research paper

Introduction Reproductive isolation is relatively the strongest influence in microevolution in comparison to others. Reproductive isolation is the existence of biological factors that impede on two species from reproducing offspring properly. These biological barriers serve as block aids in the sequence of reproduction. The reproductive process does not hold longevity in the passing down of certain genes to the next generation. If certain characteristics traits are not needed for survival or cause hindrance in trying to adapt in certain environments, they will be less viable. Many environments can require specific traits for species to survive in their climate, ecosystem, or location. These reproductive barriers are categorized into two groups- prezygotic and postzygotic. Prezygotic barriers serve as blockades before fertilization and postzygotic barriers serve as blockades after fertilization. “Recent findings suggest that postmating prezygotic isolation (i.e. gametic barriers) could be an important factor preventing hybrid formation. Competitive gametic barriers emerge when a female is inseminated by a conspecific and a heterospecific male. We examined whether sperm proportions after double matings and copulation duration impede hybrid formation. For this, we used females of Ischnura graellsii that mated with one conspecific and one heterospecific (I. elegans) male and vice versa, and calculated paternity of the second male by using RFLPs. Values of paternity (although preliminary because of a small sample size) suggest no bias in paternity towards conspecific males. However, proportion of sperm stored in the bursa and spermatheca of the female was biased towards the conspecific male when the heterospecific male was the first male, while copulation duration did not differ between conspecific and heterospecific males. Our results suggest that the relative sperm volumes may play a role as a gametic barrier in this species. However, cryptic female choice mediated by the preferential use of the conspecific sperm, although not detected, could not be discarded owing to small sample sizes in some cases.” (Taylor & Francis Online, 2013) Gametic Isolation refers to the barriers occurred from sperm and egg fertilization complications. Different species of animals require different amounts or volumes for successful fertilization. Not all species of dog, for example produce the same amount of sperm because they don’t all require the same amount. Two different species could have a successful breeding, but because the needs for reproduction aren’t the same, then the two will not be able to reproduce offspring together. Prezygotic Barriers “Prezygotic barriers prevent members of different species from mating to produce a zygote, a single-celled embryo. Some example scenarios are below: Two species might prefer different habitats and thus be unlikely to encounter one another. This is called habitat isolation. Two species might reproduce at different times of the day or year and thus be unlikely to meet up when seeking mates. This is called temporal isolation. Two species might have different courtship behaviors or mate preferences and thus find each other "unattractive". This is known as behavioral isolation. Two species might produce egg and sperm cells that can't combine in fertilization, even if they meet up through mating. This is known as gametic isolation. Two species might have bodies or reproductive structures that simply don't fit together. This is called mechanical isolation.” (Khan Academy) To further elaborate, many species of animals are not permitted to produce viable offspring for various reasons. Habitat isolation refers to the specific location of two species of animals that prevents them from ever encountering one another. For instance, a bald eagle, native to North America will most likely never come in contact with a Hoatzin which is Native to South America. The two species of bird will never even meet to even attempt reproduction because of location. Even if they were to be in the same area and given the opportunity to reproduce, they won’t even be drawn to one another for mating purposes. This brings me to my next point- behavioral isolation. The two species vary greatly in color, size, and feather pattern as well as shape. They are not to used to seeing anything of the nature of each other. So, their first instinct would not be to mate, they will more than likely fight, the eagle may try to eat the Hoatzin, or they will avoid each other at all costs because they are unfamiliar with each other. They also are known to mate or breed at different times of the year which can also serve as a prezygotic barrier. The Hoatzin species of bird mate during the rainy months of the year while Bald Eagles mate during the colder months of the year like February. Postzygotic Barriers “When prezygotic isolation mechanisms fail to keep species in reproductive isolation from each other, the postzygotic isolations will take over and ensure that speciation is the preferred route for evolution and diversity among species will continue to increase as natural selection acts. In postzygotic isolation, hybrids are produced but tend not to be viable. They may not survive long enough to be born or have major defects. If the hybrid makes it to adulthood, it is often sterile and cannot produce its own offspring. These isolation mechanisms ensure that hybrids are not the most prevalent and species remain separate.” (ThoughtCo. 2018) Postzygotic barriers really secure the fact that reproductive isolation is the strongest form of microevolution. If the two species of animals are able to get past the prezygotic barriers, they still can fail. What I mean by this is, if they can breed and produce offspring, the offspring will often be nonviable. A perfect example is the cross breeding of a male donkey and a female horse. They can produce offspring together, but the offspring which is a mule, will be sterile meaning it cannot reproduce any further offspring. The mule gets 32 chromosomes from the female horse and 31 chromosomes from the male donkey. The number of chromosomes have to match up completely in order for offspring to be fertile. Reproductive Isolation VS Genetic Mutation “A new USC Dornsife study suggests a reason why that prediction has been so challenging, even for the most-studied diseases and disorders: The relationship between an individual's genes, environment, and traits can significantly change when a single, new mutation is introduced. "Individuals have genetic and environmental differences that cause these mutations to show different effects, and those make it difficult to predict how mutations will behave, " said Ian Ehrenreich, a lead author and biologist at the USC Dornsife College of Letters, Arts and Sciences. "For example, mutations that break the cell's ability to perform DNA mismatch repair are linked to colorectal cancer, but some individuals that harbor these mutations never develop the disease."” (Science Daily, 2018) Genetic Mutation also serves as form of microevolution. Genetic mutations fail short in comparison to reproductive isolation because they are less likely to occur and doesn’t have nearly as many influences. Genetic mutations are influenced by previous diseases and gene type. Where as reproductive isolation has influences from prezygotic and postzygotic barriers which outnumber the few factors that cause genetic mutations. Also, speciation with reproductive isolation is way more likely to happen because there are so many possible opportunities for microevolution. Migration “Migration is a complex mode of dispersal, promoting the colonization of new areas, but also their regular re-colonization and gene flow. Spatial segregation — the linchpin of most speciation theory — becomes less and less likely with increasing migratory tendencies. Achieving true geographic isolation from other populations, thereby allowing differentiation to occur in the absence of gene flow, seems particularly unlikely among long-distance migrants, whose movements regularly encompass entire continents and oceans.” (Nature, 2000) Migration acts a very strong form of microevolution. The species can travel to a different geographic location for various reasons. There could be better food sources, or they may be trying to avoid predators. Migrating to new areas can lead to the formation of certain traits or genes needed for survival. For example, a larger beak size and shape may be needed for birds to obtain food in a different habitat. The food sources are very abundant, but the birds with the larger beaks are only able to obtain the food. Over a period of time, the birds with the specialized characteristics need to the environment will produce more offspring because of natural selection. The strongest birds will survive and produce the most offspring because they are most equip for the environment. Genetic Drift “Genetic drift is a random change in allele frequencies. These random changes in allele frequency can accumulate over time. Across many generations, the frequency of an allele can gradually increase, gradually decrease, or fluctuate back and forth. In other words, the frequencies of different alleles seem to ``drift’’ up and down, without any direction. This is why the random change in allele frequencies is called \term {genetic drift}. Over time, genetic drift can make once rare alleles common, or eliminate alleles altogether.” (John Hawks Weblog, 2011) Genetic drift is really a random change in frequencies of alleles. It is inevitable, but there is no evident cause or force for the changes in frequencies. This further proves that reproduction isolation is the strongest force of microevolution. The effects of genetic drift cannot compare to those of reproductive isolation because it works primarily in smaller populations and takes far longer to have a real noticeable impact on the species. Microevolution VS Macroevolution “Charles Darwin and Alfred Russel Wallace based their insight that organisms evolve by natural selection on four principles1,2: first, that organisms have “individual variations” that are faithfully transmitted from parent to offspring; second, that all organisms produce more offspring than are required to replace themselves in the next generation; third, that limited resources create a “struggle for existence” that regulates population size, such that most offspring die without reproducing; and fourth, that the individuals that survive and reproduce are, on average, by virtue of their individual variations, better suited to their local environment than those that do not. Given these four principles, evolution by natural selection (Darwin's 'principle of descent with modification') naturally follows. Such adaptive modifications within populations over time are now referred to as microevolution. Darwin anticipated that microevolution would be a process of continuous and gradual change. The term macroevolution, by contrast, refers to the origin of new species and divisions of the taxonomic hierarchy above the species level, and also to the origin of complex adaptations, such as the vertebrate eye. Macroevolution posed a problem to Darwin because his principle of descent with modification predicts gradual transitions between small-scale adaptive changes in populations and these larger-scale phenomena, yet there is little evidence for such transitions in nature. Instead, the natural world is often characterized by gaps, or discontinuities. One type of gap relates to the existence of “organs of extreme perfection”, such as the eye, or morphological innovations, such as wings, both of which are found fully formed in present-day organisms without leaving evidence of how they evolved. Another category is that species and higher ranks in the taxonomic hierarchy are often separated by gaps without evidence of a transition between them. These discontinuities, plus the discontinuous appearance and disappearance of taxa in the fossil record, form the modern conceptual divide between microevolution and macroevolution.” (Nature, 2009) Microevolution is essentially Macroevolution on a smaller scale. Microevolution occurs through adaptation. There are many ways that microevolution can occur. Reproductive isolation, genetic mutation, migration, and genetic drift all play key roles in microevolution. They all affect genetic frequency to some extent. Some have greater effects than others, while some need more time and closer observation to take notice of the changes. Macroevolution is harder to analyze because the changes take such a long time to notice. The forces of macroevolution affect genetic frequency just as microevolution. The main different is that macroevolution creates an entire new species. The time it takes for this to occur is usually longer than that of the average human lifespan so, studying these creates by one specific group or person is nearly impossible because the testing and data will often outlive the one studying the specimen or the changes in genetic frequencies. The primary mode of studying the types or levels of macroevolution is to compare the relationships between different species. Conclusion “Reproductive isolation can take many forms. It may involve the presence of distinctive morphological, behavioural or other signals that are involved in mate recognition. It may involve ecological factors that affect the fitness of hybrid offspring in nature. For example, hybrid genotypes may have lower survivorship than pure genotypes of either parental form. Reproductive isolation may involve genetic incompatibilities between species that result in the sterility or inviability of hybrid offspring. There are many potential barriers to gene flow, and they have been catalogued at length in recent treatments of speciation” (Oxford Academic, 2016) Reproductive isolation is the most affective form of microevolution in my opinion. It has many forms and can impact and population or species more visibly than any other form of speciation. It is evident through the information presented in the research. Literature Cited Rosa Ana Sánchez-Guillén, Alex Córdoba-Aguilar & Adolfo Cordero-Rivera (2013) An examination of competitive gametic isolation mechanisms between the damselflies Ischnura graellsii and I. elegans, International Journal of Odonatology, 16:3, 259-267, DOI: 10.1080/13887890.2013.821868 Scoville, Heather. "Prezygotic vs. Postzygotic Isolations." ThoughtCo, Sep. 3, 2018, thoughtco.com/prezygotic-vs-postzygotic-isolations-1224814. University of Southern California. "Genetic mutations thwart scientific efforts to fully predict our future: Effects from genetic mutations can vary dramatically from individual to individual." ScienceDaily. ScienceDaily, 17 September 2018. Scoville, Heather. “How Do Prezygotic and Postzygotic Isolations Drive Evolution?” Thoughtco., Dotdash, 3 Sept. 2018 “Species & Speciation.” Khan Academy, Khan Academy Reznick, David N., and Robert E. Ricklefs. “Darwin's Bridge between Microevolution and Macroevolution.” Nature News, Nature Publishing Group, 11 Feb. 2009, www.nature.com/articles/nature07894. Daniel L. Rabosky; Reproductive isolation and the causes of speciation rate variation in nature, Biological Journal of the Linnean Society, Volume 118, Issue 1, 1 May 2016, Pages 13–25 “Genetic Drift.” John Hawks Weblog, 5 Aug. 2011, johnhawks.net/explainer/genetics/genetic-drift.html. Survival of the Fittest By: DeMicheal Martin Table of Contents Introduction Prezygotic Barriers Postzygotic Barriers Reproductive Isolation VS Genetic Mutation Migration Genetic Drift Microevolution VS Macroevolution Conclusion Literature Cited