Extinction is a natural process.  Before humans walked the Earth, new species were diverging from other species and older species were dying out.  Extinction assumes that no members of a species are still alive.  Extirpation is when a given species can no longer be found in a particular region and ecologically extinct is when a few individuals are left and they no longer play a role in their ecosystem.  

Extinction rates are almost never constant.  Most of the species that have existed in the past are not here today.  A mass extinction occurs when at least 50% of the species and 30% of the families go extinct.  These events can occur for a variety of reasons.  The Ordovician extinction occurred because of a massive ice age.  The Permian extinction occurred because of the effects of a volcanic eruption.  The Cretaceous extinction happened 66 million years ago because an asteroid hit the Earth.  Every time a mass extinction occurs, more room opens up for biodiversity.  It will come back, even if it takes a million years.  

Based on genetic data, the American and Japanese firefly are thought to have diverged from each other over 100 million years ago.  If this conclusion is correct, it means that fireflies survived the Cretaceous mass extinction! 

The current extinction rate is predicted to be 1000x what it was during those mass extinction events.  The International Union for Conservation of Nature keeps track of all known species on Earth by asking experts what the status is for each species.  Are we currently in a mass extinction?  Only time will tell.  It is estimated that about 41% of insect species are at risk for extinction, including fireflies. 

It is hypothesized that humans are a major cause in the extinction of these species.  According to fossil records, large terrestrial vertebrates tend to go extinct with the migration of humans to an area.  Our ancestors could have hunted them directly or brought disease directly through them or indirectly through their livestock.  

Species can go extinct due to a number of reasons.  First, a decline in genetic diversity leads to a decline in evolutionary potential or more specifically, reduced adaptability, reduced fitness, and an increased chance to be wiped out by disease.  Second, the species could lose suitable habitats.  They cannot survive without a niche to exist in.  Third, species could be overharvested, due to humans or hunted by another predator.  Next, an introduced species (human or natural origin) might wipe out a native species.  Lastly, climate change has a serious impact on the health of biodiversity.  

The driving forces behind firefly extinction are habitat loss, artificial light, and pesticides.  The habitats of fireflies are being paved over for human expansion.  Fireflies are not well suited for migration as adults only live for 2-3 months to mate and the larvae are practically immobile.  Light pollution increases with an increase in human construction.  Artificial light at night interferes with flashing patterns of fireflies.  They use unique patterns to each species in order to attract food and mates but these patterns are being drowned out by the light.  Lastly, insecticides used in agriculture run off from the fields and into firefly habitats.  The pesticides used harm firefly larvae ultimately decreasing the firefly population. 

Extinction is not the end all be all for life on Earth.  If conservation efforts of some species are unsuccessful (we can’t save every species), more room is opened for increased biodiversity in the future.  Fireflies can be conserved by preserving their habitats, shutting off artificial light at night, and limiting pesticides and fertilizers used for crops.  

When a species colonizes a new habitat, they could either be invading or have been introduced to that habitat.  An introduced species is established where that species has not been established previously.  On the other hand, an invasive species is an introduced species that has negative consequences to that habitat or surrounding ecosystem.  Fireflies are generally not seen as an invasive species as they do not disperse on their own.  However, it is common that people want to introduce fireflies to a region to have in their backyards. 

The process of introducing species to a new ecosystem does not have to rely on human activity.  In fact, it’s been happening way longer than humans have been around.  But the more humans travel around the globe, the easier it is for other species to travel as well.  Most introductions of species to a region are accidental.  

During the 1960s in Japan, fireflies were intentionally introduced into a town called Matsuo-kyo.  In 2010, researchers concluded that the introduced species, L. cruciata, had a strong ecological impact on native species in Matsuo-kyo.  They looked at flash patterns, which are specific to each species of fireflies, in order to determine what happened to the introduced and native species since the 1960s.  They concluded that the introduced species either driven the native species away or to extinction because the flash patterns in 2010 matched the patterns of L. cruciata.  

When a species invades a region, it could have a hard time establishing itself or it could thrive with ease.  Many different factors play a role in whether or not a species can easily be introduced to a new habitat.  Species that are generally going to have an easier time being established in a new habitat are r-selected (short generation times, lots of offspring), are plastic (can change its own morphology based on environmental conditions), and are ecologically competent (able to tolerate a range of environments).  

The type of habitat also determines whether or not it can be invaded.  If the habitat is similar to the species native habitat, the species will have no difficulty thriving in that habitat.  Next, the more an environment is disturbed, the more room opens up for non-native species.  Lastly, lower stress environments are also more likely to be invaded as the pressures and costs of living are affordable.  Generally, the more biodiverse a habitat is, the harder it is for a species to invade that area because all of the resources are being used by already established populations.  Fireflies have a difficult time dispersing to new areas because they are only in their adult form for a few months during the summer and the larvae cannot travel significant distances at all.  

If you want to attract fireflies to your backyard, you can do a few things to create the right environment.  Light pollution interferes with mating patterns of fireflies so be sure to shut off all outside lights.  Firefly larvae feed on insects and worms that live in rotten wood and leaf litter.  Bringing in leaf litter and a few logs of wood may help to increase the chances of fireflies colonizing, especially if your backyard does not have many trees.  

Invasion or introduction happens in 3 stages: arrival, establishment, and dispersal.  First, the species has to arrive in the new location.  Next, they have to become established there, or be able to find resources and shelter in order to grow, reproduce, and survive.  Lastly, they need to be able to spread in order to maintain the population.  

When a species successfully colonizes a new habitat, they may interfere with native species that currently exist there.  The invasive species may be able to coexist with the native species.  In other cases, the native species is outcompeted and becomes displaced by the new species.  A more extreme situation is when the native species is outcompeted and goes extinct.  

Invasive and introduced species can become very problematic very quickly if not managed.  Invasive species cause more than 100 billion US dollars in damage per year in the US alone.  Damaging situations can be avoided by many interventions.  Boats, cars, trains, or planes can be inspected for known invasive species and contained if contaminated.  If the invaded species has already been established in a new habitat, they might be able to be eradicated.  Worst case scenario, the invasive species might be able to be stopped from spreading if barriers are put in place to prevent further dispersal.  Fortunately, fireflies do not cause damage in the areas they colonize and may even benefit biodiversity by feeding on slugs.  

Not every organism exists peacefully.  It may not be obvious, but most if not all creatures compete with other individuals for resources.  Competition is a negative interaction between two or more individuals that depend on the same limiting resources to survive and reproduce.  Not every individual gets the same amount of resources.  Competition can either be intraspecific, between the same species, or interspecific, between different species.  

An example of intraspecific competition in fireflies is Photinus males imitating Photuris females that imitate Photinus males in order to scare off other Photinus males, reducing mating competition.  The example of interspecific competition in fireflies is not so friendly.  Photuris females mimic Photinus females to attract Photinus males.  Instead of finding a mate, the Photinus males show up to a hungry Photuris female awaiting her meal.  The biggest competition that exists within firefly populations are competitions between males for mates. 

Resources are considered to be anything an organism interacts with that increases the growth rate of a population if it becomes more available.  Renewable resources are constantly regenerated while nonrenewable resources are fixed at a certain amount throughout an individual’s lifetime.  Liebig’s Law of the Minimum states that populations increase until the supply of the limiting resource prevents further growth.  Fireflies are predators and are usually limited by habitat space.  In order to increase the firefly population, one can stop using fertilizer that tends to eliminate insects or worms, decrease light pollution, and increase the amount of natural litter (plant growth, trees, etc) in an area.  

The competitive exclusion principle states that two species cannot coexist indefinitely when they are limited by the same resource.  If there are not enough resources to sufficiently supply two populations, one of those populations will go extinct.  Following, the more closely related the two populations are, the more similar those populations are and the higher chance they will be competing for similar resources.  

The logistic growth model can be altered to incorporate competition into the model:

Without going into too much detail, the population variable that directly influences the carrying capacity now includes a second population and how that population affects the first population.  For example, if ⍺ = 0.25, it would take 4 individuals of species 1 to use the same amount of resources as 1 individual of species 2.  This model is useful because it allows scientists to predict the outcome of competition.  

In competitive circumstances, both populations can persist or one population will become extinct.  Using the model above, initial conditions determine which species persists.  If both populations start near the carrying capacity, it is likely that one population will go extinct, as the habitat can only provide enough resources for one of the populations.  On the other hand, if both species start out with small populations, it is likely that an equilibrium will be reached.  Coexistence is favored when each species persists at low levels of different resources.  

There are three modes of competition: exploitative, interference, and apparent.  Exploitative competition is when one population consumes the majority of an essential resource so much so that other populations can no longer persist.  This type of competition is generally what is thought of as “competition.”  Interference competition is when individuals defend resources from competitors and apparent competition is when two species compete due to the presence of a common enemy.  Using the examples of interspecific and intraspecific competition from earlier, both situations are examples of exploitative competition.  The males that mimic the flashes are lowering the number of females available for mating and the Photuris females that lure the  Photinus males to their deaths are eliminating competition from the Photinus fireflies in general.  
Allelopathy is the use of chemicals to interfere with their competitors.  Fireflies contain defensive chemicals that make them predators avoid them.  The predators are able to remember to avoid fireflies due to their luminescence.

Fluctuation in population numbers has many causes.  Populations could increase or decrease due to abundance of food, disease, environmental factors, predation, or competition.  How much each population varies due to these factors also varies, depending on body size and plasticity.  Larger species do not vary as much but smaller species, such as fireflies, have significant variation.  Firefly population sizes vary from year to year depending on the weather (fireflies prefer moist environments), availability of food (fireflies are predators that eat small insects, snails, and earthworms), and availability of habitat (fireflies prefer places to protect themselves during the day).  

There is a limit to how many individuals of a population a certain habitat can hold.  An overshoot occurs when a population grows beyond its carrying capacity – the maximum number of individuals that habitat can hold.  A die-off is when a population declines substantially below its carrying capacity.  A population tends to grow significantly when there is an abundance of food and little to no predators.  For example, if the population of frogs or birds that prey on fireflies experienced a die-off, we would expect an increase in size or maybe even an overshoot of the firefly population.  A population cycle is the regular oscillation of population size over time and consists of growth and die-offs.  

Sometimes, a population may not respond fast enough to changing environmental conditions.  This results in delayed density dependence, where a population responds to environmental conditions at a time A when the carrying capacity is higher but when the offspring are born at tie B, the carrying capacity might be much lower, resulting in an overshoot and a die-off.  This can be modeled by slightly editing the logistic growth equation adding the length of the delay.  

Depending on the growth rate and the length of the delay, oscillations either continue to happen or dampen.  Oscillations occur because of biological reasons.  Some species have the ability to store resources which allow them to grow beyond carrying capacity.  Oscillations are dangerous for small populations as they are more vulnerable to extinction due to random chance events.  

Stochastic models incorporate the random chance events in population growth rate into growth models.  Demographic stochasticity involves random variation in birth and death rates in individuals while environmental stochasticity includes random variation in birth and death rates due to the environment.  A demographic example would be differences in survival rate between individual fireflies.  An environmental example would be if one population of fireflies had access to insects as food while another population does not.  

Not every available habitat suitable for a species is occupied.  Whether or not a patch is occupied depends on the size of the patch, distance between an occupied patch, and the availability of the species to move between patches.  This is known as the theory of island biogeography.  Larger patches near occupied patches have a higher probability of being occupied.  Smaller patches have less resources than larger patches.  Individuals that attempt to migrate to a patch that is farther away takes on more of a risk while migrating.  It is unknown whether or not firefly populations can migrate but it has been observed that when a field that contains a firefly population gets paved over, the fireflies do not migrate, they just disappear. 

Fireflies have a limited range of habitats that they are able to thrive in.  More technically, a niche is the range of environmental conditions, both biotic and abiotic factors, that a species can tolerate.  Each species has a fundamental niche and a realized niche.  Fundamental niches contain the entire range of conditions that a species can thrive in.  For example, the fundamental niche for fireflies is any warm and wet environment.  Realized niches are more specific and are the habitats that we find the species in.  Fireflies are found in damp areas such as forests, streams, or marshes.  

Suppose you want to attract fireflies to your backyard to entertain your family.  First, you would have to make sure your backyard is a suitable environment for fireflies.  In order to do so, you decide to use ecological niche modeling, which is the process of determining suitable habitat conditions for a species.  If you lived near a stream or have a moist environment in your backyard, you might either find fireflies or be able to attract them!  Your backyard would then be part of the ecological envelope, or range of ecological conditions that is assumed to be suitable for fireflies.  

There are many characteristics of how a species distributes within a niche.  First, the geographic range uses the realized niche to draw a boundary around where we find a species.  However, not every square mile of the geographic range will contain the species, it’s just the range where you might find the organism.  Fireflies can be found almost everywhere in the US and Canada as seen in the following map: 

The geographic range of fireflies is the entire region shaded any color of green.  This range can be described as cosmopolitan, or a large geographical range.  In opposition, species that are only found within a limited range are called endemic.  The “blue ghost” species of fireflies can be considered endemic since they are only found in the Carolinas in the United States.  

The darker shaded regions on the map above have a higher abundance and density of fireflies.  The abundance of a species is the exact number of fireflies found in that species.  The density of a species is how concentrated individuals are in a given area, or number of individuals per unit area.  The Great Smoky Mountains along the Tennessee – North Carolina border in the United States contain a high density of fireflies.  These fireflies can even sync their flashes together!  

Abundance and density are related to geographical range via resources available and utilized by each individual.  The larger the geographical range, the more species (abundance) that range can hold due to more resources available for the species.  Similarly, the smaller the body mass, the less resources each individual uses.  This allows for more individuals to thrive in a given range.  One time on a service trip to Kentucky, my group spent an evening catching fireflies and keeping them in a jar.  We were able to keep over 100 fireflies alive for a couple days before releasing them within the jar since adults do not eat much.  Fireflies are able to  thrive in a very small density.  

The density of a species does not tell us the full picture.  The individuals might not have equal dispersion.  They could be clustered in a few small areas or spread out evenly or randomly.  For example, if fireflies existed in a forest that also contained a swampy area, we might expect the dispersal to be clustered with the majority of fireflies living in the swampy area. 

Dispersal describes the movement of individuals from one area to another.  This feature is responsible for gene flow, but only as long as individuals can survive while moving to a new habitat.  Habitat corridors are stretches of suitable habitat that increase the chances for dispersal.  An example of this might be a stream connecting two ponds.  Fireflies living around the pond upstream can disperse downstream to the other pond!  

Scientists have multiple ways of estimating the density of individuals in a given habitat.  Area-volume surveys select a square boundary, count all of the individuals found in that boundary, and extrapolate that number to cover the entire geographic range.  Line-transect surveys draw a line and count individuals along that line.  This method is probably best for estimating population sizes for fireflies since they move around a lot.  A last method for estimating population distributions is mark-recapture surveys.  Scientists capture an individual, attach a marker such as a GPS, and track the individual over time.  This method is used for larger organisms such as sharks.   

Not all habitats are equal in quality.  Habitats can be considered good in quality when they contain abundant resources, limited predators, and spaces for shelters.  The ideal free distribution describes how many individuals can thrive in a habitat depending on its quality.  High quality habitats can host more individuals than a lower quality habitat.  If there are too many individuals in a high quality area, it would be worth it to a few individuals to disperse to a lower quality habitat that has not yet been inhabited by members of that species.  

Three metapopulation models describe the dispersal of individuals based on habitat quality.  In the basic metapopulation model, patches of suitable habitat exist within a matrix of unsuitable habitat.  In this type of model, we do not expect much dispersal since each good habitat is of equal quality.  In the source-sink metapopulation model, patches of varying quality are embedded in a matrix of unsuitable habitat.  Now, this model takes into account different types of suitable habitat.  In this model, more dispersal occurs from the source (highest quality habitat) to the sink (lower quality habitat but still suitable) than vice versa.  Let’s return to the stream connecting two ponds.  If the pond downstream was smaller than the pond upstream, we would expect more fireflies to travel downstream than upstream.  Lastly, the landscape population model adds varying quality to unsuitable habitats.  The amount of dispersion in this model is relatively similar to the source-sink model, but more routes are available from the source to the sink since not every unsuitable habitat is of the same quality.

A phylogeny visually represents how different species are evolutionarily related.  However, phylogenies depict hypotheses of evolutionary history to account for extinct species. Even though we don’t have a complete picture of evolutionary history, phylogenies can provide us with a useful tool to test other hypotheses of evolution.  

First, let’s go over how to read a phylogeny.  Follow along using this phylogeny:

A clade considers a group of species and their most recent common ancestor.  The section Lampyridae represents a clade.  A node is a point where species branch off.  The most recent node on this phylogeny is between Aquatica ficta and Asymmetricata circumdata.  Newer species are listed at the tips of each branch, while the ‘root’ of the phylogeny is the oldest in the  phylogeny of consideration. Branch lengths provide information on morphological or genetic differences, not time.  If you want to include more chronological information, a chronogram uses branch lengths to determine the difference between extant and extinct species.  

Traditionally, two species were put in the same clade if they were morphologically similar.  For cases of convergent evolution, scientists used the idea of parsimony. Parsimony hypothesizes that the evolution of a new morphology is an unlikely event.  For example, if researchers found fireflies in North America and Europe, they would be placed underneath a common ancestor on a phylogeny under the assumption that Luciferase, the bioluminescent protein, evolved once when the continents were not separated by an ocean instead of twice.  There are limitations to this method. Since the tree was built on evolutionary principles, you can’t use the tree to study evolution, as this would be circular reasoning.  
Over time, there have been many improvements to the process of building phylogenies. A more modern approach to assembling phylogenies involves the use of genetic data and “likelihood” to put together phylogenies.  Based on the molecular data, each possible phylogeny gets a likelihood score for the chance that this given tree is the correct one. The trees with the highest likelihood score are published in scientific papers. But what about confounding variables such as morphology and resource availability?  Phylogenetic Generalized Least Squares (PGLS) regression is used to account for the confounding variables and put species in their correct relationships with other species.  Genomics revealed that Luciferase has evolved independently at least twice.  By sequencing the genome of an American and a Japanese firefly that diverged over 100 million years ago and a Caribbean firefly, they were able to distinguish genetic markers surrounding each gene for Luciferase.

Everyone can tell the difference between a cat and a dog.  But what about a leopard and a cheetah? Or two dragonflies species?  Two fireflies species? Distinguishing two species is no easy task, especially if the organisms are morphologically similar.  Scientists define a species as a group of organisms that are reproductively isolated from anyone outside the group. There are many examples of reproductive isolation.  For example, geographical isolation involves the distance between populations or organisms that live on opposite sides of the world from each other. Another example would be gametic isolation, in which case sex cells between two different organisms cannot combine to form a new offspring.  

If you can distinguish between two very similar species, can you point out when one species splits into two different species?  Sometimes speciation events can be difficult to identify and steps toward speciation may happen in a different order for different species.  For example, mountain cats might have diverged from desert cats because mountain cats began to occupy a new habitat compared to the rest of desert cats. In contrast, an offspring of two plants might experience an error in meiosis, resulting in the offspring plant being unable to fertilize a cousin plant of the same species as the parent generation. There are four species concepts that suggest when speciation is occurring in a population. 

First, the phylogenetic species concept considers all descendants of a common ancestor.  This is useful to consider the evolutionary history of alleles and is easily available since genetic material is, in many cases, readily available for testing.  However, genetic material for testing may be hard to acquire if the species of interest is extinct. Second, the ecological species concept distinguishes species based on different habitats.  For example, a species of firefly might prefer the top of trees and another species might prefer grass. Even if they are in the same forest, the two populations of fireflies occupy different niches and would be considered distinct species.  This species concept gives us information on how each species interacts with their environment. 

Next, the morphological species concept outlines that two organisms are not from the same species if they have morphological differences between them.  In southeastern US, the Phausis reticulata fireflies glow blue while the Photinus carolinus fireflies flash yellow.  These organisms have a different version of Luciferase, resulting in a different fluorescent ‘shine’.  Lastly, the biological concept isolates two species when they can no longer reproduce with one another.  This concept is the hardest to test because it’s only applicable to living creatures and just because two individuals can mate, doesn’t mean they will.  

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This is a great example of distinguishing between two species. If you identified these two organisms as belonging to different species, you would be correct, based on the morphological species concept!  The firefly on the left is part of the genus Luciola, from Japan and the firefly on the right is part of the genus Photinus, commonly found in the United States.  

While the ecological species concept and the biological species concepts are helpful tools for identifying speciation, the phylogenetic concept is most used in scientific writing to define species.  This is most likely because sequencing the genomes of two organisms provides a comparable genetic code for ecologists to quantify the number of differences in genomic sequence between two different organisms.  A month ago, researchers used this concept to distinguish four species of fireflies in Japan!  Currently, there are over 2,000 species of fireflies classified in the world today!  

By defining what a species is, scientists can use this information to better human societies.  Researchers could warn human communities if one pattern of species is dangerous but another similar pattern is not.  They could also use information gained from evolutionary studies with species concepts to speed up the evolution process of breeding higher quality crops and animals!

According to an Appalachian legend, the fireflies that glow blue are the ghosts of Confederate soldiers that passed in that area.  This glow of Phausis reticulata can only be observed for a month during early summer as males search for females to mate.  Males glow as they search for a female, releasing a sex pheromone that triggers the female to glow.  This allows females to only glow for as short of a time as possible, to not attract predators, ultimately increasing their chances of reproduction and survival.

The survival and reproduction of an organism determines the continuation of the species.  This is also known as an organism’s fitness. Phenotypes evolve because of the fitness a specific trait incurs over another trait.  The higher one’s fitness, the more likely its genes are going to be passed on to the next generation. For example, females in the species Phausis reticulata glow blue for approximately a minute instead of flash and do not fly since they retain their larval form.  Flying exposes the firefly to predators such as frogs and birds. Not being able to fly allows the female to stay hidden.  Blue light is also harder to detect than other wavelengths. The cells in the eyes of an organism require more energy to detect the wavelength of blue light.  This is because the lower the wavelength, the more energy is required to detect it. These two unique traits of these female fireflies allow them to survive longer and reproduce. 

When animals as a species evolve over a large period of time, there are specific reasons why you don’t see some species such as fireflies become like eight feet tall.  Evolutionary constraints determine relatively how big an organism can grow to or how many legs it can have. They are restrictions on behavior, morphology, and physiology on the evolution of an organism.  A physical constraint is that fireflies cannot be larger than they are because they lack a closed circulatory system and rely on diffusion to transport oxygen and other nutrients to tissues. 

Another type of evolutionary constraint is antagonistic pleiotropy.  This occurs when one gene has a function that benefits an organism’s fitness and another function that hurts an organism’s fitness.  The effects of the gene must be balanced and not too detrimental for the organism in order not to be selected against. 

The last type of constraint we will discuss is evolutionary contingency.  Broadly speaking, contingency refers to the chance that the evolutionary events played out as they did.  If the history of life were replayed on earth, the biological forms of beetles, reptiles, mammals, etc., as we know today, may not exist in the replay.  In another world, fireflies might breathe fire instead of glow. There are only certain outcomes an offspring can have based on the parents genotype. For example, fireflies can’t give rise to an organism that looks like a cat.  The firefly is constrained to resemble their parents.  

Adaptive fitness landscapes provide a visualization for the fitness of an organism with variations in two phenotypes.  Fitness is shown through peaks. As seen below, only fireflies with certain combinations of color and ability to fly have higher fitness.  

Constraints control which parts of the fitness landscape can have individuals.  Individuals on the peaks tend to survive longer and have more offspring. A firefly that glowed blue and was able to fly might not be seen by males looking for a mate.  These females would not have any fitness and are not shown as a peak on the adaptive landscape. Even though constraints limit the types of features organisms can have, bizzare traits exist in nature as a result of evolution. 

The Hardy-Weinberg equation can be used to calculate the expected genotype frequencies for the next generation of a population given current allele frequencies.

If these expected values are similar enough to the actual observed counts using a Chi-squared goodness of fit test, we can say that the population is in Hardy-Weinberg equilibrium. If the expected values are notably different, however, we can say that some form of evolutionary influence is taking place and at least one of the Hardy-Weinburg assumptions have been broken. Selection in populations is one notable way in which allele frequencies can change over generations. 

Sexual selection is certainly present in some firefly populations. Specifically, Photinus, P. pyralis and P. macdermotti are two firefly species that have been studied for their unique sexual selection process. It has been found that female Photinus fireflies preferentially mate with males based on their bioluminescence pattern. Females favor males’ courtship signals with faster pulse rates. However, male signal attractiveness had little to no bearing on reproductive success and male paternity share as female Photinus fireflies mate with multiple males. So, males have evolved to produce varying spermaphore sizes and numbers to increase their chances of mating. Protinus fireflies deliver their sperm in bundles called spermatophores. Virgin males produce larger spermatophores, which contain more sperm, than males that have previously copulated. Males with relatively larger spermatophores have been shown to have higher fitness through increased paternity share regardless of whether they were the female’s first or second mate, increasing their likelihood of successfully mating. Females benefit from mating with males with larger spermatophores. It is thought that females that choose their mates based on spermatophore size might gain direct benefits such as improved fecundity and longevity. 

Natural selection occurs due to a difference in the survival and reproductive success of phenotypes. Many different types of selection exist. To name a few examples, sexual selection occurs when individuals in a population pick mates based on traits. Selection follows because only the individuals with the desired traits reproduce. Only their genes get passed on to the next generation. Artificial selection happens when an outside influencer breeds another animal for a specific trait. Only the genes of the animals with the desired or most pure trait get passed onto the next generation.

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Spermatophore size is also thought to be heritable. If phenotypes did not have a genetic basis, there would be nothing for selection to act upon, and the next generation of fireflies would have the same variation in spermatophore sizes as the previous generation. Natural selection does not have to occur for evolution to take place. Major events that affect allele frequencies cause evolution within a population. If a natural disaster were to take place in an environment that contained flightless and normal fireflies wiping out most of the fireflies able to fly because of a change in the atmosphere, evolution would occur since the fireflies that were able to fly, are no longer able to survive and reproduce relative to the fireflies that cannot fly.          

There are different ways natural selection can affect the average of a trait. Selection can be stabilizing, disruptive, or directional. Stabilizing selection reduces the variance of the trait but the mean stays the same. In this case, those with more extreme phenotypes struggle to survive or reproduce. Disruptive selection is the opposite of stabilizing selection. While the mean stays the same, the variance increases. Those with average values of the trait struggle to reproduce favoring those with more extreme traits. Lastly, directional selection favors one extreme over another, changing the mean of the trait but the variance stays the same. For example, females of Protinus select males with larger spermatophores to mate with. Over time, we expect the size of the spermatophores to increase as larger spermatophore size confers greater paternity share. This sexual selection is directional since the females prefer males with an extreme trait. 

All types of selection described above lead to microevolution: small changes in a population over a short period of time. Macroevolution describes larger scale changes such as the development of a whole taxonomic groups over a longer period of time. It is not known where exactly fireflies came from but it is thought that fireflies and click beetles, another luminescent beetle, acquired the gene Luciferase independently. 

The firefly genome contains coding for the protein Luciferase, which allows some, but not all, members of most firefly species to glow at night. Genes are segments of DNA that encode for proteins. A segment of DNA is transcribed into RNA, then translated into its respective protein to be modified and used by the cells. This movement of genetic information is called the central dogma. DNA is the starting point of this information flow because it holds genetic information in sequences of nucleotides and because of its notable stability. DNA of an organism is organized into different numbers of chromosomes that are tightly packed within the nucleus of a cell. 

Not all genomes are the same. Genes between individuals of the same species can vary at a single base pair to the entire gene having different patterns or sequences of base pairs. The amount of genetic variation between individuals in a species or population can be quantified in two ways: ploidy or the F-test statistic (Fst). Ploidy is the number of sets of chromosomes in an organism, whereas the Fst compares variation between two subpopulations. The higher the Fst value, the more variation exists between the two subpopulations. For example, a species of fireflies in which fluorescence is exclusive to males will have a higher Fst value than a species where both males and females glow.

The majority of DNA in the genome is non coding, or “junk” DNA. Despite this misleading nickname, it has important functions for the genome. Regulatory elements aid in transcription by promoting or inhibiting the creation of RNA. For example, some regulatory elements can shut off the Luciferase expression in cells that are not needed for glowing and other regulatory elements can increase the transcription and translation of the luciferase gene in the light organ of the firefly.

Another classification of DNA is transposable elements. These regions of the DNA are able to move, potentially copy themselves, and insert themselves into other areas of the genome. This can either help, hurt, or have no effect on the organism. When looking at the DNA you might expect that as an organism becomes more complex, it would have more DNA stored in a larger number of chromosomes. However, this is not the case. For example, a koala usually has 16 chromosomes whereas a firefly usually has 19 or 20 chromosomes.

The firefly is a diploid organism, meaning that it contains two sets of each chromosome. The exact number of chromosomes a firefly has varies depending on its sex and species. Normally, fireflies carry two pairs of nine chromosomes and an extra sex chromosome or two, adding up to 19 or 20 chromosomes. Fireflies are members of the XO sex determination system which means that males have one X chromosome where as females have two X chromosomes. There are some species of fireflies in the subfamily Luciolinae where males have 15 or 17 chromosomes. In another species within the subfamily Photurinae, males have 20 chromosomes and females have 21 chromosomes, resulting from a singular extra called a ‘B chromosome’ that contains some extra genes, which will be explained later. 

The genome of the firefly species can vary from around 500,000,000 base pairs to 1,300,000,000 base pairs. Even within the same species, such as in Pg.decipiens, the genome between individuals can vary as much as 300,000,000 base pairs. The genome of an individual firefly contains many repetitive sequences. This occurs in as low as 10% of the firefly species Photinus sabulosus, but it can also go as high as 56% in the species Photinus obscurellus. Repetitive sequences have many purposes, such as providing extra dosage or copies of a gene to protect the organism against random mutations in one of the genes and to provide more genetic variation between individuals of the same species. 

It is thought that much of the genome size variation in firefly species can be attributed to B chromosomes. B chromosomes are nonessential chromosomes that are composed of repeating sequences of DNA that have multiple forms across a wide variety of species and populations. These B chromosomes are responsible for the variation in genome size in a number of species such as certain strains of rice and grasshoppers. The extent to which B chromosomes play a role in the variability of firefly genome size is not entirely known but is a topic of current research. Another hypothesis of current investigation in the variability of firefly genome size is the recent development of triploid genomes in some females. Typically, organisms with triploid genomes are sterile. It is not known if these firefly species with triploid genomes are sterile or not, especially since all observed triploid individuals were female.

Heritability can be quantified into broad-sense and narrow-sense heritability, which can further be used to determine the phenotype of offspring based on the average phenotype of the parents. Broad-sense heritability quantifies the resulting phenotype from all heritable elements. Narrow-sense heritability uses additive heritable elements. Narrow-sense heritability is used in the Breeder’s equation to determine the phenotype of the offspring of two parents. The Breeder’s equation is particularly practical in predicting the outcome if one bred two organisms together. While intentionally breeding fireflies for a specific trait is not commonly done, there are people attempting to repopulate certain areas in the U.S. with fireflies! On the other hand, researchers take advantage of the luciferase gene to make other animals glow, and the Breeder’s equation could be applied to predict the brightness of an offspring’s glow.