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Gigantism and Dwarfism

Q: What are the reasons for gigantism and dwarfism in living creatures? How did these factors guide evolution on Earth?

Krishna: There are several reasons for gigantism and dwarfism.

More specifically they are overdevelopment and underdevelopment of the skeleton, respectively. Dwarfism and gigantism are conditions of abnormal skeletal growth. In general, skeletal growth is a complex process and can be disturbed in many ways.

Gigantism is a condition characterized by excessive growth and height significantly above average. Gigantism can appear in a child of any age, from baby to teenager.

Robert Pershing Wadlow (February 22, 1918 – July 15, 1940), also known as the Alton Giant and the Giant of Illinois, was a man who was the tallest person in recorded history for whom there is irrefutable evidence.

The main symptom is accelerated growth, which means the child will be unusually tall for their age. Other features include: large head, prominent forehead, protruding jaw, coarse-looking facial features, such as a broad nose, very large hands and feet, with thick fingers and toes, excessive sweating, a very large appetite, general weakness, Some people also get headaches, nausea, problems with vision and delayed puberty (1).

Gigantism and dwarfism in human basically depend on the quality and quantity of food eaten during the early growing age. Gigantism and dwarfism in human basically depend on the quality and quantity of food eaten during the early growing age.

Excessive amounts of growth hormone (GH) cause the signs and symptoms of gigantism. (1)Children with gigantism can also develop certain symptoms as their pituitary tumor applies pressure to their nearby brain and nerve tissues. The main sign of gigantism is excessive growth. Children with gigantism grow rapidly in height. From the time someone is born, the way their body grows is controlled by hormones produced by the pituitary gland in the brain. The most important hormone for growth is called growth hormone, also known as human growth hormone, HGH or GH. Most children with gigantism have too much growth hormone, which makes them grow too much, too fast.

Gigantism is almost always caused by a benign tumour, also known as an adenoma, growing in the pituitary gland. Usually, there is no obvious reason for this, although it may be due to rare genetic conditions.

There are also rare genetic conditions that can cause gigantism without the child having an adenoma. Examples include Sotos syndrome, Beckwith-Wiedemann syndrome, and Weaver syndrome.

A similar condition to gigantism, known as acromegaly, can affect adults. Like gigantism, acromegaly causes abnormal growth, but instead of making the person grow tall, it causes other symptoms, such as changes to facial features and enlarged hands and feet.

Dwarfism is a condition wherein an organism is exceptionally small.

Colombia's Edward Niño Hernandez is the shortest man living right now . At 72.1 cm (2 ft 4.38 in)

Dwarfism causes a person to be very short in stature. Dozens of medical conditions can cause dwarfism. Dwarfism also has many reasons to get established:

The most common cause of dwarfism is a disorder called achondroplasia, which causes disproportionately short stature. This disorder usually results in the following: An average-size trunk. Short arms and legs, with particularly short upper arms and upper legs.

Dwarfism can also be due to metabolic disorders or malnourishment. A group of conditions called skeletal dysplasias is the most common cause of dwarfism. Skeletal dysplasias cause the bones to grow abnormally, resulting in a small stature (2). Skeletal dysplasias cause the bones to grow abnormally, resulting in a small stature. This abnormal growth can also result in uneven growth that produces a body of unusual proportions. The three most common types of skeletal dysplasias are achondroplasia, spondyloepiphyseal dysplasia congenita, and diastrophic dysplasia.

Early in childhood, much of the cartilage a person is born with transforms into harder bone. In achondroplasia, this process does not happen as effectively. Achondroplasia primarily affects the bones of the arms and legs. Achondroplasia is a genetic disorder present at birth. It is a genetic disorder whereby the limbs are diminutive.

Spondyloepiphyseal dysplasia congenita (SEDc) is a genetic mutation that causes a short torso, short arms, and short legs.

Diastrophic dysplasia is the result of a gene mutation. It affects cartilage and bone development, causing very short arms and legs and a short stature.

Other causes of dwarfism may be due to (2):

organ failure that undermines the body’s ability to produce hormones or metabolize nutrients

insufficient levels of certain hormones, particularly human growth hormone (HGH). Growth hormone deficiency is responsible in these cases of dwarfism.

malnourishment or lack of sufficient food, resulting in growth problems

These are all secondary causes of dwarfism. This means they are not genetic, and may be reversible with early detection and prompt treatment.

How do these factors ( gigantism and dwarfism) guide evolution on Earth?

This is a huge subject but I will mention the important points briefly.

Insular dwarfism, a form of phyletic dwarfism, is the process and condition of large animals evolving or having a reduced body size[a] when their population's range is limited to a small environment, primarily islands. This natural process is distinct from the intentional creation of dwarf breeds, called dwarfing. This process has occurred many times throughout evolutionary history, with examples including dinosaurs, like Europasaurus and Magyarosaurus dacus, and modern animals such as elephants and their relatives. This process, and other "island genetics" artifacts, can occur not only on islands, but also in other situations where an ecosystem is isolated from external resources and breeding. This can include caves, desert oases, isolated valleys and isolated mountains ("sky islands"). Insular dwarfism is one aspect of the more general "island effect" or "Foster's rule", which posits that when mainland animals colonize islands, small species tend to evolve larger bodies (island gigantism), and large species tend to evolve smaller bodies. This is itself one aspect of island syndrome, which describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts (5).

Evolutionary “Island Rule” (3)

It is an old-standing theory in evolutionary ecology: animal species on islands have the tendency to become either giants or dwarfs in comparison to mainland relatives. Since its formulation in the 1960s, however, the ‘island rule’ has been severely debated by scientists. In a new publication in Nature Ecology and Evolution on April 15, 2021, researchers solved this debate by analyzing thousands of vertebrate species. They show that the island rule effects are widespread in mammals, birds, and reptiles, but less evident in amphibians.

Dwarf hippos and elephants in the Mediterranean islands are examples of large species that exhibited dwarfism. On the other hand, small mainland species may have evolved into giants after colonizing islands, giving rise to such oddities as the St Kilda field mouse (twice the size of its mainland ancestor), the infamous dodo of Mauritius (a giant pigeon), and the Komodo dragon.

In 1973, Leigh van Valen was the first that formulated the theory, based on the study by mammologist J. Bristol Foster in 1964, that animal species follow an evolutionary pattern when it comes to their body sizes. Species on islands have the tendency to become either giants or dwarfs in comparison to mainland relatives. “Species are limited to the environment on an island. The level of threat from predatory animals is much lower or non-existent,” Most probably because of the limited resources available on islands.

Scientific studies indicate that the magnitude of insular dwarfism and gigantism is more pronounced in smaller, more remote islands for mammals and reptiles.

They also found an effect of climate and seasonality on the island rule. Small mammal and bird species grew larger and large species stayed the same size to conserve heat in colder, harsher insular environments. Furthermore, when seasons are present, availability of resources become less predictable for reptiles, leading smaller reptile species to become larger (4).

The history of evolution is full of species of unusual size, big and small. But the factors that lead to evolutionary miniaturization or embiggening can be quite varied and are not all fully agreed-upon by scientists. This video posted below discuss what it means for life to evolve huge or tiny, what trends researchers have identified in body size evolution, and which of those trends actually holds up to further scrutiny.

Body size is of fundamental importance in understanding macroevolutionary patterns, both for extant taxa and for those with a fossil record. Some researchers describe four different kinds of body-size evolution: autapomorphic giantism, autapomorphic nanism, phyletic giantism, and phyletic nanism. The terms giantism and nanism are preferred here rather than the frequently, although incorrectly used equivalents, gigantism and dwarfism, respectively. Researchers assert that without a known phylogeny, it is difficult or impossible to differentiate these four different kinds of body-size evolution. Case examples are presented for two groups: varanid lizards (family Varanidae) and fossil horses (family Equidae). Previous hypotheses of body-size evolution within the Varanidae suggested that there were several cladogenic events in which some groups and isolated species became large. The most recent phylogeny of Varanidae based on mtDNA suggests otherwise. Mapping the known total body lengths onto the phylogeny indicates that varanids were already getting large early in their evolutionary history, with the crown group, Odatria, becoming secondarily small on mainland Australia. Although hypothesized as a giant island varanid, the komodo dragon (Varanus komodoensis) is discovered to be nested within a clade in which the basalmost taxon (V. salvadorii), also endemic to an island, reaches body lengths similar to those of the komodo dragon. Review of the Varanidae suggests that caution should be taken when characterizing taxa as island giants/dwarfs without first reviewing a phylogeny. Fossil horses (family Equidae) are frequently cited in the literature, as well as depicted in museums, as prime examples of Cope's rule, i.e., a gradual trend toward body-size increase over time. Several recent parsimony analyses have resolved many of the phylogenetic interrelationships of North American fossil horses and have elucidated their patterns of body-size evolution. In light of these new analyses, there is no evidence for Cope's rule in fossil horses. In fact, the evolution of large body size occurred multiple times in fossil horses and exemplifies autapomorphic giantism. Body-size decrease, oftentimes considered the exception to Cope's rule, is actually widespread within multiple clades of fossil horses and is characterized by both autapomorphic and phyletic nanism. The result of this analysis suggests that studies of body-size evolution must be intimately tied to a phylogeny before distinct patterns, if any, can be discerned. Cope's rule is not applicable to the two case examples presented herein, calling into question the most frequently cited mode of body size evolution (6,7).

I can go on and on as we have hundreds of research papers to quote but I think this is enough.

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