a zygote which develops into a sporangium, which in turn produces haploid spores. The diploid stage is small and short-lived compared to the haploid stage, i.e. haploid dominance. The advantage of diploidy, only exists in the diploid life generation. Bryophytes retain sexual reproduction despite the fact that the haploid stage does not benefit from heterosis; this may be an indication that the sexual reproduction has advantages other than heterosis, such as genetic recombination between members of the species, allowing the expression of a wider range of traits and thus making the population more able to survive environmental variation.
Allogamy is the fertilization of the combination of gametes from two parents the ovum from one individual with the spermatozoa of another. Self-fertilization known as autogamy, occurs in hermaphroditic organisms where the two gametes fused in fertilization come from the same individual, e.g. many vascular plants, some foraminiferans, some ciliates. The term "autogamy" is sometimes substituted for autogamous pollination and describes self-pollination within the same flower, distinguished from geitonogamous pollination, transfer of pollen to a different flower on the same flowering plant, or within a single monoecious Gymnosperm plant. Mitosis and meiosis are types of cell division. Mitosis occurs in somatic cells. Mitosis The resultant number of cells in mitosis is t
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction. Different characteristics tend to exist within any given population as a result of mutation, genetic recombination and other sources of genetic variation. Evolution occurs when evolutionary processes such as natural selection and genetic drift act on this variation, resulting in certain characteristics becoming more common or rare within a population, it is this process of evolution that has given rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms and molecules. The scientific theory of evolution by natural selection was proposed by Charles Darwin and Alfred Russel Wallace in the mid-19th century and was set out in detail in Darwin's book On the Origin of Species. Evolution by natural selection was first demonstrated by the observation that more offspring are produced than can survive.
This is followed by three observable facts about living organisms: 1) traits vary among individuals with respect to their morphology and behaviour, 2) different traits confer different rates of survival and reproduction and 3) traits can be passed from generation to generation. Thus, in successive generations members of a population are more to be replaced by the progenies of parents with favourable characteristics that have enabled them to survive and reproduce in their respective environments. In the early 20th century, other competing ideas of evolution such as mutationism and orthogenesis were refuted as the modern synthesis reconciled Darwinian evolution with classical genetics, which established adaptive evolution as being caused by natural selection acting on Mendelian genetic variation. All life on Earth shares a last universal common ancestor that lived 3.5–3.8 billion years ago. The fossil record includes a progression from early biogenic graphite, to microbial mat fossils, to fossilised multicellular organisms.
Existing patterns of biodiversity have been shaped by repeated formations of new species, changes within species and loss of species throughout the evolutionary history of life on Earth. Morphological and biochemical traits are more similar among species that share a more recent common ancestor, can be used to reconstruct phylogenetic trees. Evolutionary biologists have continued to study various aspects of evolution by forming and testing hypotheses as well as constructing theories based on evidence from the field or laboratory and on data generated by the methods of mathematical and theoretical biology, their discoveries have influenced not just the development of biology but numerous other scientific and industrial fields, including agriculture and computer science. The proposal that one type of organism could descend from another type goes back to some of the first pre-Socratic Greek philosophers, such as Anaximander and Empedocles; such proposals survived into Roman times. The poet and philosopher Lucretius followed Empedocles in his masterwork De rerum natura.
In contrast to these materialistic views, Aristotelianism considered all natural things as actualisations of fixed natural possibilities, known as forms. This was part of a medieval teleological understanding of nature in which all things have an intended role to play in a divine cosmic order. Variations of this idea became the standard understanding of the Middle Ages and were integrated into Christian learning, but Aristotle did not demand that real types of organisms always correspond one-for-one with exact metaphysical forms and gave examples of how new types of living things could come to be. In the 17th century, the new method of modern science rejected the Aristotelian approach, it sought explanations of natural phenomena in terms of physical laws that were the same for all visible things and that did not require the existence of any fixed natural categories or divine cosmic order. However, this new approach was slow to take root in the biological sciences, the last bastion of the concept of fixed natural types.
John Ray applied one of the more general terms for fixed natural types, "species," to plant and animal types, but he identified each type of living thing as a species and proposed that each species could be defined by the features that perpetuated themselves generation after generation. The biological classification introduced by Carl Linnaeus in 1735 explicitly recognised the hierarchical nature of species relationships, but still viewed species as fixed according to a divine plan. Other naturalists of this time speculated on the evolutionary change of species over time according to natural laws. In 1751, Pierre Louis Maupertuis wrote of natural modifications occurring during reproduction and accumulating over many generations to produce new species. Georges-Louis Leclerc, Comte de Buffon suggested that species could degenerate into different organisms, Erasmus Darwin proposed that all warm-blooded animals could have descended from a single microorganism; the first full-fledged evolutionary scheme was Jean-Baptiste Lamarck's "transmutation" theory of 1809, which envisaged spontaneous generation continually producing simple forms of life that developed greater complexity in parallel lineages with an inherent progressive tendency, postulated that on a local level, these lineages adapted to the environment by inheriting changes caused by their use or disuse in parents.
These ideas were cond