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I am creating a fantasy world where on one continent there are humans, dwarves and gnomes. All 'species' can interbreed (much like Neanderthals and Homo Sapiens), thus they need to have close similar ancestors for it to make sense. The planet is like earth in terms of atmosphere, gravity etc.

Is there any logical reason as to why humans would (d)evolve to become physically smaller to represent a dwarf and or gnome without needing to involve magical influence?

I have already considered:

  1. Magical influence. (Perhaps a magical crystal that enhances gravity?)
  2. Living in a cramped environment (low ceiling, thin hallways) for a longer period of time.
  3. Diet?
  4. A birth defect becomes common?

I'd love to get some input on this.

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@LiJun gives a great answer and overviews real-world examples of this type of thing happening. I would like to add that there are certainly evolutionary pressures which could explain why smaller species would be selected in certain regions:

  • Food - smaller people need less food. Especially in mountainous areas where temperatures drop during winter and farming is difficult if not impossible, a species which needs less food (and succeeds in digesting nuts and alcohols) is more likely to survive.
  • Predator Avoidance - in forested areas where food is plentiful, extremely small sized animals (gnomes, squirrels, rabbits, etc) often out-last predators by burrowing into trees or holes where larger prey cannot fit. Larger animals can't hide, but have to move (panthers can climb a tree, but monkeys swing from trees and panthers can't). In forested areas where larger predators are common, gnomes would have better success as they can live inside a tree while humans can't.
  • Growth Rate - while humans take 18-25 years to physically and psychologically mature, smaller animals can reach peak size much faster; this means faster breeding, and therefore larger overall community size in an area. When speaking of creatures with higher intellect / consciousness, this means more minds to solve complex problems.
  • Relative Strength - according to the square-cube law, smaller animals have less overall strength but larger relative strength. That is, an ant can carry objects many times it's size while an elephant cannot, but the elephant as more total strength. So smaller animals (gnomes, dwarves) would have more relative strength. Just as ants use this to their advantage to collect and gather resources for the colony, so could gnomes and dwarves find advantages for their relative strength.
  • Hibernation - as DrBob points out in his answer to another question, smaller animals can burrow and hibernate much easier than larger animals. In fact, only bears seem to be the larger animals who have figured out hibernation, and there is even debate over whether or not it's a "true" hibernation. In extreme climates, small animals who can hide and hibernate for months will have a significant advantage over larger animals.

All these are reasons why dwarf/gnome-like "sister species" to humanity would be naturally selected over traditional humans.

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  • $\begingroup$ Thank you for the great reply! Any idea on why Dwarves would be bulky and strong while Gnomes remain more frail yet agile, even though they live in the same environment and have the same amount of food available? Or do they both require their own unique/specific environment for this to make sense? $\endgroup$ – Finn Oct 15 at 12:30
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    $\begingroup$ @Finn for the same reasons why humans, dogs, spiders, horses, bears, squirrels, rabbits, and flies can cohabit the same space: their biology can direct them toward different food sources (maybe a mushroom can feed three gnomes for a week, but is poisonous to dwarves), or different skill sets (dwarves focus on mining and tool-forming, while gnomes work "smarter not harder" - maybe your gnomes work on cooking, chemistry, and making resources to last [making bread, smartly-engineered homes], while gnomes focus on brute-force foods [meat,veggies] and tools [axe, spear], etc) $\endgroup$ – cegfault Oct 15 at 12:34
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    $\begingroup$ I mean, it really could be anything; I'd encourage you to have fun and play with it; look up how different animals of these relative sizes currently act in our world and it should give you plenty of ideas for your world. $\endgroup$ – cegfault Oct 15 at 12:36
  • $\begingroup$ Consider the environmental factors that lead to earth having Gorillas, Humans, and Orangutans? [Someone with more familiarity/time can probably find more closely related primates as a better example.] - Also consider cultural impacts, and compare the look of humans at a Highland Games or Strongman competition to those who gather at a sprinting or fencing event. What leads to distinct looking peoples may have more than a single major source worth accounting for. $\endgroup$ – TheLuckless Oct 15 at 23:04
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maybe try check Homo_floresiensis and Pygmy peoples

from:https://prehistoric-fauna.com/Homo-floresiensis enter image description here

from:https://en.wikipedia.org/wiki/Pygmy_peoples#Rampasasa

enter image description here

theory regarding Homo Floresiensis

Aside from smaller body size, the specimens seem otherwise to resemble H. erectus, a species known to have been living in Southeast Asia at times coincident with earlier finds purported to be of H. floresiensis.4 These observed similarities form the basis for the suggested phylogenetic relationship. Controversially, the same team has reported finding material evidence (stone tools) on Flores of a H. erectus occupation dating back 840,000 years ago, but not remains of H. erectus itself or transitional forms.[36]

To explain the small stature of H. floresiensis, Brown et al. have suggested that in the limited food environment on Flores, H. erectus evolved a smaller body size via insular dwarfism.3 This form of speciation has been observed in other species on Flores also, as a result of selective pressures acting on island populations – including several species of the proboscidean genus Stegodon, an elephant-like creature. (A dwarf stegodont species of Flores, Stegodon sondaari, became extinct by about 850,000 years ago and was replaced by another species of normal size, Stegodon florensis, which then also evolved into a dwarf form, Stegodon florensis insularis, which disappeared about 12,000 years ago.)[37][38] This theory posits that the tropical rainforests on the island are a calorically impoverished environment, causing a dietary strain on hominins, especially in the absence of agriculture. Because of reduced resources, smaller individuals have an advantage because of their lower energy requirements.[39] This selection can also greatly affect sensory organs such as the brain, which could explain the small endocranial volume present in H. floresiensis.

The insular dwarfism hypothesis has been criticized by Teuku Jacob and colleagues[28] who argue that LB1 is similar to the pygmy peoples who populate a Flores village, Rampasasa,[40] – and who point out that size can vary substantially in pygmy populations. Contradictory evidence has emerged.[41] A 2018 study concluded that the Rampasasa pygmies are unrelated to H. floresiensis and represent an independent evolution of small stature via insular dwarfism.

theory regarding pygmy short stature

Various theories have been proposed to explain the short stature of pygmies. Some studies suggest that it could be related to adaptation to low ultraviolet light levels in rainforests.[9][10] This might mean that relatively little vitamin D can be made in human skin, thereby limiting calcium uptake from the diet for bone growth and maintenance, and leading to the evolution of the small skeletal size.[11]

Other explanations include lack of food in the rainforest environment, low calcium levels in the soil, the need to move through dense jungle, adaptation to heat and humidity, and as an association with rapid reproductive maturation under conditions of early mortality.[12] (See also Aeta people § Demographics.) Other evidence points towards unusually low levels of expression of the genes encoding the growth hormone receptor and growth hormone compared to the related tribal groups, associated with low serum levels of insulin-like growth factor-1 and short stature.[13]

regarding Insular_dwarfism

Insular dwarfism, a form of phyletic dwarfism,1 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 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.

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Like you said diet a birth disability that becomes more common. It could be that the sun gets too powerful and we are forced underground and for a while there are low ceilings and collapses happen so the eventually there will be an adaptation in the human genes.

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  • $\begingroup$ Welcome to the Worldbuilding Stack Exchange! Could you please elaborate on your answer, ideally including sources to support your statements? Having people forced to stoop does not mean that their offspring will be shorter - there must be a reason for shorter people to receive more offspring in comparison the the shorter people. $\endgroup$ – A Lambent Eye Oct 15 at 10:36
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Have you heard of speciation?

Any valid reason for populations separating (mining community stops exchanging genes with other communities, so they're all short but strong) would start this process, and you just need your divergent population to be generally shorter than your main population for them to retain this characteristic over time.

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  • $\begingroup$ Do you believe enhanced gravity (within reason) could speed up this process? $\endgroup$ – Finn Oct 15 at 9:58
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    $\begingroup$ @Finn I'm not sure how enhanced gravity would influence size, but shorter and stockier bodies would seem like the logical direction. en.wikipedia.org/wiki/Gravitational_biology lists some known effects of gravity on living organisms. I'm not sure what you mean by "speed up this process" though. $\endgroup$ – Whitehot Oct 15 at 11:39
  • $\begingroup$ thanks for your reply and the link. By "speed up the process" I mean the evolutionary process that would be required to turn a human into a "fantasy" dwarf. I am concerned about the speed of this process as procreation between both dwarves and humans have to remain possible. $\endgroup$ – Finn Oct 15 at 11:59
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    $\begingroup$ @finn I think that having increased gravity would tend to make shorter and stockier people. It would need to be a very particular gravitational setup so that one group of your population "shrinks" and the other doesn't, with a big difference in gravity between the two regions. $\endgroup$ – Whitehot Oct 15 at 12:08
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    $\begingroup$ @Finn if you're interested, you could also read up on sterile hybrid offsprings between close species, like Ligers and Mules: en.wikipedia.org/wiki/Hybrid_(biology) $\endgroup$ – Whitehot Oct 15 at 12:14
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Your small humans are stunted.

rachitic dwarf**

https://commons.wikimedia.org/wiki/File:Female_dwarf,_the_result_of_rickets,_c._1912_Wellcome_L0033871.jpg

Depicted - a woman with stunted growth due to rickets, or vitamin D deficiency during childhood. Nutritional deficiencies and chronic disease have a great impact on growth during childhood. People used to be a lot smaller on average because childhood diseases and nutritional deficiencies were common.

https://www.bbc.com/news/health-23896855

Data was collected on hundreds of thousands of men from 15 European countries.

For British men, the average height at age 21 rose from 167.05cm (5ft 5in) in 1871-75 to 177.37cm (5ft 10in) in 1971-75.

A public health expert said height was a "useful barometer" but it was crucial to focus on improving health overall.

...a high rate of illnesses such as respiratory diseases or diarrhoeas - which caused many infant deaths - would also affect survivors' development and therefore their subsequent height.

Infant mortality rates fell significantly throughout the period studied. https://www.bbc.com/news/health-23896855

Persons afflicted by disease and deficiency in childhood are often cognitively normal. Your small people can use their brains to work around their handicaps - for example this lady walks with sticks, and I bet she also wears custom shoes. One benefit of human intelligence is that we can quickly come up with technologic and cultural workarounds for problems that would require thousands of generations for an evolutionary workaround. Maybe your small people are also very clever - they need to be because they are frain and weak, but that cleverness is helpful for other endeavors as well.

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