In mammals, the fertilized embryo instead develops within the female, receiving nutrition directly from its mother. The diagrams compare evolution of allele frequency in a sexual population top and an asexual population bottom. The vertical axis shows frequency and the horizontal axis shows time. Sexual dimorphisms in animals are often associated with sexual selection — the competition between individuals of one sex to mate with the opposite sex. Because of their motility, animal sexual behavior can involve coercive sex. Male cones are smaller and produce pollen which is transported by wind to land in female cones.
In humans, biological sex is determined by five factors present at birth: These gametes combine to form embryos which develop into a new organism. Each cell in the offspring has half the chromosomes of the mother and half of the father. This process of cell division is called mitosis. Animals which live in the water can mate using external fertilization , where the eggs and sperm are released into and combine within the surrounding water. In female mammals the vagina connects with the uterus , an organ which directly supports the development of a fertilized embryo within a process called gestation. In humans and other mammals this male organ is the penis , which enters the female reproductive tract called the vagina to achieve insemination —a process called sexual intercourse. In field crickets , for example, insects with a single X chromosome develop as male, while those with two develop as female. The biological cause for an organism developing into one sex or the other is called sex determination. In pines and other conifers the sex organs are conifer cones and have male and female forms. In sexual reproduction, there are special kinds of cells that divide without prior duplication of its genetic material, in a process named meiosis. In addition to animals, plants, and fungi, other eukaryotes e. When a pollen grain lands upon the stigma on top of a carpel's style, it germinates to produce a pollen tube that grows down through the tissues of the style into the carpel, where it delivers male gamete nuclei to fertilize an ovule that eventually develops into a seed. Conversely, there are also species in which there is no sexual specialization, and the same individuals both contain masculine and feminine reproductive organs, and they are called hermaphrodites. Sexual dimorphisms in animals are often associated with sexual selection — the competition between individuals of one sex to mate with the opposite sex. This mode of reproduction is called asexual, and it is still used by many species, particularly unicellular, but it is also very common in multicellular organisms, including many of those with sexual reproduction. These gametes are the cells that are prepared for the sexual reproduction of the organism. In many wrasses the opposite is true—most fish are initially female and become male when they reach a certain size. In many organisms, the haploid stage has been reduced to just gametes specialized to recombine and form a new diploid organism; in others, such as cryptogamic plants the gametes are capable of undergoing cell division to produce multicellular haploid organisms. An individual that produces exclusively large gametes is female, and one that produces exclusively small gametes is male. The more familiar female cones are typically more durable, containing ovules within them. Genetic sex-determination, because it is determined by chromosome assortment, usually results in a 1: This double-chromosome stage is called " diploid ", while the single-chromosome stage is " haploid ". The plants attract these insects or larger animals such as humming birds and bats with nectar-containing flowers. Chromosomes are passed on from one generation to the next in this process. Life has evolved from simple stages to more complex ones, and so have the reproduction mechanisms.
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