Human variation and adaptation

Homeostasis

All organisms, including humans, must maintain a baseline of normal functions within their cells, tissues, and organs to survive. may be disrupted by a variety of both external and internal stimuli known as stressors.

Can include the physiology (function), morphology (form), and/or behavior of organisms in response to their environments.

Phenotypic plasticity

ability for one genotype to produce more than one phenotype depending on environmental conditions - ex - a plant growing different-sized leaves based on light exposure

to coping with stressors

1. adjustments (behavioural, acclamatory, developmental, all tend to be temporary)

2. 2. adaptations (permanent)

adjustments

to an organism’s nongenetic way of coping with the stressors of its environment.  ability of an organism to experience or develop an adjustment is based on its phenotypic plasticity.  

Adjustments occur exclusively on the individual level (diff individuals within a population can experience wide range of adjustments to a similar stressor). Adjustments are behavioural, acclamatory and developmental

behavioural adjustments

regarded as cultural responses to environmental stressors. (ex - turning on ac when its hot). These adjustments are temporary in nature and, since they are nongenetic, must be constantly altered to meet novel situations posed by the environment.

acclamatory adjustments

temporary, reversible changes in an organism’s physiology in response to environmental stressors.  

linked to its underlying phenotypic plasticity and the duration and severity of the stressor.

thermal stressors

can those lose or gain heat thru conduction, convection and radiation. Heat is not gained in evaporation

1. conduction -> how heat moves from a warmer body to a cooler one through direct contact.

2. convection -> when a warm body is surrounded by a cooler fluid (e.g., air or water), heat will be transferred from the warmer body to the cooler fluid.

3. Evaporation -> which occurs when a liquid, such as the water within our bodies, is converted to a gas, this energy is in the form of heat, and the effect is to cool the body.

4. radiation -> Form of electromagnetic waves is produced at a wavelength that typically lies below that which is visible to the human eye.

Thermoregulation

internal body temp is 36-37C. When an individual’s body temperature falls below 34.4℃ (93.9°F), the brain’s hypothalamus becomes impaired, leading to issues with body temperature control.  

A total loss of the ability to regulate body temperature occurs around 29.4℃, which can result in death.

body will respond to temps in two ways -> 1. increase production of heat 2.seek to retain heat

vasoconstriction

the constriction of peripheral capillaries in the skin. The decreased surface area of the capillaries through vasoconstriction results in less heat reaching the surface of the skin where it would be dissipated into the atmosphere.

also leads to the maintenance of heat near the core of the body where the vital organs are located.  

more at risk of cold-related injuries, such as frost-bite, which can lead to tissue necrosis (tissue death) in regions of the body that are most distant from the core

vasodilation

is an expansion of the capillaries within the skin leading to a more effective transfer of heat from within the body to the exterior to allow conductive, convective, radiative, and evaporative (sweating) processes to occur. Can result in heat related conditions. All humans have the same number of sweat glands. In hot environments, sweat glands become that produce great quantities of sweat. TO avoid dehydration, reduction of volume of urine that is being produced

altitude stressors

High-altitude environments, which are defined as altitudes in excess of 2,400 meters above sea level.

Challenges include decreased oxygen availability, cold temperatures, low humidity, high wind speed, a reduced nutritional base, and increased solar radiation levels.

altitude adjustments

increase the rate of breathing to allow for more quantity of air into the lungs (hypernea), is not sustainable. t

he rate of breathing begins to decrease as the person becomes acclimatized to the altitude. During the initial hyperpnea, the heart begins to beat faster but the amount of blood pushed through during each beat decreases slightly. Once the atmospheric oxygen reaches the alveoli, it spreads across the alveolar membrane and enters RBC. , it loosely binds with hemoglobin (an iron-rich protein) contained in the erythrocytes.

O2 is now free for diffusion into body cell but may not be sustained because of energy costs. With increased altitude comes an increased risk of miscarriage, lower birth weights, and higher infant mortality rates, cuz pregnant persons body is trying to maintain its own homeostasis and decrease volume of blood flow to uterus.

altitudinal developmental adjustments

occur only in individuals who spent their developmental period within a high-altitude environment.  the degree of developmental adjustment within an individual is directly related to their underlying phenotypic plasticity as well as the amount of time during the crucial growth and development period that the individual resides within the challenging environment. Growth happens more slowly & physical maturity happens later (linked to increased physiological demands, and reduced quality of nutritional base at higher altitudes)

what is an adaptation?

The long-term, microevolutionary (i.e., genetic) changes that occur within a population in response to an environmental stressor.

From evolutionary standpoint, phenotypic traits that has been acted upon by natural selection processes to increase a species’ ability to survive and reproduce within a specific environment.

In physiology, refers to traits that serve to restore homeostasis and that all adaptations are beneficial in nature.

altitudinal adaptation - examples

can happen in different manners.

Tibetan populations -> evidence of genetic exchange with denisovians (EPAS1 Gene, increased RBC production, reduces chances of stroke or heart attack, more O2 utilization).

Ethiopian populations -> increased vasodilation. Andean population -> higher hemoglobin, RBC and enlarged lung volume

skin tone adaptation

occurs along a continuum. The expression of skin tone is regulated primarily by melanin and hemoglobin. Melanin is a dark brown-black pigment that is produced by the oxidation of certain amino acids in melanocytes.  

Melanocytes are specialized cells located in the base layer. Within the melanocytes, melanin is produced in the special organelle called a melanosome. Melanosomes serve as sites for the synthesis, storage, and transportation of melanin.

Melanin

The two main forms of melanin related to skin, hair, and eye color are eumelanin and pheomelanin.

Eumelanin is a brown-to-black colored melanin particle while pheomelanin is more pink-to-red colored.  

Individuals with darker skin or hair color have a greater expression of eumelanin than those with lighter-colored skin and blonde or red hair.

melanogenesis

all humans have approximately the same number of melanocytes within their epidermis, the production of melanin by these melanocytes varies.

There are two forms of melanogenesis -> basal (dependent upon an individual’s inherent genetic composition and is not influenced by external factors.) and activated (ccurs in response to ultraviolet radiation (UV) exposure, specifically UV-B (short UV wave) exposure.).

Increased melanogenesis in response to UV-B exposure serves to provide protection to the skin’s innermost layer called the hypodermis. s

pecifically eumelanin, effectively absorbs UV-B radiation from light—meaning that it will not reach the hypodermal layer.

skin tone variations

cumulative exposure of the epidermis and underlying skin tissues to UV radiation may result in the development of abnormal cells within those tissues, leading to malignancies. With respect to skin tone, the various tonal shades occur clinally with darker skin being more prevalent near the equator and gradually decreasing in tone in more distant latitudes.

 It has been proposed that UV radiation’s destruction of folic acid, which is a form of B-complex vitamin, may have led to the selection of darker skin in equatorial regions. low levels of folic acid within the body during gestation may lead to defects in the formation of the brain and spinal cord of the fetus. This condition, which is referred to as spina bifida often significantly reduces an infant’s chances of survival without medical intervention.  

there is a relationship between lighter skin tone and vitamin D synthesis within the body. When skin is exposed to the UV-B radiation waves in sunlight, a series of chemical reactions occur within the epidermis leading to the production of vitamin D3, which is a fat-soluble vitamin that assists the body with absorbing calcium and phosphorus in the small intestine. ritical for the proper growth and maintenance of bone tissue within the body. In the absence of adequate minerals, particularly calcium, bone structure and strength will be compromised, leading to the development of rickets during the growth phase (rickets in youth)

glogers rule

This rule states that within the same species of mammals the more heavily pigmented individuals tend to originate near the equator while lighter-pigmented members of the species will be found in regions further from the equator.  applies latitudinally; however, it does not appear to hold for certain human populations near the poles.  it does not apply to the Inuit people (reflectivity of light off snow required darker skin tone to prevent folic acid degradation and consumption of vitamin d rich foods )

shape and size variations

bergman's rule -> predicts that as average environmental temperature decreases, populations are expected to exhibit an increase in weight and a decrease in surface area.

Allens rule ->  notes that longer, thinner limbs are advantageous for the radiation of excess heat in hot environments and shorter, stockier limbs assist with the preservation of body heat in cold climates.

nose shape

The selective role of climate in determining human nasal variation is typically approached by dividing climates into four adaptive zones

hot-dry, hot-wet, cold-dry, and cold-wet. role of the nasal cavity is to condition (i.e., warm and humidify) ambient air prior to its reaching the lungs.

In cold-dry climates, an individual’s nasal cavity must provide humidification and warmth to the dry air when breathing in through the nose (taller noses with smaller nostril size).

Conversely, in hot-wet environments, there is no need for the nasal cavity to provide additional moisture, so shorter noses with broader nostril size)

lactase persistence

Lactase-phlorizin hydrolase (LPH) is an enzyme that is primarily produced in the small intestine and permits the proper digestion of lactose (found in milk of mammals).

Generally, LPH production decreases between the ages of two and five and is completely absent by the age of nine. Individuals who continue to produce LPH have what is referred to as the lactase persistence trait (LCT).  commonly linked to cultures that have practiced cattle domestication. During periods of environmental stress, such as a drought, if an individual is capable of successfully digesting cow’s milk, they have a higher chance of survival than someone who suffers from diarrhea-linked dehydration due to a lack of LPH

first instances of the concept of race

first instances -> Egypt 1550 BCE, 4 groups of people that inhabited the land, based on skin colour, place of origin and behavioral traits.

In 23-79 CE -> roman philosopher Pliny the Elder created groups of people (civilized, barbarians, monstrous).

In 1000CE bible, three sons of noah -> shrem(asians) japeth(europeans), ham(africans).

Ancient greek the 'great chain of being' -> some people are rulers and some are slaves/workers.

14-18thcentury

travel, renaissance. 16th century - expanded exploration, uncivilized and not human.

18th - linneaus creating classification systems of people

Johann Blumenbach

race was defined from skin colour and head shape, believed there was fixed states and were immutable (not changing).

polygenism

multi ancestral lines - one for each different race.

monogenism

one ancestral line for all humans

Samuel Morton

pioneer of physical anth, large role in perpetuating in 1800s scientific racism to make biological determinism (that biology dictates behaviour, connection between physical characteristics and intelligence, morals, values, abilities) - no evidence for this. as both genetic and environmental factors affect intelligence

Franz Boas

early 1900s, cephalic index - took measurements of cranium sizes from those born in new york and the parents that immigrated from Europe and that head-shape did differ therefore not immutable

Galton and Hooten

proposed the idea of Eugenics - set of belief and practices that involved the controlled selective breeding of human populations with the hope of improving heritable qualities, especially through procedures like sterilization or polices regarding marriage. this idea emerged as a way to deal with criminals, diseased people, or anyone society thought were uncivilized peoples

Non concordance

describe how biological traits vary independent from each other. do not get inherited in a correlative manner. most traits in humans are clinal

continuous traits

differences between individuals in a population for a trait are a measurable along a smooth continuous gradient

what is race

a social construct based on the division of individuals into distinct groups on the basis of phenotypical variations such as skin colour, hair texture, shape of features, height. it is used to be an indicator of influence and power