Biological development
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What is biological development?
Biological development includes all processes that implement the instructions contained in DNA. August Weismann, a German biologist, distinguished the two components of organisms as germ plasm and soma. The germ plasm consists of genes passed from one generation to the next. The soma is the body produced as the organism develops.
In genetics, the word genotype indicates hereditary instructions passed from one generation to another in the genes. Phenotype is the term for functioning organisms produced by those instructions. Biological development is the production of phenotypes.
Embryology and developmental studies have been major biological problems for a long time. It was thought that adults existed as a miniature, but it is now believed that undifferentiated materials become specialized into adult structures.
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How do living things grow and change throughout their lives?
Living things grow and change through biological development, which involves implementing DNA instructions. This process relies on executive machinery modified as genetic instructions are put into operation.
Development can be quantitative, involving simple growth, or qualitative, involving changes in the system's nature. Cells differentiate to form tissues with specific functions, and this process occurs throughout an organism's life. Hormones like pituitary growth hormone, thyroid hormone, and sex hormones also regulate growth.
Both heredity and environment contribute to the traits of living things.
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What are the main stages of development in animals and plants?
Animal and plant development involves several key stages. In animals, fertilization initiates development, where sperm and egg fuse to form a zygote. The zygote undergoes cleavage, forming a blastula, followed by gastrulation, where the embryo differentiates into ectoderm, mesoderm, and endoderm. Organogenesis then occurs, leading to a recognizable species representative.
Plant development alternates between sporophyte and gametophyte generations. Fertilization precedes the development of a diploid sporophyte, which produces haploid spores that grow into the gametophyte generation. The development of a seed plant includes the formation of a triploid endosperm, which nourishes the developing embryo. Germination occurs when the seed imbibes water and emerges from dormancy, leading to seedling establishment.
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How does a fertilized egg develop into a complex organism?
After fertilization, a zygote, or fertilized egg, forms, marking the beginning of a genetically distinct organism. The zygote undergoes cleavage, dividing into smaller cells called blastomeres through mitosis.
During a process called gastrulation, the embryo develops into three tissue types: ectoderm, mesoderm, and endoderm. These tissues give rise to the skin and nervous system, connective tissues, circulatory system, muscles, and bones, and the digestive, lung, and urinary systems, respectively.
Cell differentiation is a crucial part of this process, where immature cells develop into specialized cell types with distinct structures and functions, such as muscle cells or neurons. Cell-to-cell communication is essential for coordinating cell behavior and ensuring proper development.
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What role do genes play in guiding development?
Genes play a crucial role in biological development by providing the instructions necessary for building an organism. These instructions, encoded in DNA, guide the synthesis of proteins and their assembly into appropriate structures.
During development, genes control the formation of body patterns. For instance, homeotic genes encode transcription factors that direct cells to form specific body parts. These transcription factors can activate some genes while repressing others, ensuring ordered development. Mutations in these genes can lead to incorrect development, such as the growth of legs instead of antennae in fruit flies.
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How can the environment affect the biological development of an organism?
The environment significantly influences an organism's biological development through physical and chemical factors. Temperature affects the rate of biochemical reactions, with lower temperatures slowing metabolism. Chemical factors such as gases, water, minerals, and nutrients also play a crucial role; for example, plants require carbon dioxide, water, and sunlight for photosynthesis.
Additionally, environmental pollutants can disrupt development. Contaminants in the atmosphere can harm plant and animal growth and reproduction. In aquatic systems, pollution can lead to eutrophication, causing oxygen depletion and harming aquatic life.
Moreover, sensory stimulation from the environment fine-tunes neural connections, especially during early development. Lack of appropriate sensory experiences can lead to a drop in maintained connections.
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Biological development, the progressive changes in size, shape, and function during the life of an organism by which its genetic potentials (genotype) are translated into functioning mature systems (phenotype). Most modern philosophical outlooks would consider that development of some kind or other characterizes all things, in both the physical and biological worlds. Such points of view go back to the very earliest days of philosophy.
Among the pre-Socratic philosophers of Greek Ionia, half a millennium before Christ, some, like Heracleitus, believed that all natural things are constantly changing. In contrast, others, of whom Democritus is perhaps the prime example, suggested that the world is made up by the changing combinations of atoms, which themselves remain unaltered, not subject to change or development. The early period of post-Renaissance European science may be regarded as dominated by this latter atomistic view, which reached its fullest development in the period between Newton’s laws of physics and Dalton’s atomic theory of chemistry in the early 19th century. This outlook was never easily reconciled with the observations of biologists, and in the last hundred years a series of discoveries in the physical sciences have combined to swing opinion back toward the Heracleitan emphasis on the importance of process and development. The atom, which seemed so unalterable to Dalton, has proved to be divisible after all, and to maintain its identity only by processes of interaction between a number of component subatomic particles, which themselves must in certain aspects be regarded as processes rather than matter. Albert Einstein’s theory of relativity showed that time and space are united in continuum, which implies that all things are involved in time; that is to say, in development.
The philosophers who charted the transition from the nondevelopmental view, for which time was an accidental and inessential element, were Henri Bergson and, in particular, Alfred North Whitehead. Karl Marx and Friedrich Engels, with their insistence on the difference between dialectical and mechanical materialism, may be regarded as other important innovators of this trend, although the generality of their philosophy was somewhat compromised by the political context in which it was placed and the rigidity with which their later followers have interpreted it.
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Philosophies of the Heracleitan type, which emphasize process and development, provide much more appropriate frameworks for biology than do philosophies of the atomistic kind. Living organisms confront biologists with changes of various kinds, all of which could be regarded as in some sense developmental; however, biologists have found it convenient to distinguish the changes and to use the word development for only one of them. Biological development can be defined as the series of progressive, nonrepetitive changes that occur during the life history of an organism. The kernel of this definition is to contrast development with, on the one hand, the essentially repetitive chemical changes involved in the maintenance of the body, which constitute “metabolism,” and on the other hand, with the longer term changes, which, while nonrepetitive, involve the sequence of several or many life histories, and which constitute evolution.
As with most formal definitions, these distinctions cannot always be applied strictly to the real world. In the viruses, for instance, and even in bacteria, it is difficult to make a distinction between metabolism and development, since the metabolic activity of a virus particle consists of little more than the development of new virus particles. In certain other cases, the distinction between development and evolution becomes blurred: the concept of an individual organism with a definite life history may be very difficult to apply in plants that reproduce by vegetative division, the breaking off of a part that can grow into another complete plant. The possibilities for debate that arise in these special cases, however, do not in any way invalidate the general usefulness of the distinctions as conventionally made in biology.
The scope of development
All organisms, including the very simplest, consist of two components, distinguished by a German biologist, August Weismann, at the end of the 19th century, as the “germ plasm” and the “soma.” The germ plasm consists of the essential elements, or genes, passed on from one generation to the next, and the soma consists of the body that may be produced as the organism develops. In more modern terms, Weismann’s germ plasm is identified with DNA (deoxyribonucleic acid), which carries, encoded in the complex structure of its molecule, the instructions necessary for the synthesis of the other compounds of the organism and their assembly into the appropriate structures. It is this whole collection of other compounds (proteins, fats, carbohydrates, and others) and their arrangement as a metabolically functioning organism that constitutes the soma. Biological development encompasses, therefore, all the processes concerned with implementing the instructions contained in the DNA. Those instructions can only be carried out by an appropriate executive machinery, the first phase of which is provided by the cell that carries the DNA into the next generation: in animals and plants by the fertilized egg cell; in viruses by the cell infected. In life histories that have more than a minimal degree of complexity, the executive machinery itself becomes modified as the genetic instructions are gradually put into operation, and new mechanisms of protein synthesis are brought into functional condition. The fundamental problem of developmental biology is to understand the interplay between the genetic instructions and the mechanisms by which those instructions are carried out.
In the language of genetics the word genotype is used to indicate the hereditary instructions passed on from one generation to another in the genes, while phenotype is the term given to the functioning organisms produced by those instructions. Biological development, therefore, consists of the production of phenotypes. The point made in the last paragraph is that the formation of the phenotype of one generation depends on the functioning of part of the phenotype of the previous generation (e.g., egg cell), as the mechanism that begins the interpretation of the instructions contained in the new organism’s genotype.
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