Evolution and Its Theories Discussion

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Can you help me understand this Anthropology question?

In simplest terms, evolution can be defined as a change in allele frequency over time.  A fact of nature is that allele frequencies for any given gene are always in a state of flux and for that reason evolution is a fact of nature.  At the same time we have evolutionary theories that help us explain why allele frequencies change in the first place.  For this section we will explore these evolutionary mechanisms.  

The purpose of this discussion board is meant for you to share an article or a news story that specifically deals with how evolutionary mechanisms have and are shaping the world around us.  It can be a story that looks at the fossil record or it can be one that explores current examples of evolution. 

You may look for an example of natural selection, mutation, gene flow, genetic drift, sexual selection.

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EVOLUTION OUR VIEWS OF NATURE ARE INFLUENCED BY THE AMOUNT OF BIODIVERSITY FOUND WITHIN OUR RESPECTIVE ENVIRONMENTS As humans it is hard for us not to ask questions about our respective environments. We want to understand why nature is the way it is and we do so by way of our world views. The respective world views that we have are products of our environment. Since there are a lot of different environments people over time have developed different explanations for the organisms that live within their context. As a result, explanations on nature’s biodiversity are highly variable and have been influenced by a groups interaction and interpretation of their environment. For example, peoples who lived in the desert regions of North America will have world views that are not compatible with people living in the jungles of Central America. Predictably, people that come from a place where there is little biodiversity will have a world view that suggest an unchanging universe while those that come from places were there are great amounts of biodiversity will have a tendency to have explanations that are evolutionary in nature where organisms are seen as changing and relating to each other. Images on the right show North American cultural areas match North American biomes OUR VIEWS OF NATURE ARE INFLUENCED BY THE AMOUNT OF BIODIVERSITY FOUND WITHIN OUR RESPECTIVE ENVIRONMENTS Evolutionary thinking is often thought of as a new way of thinking about nature. In it self, it is not a new idea since evolutionary or evolutionary compatible worldviews can be found embedded within different societies. Evolutionary thinking appears to be new only in the sense that static views of nature have generally dominated Western thinking where organisms are thought of as being their own unique creations. Evolutionary thinking in a modern sense, is a product of the era of exploration and enlightenment where people began to recognize that life on this planet was unusually diverse. This understanding lead people to investigate and propose explanations for why the planet abounds with biodiversity. These explanations are collectively termed evolutionary explanations. Consider the following example: An inquisitive person traveling around the world would come across different “cat” types. They would be faced with two options when trying to make sense of their observations. The cat types are either related to each other or are not related to each other. If they are related to each other then they must have a common ancestor. Many naturalists began to see repeating forms within nature. The more they saw of these repeating forms the more it became clear that organisms that shared similarities with each other must be products of similar ancestry. The skulls above are of feline forms. Notice that there is little difference between the cat types. The skulls below are of a mouse, beaver and capybara. Notice the similarities in skull dimensions. In all cases the similarities are due to the fact that they share similar structural genes with similar regulatory genes since they had a common ancestor The discovery that there was great diversity in forms lead thinkers to develop ways of organizing nature. Before taxonomic systems developed the names used to describe species were essentially long Latin descriptions of the organism. Furthermore, these early classification systems were not consistent since different individuals examining the same organisms would end up with different descriptions and thus different names. The same wild rose being named by two different observers. Rosa sylvestris inodora seu canina Rosa sylvestris alba cum rubore, folio glabro As interest in collecting specimens developed so did the amount of known species. The thousands of newly discovered species resulted in the need to better organize life. The ultimate result was a creation of taxonomic systems that used a universal naming system that allowed naturalist to better organize nature. Page from Carol Linnaeus “System of Nature”. Four thousand animals and seven thousand plants were cataloged in the last edition. Taxonomic systems allow naturalists to see relationship between species All those creating taxonomic systems typically began as individuals that had static view of nature. As they made more and more detailed natural observations they began to see that species grouped into categories. Within the created categories, subdivisions could be made and those subdivisions could be divided into further subgroupings. Furthermore, within each species one was able to see an immense amount of variation. The immensity of nature’s diversity forced the creation of taxonomic trees. When Linnaeus categorized humans he places them within a subcategory of mammals that he called primates. His categorization was based on the exclusive morphology that primates share with each other. Commonality of form began to suggest common descent. As naturalists are discovering vast amounts of organisms, careful studies by anatomists began to be conducted on the discovered organisms. Anatomists came to the realization that the internal parts of organisms are continuously repeated. Geoffroy Saint-Hilaire famously said that “All animals are formed of the same elements, in the same number; and with the same connections… however they differ in form and size”. This idea suggests that there was a primitive and general design that suggested that species overtime evolved from a common form. Saint-Hilaire among others suggested that the environment was involved in transforming species. Image on the right shows the relationship between the environment and body form among lagomorphs. Notice the change in ear length and leg length in relation to the environment. The temperature of a region essentially becomes a predictor of body form. Fossils of extinct species increase the size of evolutionary trees A byproduct of the industrial revolution was the discovery of thousands of previously unknown fossils. Knowledge of the newly found extinct species increased the size of the taxonomic trees. To many naturalists it was becoming evident that species overtime underwent change and in some cases extinction. Images on the right are of the extinct North American lion (Top) and the extant African lion (Bottom). The North American lion was one of many extinct species discovered that informed naturalists of evolutionary change. Naturalist begin to develop mechanisms to explain change By the early 1800’s the role of the environment in creating the biodiversity seen in nature was well established. The connection was understood but no mechanisms to explain how biological change happened was offered. The first to propose a mechanism of change was Jean-Baptiste Lamarck. Lamarck reasoned that the variation seen within a species is a result of the behaviors an organism has in response to its environment. In Lamarck’s view giraffes with longer necks had longer necks because of the actions that they did in life…such as reaching for leaves in tall trees. If a giraffe stopped eating leaves from tall trees its neck would get shorter and its shortened neck would be inherited by its offspring (these are pre-inheritance pre-genetics type of ideas) Lamarck’s idea was a use and disuse hypothesis. Lamarck was the first to propose an explanation for change. Darwin’s Natural Selection Among evolutionary thinkers Charles Darwin is surely the most famous. He is not famous for inventing evolution since those ideas existed long before him. He is famous for natural selection, an evolutionary mechanism that explains how organisms change overtime. Like others, he was aware that the environment is influential in the morphology of an organism however he wasn’t sure how it worked. Darwin’s insight came as a result of the observations and collections of species he obtained during a five year voyage aboard a British navy ship. Some of Darwin’s biographers have suggested that his line of questioning arose as a result of becoming aware that the birds he collected in the Galapagos islands were varieties of finches rather than different types of birds. More striking to him was the fact that the birds graded into each other. The difference between them was simply beak size. Darwin didn’t understand initially why it was possible for the environment to alter the beaks considering that the environment within the Galapagos archipelago is the same. Galapagos finches. Each of the islands has its own finch variety. Darwin’s Natural Selection Darwin came to understand that within populations particular variants were better suited to survive and reproduce than others. He reasoned that nature impedes population growth but not all within nature would be equally hindered. Those individuals that had traits that gave them differential advantage were being selected by nature. He equated this selection with the type of selection that breeders would make when creating new breeds. Darwin’s idea is dependent on a few variables. 1. Variation – Individuals within a population vary from one another. Within a finch population individuals will have different sized beaks. In humans any trait that you view varies between individuals. Siblings within a family may share traits with each other but they will not be clonal. Finches also show the same differences. Natural selection 2. Inheritance - The variation within a population is inherited. Both of these birds are members of the same finch population. However their beaks are slightly different in size. The thickness in beak size is associated with variations in regulatory genes. The bird on the right has a bigger beak because of the specific alleles it inherited from its parents. Its offspring will likewise inherit its alleles for greater beak growth. Natural selection 3. Selection - Under different conditions some variants will be better adapted than others. This particular type of finch eats seeds. Those with larger beaks have access to larger seeds. If environmental conditions change in the direction of favoring plants that produce larger seeds then it’s possible to predict that finches with larger beaks would have a higher probability of surviving. Seeing natural in today’s Galapagos finches In the 1970’s a group of researches began to study the finches from the Galapagos. In 1976 a drought began that altered the food supply available to finches. Plants that produced tougher and bigger seed survived the drought better than plants that produced seeds with thinner shells. The change in food supply lead to a major die off of the population. 80 percent of the population died by 1979 while the average beak size increased by 10 percent. In this scenario, individuals that had larger beaks were able to feed more successfully than birds with smaller beaks. The individuals with the larger beaks had greater levels of fitness. To be fit means that you are able to survive to reproductive age and reproduce. The more effective an organism is at passing on their genes to the next generation the greater fitness they have. Natural Selection and malaria When we see an unusual amount of a particular trait within a population we always ask whether or not natural selection is involved. Take for instance the trait for sickle cell. Within the U.S. the sickle cell allele seems to be a nuisance in the sense that it is only known within the context of anemia. What we find is that there are populations around the world that seem to have a higher frequency of the sickle cell trait. Why would a trait that produces anemia be found in high frequencies within a population in the first place? Left image – Red blood cells that have sickled. Red blood cells sickle when they experience low oxygen stress. The problem with the sickling of red bloods cells is that their shape will be inefficient at carrying blood through small blood vessels. They simply clog up the circulatory system. Map of sickle cell prevalence Natural Selection and sickle cell What researches have discovered is that the allele for sickle cell is a trait that increases your survivability in areas were malaria is endemic. Those that are heterozygous for the allele will have genetic immunity to malaria while that are homozygous normal will have a high probability of dyeing from malaria. Those individuals that are homozygous for the sickle cell allele will likely die of anemia but not of malaria. those Malaria kills hundreds of thousands of people per year. Those with the sickle cell allele will not die from malaria and as a result will be able to survive to reproductive age a reproduce more effectively than somebody that does not have the trait. Over the course of time the sickle cell allele become more common within malaria stricken populations. When sucking blood a mosquito introduces malaria plasmodium into blood stream. Image on far right shows plasmodium merozites reproducing in red blood cells and inadvertently destroying it. Natural Selection and malaria Nature teaches us that there are always multiple solutions to a problem. When an organism faces a strong selective pressure from lethal pathogens individuals that have unusual alleles that yield greater fitness will have a higher probability of survival even when those alleles carry potential harm. As mentioned before individuals that are homozygous for sickle cell will succumb to anemia and die from the trait. Thalassemia is also another traits that increases your chances of surviving malaria but like malaria individuals that are homozygous for the trait will suffer from anemia. G6pd is another trait that is found in high frequencies in populations endemic to malaria. Like in all other cases G6pd can increase the probability that a population will survive but like other forenamed alleles there is tradeoff…small proportion of the population will die. Map of G6pd distribution. Natural selection and clines Clines show the effects of selective pressures on traits. Consider ultraviolet light as a selective pressure. We all know that too much sun can cause skin cancer. The melanin that you produce in your skin is effective at absorbing ultraviolet rays. The more melanin you have the more effective you are at protecting your skin. What is observed within the worlds indigenous populations is that there is a direct correlation between the amount of UV light a particular region gets per year and the relative skin tone of that particular population. The skin color found among different human populations is not the result of having different amounts of melanocytes, melanin producing cells, but rather is a produce of how active those cells are in producing melanin. Amount of UV light per year. Predicted skin tone of populations based on UV index Observed skin tone of population Darwin was the first to talk about an evolutionary mechanism which he called natural selection. His goal was to attempt to understand how organisms evolved. It was already understood by many that organisms changed over time but it wasn’t understood how they changed over time? All Darwin did was provide a mechanism for how organisms changed. Darwin also introduced sexual selection which explains the sexual dimorphism displayed within species. Over time other evolutionary mechanisms were introduced that allow us to understand the reasons for why an organisms genetic makeup changes over time. This week’s lecture will deal with the lesser known but equally important evolutionary mechanisms; mutations, genetic drift and gene flow. • Before we discuss the other evolutionary mechanism we should clarify the following terms: • Evolution are the changes in allele frequencies over time. • Evolutionary mechanisms are the tested theories that explain what we observe. Evolution and Allele frequencies Within a population different alleles will be expected to be found. Consider the following thumbs. If the thumbs below are representative of a population one sees that 80% of the population has the dominant allele expressed, however, the most common allele in the population is the recessive allele. The ratios of all the alleles for a gene is called the allele frequency. In the example below we find that 40% of the population has the dominant allele while 60% of the population has the recessive allele. • The big question is: Will the allele frequency in this present generation be the same in the next generation? Defining evolution Every generation experiences changes in allele frequencies. An allele that is common in one generation is not guaranteed to be as common in the next generation. Our original hypothetical population had 40% dominant alleles to 60% recessive alleles. The preceding generation has the inverse relationship. The fact is that all natural populations experience these changes from one generation to the next (unless people clone themselves) These observable changes in allele frequencies are what we call evolution. Is there any way to stop the genetic changes that occur within populations? Even in a lab setting it is absolutely impossible to do so. The closest we might come so stopping evolution is by creating clones. Evolution is defined as any change in allele frequency from one generation to the next. This is why evolution is a fact of nature not a belief. Evolutionary mechanisms are the explanations for why allele frequencies change from one generation to the next. Many get confused on this point! Evolutionary Mechanisms: Mutations • Mutations introduce new alleles into a population. When new alleles are introduced into a population they change the allele frequency of a population. Mutations can occur within any type of cell but only mutations that occur on gametes are important for evolution since they are the only ones that can be passed down to your offspring. Individuals will pass an average of 50 mutations to the next generation. Of the 50 mutations at least 3 of them will be discernable since they effect the visible phenotype. If each individual within a population passes an average of 50 mutations per generation how many mutations will we be collectively passing on to the next generation in the state of California? You can imagine how the influx of millions of new genetic variants on a yearly basis can have the potential to change alle frequencies of many traits within the population. Most mutations will be neutral and may simply not be noticed unless they are looked for. Some of these neutral mutations may turn on to be beneficial mutations. Consider a mutation on CCR5 gene that confers resistance to HIV. This particular mutation has existed for generations but only became noticeable when it was discovered that it made people immune to HIV. This mutation is now seen as one that increases fitness. Other mutations will decrease fitness by the very fact that they cause disease. Mutations and natural selection Human blood groups are the result of mutations. Blood type O is the most common allele in the world and its also the original human blood type. It is estimated that before forty thousand years all humans had type O blood. Type A blood arose as a mutation around forty thousand years ago. Type B blood is the most recent addition to the human family arising as a mutation around nine thousand years ago. The ABO alleles are generally seen as neutral accept when we view them in the context of disease. We know from the last smallpox epidemic that people with type A blood had a four times higher chance of dying than someone with type O blood. Likewise when plague is a problem those with type O blood are more vulnerable while those with type A are more resilient. Depending on circumstances different mutations will have greater differential fitness in different environments. The different types of mutations Not all mutations result in changes in amino acids sequence but when they do they have the potential of altering the function of the resultant proteins and their by introducing new traits into a population. Point mutation - one nucleotide is exchanged for another.. Deletions One or more nucleotides are removed Insertions Nucleotide s may be introduced Evolutionary mechanism: Gene Flow As mentioned before, any and all mutations arise randomly within any human population. If a mutation appears within an individual and that individual has offspring then that mutation with time may spread from its point of origin. Other than those rare situations where the genes are supposed to stay exclusively within the family, there is a tendency for those alleles to eventually spread to neighboring populations. An allele that appears in one village will eventually make its way to a neighboring village. With time the allele will be found miles from its point of origin and will also be able to give us a history of its spread. • Gene flow is seen as an evolutionary mechanisms on the grounds that it introduces new alleles into a population and therefore alters the allele frequencies within that population. Evolutionary mechanisms: Gene flow Consider the allele for Blood type B. We know that it is relatively young and we could pinpoint were it began. The map below shows blue regions were blood type B is unusually high. As you go in a westward direction you find that the allele for B becomes less and less common being least common in the Basque region of Spain. Over the course of nine thousand years Blood type B spread in all directions and its clinal distribution gives us a view of its expansion. Gene flow • Gene flow can give us insight into historic events. In this example one can see how the medieval Viking expansion left a genetic track in continental Europe. Clinal distribution of CCR5 Delta 32 mutation. Map showing historic Viking expansion Gene Flow and genetic distances The closer populations are to each other the more genetically alike they are. This has to do with the fact that neighboring populations will experience greater gene flow. Its possible to estimate how close individuals are based on how close their ancestors lived to each other. Consider the following maps. The one on the left is a map of Europe with its national boundaries. The map on the right was composed by looking at nine hundred thousand nucleotide positions. The intention of the authors of that study was not to create a map of Europe, interestingly enough their genetic data did create a map of Europe. Genetic Drift Sometimes alleles are found in extraordinary high frequencies within a population for no reason other than chance. When a population is small, allele frequencies can change erratically. Consider the clubbed thumb vs the non-clubbed thumb allele. Imagine if there is a village that consists of only 100 individuals. If all of them are of reproductive age what are the chances that they will all have the same number of descendent? Its very unlikely! Some will have more offspring than others and some may not even reproduce. Allele frequencies will be expected to change quick within such a population. Human history has been characteristically composed of small populations with the last few centuries being an exception. • In genetic drift, alleles that become more common have nothing to do with selective advantage or disadvantage within that environment. Its simply a matter of chance. GENETIC DRIFT: FOUNDER EFFECT The founder effect is a special type of genetic drift. When a small group of individuals move away they take with them a subset of the genetic diversity found within the original population. If the new population remains small and has little influx of alleles from the outside the particular alleles present in the original founders will become unusually disproportionate within the foundling population. Alleles that were rare in the parent population might be common in the daughter population. Populations that experience extreme founder effects are known for their disproportionately high genetic disorders. Circles on the right represent the alleles in the parent population. The red alleles is common while blue is rare. Small group leaves taking with it a particular set of alleles that is not representative of a founding population. Once rare alleles become more common in founding population. The Amish and the founder effect There are many examples of founder effects but perhaps the best known and best studied are the Amish. The Amish are descendants of a small group of Swiss Germans that settled in the US a few centuries ago. Over the course of generations they have multiplied and now number in the hundreds of thousands. The Original Amish carried with them a small set of the genetic variation from their parent population. After they arrived they isolated themselves genetically from the rest of the world. Over the course of time all Amish have become related to each other. The Amish brought with them a unique set of alleles which are disproportionately present within their population. Unfortunately, many of these alleles are unusually rare genetic disorders. Below is a partial list of rare genetic disorders found among the Amish. Gerodermia Osteodysplastica 3-beta-Hydroxysteroid dehydrogenase 3-Methylcrotonyl-CoA carboxylase 2 deficiency 3-Methylglutaconic Aciduria, Type V Adducted Thumbs Syndrome Albinism, oculocutaneous, type 1B Amish infantile epilepsy syndrome Ataxia Telangiectasia Bardet-Biedl Syndrome Bartter Syndrome, Type 3 Cardiomyopathy, dilated, with AV block Cartilage-hair hypoplasia Cerebral vasculopathy and early onset stroke cerebral vasculopathy and early onset stroke Charcot-Marie-Tooth Disease, Type 4A Cholestasis, progressive familial intrahepatic 1 Chronic Granulomatous Disease Cockayne Syndrome, Type B Coclear deafness, myopia, and intellectual Cohen Syndrome Congenital Adrenal Hyperplasia, due to 11Congenital Sodium Diarrhea Cortical Dysplasia-Focal Epilepsy Syndrome Corticosterone Methyloxidase Type 1 Deficiency Cystic fibrosis Cystinosis Desmoid disease, hereditary Ellis-van Creveld syndrome Endocrine-cerebro-osteodysplasia Epidermolysis Bullosa Letalis Galactosemia Genetic Defect in Thyroid Hormonogenesis 2A Gitelman Syndrome Global Developmental Delay and Autism Glutaric acidemia I Glutaric Aciduria, Type 3 Hashimoto Thyroiditis Hemophilia B Homocystinuria Hypercholanemia, familial Hypertension Hypothyroidism and muscular hypertrophy Infantile parkinsonism-dystonia Interleukin-7 receptor alpha chain deficiency ITCH E3 Ubqiquitin Ligase Related Syndromic Jackson-Weiss Syndrome (JWS) Jervell and Lange-Nielsen syndrome Lethal neonatal rigidity and seizure syndrome Major Affective Disorder 1 Mast Syndrome McKusick-Kaufman Syndrome Mental retardation, non-syndromic Microcephaly with chorioretinopathy Microcephaly, Amish type Moyamoya disease 1 Muscular dystrophy, limb-girdle, type 2A Muscular dystrophy, limb-girdle, type 2E Nanophthalmos 2 Nemaline myopathy 5 Nephrotic Syndrome, Type 2 Oculocerebral Syndrome with Omenn syndrome Osteogenesis Imperfecta, Type 1 Pantothenate kinase-associated Parkinson Disease Pelizaeus-Merzbacher-like syndrome Phenylketonuria Posterior column ataxia with retinitis Primary Ciliary Dyskinesia, Type 3 Primary Congenital Glaucoma 3A Prolidase deficiency Propionic acidemia Pyruvate kinase deficiency of red cells Refsum Disease, Infantile Form Senior-Loken Syndrome 1 Severe Combined Immunodeficiency, T CellSevere Combined Immunodeficiency; due to Sitosterolemia Spastic paraplegia 20, autosomal recessive Sudden infant death with dysgenesis of the Symptomatic epilesy and skull dysplasia TMCO1 defect syndrome Trichothiodystrophy, Nonphotosensitive 1 Usher Syndrome due to HARS deficiency Genetic drift: bottleneck The bottleneck effect is a special type of genetic drift were a population is reduced by fifty percent or more by an event such as a natural disaster, famine, overhunting or war. Consider the overhunting of the American bison. Within a short span of time the bison population dropped from 60,000,000 to around 500 individuals. Those that survived were simply lucky. Unfortunately the genetic variation found within the original population was dramatically reduced and now the population is highly inbred. The end result is similar to what you would see in the founder effect (except for the potential for extinction). Currant bison population is highly inbred and vulnerable to disease. Mountain of bison skulls Genetic drift: bottleneck The island of Pingelap in the South Pacific is known for being the island of the color blind. In this island forty percent of the population carries the allele for achromatopsia which renders homozygous individuals completely colorblind. In the US one out of every thirty three thousand will be effected but in Pingelap one out of every ten is effected. This condition is found in such large number because of a typhoon that devastated the island in 1775. The typhoon killed all but twenty individuals. One of the survivors had the allele for achromatopsia. This allele is not found in high frequencies because it had a selective advantage in this population but rather it was simply a matter of chance. Pingelap Sexual selection When working on natural selection Darwin saw that his theory worked well when explaining the long necks of giraffes or a lions large teeth and powerful jaws. Under natural selection alleles that confer an advantage should be selected for while those that hinder your existence should be selected against. When viewing most species Darwin began to notice that there was significant male vs female differences or sexual dimorphism. These differences didn’t fully initially catch his full attention. After all, it made sense to him why an elk might have antlers but like others he wouldn’t have asked why only males seem to have antlers. It makes sense for an elk to have antlers since such a trait would have an advantage in nature when dealing against predators. However, if having antlers was advantages against predators then why don’t both sexes have antlers. Sexual selection What really made Darwin question the logic of male vs. female differences was the fact that many of the traits that he found among males didn’t seem to have any advantage at all. Consider the bird pairs below. The camouflaged ones are females while the highly visible ones are males. Within the context of natural selection camouflage has an advantage and it should be equally shared by males and females. However, it’s the females that seem to have this advantage. In contrast, males often times have ornaments, such as colorful feathers or usually extravagant feathers that make them highly vulnerable to predation. Sexual selection Peafowls are a great example of Darwin’s dilemma since the males have unusually exaggerated trains that don’t follow the logic of natural selection, since males with the longest trains are more likely to get eaten by predators. The males tail feathers attract allot of attention and are metabolically expensive to produce while being relatively heavy. Why have flashy costly feathers that make it hard to escape? In Darwin’s mind, those individuals that were able to survive would have the most offspring over their lifetime. Consequently, all males over time should have small feathers. Within the context of natural selection unusually large feathers would progressively reduce in size since those with the smallest feathers, the survivors, should be having the most offspring. Alleles that are a predicament to ones survival should be found in low frequencies. Males have the long bright feathers while females have short and dull feathers. Sexual selection Darwin eventually realized that the length of the feathers in peacocks was associated with in individuals reproductive success but not necessarily with their survivability. Observations on captive peafowls suggested that within any population, males with the longest feathers will always have more mating opportunities and will sire exponentially more offspring. Those with the shortest feathers will sire far less offspring or none at all. From the perspective of a peacocks, having expensive, long and dangerous feathers might shorten the peacocks lifespan, but in that short lifespan the individual that was more likely to get killed would have had most of the offspring within the population. Males with smaller feathers would have survived more years but they would have had minimal chances at passing on their alleles to the next generation. Sexual selection Darwin’s suspicions lead him to experiment with captive peafowl populations. He looked for the males that were the most endowed and shortened their feathers or removed eye spots. He also altered the peacocks with the short feathers by artificially lengthening their feathers. The peacocks that once had the longest feathers were no longer popular with the peahens while the unpopular short tailed peacocks received all the mating's because of their newly elongated feathers. Others have repeated Darwin’s experiment on peafowls and other birds and have shown similar results. • Why do the males of some species have costly and in some cases potentially dangerous ornaments? • Female choice vs. male competition Female choice Females of most species don’t mate with just any random male. They have a tendency to be choosy. Females will have a tendency to choose males based on particular phenotypes. Females might choose males based on length of feathers, size of body, or how bright their colors are. Females have to be more choosey because of their limited reproductive potential. Human females, as an example, can only reproduce a limited number of times in their lives while a human male has no limit to how many times he could potentially reproduce in his lifetime. Also consider the investment. A human female has to invest a lot of time and energy to reproduce while a male doesn't. In this context, a females choosiness is important since the quality of the male is going to increase her offspring's chances of survival. What about the peafowls? Why are peahens more attracted to peacocks with longer feathers? Why do longer feathers matter in the first place? It turns out that longer feathers are a measure of the genetic health of an individual. Among peacocks, the healthiest males are the ones that grow the longest feathers while the sickly males grow shorter feathers. Sickly males are simply not able to allocate resources to making metabolically expensive feathers since their resources are being spent fighting off pathogens. Male competition Within most species half of those born will be female and the other half will be male. Females maybe choosy but males will compete with each other and the winner may not necessarily be the females first choice. Among species were males compete physically with each other one is bound to find that males are much larger than females in size. Size matters when competing against other males Males on the right and female on the left. Sexual selection in humans Among humans, males are on average 10% larger than females. One can see some differences when observing human skulls. Males will have a tendency to have more muscle marking than females. However, sexual dimorphism (male female morphological differences) within our species is not as obvious as these skulls suggest. Before puberty male and female skulls look the same (the middle skull). After puberty sexual dimorphism generally becomes more pronounced (Female on left and male on right). However, not all males will end up looking like this unusually robust skull on the right. Within our species there is a tendency to see some degree of continuation within the sexes. Sexual selection in humans Among mammals, a females breasts increase in size during lactation. Humans seem to be the exception. Human breasts begin to increase in size on the onset of puberty. The increase in size is associated with an increase in fatty deposits that are hormonally induced rather than products of lactation. A similar increase in fatty deposits occurs around the pelvic area in human females. Female pelvic girdles are shaped differently but they are not generally larger than males. The hormonally induced fatty deposits found around the hip area make them appear wider but in reality they are not. However, the pelvic inlet is significantly wider in human females due to differences in skeletal growth that is driven by hormonal differences between males and females. • The gorilla on the left has swollen breasts that are products of lactation. If a gorilla never has offspring her breasts will look indistinguishable from males.
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Evolution and Its Theories
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Evolution and Its Theories
Evolution is defined as the change in the different characteristics of populations over a
number of generations. The changes are as a result of evolutionary mechanisms which also have
an impact on the frequencies of these traits. In addition to changing the frequencies of the traits,
evolutionary mechanisms have shaped the world around us. The shaping of the world around us is
explored ...

Nalbfb96 (10965)
University of Virginia

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