Ontogeny is the developmental process of an organism from fertilization to adulthood, while phylogeny deals with the evolutionary development of a species. The process begins with the fusion of gametes and mitosis, leading to the formation of an embryo. The ontogeny of multicellular organisms involves the differentiation of cells into tissues and organs. The process continues until the organism reaches physical and sexual maturity. Understanding ontogeny helps biologists examine the effect of mutations at different stages of an organism’s life. Various disciplines in biology are defined by significant periods within the ontogenetic cycle.
Ontogeny refers to the developmental course of an organism from inception to adulthood. The cycle begins at fertilization and is completed when the organism reaches physical and sexual maturity. As a subject of study in biology, ontogeny deals with the developmental process of an individual member of a species throughout its life. The evolutionary development of an entire species is the subject of phylogeny.
The process of human ontogeny begins with the fusion of male and female gametes to form a zygote with a complete set of human chromosomes. This is said to be the beginning of an ontogenetic cycle. The single cell will grow into a single individual organism by mitosis, a series of asexual reproductions. The population of a species consists of individuals at various stages of the ontogenetic cycle.
In unicellular organisms, mitoses normally lead to the formation of independent, genetically identical individuals. In multicellular organisms, the initial products of mitosis are genetically equivalent, but the cells remain connected and subsequently differentiate to form a single individual. The ontogeny of multicellular organisms is that successive generations of cells derived from the zygote differentiate into tissues and organs. This process leads to the development of an embryo.
An embryo can continue to develop in different contexts, depending on the species. The path to the juvenile stage can also be direct or through an intermediate form. Development continues through the juvenile stage until the organism reaches its physical and sexual maturity in adulthood. At this point, ontogeny usually stops.
The ontogeny of an organism is sometimes represented as an inverted cone. At the bottom of the cone is the unicellular zygote, which contains the potential to produce an adult organism. Each successive stage of development mitigates the range of possible actions in subsequent stages and is represented by an increase in the height and diameter of the cone. A mutation early in the ontogenetic cycle will have far greater consequences than one that occurs later. This visualization helps biologists examine the effect a viable mutation might have at various stages in an organism’s life.
Understanding the evolutionary life of an organism, ontogeny necessarily touches on many aspects of biology. Various disciplines in biology are defined by significant periods within the ontogenetic cycle. Embryology deals with the period from fertilization to the beginning of independent individual development, for example. The genetic regulation of the cycle in its course is the subject of developmental genetics. Life cycle biologists identify the sequence and qualitative nature of the stages in an ontogenetic cycle.
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