Viruses play a significant role in day-to-day life, even though they are so tiny that they can’t be seen with the naked eye. The COVID-19 global pandemic is a current example of this, as the coronavirus has led to the closures of businesses, schools, and public spaces all over the world.
For billions of people around the globe, viruses are at the forefront of their minds right now. That’s why it’s important to share information about what viruses are, how they are structured, and how they spread between people. In this post, we will cover all of those vital topics and more.
What are viruses?
Viruses are a form of microbes, which are microscopic living things that exist all around the world, in air, water, and soil. Other examples of microbes include bacteria and fungi. Some microbes, like the bacteria Lactobacillus acidophilus, also live within the human body and help keep people healthy. However, other microbes can be seriously harmful to people — like some viruses.
How are viruses structured?
Viruses are the smallest type of microbes. According to the Microbiology Society, viruses are so small that “500 million rhinoviruses (which cause the common cold) could fit on to the head of a pin.”
Technically, viruses are not living organisms. Unlike some other microbes, viruses do not have cells. Instead, they contain genetic material (DNA or RNA), as well as a protective outer layer called the capsid, which is made from protein. The small size of a virus allows it to infiltrate healthy host cells, where it relies on the cell’s machinery to replicate itself. Without host cells, a virus would not be able to make copies of itself.
How do viruses affect people?
Different viruses can affect people in many different ways. For example, the influenza A and B viruses infect people with the flu every year. For many, flu symptoms will resolve themselves with rest and proper care. But for others, this virus can be deadly.
Similarly, the world is now grappling with COVID-19 (also known as coronavirus disease). People who have been infected with COVID-19 often experience respiratory illness and other symptoms like fever and cough.
Another example is the varicella-zoster virus, which causes chickenpox and shingles. These viral infections are highly contagious and cause itchy and painful blisters on the skin.
How do viruses spread?
People can transmit viruses to one another quite easily, especially in enclosed spaces. The main way that this happens is through coughing and sneezing. When a person has been infected with a virus, the droplets contained within their coughs and sneezes contain particles of the virus. If those particles come into contact with another person’s nose or mouth, they may become infected.
Viruses (including SARS-CoV-2, the virus that causes COVID-19) can also live on plastic and steel surfaces for two to three days. If someone were to touch a contaminated surface and then touch their eyes, nose, or mouth, they also run a risk of infection. This is why handwashing is critical to prevent the spread of this virus.
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While animals, plants, and fungi are some of the more commonly recognized kingdoms in biological taxonomy, there are also other less well-known kingdoms of organisms that exist and play an essential role in the biosphere.
It’s true that there is some debate among biologists regarding the classification of kingdoms and how many truly exist. However, the following six kingdoms have mainly been agreed upon by a number of prominent biologists, and the six-kingdom system is generally taught in schools across the United States.
In this post, we will offer a quick overview of each kingdom and the role it plays in the world. Take a look and discover the differences between these fascinating life forms.
Of course, animals are the most widely recognized kingdom, with human beings belonging within this classification. It is also the largest sector, with more than 1 million identified species. Animals include organisms that dwell on land and in the sea, such as mammals, amphibians, and reptiles. Invertebrates also fit within this kingdom and make up 97% of all animal species.
Plants are the second-largest kingdom of the six and are critically important to the wellbeing of the earth. Species within this kingdom create their own food via the process of photosynthesis, while also providing oxygen that many animals need to breathe. Plants also serve as food for many land and sea animals.
In previous schools of thought, fungi were categorized within the plant kingdom. However, you will find a few notable differences between the two kingdoms. For instance, fungi are not able to feed themselves through photosynthesis and must rely on the organic matter around them for sustenance. Common examples of fungi include mushrooms, yeast, and molds.
The organisms within the Protista kingdom fit within a wide-ranging spectrum. Most consist of a single cell (unicellular), but all protists are eukaryotes, just like fungi, animals, and plants. This means that their cells have a complex composition, including a nucleus and organelles. Examples of protists include algae and amoebas.
Unlike each of the four kingdoms listed above, eubacteria are single-celled prokaryotes (as are archaebacteria). This means that the nucleus within the cell is not bound by a membrane. These bacteria are found almost everywhere and include things like Escherichia coli (better known as E. coli) and Streptococcus pneumoniae (which causes strep throat).
Archaebacteria were discovered in the 1980s and joined the six kingdoms shortly after that. Like eubacteria, they are single-celled prokaryotes. However, they are in a realm of their own, because biologists discovered that they are the oldest living organisms on the planet. Scientists believe that archaebacteria derive from ancient bacteria that used to live in hydrothermal vents in the deep sea. Interestingly, they are not closely related to eubacteria.
Within each of these six kingdoms, organisms can be further classified into phyla, classes, orders, families, genera, and species. Biological taxonomy is complex and sophisticated, as scientists continue to study life forms all around the globe.
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For many men, hair loss is an inevitable but challenging part of getting older. The most common type of hair loss is male pattern balding, which is also known as androgenic alopecia.
While there has been extensive discussion and debate about alleged cures for hair loss (including pills, lotions, and hair transplant surgery), not many people know about the science behind hair loss. In this article, we will outline the medical causes behind male pattern balding, focusing on how dihydrotestosterone (DHT) impacts hair loss.
What is DHT?
Men’s bodies naturally produce androgen hormones, which create male sex characteristics such as a deep voice and increased muscle mass. DHT and testosterone are two examples of these androgens.
Of all testosterone in a man’s body, about 10% of it is converted into DHT using an enzyme called 5-alpha-reductase (5-AR). Following that process, DHT is then released into the bloodstream. Sometimes, there is an excess of DHT in the body, which can not only lead to hair loss but also coronary heart disease and prostate cancer.
What role does DHT play in hair loss?
In the bloodstream, DHT and other androgens (like testosterone) attach themselves to receptors on hair follicles. However, DHT binds to these receptors five times more than testosterone does.
When this happens, DHT reduces the size of the follicles and shortens the growth cycle of hair that originates from these follicles. Then, the follicles are unable to create enough healthy hair, so hair may appear thin and take longer to regrow.
When there are higher amounts of 5-AR in the body, more testosterone will be converted to DHT (which is considerably more potent than testosterone). As a result, more DHT in the bloodstream will equate to increased hair loss.
DHT can impact people in a variety of ways, but many men experience the adverse effects of this androgen. In fact, about half of men in the United States will have some form of DHT-caused hair loss by the time they turn 50.
How can this information be used to prevent or reverse hair loss?
Limiting the amount of DHT has proven to be effective in reducing male pattern balding. One of the most common treatments is the oral medication finasteride, which prevents 5-AR from converting testosterone into DHT. In turn, this increases the amount of testosterone in the body.
Researchers have found finasteride to be useful for hair regeneration. In one Japanese study, 87% of participants reported seeing great, moderate, or slight increases in hair growth from taking 1mg of finasteride per day.
Currently, in the United States, finasteride is available with a doctor’s prescription and sold under the brand names Propecia and Proscar. It is important to remember that this medication will only stimulate hair growth as long as an individual takes it. In other words, once a person stops taking finasteride, hair growth will cease.
Although hair loss is not considered a serious medical condition or disease, it can cause severe self-esteem and confidence issues for the men (and women) who experience it. As such, it is vital to understand the science behind hair loss and the available methods for treating it.
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For expecting parents, envisioning what their baby will look like is an exciting part of the pre-birth journey. Factors like eye color, hair color, and other physical features remain a mystery until the child is born (and can even change after birth).
However, there are ways to determine the probability of a child’s eye color, using their mother and father’s genetics. In this post, we will discuss how a person’s eye color is determined and provide examples of specific eye color outcomes for children based on their parents’ genetic traits.
How is eye color determined?
In broad terms, a person’s genetics are the determining factor in their eye color. However, the specific genes that affect eye color are more complex than some may realize. Here’s how the process works.
First, these genes produce the pigment known as melanin (which also plays a role in a person’s skin and hair color). The amount of melanin that someone has in the front layers of their iris will dictate what their eye color is. More melanin equates to darker eyes, while less melanin creates lighter eyes.
In addition to melanin, there are also two subtypes of this pigment present in the eyes: red-yellow pheomelanin and black-brown eumelanin. Depending on how much of these pigments are produced, eyes may appear in shades such as green, hazel, or amber.
So what determines how much melanin is produced? Let’s look at chromosome 15, where two genes play a significant role in pigment production. First, the OCA2 gene produces the P protein, which is present in melanocytes (the cells that create melanin).
Meanwhile, the HERC2 gene contains a strand of DNA that controls the expression within OCA2. As HERC2 reduces expression of OCA2, less melanin is produced (leading to lighter-colored eyes).
Examples of eye color outcomes
Putting all of that information into practical use, how can parents-to-be predict their child’s eye color? While there is no way to be entirely certain about the outcome, genetic data can be applied to come up with an assumption about a baby’s potential eye color.
To do this, parents can create a graph (known as a Punnett square) using each individual’s eye color and family history to get an overview of the possibilities for their child’s eye color. On the graph, each parent will be assigned a sequence of letters based on their eye color and the traditional eye color that others in their family lineage have had.
Here’s a simple example. If the mother has brown eyes and a family history of brown eyes, they could be assigned “BB” on the chart, listed on the horizontal axis. If the father has brown eyes, but most others in his family have had blue eyes, he would be assigned “Bb,” listed on the vertical axis. The result is four squares with four possible combinations – two that would indicate a likelihood of brown eyes (“BB”) and two that offer the possibility of blue eyes (“Bb”). To learn about how to create your own Punnett square, check out this resource, which also offers examples of more complex eye color combinations.
As mentioned earlier, this type of analysis doesn’t guarantee a child’s eye color but instead offers an overview of potential outcomes. It is also important to note that melanin takes some time to permeate a newborn’s eyes, which explains why many babies are born with blue eyes that later change.
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The hard outer shell of arthropods and insects like beetles is primarily made up of chitin, a naturally occurring biopolymer. The following BiologyWise article elaborates more on the structure, function, and uses of chitin.
Did You Know?
Behind cellulose, chitin is the second-most abundant natural biopolymer in the world.
If one is to observe a lobster closely, he cannot fail to notice its tough outer covering. This protective outer shell, referred to as the exoskeleton is a distinguishing feature of arthropods that include crustaceans (crabs, lobsters, shrimp), arachnids (ticks, mites, scorpions, and spiders), and even insects (beetles, grasshoppers, butterflies). Chitin, a naturally occurring biopolymer is an important component of this exoskeleton. The internal shells of cephalopods and radulae of mollusks are also primarily composed of chitin.
Chitin is essentially a linear homopolysaccharide (long chain polymer) consisting of repeated units of N-acetyl-glucosamine, which is a monosaccharide derivative of glucose. These units form covalent β-1,4 linkages. Chitin with the chemical formula (C8H13O5N)n is considered as a complex carbohydrate, whose structure resembles that of cellulose, with one hydroxyl group on each monomer replaced with an acetyl amine group.
- This skeleton on the outside of the body appears hard and rigid due to the presence of chitin that is known for its tough elastic properties. Although chitin is the dominant constituent, other compounds such as proteins and calcium carbonate also play a crucial role in the formation of exoskeleton. The main function of this chitin-containing exoskeleton is to keep the inner soft tissue safe from any sort of injury.
- Most importantly, it prevents these delicate tissues from becoming dry. In short, it acts as a watertight barrier against dehydration, which is crucial for their survival.
- The hard chitin-containing exoskeleton of arthropods also acts as a defense mechanism against predation. This outer covering can tolerate strong compressive stresses, which can provide protection from predation because predators exert a compressive force on the exoskeleton to injure their victim.
- The fungal cell wall that protects the micro-organism from the outside environment is also made up of chitin.
Chitin is released from the animal’s outer skin (epidermis) to form the protective covering. After the exoskeleton fully develops, the growth of epidermis stops. Moreover, the exoskeleton is found to be relatively rigid, since it does limit growth with the increase in the size of the animal. So when there is a mismatch between the anatomy of the arthropod and the size of the exoskeleton, it can cause suffocation. To avoid this, the animal gets rid of the exoskeleton and begins to form a new one. This process of shedding the current skeleton is done periodically, which is necessary for their proper growth.
As a Fertilizer
One of the most important benefits of chitin is its use in making fertilizers. Chitin-containing fertilizers are organic, non-toxic, and have shown to increase crop productivity. Chitin in fertilizers helps in increasing soil organisms and enzyme activities, which positively affects soil health. This in turn increases crop yield.
As a Food Additive
Chitin has a long history of use as a food additive. It is commonly obtained from crabs, and shellfish that include shrimp. Sometimes cell walls of eumycetes (a type of fungi) are used as a source for extracting chitin. Microcrystalline chitin (MCC) as a food additive can be helpful to enhance taste and flavor.
As an Emulsifying Agent
Use of food chitin can also help in creating stable food emulsions. It essentially acts as an excellent emulsifying agent, which helps to prevent the breaking of emulsion when exposed to other fluids. For instance, whipped dessert toppings often contain chitin that provides uniformity and stability to the product.
This naturally-occurring fiber-forming polymer can also help to lower cholesterol levels as found out through animal studies. Chitin molecules tend to mop cholesterol and fat in the digestive system. So chitin in the diet may help to reduce cholesterol absorption efficiency.
As a Surgical Thread
Chitin is also used for manufacturing strong and flexible surgical threads. Quite a few dissolvable stitches used to close wounds are made from chitin. These stitches start decomposing during the wound healing process. Reports also suggest that stitches composed of chitin may help to facilitate the healing of wounds.
Chitin in its supplemental form may help to reduce cholesterol. Moreover, chitin is said to have antioxidant, anti-diabetic, anti-inflammatory, antimicrobial, anticoagulant, antihypertensive, and anticancer properties. So taking it in the supplemental form may be beneficial for overall health.
Receptor-mediated endocytosis is a process in which receptors are used for importing material from extra-cellular matrix into the cells. This BiologyWise post elaborates more on this cellular process critical for the growth and development of cells.
Did You Know?
If endocytosis, an important cellular process is not taking place properly, it may cause chronic ailments like leukemia and Alzheimer’s disease.
Endocytosis is an important process used by body cells for their survival. It is essentially a cellular process in which cells ingest nutrients in the molecular form. This is how cells eat for their survival. It is a mechanism of nutrient absorption, which is critical for cellular growth.
In most cases, the material that needs to be absorbed by cells is available in the form of large molecules that are simply unable to penetrate the cell membrane. This is where endocytosis comes into picture, wherein the cell simply swallows the substance (molecule) and enters the cytoplasm. Thus, it is through endocytosis that the substances can move inside the cell.
Depending upon how endocytosis is carried out, it is classified into 3 main types, viz. phagocytosis, pinocytosis, and receptor-mediated. Here we will be discussing receptor-mediated endocytosis.
Receptor-mediated Endocytosis (RME)
This type of endocytosis makes use of receptors (attached to the cell membrane) to engulf molecules. It is also referred to as clathrin-dependent endocytosis, since clathrin (a type of protein) is crucial for the proper execution of this cellular process. In this technique, specific molecules that get bound to the receptors can only be engulfed by the cell.
- Whenever a receptor (molecule) that is embedded within the plasma membrane, detects a molecule (that it can bind to) outside the cell, it immediately hooks onto it. The molecule (ligand) attached to the receptor then travels all the way to the clathrin-coated pit.
- A clathrin-coated pit is a special area located on the plasma membrane that initiates the uptake of molecules present in the extracellular region. The pit shows a distinct polygonal lattice of clathrin chains on its inner surface. When the receptor-molecule reaches the clathrin-coated pit, it is observed that the pit folds inwards and then that part of the membrane detaches itself to form a closed coated vesicle.
- These vesicles act as tools to move molecules inside the cells. Simply put, they transport molecules within the cells. If the receptor-bound molecule is a pathogen, opsonization mechanism is activated, meaning the molecule is tagged as a pathogen for subsequent destruction. After opsonization, the protein coat of clathrin is shaken off to allow the vesicle to merge with an early endosome.
- An early endosome is an organelle, a sorting compartment that helps separate the molecule from its receptor. Once the vesicle fuses with the endosome, multiple compartments are formed within the endosome and at the same time the molecule detaches itself from the receptor.
- Chemical changes occur within the endosome to form a late endosome. The late endosome splits into two, in which one endosome contains the molecule, while the other contains the receptor.
- The molecule-containing endosome then combines with a lysosome, which is essentially a membrane-bound cell organelle that stores digestive enzymes. The main job of the lysosome (also referred as the stomach of the cell) is to promote breakdown of the molecule, which can either be a protein or fat. The lysosome can also ingest pathogens including bacteria and viruses, in turn helping to clear cellular debris.
- The lysosome is essentially involved in digesting the material present in the endosome. The receptor in the other endosome is then recycled and sent back to the cell surface.
Body cells use the mechanism of receptor-mediated endocytosis to absorb cholesterol, growth factor EGF, and the iron transport protein transferrin from the bloodstream.
Any change in the normal sequence of DNA is called a mutation. Most mutations in organisms are deleterious by nature. This post explains this type of mutation in a comprehensive manner.
Deleterious mutations do not persist in haploid populations, since they only possess one copy of genes, which if mutated, proves to be fatal to the organism. Thus, the mutated gene is not passed onto further generations.
All living beings possess genetic material that is composed of a sequence of nucleotides. Errors in this sequence are known as mutations, and they exist in the genome of all living beings. It is impossible for the genome of a biological entity to be devoid of any mutations. With each new generation, almost 100 – 200 new errors are incorporated into the genome. Considering the vast timeline of the existence of life on Earth, the amount of accumulated mutations is staggering. To put this in perspective, if we consider humans, each generation includes 100 new errors into the human genome, and at the current population growth rate, each generation of humans on the entire planet has a cumulative 100 billion mutations. Over the years, this vast aggregation of mutations has provided the raw material for the development of various genetic alleles, which increase the genetic variation and diversity, thereby providing the groundwork for the process of evolution and natural selection.
Despite the usefulness of mutations with respect to genetic variability, not all of them are desirable with regards to the overall fitness of the organism. Hence, they are segregated into three types: neutral, beneficial, and deleterious. Neutral mutations have no observable effect on the organism. They merely increase the genetic variation. Beneficial mutations provide the organism with a vital advantage for its survival and proliferation. Finally, deleterious mutations, as the name suggests, pose a threat to the fitness of the organism, as they have harmful effects of the general health of the organism. In general, when considering population genetics, mutations are usually deleterious in nature; very few are beneficial or neutral.
As mentioned earlier, these mutations reduce the fitness of an individual. But what is meant by fitness? It refers to the capacity and ability of an individual to carry out normal life activities, to live a relatively disease-free existence, and to pass on their genetic material to the next generation by reproducing. This is applicable to all living beings, from plants to animals, from microorganisms to birds, and from insects to humans. The fitness of the individual enables it to reach maturity, where its healthy constitution is conducive to reproduction, via which its genetic legacy is passed on to the next generation.
However, in case of deleterious mutation, any one or all of these stages may be compromised, causing the individual to be unfit. The mutation may cause health problems, reproductive failure, and even premature death, depending on the gene in which it occurs. With respect to humans, these three effects can be explained by the following examples.
A mutation in the LMNA gene that produces laminin, a protein that provides support to the cellular nucleus, gives rise to a condition called ‘progeria’. It is a disease that causes accelerated aging, and is characterized by sclerotic skin, baldness, bone abnormalities, growth impairment, etc. Almost all genetic disorders are caused due to the presence of deleterious mutations.
A mutation in the AZF (azoospermia factor) or the SRY (sex-determining region Y) gene gives rise to a non-functional gene-product, resulting in infertility in case of men, thereby inhibiting that particular male to be able to pass his genes to the next generation.
In case of animals like peacocks, who attract mates by the display of their bright tail feathers, any mutation that adversely affects this phenotype would result in the peacock’s failure to attract a mate (peahen) and reproduce.
Mutations that give rise to conditions like spina bifida, metabolic genetic disorders, and Marfan syndrome greatly reduce the quality and length of the affected individual’s lifespan, causing that person to die a premature death. Also, some mutations that occur in embryo development genes result in the premature death of the embryo, leading to miscarriages or still births.
Persistence of Deleterious Mutations
Despite their harmful nature, why do deleterious genes persist in the genome of organisms? This could be due to a number of reasons, such as the rate of elimination of these mutations may be low compared to the rate at which they appear. Other probable reasons are as follows.
This is the condition where the possession of two different copies of a gene (wild-type and mutant) is beneficial to the organism, rather than detrimental. An example of this is the mutation that occurs in the hemoglobin gene, resulting in the condition called sickle cell anemia (SCA). In this case, the homozygote for the mutant allele will show a deleterious effect, i.e, the individual will suffer from SCA (all RBCs will be sickle-shaped). However, if the individual is a heterozygote, the recessive nature of the condition will render him a carrier (Partial sickling of RBCs) of the condition. This is beneficial, since the malarial parasite P. falciparum that infects red blood cells and deprives them of oxygen will be unable to infect the sickled cells and lead to a malarial infection. In other words, the partial sickling of RBCs of a carrier render that person immune to malaria. On the other hand, a wild-type homozygote individual would be susceptible to the malarial infection.
No Effect on Reproductive Fitness
In some cases, the deleterious effect of the mutation is exhibited at a later stage in life, by when the reproductive stage of the organism has already elapsed. Hence, the mutations are passed on despite their harmful nature, as the effect does not interfere or exhibit itself during the reproductive stage. An example of this is the trinucleotide repeat mutations seen in the HD gene that causes Huntington’s disease. In this case, the effects of the disease are seen after the age of 40, and till then, the individual has already reproduced and passed this deleterious mutation onto the offspring. Despite this affecting the fitness of the individual, it persists, since it does not affect the reproductive fitness of the individual, but merely shortens the lifespan.
Maintained by Mutations
Some mutations may keep arising in certain genes despite the elimination efforts taken by the organisms genome. This may be due to the hyper-mutable nature of the gene, and also because the gene maybe too vital to tamper with (to prevent the induction of other accidental errors). An example of this is the NF gene, which when mutated, gives rise to a condition called neurofibromatosis, that causes tumors of the nervous system. Here, it may be difficult to remove the mutation, since any unwanted disruption in the gene sequence will only cause further damage. Also, even in case this mutation is eliminated, the gene does have a high tendency to mutate; almost 1 in every 4,000 gametes possesses new mutations of this gene.
Maintained by Gene Flow
This refers to the prevalence of a mutated gene copy in a population to its introduction by another population that has migrated to the same location. As mentioned above, the SCA mutation is beneficial to areas with rampant malaria, as is the case with the regions of the African continent. However, when the carriers residing in this area migrated to other countries with a low incidence of malaria, the SCA mutation was introduced into the populations of those countries. Therefore, human migration brought about the flow of genetic material from Africa to other countries, where this mutation, in the absence of malarial incidence, was purely detrimental.
Polyploidy of Genome
Deleterious mutations are usually recessive in nature. If a haploid organism possesses a deleterious mutation, the effect can be readily observed, crippling the organisms fitness, and resulting in its demise. However, in case the organism is a diploid or polyploid with multiple alleles of a gene, the detrimental effect can be silenced or overridden by the presence of a fully functional wild-type allele. While this prevents the expression of the mutated allele, it does not eliminate it, causing it to persist in the population, till two individuals with the same allele reproduce and give rise to an offspring that will suffer the deleterious effects of the mutation.
Although the cellular repair machinery, along with the proofreading mechanisms, try to eliminate the mutations, certain mutations are not rectified or are actively conserved (as explained above). The accumulation of mutations, by this way, over the course of several generations, leads to an effect called Muller’s ratchet, which may lead to the extinction of the species of that organism. This effect is a principle studied in reference to the extinction of species, and the effort to conserve those on the brink of extinction.
Genetic diversity is considered to be the baseline of biodiversity. It is the cumulative sum of all the genetic traits or characters present in the genome of any given population or species.
Africa exhibits higher genetic diversity levels than most other areas of the world.
The term ‘biodiversity’ refers to the amount of variation in all biological entities within ecosystems. It is often defined as being the cumulative sum of all genes, species, and ecosystems of a particular region. Hence, it can be studied on three interrelated levels: genetic, species, and ecosystem. However, the genetic level forms the baseline. Genetic diversity in organisms forms the root of any genetic adaptation or variability. It is the basis for evolution and natural selection. The genetic diversity of a species paves the way for the species to be able to adapt to its changing environment. It refers to the possession of various genetic traits in a species, that may or may not be expressed depending on the prevalent environmental conditions. However, this term should not be confused with that of genetic variation.
Genetic variation refers to the prevalence of two or more allelic forms of a particular gene in a given population, whereas, genetic diversity refers to the prevalence of a diverse range of genes in the gene pool of organisms. Hence, variation deals with individuals, whereas, diversity deals with entire populations or species. Genetic diversity can be considered to be a combination of variation as well as variability (tendency for variation to occur in a species). This combination helps biological species to be adaptable to changes in its environment. These changes could be a result of change in population sizes, weather conditions, natural disasters, arrival or departure of competing species, etc.
Sources of Genetic Diversity
In germ-line mutations, the alteration in the DNA sequence is passed onto the organism’s offspring. In time, these mutations may accumulate and cause defects in the genome, which could ultimately prove fatal, causing the line to die out. On the other hand, some mutations may be conserved by the genome, and ultimately, over time, lead to the development of a new trait, i.e., cause the evolution of a gene. This would increase the genetic diversity of that organism. In some cases, the evolution of new genes may have a drastic result, causing the formation of an entirely new species. However, this process is incredibly slow, taking place over a long period of time, although, it is the way true diversity manifests in nature and induces evolution.
It refers to the creation of a new species. The differentiating factor between the old and new species is that, they are unable to breed with each other. Speciation takes place via multiple processes, which include geographical isolation, competition, and polyploidy.
◆ Geographical isolation occurs when natural phenomenon cause a habitat to be split in such a way that a part of the population is separated from the main population, with no means of rejoining it. Eventually, the separated individuals adapt to their new surroundings, and evolve at a different rate and in a different direction than the parent population, causing them to diverge to an extent where they are no longer sexually compatible with the main population. In essence, they have evolved to form a new species. This same effect is observed when a species migrates from one habitat to a new one (immigration).
◆ Speciation by competition occurs when a particular food source becomes limited, or by the arrival of a different species in the habitat that survives on the same food source. In such a scenario, certain parts of the population will undergo changes and adapt themselves to become better at acquiring the food, and protecting it from being taken away. Hence, in terms of species propagation, the better adapted individuals have a better chance at reproducing, due to their enhanced ability to secure food in comparison to the other individuals who have diverted to other food sources. Therefore, the selective breeding of the enhanced part of the species would eventually lead to them becoming a separate species altogether
◆ Polyploidy is observed more in case of plants than in animal species. This is so, because, animal reproductive processes are much more sensitive to gross chromosomal changes as compared to plants. In plants, however, polyploidy is quite a common phenomenon. Polyploidy refers to the presence of more than two copies of a chromosome in the genetic material of the organism. In plants, instances of self-fertilization are quite common, and there are also cases where similar species have been successfully crossed with each other. Due to this, the polyploid effect can be observed instantaneously in the subsequent offspring. The most common example of this is that of common wheat (Triticum aestivum), which contains six sets of chromosomes. It is the result of the crossing between a diploid Triticum urartu and a diploid Aegilops speltoides, which resulted in Triticum turgidum, that possess 4 sets of chromosomes. Finally, Triticum aestivum was obtained by the crossing of Triticum turgidum with a diploid Aegilops tauschii.
Errors in Meiosis
During prophase of meiosis, in plants and animals, crossing over of chromatids occurs. This causes an exchange of genetic material between the two homologous sister chromatids. The intermixing of genes from the parental chromatids causes an increase in the genetic diversity of the resulting offspring. In case the crossed over chromatids do not separate and remain joined during chromosomal segregation, one daughter cell gains an additional chromosome, while the other is devoid of one. This increases the diversity in one cell while, at the same time decreases it in the other.
During meiotic divisions, translocations and chromosomal structural changes may lead to the formation of dissimilar sister chromatids. In such an event, the load of genes being distributed to each daughter cell will vary, causing the induction of diversity in the genetic material of each daughter cell. However, in some cases, these errors might also prove fatal to the daughter cell/offspring.
Factors Affecting Genetic Diversity
Any event or gradual process that culminates in the extinction of a species causes the loss of all the genetic diversity represented by that species of plant or animal. Entire species may be wiped out due to mass extinction events like the Ice Age or the meteor and volcanic eruptions that wiped out the dinosaurs. Species may also go extinct individually due to their excessive hunting as a source of food or valuable commodity. For example, the dodo birds were hunted as a convenient food source since they were unable to engage in evasive tactics. Also, the rhino has been driven to the very brink of extinction by hunting it for its horns. Alternatively, species may experience extinction due to alterations to their ecosystem. An example would be deforestation in order to utilize land to set up cities or factories. This would lead to the destruction of that ecosystem and all the indigenous specious therein. In some cases, certain genes may also go extinct in an organism due to its inability to be transferred to the next generation.
As mentioned previously, competition for food resources may cause the adaptation and evolution of a species. However, this is the positive effect. If the negative effect is considered, there is chance that the newly arrived competing species is stronger and better adapted than the indigenous species. This would lead the new species to out-compete the old species, and alienate them from the food source. The unavailability of a food source would cause the old species to die out, and hence, become extinct. Although, this outcome can be averted by the migration of the old species to a new environment which lacks a competing species.
These events refer to situations where a considerable amount of the population die due to natural phenomena or human activities. The surviving population just represents a fraction of the original genetic diversity of the species, and hence, if the population of the species rebuilds, the overall genetic diversity will also be low. The lost diversity will require a considerable amount of time to be recovered. This is coupled with the side effects of inbreeding and genetic homogeneity, leading to an effect called the bottleneck effect.
Significance of Genetic Diversity
A high diversity is vital, since genetic diversity is directly related to biodiversity. Reduced diversity would eventually reduce the overall biodiversity of an ecosystem, whereas, greater diversity would lead to increased biodiversity due to the adapting and speciation of the species when faced with an adverse environment. If we consider an organism with higher diversity of genes, it implies that it has a higher chance of utilizing its diverse gene pool and adapting itself to change along with its changing environment. This allows it to evolve to be better suited to survive. One can equate this to possessing a fully-kitted-out tool box while carrying out repair work. In case some repair fails, one has the necessary tools to deal with it and overcome the setback. However, if the tool box contains a minimal amount of tools, the repairs becomes difficult to manage, and may even lead to further complications. Similarly, if an organism is not genetically diverse, but is homogeneous and uniform, then in the event of a change in circumstances, it will be ill-equipped to adapt to and overcome the adversity, causing it to die out. Even if such an event does not occur, the uniform gene pool will lead to the side effects of inbreeding, such as accumulation of genetic defects, loss of genetic health, and eventual death of species.
◾ The domestication of wolves into various breeds of dogs via selective breeding has greatly increased the genetic diversity of the canine genome as compared to its lupine origins.
◾ In general, woody plants like trees have a higher genetic diversity than vascular plants like grasses. This is due to the size of their geographic range, and also due to the ability to scatter their seeds over a wide area by the use of various seed dispersal methods.
◾ The cheetah population in the world has a very low genetic diversity, as a consequence of a bottleneck event that occurred 10,000 years ago. This low diversity has led to an increase in lethal genetic disorders and low reproductive success.
◾ In potato cultivation, new plants are formed as a result of asexual reproduction, and hence, are clones of the parent plant, implying that there is very low genetic diversity. This proved to be disadvantageous for the potato plant during the potato famine in Ireland in the 1840s, when potato fields were ravaged by a rot-causing oomycete called Phytophthora infests.
◾ In oceanic plankton, ocean viruses help maintain good levels of genetic diversity by a process called genetic shifting. The viruses carry genes of other organisms in addition to their own genome, and when this viral particle infects a cell, these genes are transferred into the infected cell as well, where the host genome changes and incorporates these genes, thereby gaining genes and diversity.
◾ Numerous varieties of corn have emerged as a result of hybridization between multiple related genera, and also due to the presence of mobile genetic elements called transposes. These factors have resulted in a marked increase in its genetic diversity, which can be observed in the form of the diverse colors exhibited in its kernels.
A high level of genetic diversity of a species is essential for its prolonged survival, despite the changing environment. Hence, in order to preserve and maintain the current biodiversity of the various ecosystems on the planet, appropriate conservation measures must be undertaken. Additionally, research could be carried out regarding ways to increase the genetic diversity of the species that lack it.
Reproductive isolation refers to a set of mechanisms that prevent animals of the same group from breeding. These are reproductive barriers that do not allow the species to mate and produce offspring.
Did You Know?
Reproductive isolation that occurs before fertilization is known as prezygotic isolation. However, when it occurs after fertilization and prevents the fertilized egg from turning into a fertile offspring, it is known as post-zygotic isolation.
Reproductive isolation refers to a set of conditions, that can be psychological, ecological, genetic or behavioral, which do not allow animals of closely-related species to unite and mate. In reproductive isolation, there are strong reproductive barriers that keep the related species separate. Two species belonging to the same family are unable to breed and produce offspring successfully due to various differences, which form an important aspect of reproductive isolation.
Types of Reproductive Isolation with Examples
- Here, the breeding or mating calls act as powerful reproductive barriers. If the closely-related species are unaware of their mating rituals (which are often bizarre), inbreeding does not occur. In behavioral isolation, despite being close to each other there is no sexual attraction between the male and female species. This happens because either of the species is unaware of special signals or the ritual that has to be performed before mating.
- Male hooded seals inflate their nasal cavities like a bubble gum to attract their female counterparts. The female seal will mate the one with the most visually appealing nasal balloon. So unless the male blows up his nasal, the female won’t come near its prospective mate.
- Similarly, light production in male fireflies is necessary to draw female attention. Depending upon the flash pattern produced by the male, the female will then respond and light up to signal the mate.
In mechanical isolation, the reproductive barrier arises due to incompatibility between the sex organs of male and female species. The organs do not fit correctly (probably due to variation in shape or size) or cannot be aligned due to their unique body structure, thereby preventing inbreeding. Mechanical isolation is commonly noticed among snails with their shell coiling in the opposite direction.
- Sexual receptivity in male and female species is possible only if their breeding season is the same. In temporal isolation, mating seasons of the closely-related species do not match.
- In some animal species breeding seasons differ and arrive at different times of the year. Also, some species are sexually active in the morning while their counterparts prefer nighttime for mating. These different times of the day for remaining sexually active are also one of the isolating mechanisms of temporal isolation.
- Some species of fruit flies such as Drosophila persimilis are active in the morning, whereas its close relative Drosophila pseudoobscura prefer mating in the afternoon.
- In ecological isolation, the male and female species prefer a different habitat for mating. The difference in mating sites forms the reproductive barrier in ecological isolation. As their places of breeding do not coincide, the species prefer not to mate.
- Rana aurora, one of the species of frogs found in northern California prefer rapidly moving water streams for inbreeding. Whereas, the Rana catesbeiana species prefer the still, slow-moving waters of permanent ponds.
The reproductive barrier occurs due to incompatibility between the sperm and the egg. As we all know, the interaction between a female egg and a male sperm that produces a zygote (fertilized egg) is necessary to produce offspring. However, the sperm is not drawn towards the egg due to genetic or chemical incompatibility. Gametic isolation helps prevent explosion of population.
Many species of female fish simply drop their eggs into the water. Also, the sperms of male fish of similar species are deposited into the water. The mechanism of isolation prevents formation of hybrid species and ensures that the eggs and the sperms of only the same species combine. Gametic isolation is essentially a hybridization barrier to control the population of similar species.
In this type of reproductive isolation, the closely related species mate and even the female eggs get fertilized. However, the zygote formed is immature, hence suffers an early death. So even if the inbreeding of the species takes place, no offspring is produced due to the immature zygotes.
- An offspring that is produced from two different species is essentially called a hybrid. The term ‘inviable’ means something that is incapable of surviving. In hybrid inviability, the zygote formed from combining of the sperm and egg of two different species is incapable of sustaining its own life.
- The fertilized egg formed of different species does not mature beyond its early stages of embryonic development. The mating of a female cat with a male dog or rabbit may produce a fertilized egg, but it will die shortly due to genetic differences.
In hybrid sterility, different species breed and produce an offspring, which does survive to become an adult. However, the adult suffers from very low fertility, thereby proving to be incapable of giving birth to an offspring. For instance, mating between a horse and a donkey gives rise to a species, commonly referred to as mule, which is found to be sterile.
Due to reproductive barriers, closely related species may remain separated for centuries and evolve to become completely different species. This is a perfect example of speciation wherein the two species cannot mate because they are now distinct species with absolutely no similarity.