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A 'life cycle' is a period involving one generation of an organism through means of reproduction, whether through asexual reproduction or sexual reproduction. In regard to its ploidy, there are three types of cycles:

★ '''haplontic''' life cycle

★ '''diplontic''' life cycle

★ '''diplobiontic''' life cycle (also referred to as ''diplohaplontic'', ''haplodiplontic'', or ''dibiontic'' life cycle)
These three types of cycles feature alternating haploid and diploid phases (''n'' and ''2n''). The haploid organism becomes diploid through fertilization, which joins of gametes. This results in a zygote which then germinates. To return to a haploid stage, meiosis must occur (see Cell division). The cycles differ in the product of meiosis, and whether mitosis (growth) occurs. Zygotic and gametic meioses have one mitotic stage and form: during the ''n'' phase in zygotic meiosis and during the ''2n'' phase in gametic meiosis. Therefore, zygotic and gametic meiosis are collectively term 'haplobiontic' (single mitosis per phase). Sporic meiosis, on the other hand, has two mitosis events ('diplobiontic'): one in each phase.

Contents

Haplontic life cycle

Diplontic life cycle

Life history theory

Haplontic life cycle

A 'zygotic meiosis' is a meiosis of a zygote immediately after karyogamy, which is the fusion of two cell nuclei. This way, the organism ends its diploid phase and produces several haploid cells. These cells divide mitotically to form either larger, multicellular individuals, or more haploid cells. Two opposite types of gametes (e.g., male and female) from these individuals or cells fuse to become a zygote.
In the whole cycle, zygotes are the only diploid cell; mitosis occurs only in the haploid phase.
The individuals or cells as a result of mitosis are 'haplonts', hence this life cycle is also called 'haplontic life cycle'. Haplonts are:

★ Most fungi

★ Some green algae

★ Many protozoa

Diplontic life cycle

In 'gametic meiosis', instead of immediately dividing ''meiotically'' to produce haploid cells, the zygote divides ''mitotically'' to produce a multicellular diploid individual or a group of more unicellular diploid cells. Cells from the diploid individuals then undergo meiosis to produce haploid cells or gametes. Haploid cells may divide to form more haploid cells, as in many yeasts, but the haploid phase is not the predominant life cycle phase. In most diplonts, mitosis occurs only in the diploid phase, i.e. gametes usually form quickly and fuse to produce diploid zygotes.
In the whole cycle, gametes are usually the only haploid cells, and mitosis usually occurs only in the diploid phase.
The diploid multicellular individual is a 'diplont', hence a gametic meiosis is also called a 'diplontic life cycle'. Diplonts are:

★ Animals

★ Some brown algae

★ Some fungi, e.g. brewer's yeast

Life history theory

In animal and human biology life history theory is a method of understanding evolved behaviors and strategies to optimize reproductive success.