Unit 8: EvolutionAlthough evolution is the last unit we are covering, it is probably the most important concept in biology and governs all others. Evolution is the gradual change of species over in response to selective pressure. Here we will establish the basic rules of evolution and what it means to evolve. Darwin may be regarded as the discoverer of evolution, but he did not accomplish the task on his own- we will study the naturalists who helped to develop the Theory of Evolution by Natural Selection some of whom were right in their ideas and some who were wrong but important all the same. This unit will focus heavily on exploring the different patterns that can be seen in evolution and how selective pressure can be applied, but will also draw on the previous unit as we explore genetic relationships in stable populations. We will take a look out the evolutionary history of the human race before concluding by exploring some of the theories on how our planet came to be and how life began on it.
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The Theory of Evolution
Evolution is a Scientific Theory.
In a scientific sense, a theory is a hypothesis that has been tested and verified with data after numerous, repeated experiments, observations and peer review. In this way, a scientific theory is well-substantiated and objective. This differs dramatically from our colloquial or everyday use of the word theory which is really nothing more than a synonym for "guess". Evolution by natural selection is a scientific theory: it has been supported a variety of lines of evidence including the fossil record, DNA and anatomical structures. So while many people dismiss evolution by natural selection as "just a theory", this is really not a fair assessment of evolution.
In a scientific sense, a theory is a hypothesis that has been tested and verified with data after numerous, repeated experiments, observations and peer review. In this way, a scientific theory is well-substantiated and objective. This differs dramatically from our colloquial or everyday use of the word theory which is really nothing more than a synonym for "guess". Evolution by natural selection is a scientific theory: it has been supported a variety of lines of evidence including the fossil record, DNA and anatomical structures. So while many people dismiss evolution by natural selection as "just a theory", this is really not a fair assessment of evolution.
Evolution by Natural Selection
In its simplest sense, evolution by natural selection is change over time. Whenever we are discussing evolution by natural selection, we are talking about the change of a species, not the change of an organism- evolution works on a species level, not an organismal level. There are four major conditions that must be met in order to for evolution by natural selection to occur:
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Please note, you should skip the scene from 4:12 - 4:20 as it is slightly inappropriate.
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In any ecosystem there is a finite amount of resources (food, water, space, etc.) which in turn means that all ecosystems have a carrying capacity- a defined number of organisms that can be sustained in an ecosystem. So if more organisms are born than the environment can support, a competition for resources will ensue among the organisms. Because each organism has its own unique DNA, each organism will have its own unique characteristics. Some will be better adapted to the environment than others, giving them an edge at collecting resources and surviving. We refer to these organisms as being more fit. Assuming that their traits are heritable and able to be passed on to the next generation, the organisms that are better at surviving will have more offspring, and increase the prevalence of their genes and by extension their characteristics in the gene pool. This is evolution by natural selection.
The Role of Genes and DNA
All life is made of cells and all cells contain DNA: these are some of the most tenets of life and yet they have some of the most profound impacts on an organism. An organism's genotype (its DNA) controls every aspect of its design. Within a population exists a gene pool, the sum of all genes (a specific region of DNA that codes for something, usually a protein) in that population. Genes and DNA can mutate and change over time allowing new traits to develop. The more diversity present in a gene pool, the easier it becomes for a population to adapt to environmental change and survive (note: it is easier for the population to survive, not necessarily the individual organisms). These new traits can be advantageous (+), detrimental (-) or neutral (0) to the organism and because they exist within a gene they are heritable and able to be passed down to future generations. When natural selection is applied to a population/species, organisms with advantageous traits become more fit. This ultimately will increase the prevalence of advantageous traits in the gene pool and population/species. On the other hand, organisms that possess detrimental traits will have reduced fitness, lowering the prevalence of these disadvantageous traits in the gene pool. Finally, organisms with neutral mutations will not have their fitness altered.
All life is made of cells and all cells contain DNA: these are some of the most tenets of life and yet they have some of the most profound impacts on an organism. An organism's genotype (its DNA) controls every aspect of its design. Within a population exists a gene pool, the sum of all genes (a specific region of DNA that codes for something, usually a protein) in that population. Genes and DNA can mutate and change over time allowing new traits to develop. The more diversity present in a gene pool, the easier it becomes for a population to adapt to environmental change and survive (note: it is easier for the population to survive, not necessarily the individual organisms). These new traits can be advantageous (+), detrimental (-) or neutral (0) to the organism and because they exist within a gene they are heritable and able to be passed down to future generations. When natural selection is applied to a population/species, organisms with advantageous traits become more fit. This ultimately will increase the prevalence of advantageous traits in the gene pool and population/species. On the other hand, organisms that possess detrimental traits will have reduced fitness, lowering the prevalence of these disadvantageous traits in the gene pool. Finally, organisms with neutral mutations will not have their fitness altered.
For example, imagine an organism that feeds on the leaves of tall trees. Now imagine that a mutation allows some of these organisms to have longer necks than others. Natural selection will favor the organisms with the mutation for longer necks since they will be more successful at acquiring food and ultimately reproducing. The organisms with longer necks will have higher fitness and go on to dominate the gene pool. This is how giraffes came to be the organisms we see today. In contrast, imagine a mutation causes a particular organism to be brightly colored. Organisms with this mutation do not blend in easily with the environment and are more easily identified by predators. Natural selection will not favor the organisms with this mutation and these organisms (and the mutated gene) will ultimately die out.
The Naturalists
James Hutton and Charles Lyell
Today we know and frankly take fro granted that the world is an ancient place that has existed for millions of years and has changed many times over the course of history. But for a long time, this was not the case; people assumed that the world was relatively new, only a few thousand years old and a very stable, static place. In short, species never changed, they had existed in their present form since the creation of the Earth. But in the 1700 and 1800s, new evidence began to surface that challenged these ideas and and assertions. One of the most obvious would be the discovery of fossilized organisms that did not resemble any species living today. This implied that the world was not a static place, but just like the species that occupied it, tad been changing over time.
Today we know and frankly take fro granted that the world is an ancient place that has existed for millions of years and has changed many times over the course of history. But for a long time, this was not the case; people assumed that the world was relatively new, only a few thousand years old and a very stable, static place. In short, species never changed, they had existed in their present form since the creation of the Earth. But in the 1700 and 1800s, new evidence began to surface that challenged these ideas and and assertions. One of the most obvious would be the discovery of fossilized organisms that did not resemble any species living today. This implied that the world was not a static place, but just like the species that occupied it, tad been changing over time.
When people began to recognize that the Earth was not static, it still took a long time to determine exactly how it changed over time. One of the earliest schools of thought on the matter was known as catastrophism. According to catastrophism, the world was shaped by sudden and violent events throughout history. Further, the Earth was developing in a particular direction: it started as a hot molten core and was cooling to become more stable. This idea that there was a goal to any sort of change or even a higher power at play was pervasive at the time and would reappear in some of the earlier forms of evolution developed by Lamarck, but not Darwin.
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James Hutton and Charles Lyell were some of the first to challenge this idea of catastrophism. Hutton developed the idea of gradualism; which argued that the Earth changed slowly over time through a series of imperceptibly small changes. Later, Lyell built off of Hutton's ideas to develop his own theory called uniformitarianism. According to this concept, the Earth's crust has been changing gradually and consistently due to continuous and uniform processes. In a broader sense, uniformitarianism argues that the natural laws and processes that govern the universe have always operated in the same way. In other words, natural processes are universal, all forces and interactions operate in the same manner throughout the universe, a central theme in modern science. The concept of uniformitarianism and its implications for geology helped to lay the foundation for the development of the theory of evolution.
Lamarck
Darwin was not the first to propose a theory of evolution. Lamarck had also observed the same evidence Darwin had and came to a similar conclusion: all life changed over time. At the time this was a revolutionary concept. However, Darwin and Lamarck differed in their ideas about how organisms changed over time. Lamarck suggested that organisms wanted to become more complex and worked to change themselves. In short, Lamarck believed that individuals, not species evolved and that evolution was goal oriented (similar to the development of the Earth according to catastrophism). So when a giraffe becomes hungry, it stretched its neck to eat leaves and that trait |
was passed down to the giraffe's offspring. This was termed, the inheritance of acquired characteristics. In reality, Lamarck's theory has several major issues (but it was a good first try). Species evolve, not individuals and traits acquired during the lifetime of an organism cannot be passed on to offspring. An organism's genotype is set at birth and cannot be altered as Lamarck suggested.
Charles Darwin (1809-1882)
Charles Darwin was an English naturalist and geologist. Darwin is best know for his development of the theory of evolution by natural selection. He traveled the world aboard the HMS Beagle observing the diversity of life. As Darwin traveled the world on the HMS Beagle he made a series of observations that contributed to his theory. For instance, on the Galapagos Islands, Darwin observed a wide diversity of finch and tortoise species that he later concluded had all descended from a common ancestor. The Galapagos Islands were remote meaning that some ancestral organism would have arrived to colonize the area, but in doing so found a plethora of niches to exploit. This caused a rapid speciation as the organisms changed to better suit their environment. |
This explained why these seemingly different species were also so highly related. Building off the work of Hutton and Lyell, who demonstrated that the Earth changes over time (deep time), Darwin proposed that life descended from a common ancestor and over time changed to be better adapted to its environment. His most famous book, On the Origin of Species was groundbreaking and Darwin waited years before publishing it. He feared tremendous pushback by the public against his ideas and only published his work when he was approached by Alfred Wallace, another naturalist, who had arrived upon similar conclusions about life.
Alfred Wallace
Shortly after Darwin completed his theory of evolution by natural selection, Alfred Wallace stumbled upon the same conclusions developed the same theory independently from Darwin. Like Darwin, Wallace had traveled and observed much of the world in particular the Amazon and Malay Archipelago which lead him to the same conclusions. Today, Wallace is known as the father of biogeography, the study of the distribution of plants and animals. He even has his own effect, known as the Wallace Effect, which states that natural selection encourages the formation of barriers to hybridization as seen in the biological species concept. This is because hybrid organisms (the offspring of parents from different species) cannot propagate because hybrid organisms are sterile and from an ecological or evolutionary perspective are a waste of resources. Remember, the entire point to life is to pass your genes on to the next generation- you cannot do that if your offspring are sterile.
Additionally, hybrid organisms will be caught in between two niches and cannot succeed at either. Consider for example a lion and tiger mating to form a liger. The liger will be well rounded in its traits, a blended intermediate of both parents. The hybrid could fill the role of both of its parents, but not as well as the respective parent. The lion and tiger have each had generations to become adapted to their environment, natural selection has favored only the strongest individuals. The liger has not had that opportunity and so the lion will always outcompete the liger in the lion's habitat and the tiger will always outcompete the liger in the tiger's habitat. In short, the liger cannot ever be as successful as it parents. Ultimately, Wallace was the one who convinced Darwin to publish his work and become synonymous with evolution.
Shortly after Darwin completed his theory of evolution by natural selection, Alfred Wallace stumbled upon the same conclusions developed the same theory independently from Darwin. Like Darwin, Wallace had traveled and observed much of the world in particular the Amazon and Malay Archipelago which lead him to the same conclusions. Today, Wallace is known as the father of biogeography, the study of the distribution of plants and animals. He even has his own effect, known as the Wallace Effect, which states that natural selection encourages the formation of barriers to hybridization as seen in the biological species concept. This is because hybrid organisms (the offspring of parents from different species) cannot propagate because hybrid organisms are sterile and from an ecological or evolutionary perspective are a waste of resources. Remember, the entire point to life is to pass your genes on to the next generation- you cannot do that if your offspring are sterile.
Additionally, hybrid organisms will be caught in between two niches and cannot succeed at either. Consider for example a lion and tiger mating to form a liger. The liger will be well rounded in its traits, a blended intermediate of both parents. The hybrid could fill the role of both of its parents, but not as well as the respective parent. The lion and tiger have each had generations to become adapted to their environment, natural selection has favored only the strongest individuals. The liger has not had that opportunity and so the lion will always outcompete the liger in the lion's habitat and the tiger will always outcompete the liger in the tiger's habitat. In short, the liger cannot ever be as successful as it parents. Ultimately, Wallace was the one who convinced Darwin to publish his work and become synonymous with evolution.
Forms of Selective Pressure
Natural Selection
In order to evolution to occur, some type of selective pressure must be applied. The most obvious selective pressure is of course natural selection, the idea that —the natural environment determines which organisms within a population or species will be the most fit. Natural selection centers around differences in the phenotypes of various members of a population or species. Those organisms with the most advantageous traits,
In order to evolution to occur, some type of selective pressure must be applied. The most obvious selective pressure is of course natural selection, the idea that —the natural environment determines which organisms within a population or species will be the most fit. Natural selection centers around differences in the phenotypes of various members of a population or species. Those organisms with the most advantageous traits,
phenotypes that enhance their survival, will be better at survival and reproduction allowing them to dominate the gene pool. Under natural selection, those individuals with the highest fitness (the ability to survive and reproduce) will dominate while others with lower fitnesses will be bred out. Natural selection includes both biotic (ie. predation or other forms of symbiosis) and abiotic factors (ie. available sunlight and precipitation).
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Artificial Selection
Natural selection is not the only type of selective pressure that organisms can be exposed to and drive evolution. Under artificial selection, humans select for the phenotypes they deem the most advantageous. These phenotypes do not necessarily increase the fitness of the organism (the hallmark of natural selection) and in many cases actually lower the fitness of the organism. Artificial selection has been performed for many thousands of years most obviously with domestication. Humans selected for the wolves that were the most docile, dependent and loyal and the crops that offered the highest yield and developed new species in this manner by influencing the evolution |
of the species. Again, all that is required for evolution to occur is a shift in the allelic frequency of the gene pool, which is exactly what humans were and are doing.
Sexual Selection
Sexual selection is a subset of natural selection. Under this form of selection, males and/or females exhibit preferences in the opposite sex. Most of the time, it is the female will select a male based on traits such as size, coloration or a mating ritual. These traits may or may not increase the fitness of the male, but are required to secure a mate and have offspring. This type of selection often leads to sexual dimorphism in which males and females of the same species will have different appearances including size, coloration or some other distinguishing feature(s).
Sexual selection is a subset of natural selection. Under this form of selection, males and/or females exhibit preferences in the opposite sex. Most of the time, it is the female will select a male based on traits such as size, coloration or a mating ritual. These traits may or may not increase the fitness of the male, but are required to secure a mate and have offspring. This type of selection often leads to sexual dimorphism in which males and females of the same species will have different appearances including size, coloration or some other distinguishing feature(s).
Genetic Drift
There are occasions in which certain phenotypes and alleles are favored at random, with no particular reason. This is known as genetic drift and is more common in smaller populations. In short, the allelic frequency of the gene pool shifts not because of any particular advantage or disadvantage the alleles might convey but because of random circumstances. In general there are two main ways genetic drift can occur: the bottleneck effect and the founder effect. During a bottleneck effect, some form of disaster or catastrophic event devastates a population. The survivors have no particular advantage other than they were in the right place at the right time, they were "chosen" at random. Obviously there is not natural selective force at play here. |
The founder effect is pretty similar to a bottle neck effect in that certain organisms are selected at random, but there is no death in the population. During a founder effect, a group of organisms establishes a new population in a new environment and thereby dominates the gene pool. Again, these organisms have no particular selective advantage, they were simply in the right place at the right time. This can be seen with the Galapagos finches and in fruit flies in Hawaii. In both the founder and bottleneck effect, the randomly selected group of colonizers or survivors is not reflective of the original population as whole in regards to the gene pool and genetic diversity.
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Patterns of Selection
Natural Distribution of Phenotypes
As we already know, variation in a population or group of organisms is natural and in many ways very healthy for the stability of the species. Populations with greater genetic diversity and by extension wider ranges of phenotypes are more adaptable and can more readily respond to environmental changes whether they are biotic or abiotic. Generally speaking, the distribution of phenotypes within a group of organisms will resemble a bell curve: more extreme phenotypes will be less common and more intermediate ones will be more prevalent. Bear in mind that the figure to the left is a generalized model, not every population will have that exact distribution curve in regards to the phenotypes present. Some curves may be much wider, but short or taller and thinner. But they will also respond to evolutionary pressures in similar ways as described below. |
Directional Selection
During direction selection, selective pressure favors on of the extreme phenotypes at the edge of the curve and causes the species to evolve in that direction. Alternatively, one of the extreme phenotypes near the end of the curve could be selected against. Either way the result is the same and the population evolves toward one of the extreme phenotypes. For example, imagine we were looking at finches and |
focusing on beak size. Under directional selection one extreme beak phenotype will be favored (but not both). This could be either a very large or very small beak. Either way the population will shift in the direction of the favored "extreme" beak type.
Stabilizing Selection
During stabilizing selection pressure favors the intermediate phenotypes toward the middle of curve while both of the more extreme types of phenotypes (near the ends of the curve) are selected against. In this way the population is "stabilized" as all phenotypes transition to the intermediate type. Imagine again we are focusing on finch beaks. However, now the extreme beak types (really big and really small) are selected against, while the medium phenotype or medium sized beaks are favored. The population will shift and more finches will have the medium phenotype. |
Disruptive Selection
In disruptive selection, selective pressure favors the two most extreme phenotypes but those phenotypes in the middle (intermediate) are selected against. In this way, the population is "disrupted" as two unique phenotypes begin to emerge. Disruptive selection and lead to the start of divergent evolution. Once again, imagine our finches and beak size. However, this time both extreme phenotypes (really small and really big beaks) are favored and the intermediate phenotype (medium sized beak) is selected against. The population will begin to diverge as more and more individuals have the extreme phenotypes. |
Types of Evolution
Macroevolution vs. Microevolution
As you know, evolution refers to the change of a species through time. However, evolutionary biology can actually study evolution occurring at various levels. For example, major evolutionary changes such as those that influence organisms at the species or higher levels of taxonomical organization are known as macroevolution. Smaller steps in evolution such as those that influence individual organisms or populations (in other words below the species level) are examples of microevolution.
As you know, evolution refers to the change of a species through time. However, evolutionary biology can actually study evolution occurring at various levels. For example, major evolutionary changes such as those that influence organisms at the species or higher levels of taxonomical organization are known as macroevolution. Smaller steps in evolution such as those that influence individual organisms or populations (in other words below the species level) are examples of microevolution.
The image to the left does a nice job in summarizing the differences between micro and macroevolution. We can see that changes in genetics, the development of individual organisms, populations and/or migration, are all forms of microevolution. Specific examples of each would include the following:
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Macroevolution occurs whenever there are changes in vicariance (the geographic isolation of a species), historical/developmental constraints, and/or species selection. Specific examples of each would include the following:
- Vicariance (E): Geographical isolation.
- Historical/Development Constraints (F): Environmental changes that cannot be adapted to and cause extinction.
- Species Selection (G): Changes that affect speciation rates and type.
Coevolution
Coevolution (also called parallel evolution) occurs when two species evolve together, in response to one another. A change in one generates a reciprocal change in the other. An example of coevolution would be Darwin's moth and the Madagascar Star Orchid. Darwin's moth has evolved a long proboscis (a long mobile nose) to help it reach the pollen found deep inside the Madagascar Star Orchid. Both species benefit from the interaction: the moth gets food in exchanging for helping to fertilize the lily. |
Each species relies on the other and the evolution of one has had a major impact on the other. Coevolution is driven by symbioses, especially mutualisms and antagonisms such as with predator-prey arms races. In such races, both the predator and prey and attempting to get the upper hand against the other, there is a selective pressure for the predator to be more effective at hunting and for the prey to become more effective at evading. A great example of this can be seen with bats and moths. Bats cannot see very well and use echolocation to hunt their prey. The bats emit sound waves that bounce off objects and return to the bat which can they use the data to detect food. The moths in response have evolve the ability to broadcast their toxicity using ultrasonic sound, effectively telling the bats "stay away, I'm dangerous." Some moths can even use their ultrasonic sound to disrupt the bats echolocation and hide their position.
Divergent Evolution
Divergent evolution occurs when evolutionarily related species evolve to become less similar and more distinct. Divergent evolution usually results from a geographical barrier emerging and isolating populations. Without the ability to interbreed, the two population evolve under different pressures and become separate species. Divergent evolution is closely linked with the process of speciation in which new species are generated. When populations of the same species become isolated from one another they begin to evolve in different ways through divergent evolution as they respond to different selective pressures in their own environments. |
Convergent Evolution
Convergent evolution occurs when evolutionarily unrelated species evolve to become more similar. This is because they share similar ecological niches and therefore evolve under similar selective pressures despite being unrelated genetically. One example of convergent evolution would be marsupials and placental mammals. Although these two groups of organisms live in different ecosystems and are unrelated, they evolved under similar selective pressures and evolved similar body designs. The development of wings is also an example of convergent evolution. As you can see in the figure to the right, birds, Pterosaurs and bats all share similar wing structure and design despite being unrelated and not sharing a common ancestor. Each species evolved under similar selective pressure that caused them to develop similar morphologies. |
Gene Flow
In genetics and evolution, gene flow refers to the exchange of genes or alleles between different populations, which is accomplished through immigration and emigration between various populations of the same species. Gene flow is a stabilizing force, it prevents populations from diverging or becoming distinct from one another. By exchanging genes between the various gene pools, gene pools cannot differentiate which would lead to speciation. For example, imagine that in population #1, a particular allele has gone locally extinct. This could have massive consequences for the evolution of the population and species as a whole. But if the "lost" allele still exists in population #2, it is possible to restore the missing gene to population #1 through gene flow. Disrupting gene flow, such as with a geographic barrier causes the populations to become isolated, the first step toward speciation. |
Speciation
As the name suggests, speciation is the process by which new species develop and results from divergent evolution. In order for speciation to occur, populations must become isolated from one another in some way. This isolation will prevent deneflow between the populations due to some type of barrier disabling any interbreeding between the two populations. Now isolated, each population will develop under the selective pressures of its respective environment. Over time, the populations will differentiate further and further to the point that they can no longer interbreed even in the absence of a barrier and will no longer be considered the same species. There are two modes of speciation: allopatric and sympatric speciation. During allopatric speciation, populations become physically isolated from one another and develop into nice species over time (geologic or deep time). Allopatric speciation requires a geographic barrier like a mountains range or ocean to isolate species. |
In contrast, sympatric speciation occurs when organisms becomes isolated while occupying the same area. In the case of sympatric speciation the isolation is not physical but the result of the organisms occupying different niches in their environment. As a result, the groups of organisms within the population interact less and begin to adapt to separate selective pressures ultimately resulting in the formation of unique species. An example of this would be flies that eat fruit. While the organisms may live in the same area, they may consume different types of fruit and in doing so occupy different niches. Now each group will respond to its food source and evolve independently of the other group.
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Adaptive Radiation
In many ways, adaptive radiation is speciation in overdrive. Stated perhaps more scientifically, adaptive radiation is the rapid diversification of an ancestral species into a wide variety of new forms. Adaptive radiation is very common with founders, organisms that colonize a new area or one not previously occupied by a species. When the founders arrive in the new habitat, a range of niches are available to be filled, each with its own selective pressures. As the founders begin to occupy these new niches, each group of organisms begins to evolve under unique pressures resulting in the proliferation of a number of new species that all share a common ancestor. A classic example of this is Darwin’s finches from the Galapagos. Originally, there were no finches on the Galapagos islands, but when the ancestral finch arrived on the islands from mainland South America and found a variety of new, open niches things changed rapidly. The ancestral birds quickly differentiated into a variety of new species to take advantage of the open resources. |
Altruism
In biology, altruism refers to any behavior by an individual that benefits another organism at the expensive of the self. This is very different from any of the symbioses we discussed earlier like antagonism, as the organism is deliberately performing a self-sacrificing action that reduces its fitness to benefit other individuals. Such behavior should be selected against in nature by natural selection as the goal is to maximize fitness under selective pressures. As much of an oddity as this might seem, altruism can be seen many times in nature:
Kin selection works to better address that flaw. Under this model, altruistic individuals do not support individuals indiscriminately. Instead, they only support organisms that they are closely related to, their relatives. Relatives would have a higher chance of sharing the altruism gene, meaning they would all work to support each other. And even if the individual dies, its genes could still be passed on through surviving relatives would would share a high degree of the genotype, including the altruism gene.
In biology, altruism refers to any behavior by an individual that benefits another organism at the expensive of the self. This is very different from any of the symbioses we discussed earlier like antagonism, as the organism is deliberately performing a self-sacrificing action that reduces its fitness to benefit other individuals. Such behavior should be selected against in nature by natural selection as the goal is to maximize fitness under selective pressures. As much of an oddity as this might seem, altruism can be seen many times in nature:
- Vampire bats regularly regurgitate blood and donate it to other members of their group who have failed to feed that night, ensuring they do not starve.
- In numerous bird species, a breeding pair receives help in raising its young from other ‘helper’ birds, who protect the nest from predators and help to feed the fledglings.
- Vervet monkeys give alarm calls to warn fellow monkeys of the presence of predators, even though in doing so they attract attention to themselves, increasing their personal chance of being attacked.
- In social insect colonies (ants, wasps, bees and termites), sterile workers devote their whole lives to caring for the queen, constructing and protecting the nest, foraging for food, and tending the larvae. Such behaviour is maximally altruistic: sterile workers obviously do not leave any offspring of their own—so have personal fitness of zero—but their actions greatly assist the reproductive efforts of the queen.
Kin selection works to better address that flaw. Under this model, altruistic individuals do not support individuals indiscriminately. Instead, they only support organisms that they are closely related to, their relatives. Relatives would have a higher chance of sharing the altruism gene, meaning they would all work to support each other. And even if the individual dies, its genes could still be passed on through surviving relatives would would share a high degree of the genotype, including the altruism gene.
Cladograms and Phylogenetic Trees
Cladograms
A cladogram is a diagram used to illustrate the relationship between various species or groups of organisms. A cladogram uses a series of branching lines to illustrate the position of various clades, groups of organisms that consist of a common ancestor and all of its descendants. This means that each clade is monophyletic as opposed to paraphyletic (containing a single common ancestor but not all of the descendants). In a cladogram, organisms are typically positioned according to their characteristics. These could include physical characteristics, but also DNA evidence which is more reliable. In a cladogram, time is a major |
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factor and more ancient divisions will be found toward the left of the cladogram and more recent divisions are toward the right. Each node where the lines diverge represents a shared common ancestor between the two species. The example to the left illustrates what a cladogram looks like. Notice that the lines are uniform in regards to their spacing and there is no mention of what criteria is used to differentiate between the species.
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Phylogenetic trees
Phylogenetic trees are very similar to cladograms with a few small, but important differences. The drawback with cladograms is that while time is a factor, it is not measured very well. While the branching in a cladogram does correspond with how long ago the groups diverged, it is only relative and does not provide a concrete determination of time. The branches are evenly distributed rather than reflecting how much time has actually passed. Phylogenetic trees allow us to more definitively see how a species changed over time: the branchings are proportional to the amount of time that has actually passed. This can be seen in the example to the left. |
Additionally, phylogenetic trees mark the appearance of various traits over time, allowing us to better evaluate the evolutionary history of the groups. Cladograms are more of a hypothesis and less well defined than phylogenetic trees. Ultimately, the differences between a cladogram and phylogenetic tree are pretty superficial and the terms are often used interchangeably. Focus on the bigger picture, that both cladograms and phylogenetic trees help to organize species into clade according to the evolutionary history, genetics and physical traits.
Human Evolution
Our Place on the Tree of Life
Modern humans have been around for roughly 200,000 years. Today, all humans belong to the species Homo sapiens and the family Hominid, but what exactly does this mean? We use taxonomy and phylogeny to categorize and organize different species. Under this so called "tree of life" species are organized into various levels according to their characteristics and genetic relatedness. The highest level of organization is the domain of which there are 3: Archaea, Bacteria and Eurkarya. Because humans are eukaryotes we belong to the Eukarya domain. Humans are also animals and therefore fall within the kingdom Animalia, which excludes other types of eukaryotes such as plants or fungi. Within the animal kingdom, humans occupy the chordata phylum. |
Chordates are animals that have a notochord, a dorsal nerve cord as well as several other distinguishing characteristics observed in vertebrate organisms (compared to invertebrates). The phylum chordata includes 7 different classes of animals including jawless fish (lamprey), cartilaginous fish (sharks and rays), ray-finned fishes, amphibians, reptiles, birds and mammals. Humans of course are mammals and occupy the order Primates within the mammal class. The order Primates includes all monkeys and apes and can be further narrowed into the family Hominidae that includes the great apes (chimpanzees, orangutans, gorillas) and humans. The family Hominidae
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includes 4 genera and 7 different species currently living. Members of this family are known as hominids. And finally, humans belong to the genus Homo which includes several other closely species that are now extinct. In other words, Homo sapiens are the only living representatives of the genus Homo.
Human Family Tree
Homo sapiens are a relatively new type of hominid in the very ancient family Hominidae. There have been many other genera and species that have died out over millions or hundreds of thousands of years. You will notice in the family tree to the right, the ancestors of modern humans fell into one of four genera, only one of which is still in existence today. These genera included Ardipithecus, Australopithecus, Paranthropus and Homo. Obviously, we are more closely related to the other members of the Homo genera and less closely related to the others. Our earliest ancestors first developed in Africa and had an arboreal lifestyle before transitioning to a terrestrial one. Below we will explore a few our most notable ancestors. |
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Australopithecus afrarensis
One of our oldest relatives, Australopithecus afrarensis lived between 3.85 and 2.95 million years ago. These species had a lifetime of 900,000 years, that's 4.5x longer than our species has existed so far. Australopithecus afrarensis occupied Eastern Africa including Ethiopia, Kenya and Tanzania.Australopithecus afrarensis was much smaller than modern humans with males reaching 4' 11" and 92 lbs and females reaching 3' 5" and 64 lbs on average. The first fossil of this species was discovered in 1974 and nicknamed "Lucy." |
Homo habilis
Beloning to the Homo genera, Homo habilus is known as the "tool user" or "handyman" a testament to the species' ability to create and use tools to better survive. Homo herbals existed from 2.4-1.4 million years ago and occupied Eastern and Southern Africa. Much like Australopithecus afraensis, Homo habilus was smaller than modern humans ranging in height between 3' 4" - 4' 5" and weighing 70 lbs on average. The species was discovered in 1960. |
Homo erectus
Homo erectus lived from 1.89 - 143,000 years ago and occupied Northern, Eastern and Southern Africa as well as Western Asia. As we can see, the origins of modern humans began in Africa and began to slowly expand off of the continent to the rest of the world. Homo erectus was taller than the previous two species, reaching heights of 4'9" - 6'1" and weights of 88-150 lbs. Homo erectus' name stems from its ability to walk upright (erect) and important transition from arboreal to terrestrial life. This species was discovered in 1891. |
Homo neanderthalensis
Homo neanderthalensis is one of our most recent ancestors existing from 400,000-40,000 years ago. In fact, we actually lived alongside their hominids and on occasion interbred with them. Homo neanderthanlensis occupied Europe and southwestern to central Asia. Individuals shared a similar stature with modern humans with males reaching heights of about 5' 5" and 143 lbs and females being roughly 5' 1" and weighing approximately 119 lbs. Neanderthals were highly intelligent evidenced by their prowess as skillful hunters, practicing of burial rituals and the ability to create symbolic objects one of the first steps toward written language. Neanderthals were discovered in 1829. |
Hardy-Weinberg Equilibrium
Not all populations are evolving all the time, some maintain genetic equilibrium during which allelic frequencies do not change. When populations experience equilibrium, we can calculate the frequency of various alleles in the population since they are not changing. In order for a population to be held in equilibrium, certain conditions must be met. These conditions prevent any change in allelic frequency.
- There must be random mating. Organisms must mate at random and therefore there is no sexual selection going on.
- Large population size: This reduces or prevents genetic drift from occurring.
- No gene flow: Populations are isolated and individuals cannot immigrate or emigrate between them and introduce new alleles.
- No mutations: No new alleles appear in the population which would shift the allelic frequencies.
- No natural selection: If certain traits are favored, alleles will be selected for or against in the population. This would cause a change in allelic frequency and disrupt equilibrium.
Hardy and Weinberg developed a mathematical formula to determine the likelihood of having various alleles under genetic equilbirum. This is summarized as p^2 + 2pq + q^2 = 1.
- p = the frequency (%) of the dominant allele
- q = the frequency (%) of the recessive allele
- p + q = 1 (100%)
Remember that the term genotype refers to the allelic combination of an organism for a specific gene. An organism's genotype controls its phenotype or physical traits. For our purposes here with genetic equilibrium, Mendelian rules apply; alleles are either dominant or recessive. The dominant allele is always expressed, but the recessive allele is only expressed in the absence of the dominant allele. There are three major types of genotypes:
- Homozygous dominant: the genotypic condition in which both alleles for a gene are dominant. Since p is used to calculate the frequency of the dominant allele, the percentage of homozygous dominant in a generation or population = p^2.
- Heterozygous: the genotypic condition in which one dominant allele and one recessive allele are present for a specific gene. Since p and q represent the frequency of the dominant and recessive alleles respectively, the percentage of heterozygotes in a generation or population = 2pq
- Homozygous recessive: the genotypic condition in which both alleles for a gene are recessive. Since q represents the frequency of the recessive allele, the percentage of homozygous recessive in a generation or population = q^2
Evidence for Evolution
Deep Time Evidence
Evolution by natural selection requires hundreds of thousands to millions of years to occur. We call this kind of time deep time or geologic time. For a long time, believed that the Earth was only a few thousand years old, which would have not provided enough time for the type of evolution that Darwin suggested to occur. Today we know better: the Earth is roughly 4.5 billion years old and life began 3.5 billion years ago, more than enough time for evolution by natural selection to occur. The Fossil Record
Fossils are the preserved remains of organisms from the distant past. These include actual body parts or even the impressions of footprints. Fossils are vital to helping to establish the long history of life on Earth, but are very rare because they only form under very specific conditions. Fossilization requires rapid burial to prevent the decomposition of the sample. The rarity of the fossil formation ensures that the fossil record will never be complete, but it does help to paint a picture of the past. |
Genetic Evidence
All living organisms have the same basic cellular design, use carbon as a backbone for building organic molecules and perhaps most importantly use DNA to hold genetic information. The universality of these characteristics are extremely powerful and suggest the descendent from a common ancestor for all living things, exactly as evolution by natural selection predicts. The likelihood that each species would independently develop all of these traits with such a high degree of overlap is almost impossible to imagine.
All living organisms have the same basic cellular design, use carbon as a backbone for building organic molecules and perhaps most importantly use DNA to hold genetic information. The universality of these characteristics are extremely powerful and suggest the descendent from a common ancestor for all living things, exactly as evolution by natural selection predicts. The likelihood that each species would independently develop all of these traits with such a high degree of overlap is almost impossible to imagine.
Molecular Clock
One of the incredible things about DNA is that the rate of mutation is surprisingly consistent. This fact allows scientists to compare the genomes of two or more species and analyze the number of differences between their DNA. In this way, DNA can serve as a molecular or evolutionary clock, allowing us to estimate how long ago the species diverged. For example, imagine species A shares 98% of its genome with species B and 65% of its genome with species C. Using our evolutionary clock, we can estimate that species A and B diverged more recently than A and C because A and C have a greater number of mutations or differences between their DNA.
One of the incredible things about DNA is that the rate of mutation is surprisingly consistent. This fact allows scientists to compare the genomes of two or more species and analyze the number of differences between their DNA. In this way, DNA can serve as a molecular or evolutionary clock, allowing us to estimate how long ago the species diverged. For example, imagine species A shares 98% of its genome with species B and 65% of its genome with species C. Using our evolutionary clock, we can estimate that species A and B diverged more recently than A and C because A and C have a greater number of mutations or differences between their DNA.
Hox Genes
Hox genes are a group of genes that control the body design of an organism, first expressed during embryonic development.These genes are conserved in a variety of organisms including vertebrates (mammals, birds, reptiles, amphibians and fish) and even fruit flies (Drosophila). The presence of these genes in so many diverse organisms (much like the shared cellular structure, use of carbon and DNA in all forms of life) implies descent with modification from a common ancestor. |
The head-to-tail organization of the body is under the control of different Hox genes. Flies have one set of eight Hox genes (represented by the different colored boxes), while humans have four sets of Hox genes. In both organisms, the activity of the gene matches its position in the DNA: genes active in the head lie at one end, while those that are active in the tail lie at the other end and genes that affect median regions lie in the middle.
Structural/Anatomical Evidence
Morphology (the study of the structure and form of an organism) can also be used as evidence for evolution. Evolution can only act on whatever is present in a group of organisms. So when we look around at the various adaptations of organisms and begin to see patterns and trends, evolution helps to explain these observations. Below are the three main types of structures used as evidence for change of species over time.
Morphology (the study of the structure and form of an organism) can also be used as evidence for evolution. Evolution can only act on whatever is present in a group of organisms. So when we look around at the various adaptations of organisms and begin to see patterns and trends, evolution helps to explain these observations. Below are the three main types of structures used as evidence for change of species over time.
Homologous Structures
Homologous structures have the same origin and basic design, but have been modified to have different functions and better suit the needs of the organism in its habitat. In other words, these are structures that share a similar structure, but have been modified to fulfill different tasks depending on the needs of the organism. For example, the flipper of a seal and arm of a human have the same bones (ie. radius, ulna, humerus, etc.) but these have been modified to better suit the organism. Such structures have evolved under divergent evolution. |
Analogous Structures
Analogous structures serve similar functions, but have different designs and are unrelated evolutionarily. This is because the organisms live in similar niches thereby fulfilling similar roles in their environment and have evolved structures that appear similar in use, but unrelated in terms of their structure and design. These are structures that have evolved through the process of convergent evolution. Examples include wings in birds, insects and bats. |
Vestigial Structures
Vestigial structures have become reduced (made smaller) because they are no longer used by the organism (in terms of deep time). An organism only has so many resources, if a structure is not being used, there is a selective pressure to stop investing resources and ultimately reduce it. In short, the presence of vestigial structures implies that species have indeed changed over geologic time. Examples include the hind limbs of whales and the appendix in humans. Whales are descended from large, rat-like organisms that lived on land on four legs. Over time, these organisms evolved to become more aquatic and exploit a new niche. The hind limbs became less and less helpful and used less often. Over time they shrank or became reduced leaving the small remnants we see today. |
Biogeographical Evidence
We learned from Hutton and Lyell that the Earth is constantly changing. Through the process of planet tectonics the original supercontinent land mass Pangaea has broken up into the continents we see today. In doing so, species have become isolated over deep time, a perfect recipe for speciation and evolution. As we look around the world today, we can see evolutionarily related species on different landmasses. For example, the Big Cats (lions, jaguars, puma, etc.) are scattered around the world, but share a high degree of genetic relatedness and a common ancestor. That ancestral species was divided and isolated by continental drift. The independent populations could no longer experience gene flow and began to diverge and eventually speciate entirely. |
The Primitive Earth and History of Life
Proto-Earth
The Earth formed roughly 4.5 billion years ago with life developing about 1 billion years later. But the early Earth was very different from the Earth we know today. For instance, the Earth's crust did not solidify until about 4.2 billion years ago. This meant that the surface of the Earth was a liquid sea of molten lava for about 300 million years. At this time, the Earth was being bombarded by debris from space. In fact, one major impact was so forceful, a huge amount of the Earth was ejected into space and would eventually form the moon. The object (known as Theia) was a planetary sized body. |
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The atmosphere was also very different. Today the composition of the atmosphere is as follows: 78% nitrogen, 21% oxygen, 1% argon and the remaining 2% consists of a variety of different gases including water vapor and carbon dioxide. But, billions of years ago the atmosphere was mostly carbon dioxide, nitrogen and water vapor with no oxygen. The oxygen would not appear until life began to appear and the process of photosynthesis become more prevalent.
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Miller-Urey Experiment
The question to how did life begin has been puzzling scientists for a long time. Stanley Miller and Harold Urey attempted to answer this question in 1952 in their laboratory, by investigating the origin of the organic macromolecules (carbohydrates, lipids, nucleic acids and proteins). They attempted to recreate the early Earth's environment to form these simple organic macromolecules, the basic building blocks of life. The experiment utilized water (H2O), ammonia, (NH3), methane (CH4) and hydrogen gas (H2) to simulate the early atmosphere sealed in a flask. The flask was heated to generate water vapor and exposed to discharging electrodes to simulate lightening. After a week of operation, Miller and Urey were able to recover amino acids indicating that it was possible for organic compounds to develop from inorganic chemical without the assistance of living organisms as a catalyst. In short, it was possible for life to begin on the Proto-Earth. Today, we |
realize that Miller and Urey did not configure their experiment exactly right, it is difficult to pinpoint the exact concentration of the various gases in the atmosphere at that time since so little evidence still exists. None- the-less, the experiment inspired many others that have all demonstrated that the idea is at least possible and brings us one step closer to understanding the origin of life on Earth.
RNA World Hypothesis
We know that DNA is the molecule of heredity, the storage form of genetic material in all living things. However, this may not have always been the case. RNA is a much simpler structure and much more autonomous than DNA. RNA is capable of catalyzing its own synthesis and supporting many other molecules. RNA can also store genetic material as evidenced by retroviruses. The RNA world hypothesis suggests that RNA developed before DNA and existed as the storage |
form for genetic material in the first cells. It was only later when DNA was developed and later replaced RNA. Why did RNA get replaced considering it has so many advantages? RNA is single stranded meaning its nitrogenous bases are exposed. This makes RNA more reactive and less stable, which is detrimental to the cell. DNA on the other hand is single stranded, none of the nitrogenous bases are exposed which makes it more stable than RNA. Today the two molecules work closely together to support one other.
Microspheres Hypothesis
Where the Miller-Urey experiment tried to answer where the first organic macromolecules cam from and the RNA World Hypothesis investigates which came first RNA or DNA, the microspheres hypothesis explores the origin of the first cells. This hypothesis notes that when amino acids (the monomer subunits of proteins) are heated they can form proteinoid microspheres, small bubbles formed from organic molecules. —These microspheres display some of the most basic characteristics of life and may have been the foundation from which the earliest cells were built. For instance, modern proteinoid microspheres exhibit a simple, but selectively permeable membrane, have some primitive ability to store and utilize energy and even exhibit |
basic replication through division to form two, smaller microspheres. The hypothesis asserts that these microspheres may have served as the blueprint for the first cells to develop.
Panspermia Hypothesis
Throughout its 4.6 billion year long life span, the Earth has been struck by celestial bodies such as asteroid and meteors many, many times. The Panspermia hypothesis proposes that one of these objects carried life or organic molecules to Earth from other region in space. In essence, life is relatively in the universe and Earth is just the result of a colonization event billions of years ago. The Panspermia hypothesis dodges the question of how did life begin on Earth and replaces it with, how did life originate in the universe. If this is the case, the question is far more difficult to answer. |