Sitting on a park bench one day, you notice a Great Dane and Chihuahua happily sniffing and circling each other in mutual greeting. Staring, you notice how different they are and wonder how it is even possible that they are still part of the same species. This difference you see is known as variation. The World Canine Organization recognizes 339 individual breeds of dogs, all of which exhibit unique forms of variation. While many of the variations we see in those 339 breeds are the result of domestication and purposeful breeding, variations within a more natural environment still occur. Differences in physical and genetic makeup exist across all organisms. These differences or variations allow for a species or population group to become better adapted to certain environmental stressors. There are four ways in which variation can occur: mutation, gene flow, genetic drift and natural selection.
Thanks to creative sci-fi films, the word ‘mutation’ may prompt us to picture genetic monsters or creatures with two heads, one eye and an array of superpowers. However, the majority of gene mutations are subtle or even undetectable. Gene mutations occur when errors are recorded within DNA during the cellular replication process. There are two classifications of gene mutations: somatic and hereditary.
Somatic mutations are the more common of the two, and occur during the lifetime of an individual. They are typically caused by environmental factors, like UV rays from the sun, or are the result of spontaneous errors made when adult cells divide. While somatic mutations don’t directly affect the entire genome and can’t be passed to the next generation of the individual, they will be present throughout the descendants of the original mutated cell which sometimes cause cancers. Hereditary mutations are passed from parent to offspring. These occur in the cells of the sperm or egg. If either an egg or sperm cell holds a mutation, the offspring from the fertilized egg will have the mutation present in virtually all of the cells of it’s body. Although rare, by creating new genes and alleles, hereditary mutations have the ability to introduce new variants, thus potentially increasing genetic differences between populations. So, while it is highly unlikely that you are a mutant with awesome powers, it is not impossible.
As humanity becomes more unified, so do our genes. When an organism migrates or moves, from one population to another, it allows its genetic material to move as well. Known as gene flow this can occur in many different ways depending on the organism itself. For example, pollen from one group of flowers may passively be transported to another group by means of insects, birds or wind, whereas humans may actively elect to move to a different region and population group.
The ability of genes to move into a new population group can be a very important source of genetic variation, especially if that gene version did not previously exist within a population. In the Amboseli basin in Kenya, mating between the yellow and Anubis baboons has allowed for the transferring of genetic makeup between the two different subspecies creating maturational and reproductive advantages. Although gene flow may seem like an easy way for organisms to transfer genetic material, barriers can block the amount of flow a population may see. There are two types of barriers that can affect gene flow: Allopatric speciation and sympatric speciation. Allopatric speciation is when gene flow is blocked by a physical barrier such as a mountain range, large body of water, or man-made obstacle. Sympatric speciation, barriers occur due to gene incompatibility. This typically arises when two different species originate from the same ancestral species but are different enough genetically to not be able to successfully reproduce. Limitations in reproductive success can be caused by incompatible genetics, fragmentation, different social behaviors or hybridization. A good example of this is the Liger.
All humans have two copies of every gene; one from their mother and one from their father. In reproduction, only one gene copy from each parent is transferred to their child. Like flipping a coin, each gene copy has a 50% chance of being inherited by you. However, the outcomes of what a child will inherit are not always the same. This is because the outcome is dependent on whether a gene pair is heterozygous (carries two different genes), homozygous (carries two of the same genes), dominant (only one gene needed to show) or recessive (both genes need to show). Known as Mendelian inheritance, certain gene pairs will be more likely to be inherited due to their physical makeup and probability. The ability to examine certain genotypic outcome probabilities can be done through by using a Punnett square.
So how can the frequency at which you receive gene pairs affect the evolution of your entire species? Well, frequencies of particular gene pairs can change from one generation to the next due to chance events that affect the reproductive success of certain individuals. Random occurrences that affect the genetic change within a population is known as genetic drift. As random occurrences continue to affect the variation of one population group, the differences between that group and another will expand creating drift. In 1811, the first specimen of the peppered moth (Biston betularia) began to appear in Manchester, England. A light bodied moth with black speckling, it was well adapted to blending in with the light colored lichen and tree bark found in the area. The majority of the moths showed this lighter dominant color, however, a darker variation of the moth existed, as well. Unfortunately for them, their dark coloration made them unable to blend into their environment, resulting in their being prone to predation . As a result, the frequency of dark coloration was about 0.01%. However, with the establishment of the Industrial Revolution, many of the trees became covered in soot resulting in a massive shift in how the environment appeared. And, like a classic underdog story, the light-colored moths became prone to predation decreasing their population, while the darker peppered moths camouflaged better in the pollution and succeeded in increasing their frequency to approximately 98%.
Natural selection, a theory developed by Charles Darwin, focuses on variations which are directly caused by environmental factors relating to an individual’s fitness within a population group. While fitness may bring to mind athletic individuals, fitness from an evolutionary standpoint instead focuses on the reproductive success of an organism. For example, if humanity was all of a sudden restricted to living in caves, taller individuals might hit their heads on low ceilings, knocking themselves out and limiting mating opportunities. While, shorter individuals who were not at risk of hitting their heads, might be able to spend more time mating, increasing their overall fitness. The better suited an organism is to its environment, the higher its probability to pass on genes to its offspring, gradually increasing the amount of that particular gene within the population. As a result, our cave population would show an increase in shorter individuals compared to taller people across the next generations.
So now as you sit back on your bench all knowing, you wonder which evolutionary force created the doggie variation you’re seeing. Is it mutation? Gene flow or drift? *Gasp*…could it be natural selection?! While you may or may not ever discover which evolutionary force created all of the wonderful dog breeds we see today, remember this, you are in a dog park surrounded by happy dogs of different sizes, shapes, and colors. Now get off your bench and play with the dogs!
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