3C and 3D Notes KEY 23-24 JM
Biology Honors Name:
Unit 3C Evolution Notes
Evolution is a change in organisms over time. It is important to note that individual organisms cannot evolve but rather populations of organisms evolve. A population is a group of organisms of the same species.
Speciation, or the formation of new species, often occurs when a single population becomes separated into two isolated populations that are exposed to different environmental conditions. If these two populations remain separated and do not mate with each other, given enough time they could evolve into two different species. (reproductive isolation)
During the long history of the earth (4.6 Billion years), there has been a great deal of change in the conditions existing on the planet, such as, climate, formation of mountains and new land masses, collisions with asteroids and comets from outer space etc. Organisms have evolved, or changed, over time in ways that have made them better able to survive in their changing environment.
Charles Darwin and his Theory of Evolution
Charles Darwin was an English naturalist/scientist who traveled the world in the 1830s on a ship named the Beagle. He was 22 years old when he set off on his 5-year voyage. His job was to collect and classify different organisms from around the world. During his voyage, he observed many interesting similarities and differences among the animals and plants that he studied. He was also interested in the adaptations that organisms seemed to have in their specific environments. In addition to his passion for studying living things, Darwin was also very interested in geology and fossils. From his observations, including those that he made on the Galapagos Islands, Darwin came up with his theory of evolution.
His theory has two important key points:
Descent with Modification 2. Natural Selection
The first point explains that organisms evolved from ancestral organisms, gradually developing adaptations that allowed them to successfully reproduce in the changing environments they inhabited. His second point explains how organisms evolved from ancestral forms. Organisms that have variations that allow them to survive in their environment will be “Selected For” and will be able to successfully reproduce and pass on those favorable traits.
Darwin’s theory has been experimentally tested and supported many times over since he first published his work in 1859. This however does not mean that Darwin had it “all” right. Parts of his theory are still being tested and there is some evidence that supports the fact that living forms did not evolve gradually as he predicted, but may have changed significantly in much shorter periods of time. It is important to understand that even though some scientists may disagree on points such as evolutionary rate, they do not disagree on whether or not natural selection occurred. It is also important to note that Darwin was not the first scientist to believe in evolution or to put forth a theory on how evolution occurs. What set Darwin apart from the others such as Lamarck was that he proposed natural selection was the primary way that new species developed from ancestral species, an idea that was eventually supported by many scientific observations and experiments.
Although Darwin’s observations were made in the 1830s, on his trip around the world, he spent the next 30 years analyzing and interpreting his findings. Even in the 1850s when his manuscript was complete, Darwin was hesitant to publicize his ideas because of the popular religious beliefs of the day. “Darwin’s view of life contrasted sharply with the conventional paradigm of an Earth only a few thousand years old, populated by unchanging forms of life that had been individually made during a single week in which the Creator formed the entire universe.” The enormous amount of scientific evidence that Darwin and other scientists since Darwin have collected runs counter to this traditional view of how life was created. It was not until another scientist by the name of Wallace was about to put forth a similar theory that Darwin was convinced that he should finally publish his book The Origin of Species in 1859.
Summary of the important points of Darwin’s Theory of Evolution by Natural Selection
More organisms are born than can survive. Due to competition between organisms for food, light, water, predation – only some will survive.
There are variations among organisms. Some variations aid survival that leads to greater reproductive success, and some reduce the chance of survival and reproduction.
Organisms with favorable variations (characteristics) are selected for and will tend to leave more offspring than those with less favorable variations. This is natural selection, where nature or the environment “selects” those organisms that will be better at survival and therefore have better reproductive success.
Organisms that survive will pass on their favorable characteristics to their offspring (good variations are inherited and become adaptations).
Gradually, over time, different populations of organisms can change and evolve into different species.
Although Darwin knew that there were variations that were selected for or against, and that these variations were passed to offspring, he did not know how the variations arose, or how they were passed on. Today we can answer both of these questions.
Ultimately, variations arise from random mutations in DNA. These changes in the DNA translate into changes in the proteins that organisms produce. This of course means that the traits / characteristics that the proteins produce will also change.
Variations are passed on because DNA replicates prior to cell reproduction, so that when cells divide each new cell receives the same DNA. In sexual reproduction, when gametes are produced they will carry the genetic information to the next generation.
Some additional important information of Evolution by Natural Selection
Evolution has no direction; the process is random, and depends upon the availability of existing variations in organisms, and the influences of the environment on these organisms. The variations that are present in a population are produced by random or chance mutations. Organisms cannot change themselves at will to help them better adapt to a change in their environment. They either already have the helpful characteristics, or do not have the helpful characteristics that will enable them to survive.
Natural selection cannot fashion perfect organisms. Natural selection can only act on variations that are available in the population, and the best possible solution to a problem is not always available. Remember that variations originate by random changes in the genome. Adaptations are often compromises. Each organism must do many different things. For example, we humans are extremely versatile and athletic due to our opposable thumbs, upright posture, and flexible limbs, but because of these structures humans are prone to sprains, torn ligaments, dislocation, and back pain. In a way, these are compromises that have been made for agility. Evolution does not produce perfection.
A common misconception about evolution is to see evolution as always following a progression from simple to complex, from simple organisms to more advanced organisms at the top of a ladder of ascent as more and more “perfectly” evolved. In reality, as mentioned before evolution is not directed, goal-oriented, working its way up some ladder of perfection. A better metaphor would be to compare evolution to a many branched tree, with many dead branches and twigs, but with the still-living branches having survived the twists and turns of the changes in Earth’s history. Bacteria, amoebas, or extinct dinosaurs should not be thought of as “lower” forms of life than chimpanzees or humans. Each possesses (or, in the case of dinosaurs, possessed, for well over 100 million years) a unique set of adaptations that defines it as a successful organism. Do not confuse the level of complexity with the degree of adaptation. Many species of bacteria are probably better “adapted” for life in their ecosystem than we are. They have been around for at least 3.9 billion years!!!
“Survival of the fittest” can be used as a synonym for Natural Selection. It is a phrase used to describe the reproductive success of an organism; or more correctly, a population of organisms. If an organism possesses adaptations that make it strong or more competitive, but does not reproduce itself, its fitness is 0. Without reproductive success, the passing of genes from parent to offspring, the population will eventually go extinct.
In natural selection, there are many conditions that can act as selective pressures, including environmental changes such as temperature, drought, rainfall, meteor collisions, environmental toxins, earthquakes, volcanoes, etc. Selective pressure can also come from competition among organisms, predation, disease, and parasitism.
Lamarck’s Theory of Evolution
Before Darwin, there was a French scientist named Lamarck who also proposed that organisms evolve, or change, over time. However, his theory on how they change was very different from Darwin’s, and is not accepted today. His idea was that organisms acquired, or gained, characteristics during the course of their lifetime, in order to live better in their environment. If they used a particular characteristic, it would become better adapted for its function and would be passed on to the next generation. If it was not used, it would disappear. This became known as The Theory of Acquired Characteristics through use or disuse. A famous example of this would be the evolution of long necks in giraffes: Lamarck thought that as the giraffes stretched their necks to reach and eat the tree leaves they grew longer and longer throughout the course of their lives. He also thought that the long neck characteristic could then be passed on to the next generation. This, of course, is not true. If a human lifts weights, then his or her children should not be more muscular if they hadn’t lifted weights.
Evolution is a continual process that occurs over long periods of time. Organisms have been evolving on earth for over 3.8 billion years, into all of the different organisms that inhabit the earth today. For this reason, it is difficult to observe evolution directly. There are, however, some examples that can be used to illustrate how evolution by natural selection occurs. These examples show how the changing environment is closely tied to the change in organisms over time.
Example 1: Industrial Melanism (the evolution of color change in moths)
This species occurs in 2 varieties throughout England: one form is light with dark spots, the second uniformly dark.
During the day, both varieties rest on trees and rocks covered with light colored lichens. Light individuals are camouflaged against the background, while dark moths are easily seen by predators such as birds.
Before the Industrial Revolution, which polluted the air, dark Peppered Moths were very rare, due to predation.
Pollution eventually killed the lichens, and the trees were darkened by the late 1800s. Under these new conditions, the dark moths were less conspicuous, and the light moths became more obvious.
The number of dark moths increased, and by the early 1900s, the population of moths was almost entirely of the dark variety.
Natural Selection acted upon the EXISTING VARIATIONS in the population; the dark moths were favored, and were able to leave more offspring.
Example 2: Antibiotic Resistance in Bacteria
Some bacteria have a natural resistance to antibiotics due to some chance mutation. The mutation could cause an existing enzyme to be altered in such a way that it is able to break down the antibiotic so it cannot kill the bacteria. It is important to note that the resistance does NOT occur in response to contact with the antibiotic.
When a population of bacteria is exposed to the antibiotic, those that do not have resistance will die, and those with resistance will survive and go on to reproduce, producing many resistant bacteria.
The resistant bacteria can then be spread to other individuals directly or it can wind up in the environment and the gene that makes it resistant can be spread to other disease causing bacteria. Because of antibiotic resistance, scientists always need to develop new antibiotics. Antibiotic resistance is a crucial ongoing problem because microbes are changing faster than we can develop new medicines to kill them. Some diseases that were once easily curable are now life threatening because some strains of bacteria are resistant to all available drugs.
Remember that in addition to proposing that natural selection was the mechanism by which evolution occurred, Darwin’s theory also explained that all organisms are related in some way to each other because they all evolved from a common ancestor (Descent with Modification).
For example, it is believed that all living things evolved from single-celled organisms that originated approximately 3.5 billion years ago. Over the last 3.5 billion years, these ancestral organisms diversified into all of the millions of organisms that exist today. For this reason, all organisms share at least some structural and chemical characteristics. Some organisms are more closely related because they share a more recent common ancestor while others are more distantly related. For example, humans are more closely related to a dog, which is another animal, than humans are to a tree, which is a plant. We are also more closely related to a horse, which is a mammalian animal, than a lizard which is a reptilian animal. Because it is difficult to directly observe natural selection, most of the evidence Darwin gathered, with the exception of Artificial Selection, supported his ideas on descent from a common ancestor.
Some of Darwin’s observations that provided evidence to support his theory of Descent with Modification:
Biogeography (the distribution of plants and animals around the world)
Artificial Selection
Fossil Records (Paleontology)
Comparative Anatomy & Embryology
Comparative Biochemistry
1. Biogeography:
One example of biogeography that helped Darwin formulate his ideas on evolution is the distribution of various Finch species with different beak adaptations on the Galapagos Islands. The various finches on the different islands are similar to each other, but with some variations, especially in beak shape. Darwin suggested that all these different finches evolved from a common ancestral finch that flew over and populated the islands. The different environments they inhabited had different food sources that required different beak adaptations. Over time, the finches evolved these adaptations, which allowed them to survive, reproduce, and pass on the trait.
This type of evolution where one species diverges into two or more different species is called divergent evolution. The term adaptive radiation is used to explain what happens in divergent evolution. It is used to describe when different populations of the same species radiate out into different environments and adapt to those environments, eventually forming different species.
SPECIATION The formation of a new species.
There are several ways a new species can arise from a preexisting species. The most common way a new species can arise is due to geographic isolation. This is where a population of organisms within a species is in some way geographically isolated from the rest of the species, therefore can no longer interbreed. These different populations will accumulate different random mutations which will produce different variations upon which natural selection can act. Because the populations will not interbreed, new alleles that arise in one population will not be exchanged with the other population. If enough time passes in two different environments, the two distinct populations will adapt to the different selective pressures, until they become genetically different enough that they can no longer reproduce with one another. At this point, they would be reproductively isolated, and considered two different species. A classic example of this is what happened to the Finches on the Galapagos Islands, which are all believed to have descended from a single ancestral species found on the South American mainland. The various species of finches have developed on the Galapagos Islands, a volcanic set of islands that arose from the Pacific Ocean relatively recently in geologic time (5 million years ago).
Geographic Isolation can result from a number of geographic barriers:
Islands
Mountain Ranges
Area of Forest separated by grasslands
Continental Drift
2. Artificial Selection:
Darwin was aware of the practice of artificial selection (selective breeding); where organisms such as plants and animals are mated in a controlled way in order to produce desired characteristics. For example, dogs have been selectively bred from a wolf ancestor, to produce all the breeds of dogs that exist today. Darwin figured that if people could alter the characteristics of organisms by selecting for the most desired trait, maybe the environment selects for the most favorable traits in nature.
3. Fossil Record:
The fossil record preserves a chronological history of the appearance of organisms throughout Earth’s history. Example: fossil fish are the earliest fossil vertebrates, followed by amphibians, reptiles, mammals and birds. Fossils can be dated by the sedimentary layers in which they are found. In this example, only fossil fish are found in the lowest layers, the oldest rock, of sedimentary deposits, while both fossil fish and fossil amphibians are found in the next layer. From this, it can be inferred that amphibians evolved after fish. The chronological appearance of organisms in the rock strata is the same throughout the entire world. It does not matter if you are looking in North America, China, or Africa; organisms always appear in the same order – reptiles are never found in strata below where fish first appear. This led Darwin to propose that different organisms evolved at different times throughout Earth’s history. In other words, all organisms were not created at the same time as religious scriptures suggest.
Since Darwin’s time, scientists have discovered many intermediate fossils between different taxonomic groups such as fish and amphibians. These intermediate or transitional fossils (fossils that possess characteristics of both groups) help support Darwin’s idea of descent with modification, whereby some fish evolved into amphibians and some amphibians evolved into reptiles. Darwin’s information was based on relative dating not absolute dating. Relative dating is done by observing where different organisms appear in rock strata, while absolute dating is based on actually determining the age of the strata using radioactive isotopes. It should be noted that quantitative evidence from absolute dating corroborates what was inferred from relative dating.
Darwin was familiar with the geological studies of Lyell and Hutton, who established the idea that sedimentary layers exposed in mountain ranges around the world were formed over a long period of time, knowing that sedimentary strata in some cases were hundreds or thousands of feet thick had to be more than 6,000 years old. Darwin himself observed the presence of marine fossils high in the Andes Mountains of South America, thousands of feet above sea level. Again, the natural events that led to this type of geological change would have taken a very long time.
Although many organisms have gone extinct through the course of evolution, all five types of vertebrates are still present and evolving. In the case of amphibians giving rise to reptiles, only some amphibians evolved into reptiles, while other species of amphibians remained amphibians and continued to evolve as amphibians. The two groups of amphibians, those that evolved into reptiles and those that remained amphibians were obviously subjected to different selective pressures.
Because the formation of fossils requires very specific conditions, and because there were probably not enough numbers of every type of organism that evolved, transitional fossils that document how one group evolved into another cannot always be found. However, transitional fossils showing the evolution of many different groups, including primates have been uncovered. The evolution of Homo sapiens from an ape ancestor has been extensively documented. The following evolutionary tree (Phylogenetic tree) shows how humans and other present-day primates evolved from a common ancestor.
This tree shows that all present day apes evolved from an ape-ancestor more than 8 million years ago. It also shows that humans and chimps share a common ancestor, and begin to evolve along separate paths, about 6 MYA. Because evolution is not a ladder, but rather more like a branching tree, the development of humans from primate ancestors means that one branch within the tree of apes split off and eventually produced a twig called Homo sapiens, while other branches of the same tree evolved along their own branches, producing descendants living side-by-side with us today – the gibbons, orangutans, chimps and gorillas, all modern apes.
4. Comparative Anatomy & Embryology:
This refers to the structural similarities found between related organisms. These anatomical similarities are called homologous structures. For example, the forelimbs of birds, bats, whales, and humans contain the same bone structure.
Homologous structures provide strong evidence in support of Darwin’s theory of evolution. Organisms share similar structures if they share a common ancestor that also had these structures. Over time, these structures would become somewhat different to serve different functions in different environments. Another way to say this is that the organisms were exposed to different selective pressures. An example of homologous structures is a whale flipper and a human arm/hand, having similar internal anatomy even though they look different on the outside.
A special type of homologous structure is a vestigial structure. Vestigial structures have no apparent function in some organisms; but similar structures that still have a function are found in other related organisms. Examples include the small, internal hind limb bones still found in some whales, and the nonfunctioning wings of some flightless birds, such as ostrich.
Vestigial structures also provide evidence to help explain the relatedness between organisms. Why would we have structures that serve no purpose now if we weren’t related to organisms that also have these same structures and still use them? Example: whales evolved from terrestrial mammals that moved into the ocean. The ancestral terrestrial mammals had hip (pelvis) bones, and over time, these bones became reduced in size in the whale because they probably hindered their survival. They remain as small, vestigial structures because they probably no longer make them less fit. If a structure does not harm the organism’s ability to survive and reproduce it will often be retained even if it no longer serves any function. Another example of this is our own tail bone (coccyx). We have one because our ancestors had a functional tail, and over time, the tail has been reduced in size but has not completely disappeared.
Comparative embryology is a science that studies the embryological development between organisms. Closely related organisms go through similar stages in their development.
Example: all vertebrate embryos go through an embryonic stage in which they possess:
Gill openings (gill slits) on the sides of their throats. As development progresses, the gill slits either develop into structures characteristic of that vertebrate, or they disappear, as in the case of humans
Tails
These are both vestigial embryonic structures in human development!
Organisms that appear very different in their adult form may appear very similar in their embryonic stages. These animals have a common ancestor that used similar developmental pathways. These pathways were retained over time because they worked. If they had not been beneficial to the survival and reproduction of these organisms they would have been selected against.
5. Comparative Biochemistry: more evidence for descent from a common ancestor.
All organisms, from bacteria to bananas to a human have many similarities in their biochemistry, specifically, their DNA and proteins. In addition to all organisms having the same genetic code (AUG – codes for met, UUU – codes for phe, etc.). All organisms share many genes in common even though a few of the nucleotides may be different. The more closely related 2 organisms are, the more similarities there will be in their genes and proteins.
An example would be if you were to compare an important protein in animals called cytochrome. There are fewer differences in this protein between more closely related animals.
Cytochrome proteins as well as many other types of proteins are found in ALL organisms, even bacteria. This supports the idea that ALL organisms are related and evolved from a common ancestor.
Comparative biochemistry is considered the best evidence for evolution, primarily because it is quantitative. You can look at the number of amino acids that differ within similar proteins in different organisms to see how closely/distantly related the organisms are. You can also look at the number of nucleotide differences within similar genes in different organisms.
Additional Notes on Evolution: Important Terms and Concepts
The rate of evolution: gradualism vs. punctuated equilibrium
Gradualism: is when changes in organisms within a species which eventually lead to the formation of new species occurs over long periods of time. In other words, changes in form occur gradually over millions of years. Darwin and a majority of scientists after him believed in a slow, graduation accumulation of small changes in organisms over time. Even though most scientists today believe that this occurs to some extent, there are probably other mechanisms involved in how organisms change, and how new species are formed. One of the reasons that scientists began looking for other explanations in addition to what Darwin presented might be a fossil for a small-necked giraffe, and a fossil for a long-necked giraffe, and very few fossils for giraffes with an intermediate neck length. These transitional fossils are sometimes, but not always missing. For example, we do have many intermediate transitional fossils that show how whales evolved from terrestrial to aquatic mammals.
Punctuated Equilibrium: explains that species may exist with very little change for relatively long periods of time, and then experience a period of rapid change in which speciation occurs. This could have to do with drastic, sudden changes in the environment, followed by long periods of little environmental change. This theory helps explain why there might be an absence of transitional fossils. Fossils only form under very specific conditions, and if speciation occurs relatively quickly, those conditions may not be present. For this reason, when asked which theory best represents the fossil record, the answer is usually punctuated equilibrium.
Even though scientists are debating the rate of evolutionary change, many evolutionary scientists believe that both of these could have occurred, and all believe that natural selection played a role in both processes.
Convergent Evolution: is when two or more species that are only distantly related (do not share a recent common ancestor) develop similar adaptations and resemble each other because they live in similar environments and are exposed to similar selective pressures. With convergent evolution, the similar characteristics shared by two organisms evolved after they split from a common ancestor. This is very different from divergent evolution where the similar characteristics are found in and inherited from a common ancestor. A classic example is the similarity in body type between sharks (fish) and whales (mammals). Structures that resemble each other due to similar selective pressures and not to recent common ancestry are called analogous structures. Sometimes it is difficult to tell whether a common characteristic is homologous or analogous. In general, the more complex two structures are, the less likely they evolved independently.
Divergent evolution leads to homologous structures; convergent evolution leads to analogous structures.
Another example of convergent evolution is the evolution of marsupial and placental mammals. Continental drift is the drifting of continents resulting from the movement of plates of the Earth’s crust and mantle. Plate movements continually rearrange geography. It is the major geographical factor correlated with the worldwide spatial distribution of life. Example: 250 million years ago, plate movements brought all landmasses together, forming one supercontinent Pangaea. Subsequently, 180 million years ago, Pangaea began to break apart, isolating the geographical distribution of many species; an example would be the separation and isolation of Australia, about 65 million years ago, which led to the evolution of its unique marsupial mammal species. Marsupials are mammals that have partial development in the uterus and partial development in a pouch outside the mother’s womb, while placental mammals have complete development inside the uterus. These different types of mammals have mainly evolved separately from each other on different continents for about 65 million years – marsupials in Australia and placentals in other parts of the world. Mammals first evolved over 200 million years ago, and it is believed that marsupials and placentals split from a common ancestor about 100 million years ago. Marsupials probably evolved in what is now North America, and migrated south into South America when many of the continents were still one land mass. Marsupials then moved from what is now South America into what is now Australia before Australia split off into a separate continent. It would seem that there was a larger representation of marsupial mammals than placental mammals at that time in South America, so when Australia formed it was initially populated primarily by marsupials. The marsupials, in isolation in Australia, then radiated out into the various habitats and evolved into a diverse group of mammals.
Apparently, in other parts of the world where placentals and marsupials coexisted (such as N/S America), the placentals outcompeted the marsupials. On these continents placentals diverged into many different placental species. The largest adaptive radiation of mammals occurred after the extinction of the dinosaurs, because so many new habitats became available. It should be noted that there are still some marsupial mammals in the Americas. North America has only 1 species, the Virginia Opossum, while Central and South America have more diverse marsupial fauna.
Because many of the habitats on Australia were similar to those in the Americas and Europe, many marsupial mammals resemble their placental counterparts on other continents. Some examples of animals that have evolved convergently due to similar selective pressures include:
The Marsupial Mole (Australia) vs. Placental Mole (North America & Europe)
Sugar Glider (Australia) vs. Flying Squirrel (North America & Europe)
Tasmanian Devil (Tasmania/Australia) vs. Wolverine (North America & Europe)
Kangaroo (Australia) vs. Patagonian Cavy (South America)
Plantigale (Australia) vs. Deer Mouse (North America)
Modes of Natural Selection
As you know, certain individuals in a population will be selected for, and will contribute their genes to the next generation. There are different types of natural selection, some of which are described below.
Stabilizing Selection
Favors intermediate phenotypes, and selects against extreme phenotypes. This reduces phenotypic variation.
This is the most common type of selection when the environment is not undergoing drastic change, since most populations are well adapted to their environment.
Ex: Human birth weights are in the 3 – 4 kg range; babies with much smaller or higher birth weight have a higher infant mortality.
Directional Selection
Favors phenotypes of one extreme, and shifts the frequency curve away from the average; it can be in either direction.
Most common during times of drastic environmental change, or if a species migrates to a new habitat with different environmental conditions.
Ex: fossils show the average size of European black bears increased after periods of glaciation, only to decrease during warmer interglacial periods.
Disruptive Selection
When opposite phenotypic extremes are favored over intermediate phenotypes.
This occurs when environmental conditions are variable in such a way that extreme phenotypes are favored.
Sexual Selection
When an organism selects a mate because of some distinct characteristic, or when a certain characteristic enables an organism to more successfully mate.
For example, in some tropical beetles females prefer to mate with males that have an elongated snout. Therefore, males with longer snouts will be more likely to reproduce and pass on their genes for longer snouts to the next generation. Over time, males will tend to develop even longer snouts.
IV. Coevolution
Coevolution is an important concept to understand when discussing the development of adaptations and speciation. Coevolution is the evolutionary change that occurs in two or more different species as a result of their selective pressures on one another. Organisms do not evolve in isolation from other organisms; remember, the environment includes both abiotic and biotic factors. For example, parasites evolve adaptations that make them better at finding and colonizing their hosts. This, in turn, causes the hosts that are harder for parasites to find or to invade successfully to be selected for. Another widespread example is the coevolution that has occurred between flowering plants and the animals that eat them, pollinate them, and disperse their seeds. For example, flowers contain nectar, a food source for birds, and birds have evolved various beak shapes that allow them to obtain the nectar. Also, as the birds feed on the nectar, pollen brushes off onto their wings and the birds can then transfer the pollen to other flowers. The transfer of pollen from one flower to another of the same species is necessary for reproduction.
V. Heterozygote Advantage / Superiority
Heterozygote superiority is when being a carrier or heterozygote is an advantage over either other genotype. In the case of sickle-cell anemia, the mutant allele is recessive, and ss results in having the disease. The normal allele is dominant, and therefore SS and Ss do not have the disease, but individuals that are heterozygous have the additional advantage of being able to resist malaria. Malaria is a disease that is carried by mosquitoes that transmit a protist parasite that infects red blood cells. This causes destruction of red blood cells and can be fatal. The phenomenon of heterozygote superiority causes an increase in the frequency of the recessive allele, even though this allele is selected against in individuals that are homozygous recessive. If there is no heterozygote superiority, and a recessive disease reduces reproductive success, you would expect the recessive allele to decrease and remain at a low frequency in the population.
Unit 3D Population Genetics Notes
In order to understand evolution, the change in organisms over time, it is important to consider the changes that occur at the level of the gene. Ultimately, evolution is only possible if there are changes in genes or alleles due to mutations.
Important terminology for population genetics:
Population: a group of individuals of the same species living in a particular area. A population is the smallest group of individuals that can evolve. An individual cannot evolve! The evolution of Darwin’s finches on the Galapagos Islands is an example of how a single population of organisms of the same species can evolve into different species.
Gene pool: all the genes (alleles) in a population that can be passed on to the next generation.
Population genetics deals with allele frequencies and genotype frequencies in a population. Usually only one trait will be considered at a time, to see if the frequencies change or remain constant over the course of many generations.
Each of the two alleles for a trait will be present in the population at a certain frequency (percent). Together their frequencies must add up to 100%.
Example: if the frequency of the B allele in the population of Livingston is 70%, then the frequency of the b allele must be 30%.
The following example shows how you could calculate the genotype and allele frequencies in a population for a single trait. For example, the trait of tongue-rolling, where T=rollers, and is dominant, and t=nonrollers, and is recessive. Of course, it would seem that these frequencies can only be calculated if you knew how many homozygous dominant and heterozygous individuals there are in a population. Phenotypically these individuals are indistinguishable, and you would not be able to determine their individual frequencies. You would, however, be able to calculate the frequency of the homozygous recessives, because they appear phenotypically different from the other two genotypes.
The numbers chosen for the example on the previous page show a population in genetic equilibrium. This means that the genotype and allele frequencies have NOT changed and will NOT change from generation to generation. It is important to note that many populations ARE NOT in genetic equilibrium, and so their genotype and allele frequencies will change over time.
Sometimes a population in genetic equilibrium is also said to be in Hardy-Weinberg equilibrium. Hardy and Weinberg were two mathematicians that demonstrated that if certain conditions were met, genotype and allele frequencies would not change from one generation to the next even though genes were being shuffled during sexual reproduction (Meiosis/Fertilization). In other words, sexual reproduction alone will not cause genotype and allele frequencies to change.
There are 5 ideal conditions that must be met in order for genetic equilibrium to be attained:
1. No Natural Selection, where one phenotype is favored over another.
2. No Mating Preferences (random mating). There is an equal chance of any male or female mating with any other member of the opposite sex. Ex: AA female would be just as likely to mate with AA male as a Aa or aa male.
3. No mutations causing the appearance of new alleles.
4. No exchange of genes between one population and another. This is known as gene flow. There would be no immigration or emigration (into/out of) a population. In other words, the population would be isolated from other populations.
5. You must look at a large population size – small populations can show what is known as genetic drift, where by chance certain alleles can be over or under represented. The allele frequencies in a small population may not be representative of the allele frequencies in a larger population. Ex: AA, Aa, aa – 25 of each, 75 total – pick out 5 – may pick out all AA’s by chance, and eliminate the Aa and aa genotypes.
Of course, these conditions are not always met for most genes. In many cases, there is natural selection, there are mating preferences, there are mutations, and there is gene flow. If any of these occur to a significant degree, the population would not be in genetic equilibrium. So the question is, are populations ever in genetic equilibrium for a particular trait? For some traits, such as tongue rolling, most of these conditions are met, so in looking at tongue rolling in Livingston, we could probably say the population is in genetic equilibrium. Even if a trait does have natural selection acting against one of its alleles, and some of the other five conditions are not met, if you look at populations over short periods of time, they might be close to a genetic equilibrium. If looked at over many generations, you would most likely see changes in allele or genotype frequencies. For example: if you were to study the change in frequency of individuals in a population that have cystic fibrosis (recessive) versus how many individuals are homozygous dominant and heterozygous, you would probably see little if any change over a few generations. Over many generations however, especially if there is a strong selective pressure against the individuals that have the disease, you would expect the frequencies to change. If a population is under very strong selective pressures against a particular phenotype then even over short periods of time the allele and genotype frequencies will change. A good example of this is what happened to the peppered moths! Since one phenotype was strongly selected against at a particular time its frequency in the population rapidly declined. When the trees were blackened by soot the dark moths that had a dominant allele D (DD or Dd) increased in number while the white moths that were homozygous recessive (dd) decreased in number. It is easy to see how the allele and genotype frequencies changed rapidly and drastically in this instance. In most cases however, when the environment changes drastically in a very short period of time, the population would probably not harbor a variation that would allow it to survive at all. Because natural selection tends to make organisms of the same species more similar than different, the variations that are present in a population would usually not allow any of the organisms to adapt to a fast changing environment.
Hardy-Weinberg
Hardy and Weinberg came up with a simple algebraic equation to show that if the population is in genetic equilibrium, genotype and allele frequencies will not change.
The Hardy-Weinberg equation is: p2 + 2pq + q2 =1
The equation was derived from the fact that:
To show genotypes in the population, you take the square of (p + q), since both males and females will be contributing dominant and recessive alleles to each generation:
(p + q)2 = (p + q) (p + q) = 12 which is equal to: p2 + 2pq + q2 = 1
Reference the data on the bottom of page 1. Since the population shown was in Hardy-Weinberg equilibrium, you can use the Hardy-Weinberg equation to calculate all genotype and allele frequencies from the frequency of the homozygous recessive in the population. For example, if you examined 500 individuals in Livingston (assuming 500 was a big enough sample size), and you counted 20 individuals that could not roll their tongue, you could then calculate all of the other genotype frequencies from that information.
Use the following steps:
1. Determine the frequency of the homozygous recessives (tt) in the population
Frequency of tt = 20 = 0.04
500
2. Determine the frequency of the recessive allele (t) in the population.
Frequency of t = √0.04 = 0.2
3. Determine the frequency of the dominant allele (T) in the population.
Frequency of T = 1-0.2 = 0.8
4. Determine the frequency of the homozygous dominants (TT) in the population.
Frequency of TT = (0.8)(0.8) = 0.64
5. Determine the frequency of the heterozygotes (Tt) in the population.
Frequency of Tt = 2(0.8)(0.2) = 0.32
Many students often wonder about the value of the Hardy-Weinberg equation, if many traits looked at are not in genetic equilibrium because of the 5 conditions are not all met.
The Hardy-Weinberg equation is usually used for:
1. A baseline from which you can measure change.
After you establish the initial frequencies of alleles and genotypes in a population, if there is no change over time, you can conclude that no evolution has taken place. Over long periods of time, you would expect there to be a change, but over short periods of time, frequencies should remain relatively stable. As a scientist, if you detect a change in frequencies, you would then have to look into what caused these changes. For example: You could determine whether there is an increase/decrease in a trait that has been established in a population. If the frequency of sickle-cell anemia changes in a population, you could investigate the cause(s) of the change.
2. Medical importance
Can predict genotypic and allelic frequencies in a model population. Even though there is no such thing as a model population in nature, over short periods of time frequencies usually remain relatively stable. For example: You can predict how many people in a population are heterozygotes (carriers) for a rare recessive allele. PKU (phenylketonuria) and sickle-cell anemia are two examples. At a given time, you can predict with some accuracy the number of carriers in the population, because although changes occur, they occur at a relatively slow rate. This is why the formula can be applied. It can be used for short-term predictions.
Let’s continue with the example of PKU in the general population – estimated to be 1 in 15,000.
q2 = 115,000 or 0.000067
q = √0.000067 or about 0.0082
p = 1 – q or (1- 0.0082) = 0.9918
2pq = 2(0.9918)(0.0082) = about 0.016 (1.6%)
In other words, in the population under study, 16 in every 1,000 persons are heterozygous carriers for the PKU gene.
Helpful information: You will be given a separate problem packet with H-W Equation questions. Be sure that when you answer the questions, you are careful to identify what is being asked. If you are asked for allele frequency the answer will be the p or q value. If you are asked for genotype frequency, the answer will be the p2, 2pq, or q2 value. And if you are asked the number of individuals that have a particular genotype, be sure to multiply the p2, 2pq, or q2 values by the number of individuals in the population.
Additional important terms and concepts dealing with Population Genetics:
1. Founder Effect and Population Bottleneck
Both of these are examples of genetic drift. In the founder effect, a small population branches off from a larger population and establishes its own gene pool in some new location; as they found this new population, by chance rare alleles may be overrepresented or lost.
Example 1: Among the Amish people of Pennsylvania, the frequency of a recessive allele which when homozygous causes polydactylism (extra fingers) is very high. The entire population of Amish is descended from a very small number of founders; by chance, one of the founders must have been a carrier of this allele; extensive inbreeding (Amish normally do not marry non-Amish) has resulted in an increased frequency of the allele in subsequent generations. Sometimes it is said that a population such as this has gone through a population bottleneck. A bottleneck can also occur due to other things, such as disease or catastrophe that eliminates a large portion of the population, and the alleles that they possess.
Populations such as this which show a high amount of inbreeding tend to expose normally hidden recessive alleles, by producing homozygous recessive individuals. This often leads to a higher incidence of an inherited disease within this inbred population.
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Note: population bottlenecks will produce less variation in a population, because many alleles have been eliminated that would have contributed to variation.
2. Importance of Hidden Recessive Alleles
Since recessive alleles are not expressed in heterozygotes, mutations that produce neutral, less favorable, or harmful alleles are hidden. This allows for a large pool of alleles that may not be suitable for present conditions to remain in the population and be available, and possibly beneficial, should the environment change. All of us harbor many recessive alleles that at some point in time may allow us to survive better due to some environmental change. This, of course, is what occurred with heterozygote superiority with the sickle allele. Consider the following scenario: a new deadly virus emerges that kills a large portion of the infected population. Some individuals harbor a recessive allele that produces a variant of the cell receptor that the virus normally binds to in order to enter/infect cells. These individuals are therefore protected against this particular microbe.