Mechanisms That Produce Change in Populations PDF

# Mechanisms that Produce Change in Populations ## Objectives At the end of this module, the learners will be able to: * **K:** Explain the basic mechanisms of evolution: natural selection, mutation, genetic drift, and gene flow/migration. * **S:** Describe the effects of evolution on the diversi...

# Mechanisms that Produce Change in Populations ## Objectives At the end of this module, the learners will be able to: * **K:** Explain the basic mechanisms of evolution: natural selection, mutation, genetic drift, and gene flow/migration. * **S:** Describe the effects of evolution on the diversity of the population. * **A:** Appreciate the major evolutionary forces that have created the variations in the species. ## Important Terms ## Evolution A population is changing in its genetic makeup over generations. An image shows a prehistoric-looking aquatic creature next to a modern-looking orca whale. ## The Levels of Organization An image shows a diagram of the levels of organization from organisms to the biosphere. The diagram shows: * **Organism:** A single impala. * **Population:** A group of impala. * **Community:** A group of impala and zebras in the same environment. * **Ecosystem:** A savanna ecosystem with both impala and zebra. * **Biosphere:** Earth ## Mutation A change in the nucleotide sequence of an organism's DNA or in the DNA or RNA of a virus. An image shows a double helix DNA model. An image shows a baby with the caption, "Physical Features". The caption describes physical features that may occur in a baby due to a mutation. ## Natural Selection The preferential survival and reproduction, preferential elimination of individuals with certain genotypes (genetic compositions), by means of natural or artificial controlling factors. Images of Charles Darwin and Alfred Russel Wallace. ### Natural Selection * **Preferential survival:** There are genes that are maintained by the organism because these are beneficial or helpful that can help the organism survive and reproduce. * **Preferential elimination:** Loss of traits or characteristics that are useless for the organism. ## Gene Pool All copies of all the genes in a population. The combination of all the genes (including alleles) present in a reproducing population or species. ## Allele Frequency Refers to how common an allele is in a population or how frequent a certain trait or characteristic occurs in population. ## Genetic Drift Change in allele frequency because of chance. Animated image shows a series of bottles containing a population of red and blue marbles. The arrangement of the red and blue marbles changes in each bottle, demonstrating that the composition of the population changes from generation to generation. ## Microevolution A change in the frequency of gene variants, alleles, in a population, typically occurring over a relatively short period of time. ## Gene Flow (Migration) The transfer of alleles from one population to another, resulting from the movement of fertile individuals or their gamete. ## Gene Flow or Migration Animated diagram shows two populations of birds. Population A is all blue birds and Population B is all red birds. Arrows show a blue bird moving into population B and a red bird moving into population A. ### Reminder: * **Allele:** One of two or more alternative forms of a gene. * **Genotype:** An organism's complete set of genetic material. * **Phenotype:** Observable characteristics ## Population Genetics * The field of Biology that studies allele frequencies in populations and how they change over time. * It is the branch of biology which focuses on inherited variations in populations of organisms. * The changes that occur in the genetics of a population or between several populations of organisms are studied here. Population genetics focuses on the reasons why there is microevolution which is also the reason why there is large scale evolution. ## Five Causes of Microevolution * Genetic drift * Assortative/Random mating * Mutation * Natural selection * Migration (gene flow) ## Mechanisms: The Processes of Evolution Biological evolution is descent with modification. This definition encompasses small-scale evolution and large-scale evolution. Evolution helps us to understand the history of life. Biological evolution is not simply a matter of change over time. Lots of things change over time: trees lose their leaves, mountain ranges rise and erode, but they aren't examples of biological evolution because they don't evolve descent through genetic inheritance. * **Small-scale evolution:** Changes in gene/allele frequency in a population from one generation to the next. * **Large-scale evolution:** The descent of different species from a common ancestor over many generations. Evolution is the process by which modern organisms have descended from ancient ancestors. Evolution is responsible for both the remarkable similarities we see across all life and the amazing diversity of that life. ## How Evolution Works? Genetic variation is fundamental to the process in which upon selective forces can act in order for evolution to occur. Evolution is possible when there is variation in the genetic makeup among organisms that affects their traits. Then, if certain individuals in a population reproduce more successfully than others so that their traits become more common within the population, the result is evolutionary change. ## Mechanisms of Change ### 1. Mutation Mutation is a change in DNA, the hereditary material of life. An organism's DNA affects how it looks, how it behaves, and its physiology - all aspects of its life. So a change in an organism's DNA can cause changes in all aspects of its life. A mutation could cause parents with genes for bright green coloration to have offspring with a gene for brown coloration. That would make genes for brown coloration more frequent in the population than they were before the mutation. Mutation is a change in a DNA sequence, usually occurring because of errors in replication or repair. Mutation is the ultimate source of genetic variation. Changes in the composition of a genome due to recombination alone are not considered mutations since recombination alone just changes which genes are united in the same genome but does not alter the sequence of those genes. An image shows a group of green beetles and a group of brown beetles illustrating that a mutation could cause parents with genes for bright green coloration to have offspring with a gene for brown coloration. ### Sources of Genetic Variation Without genetic variation, some of the basic mechanisms of evolutionary change cannot operate. There are three primary sources of genetic variation, which we will learn more about: 1. **Mutations** are changes in the DNA. A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations. 2. **Gene flow** is any movement of genes from one population to another and is an important source of genetic variation. 3. **Sex** can introduce new gene combinations into a population. This genetic shuffling is another important source of genetic variation. An image shows three stages of genetic shuffling with chromosomes being moved in an egg cell. The caption reads, "Genetic shuffling is a source of variation." ### Mutations are random Mutations can be beneficial, neutral, or harmful for the organism, but mutations do not "try" to supply what the organism "needs." In this respect, mutations are random - whether a particular mutation happens or not is unrelated to how useful that mutation would be. ### Not all mutations matter to evolution Since all cells in our body contain DNA, there are lots of places for mutations to occur; however, not all mutations matter for evolution. Somatic mutations occur in non-reproductive cells and won't be passed onto offspring. An image shows a red delicious apple with a golden section; it's caption reads, "For example, the golden color on half of this Red Delicious apple was caused by a somatic mutation. The seeds of this apple do not carry the mutation." The only mutations that matter to large-scale evolution are those that can be passed on to offspring. These occur in reproductive cells like eggs and sperm and are called germ line mutations. ### A single germ line mutation can have a range of effects: 1. **No change occurs in phenotype** Some mutations don't have any noticeable effect on the phenotype of an organism. This can happen in many situations: perhaps the mutation occurs in a stretch of DNA with no function, or perhaps the mutation occurs in a protein-coding region, but ends up not affecting the amino acid sequence of the protein. 2. **Small change occurs in phenotype** 3. **A single mutation caused this cat's ears to curl backwards slightly.** 4. **Big change occurs in phenotype** Some really important phenotypic changes, like DDT resistance in insects are sometimes caused by single mutations. A single mutation can also have strong negative effects for the organism. Mutations that cause the death of an organism are called lethals and it doesn't get more negative than that. An image shows a black cat with curled ears. There are some sorts of changes that a single mutation, or even a lot of mutations, could not cause. Neither mutations nor wishful thinking will make pigs have wings; only pop culture could have created Teenage Mutant Ninja Turtles - mutations could not have done it. ### Mutations happen for several reasons. 1. **DNA fails to copy accurately** Most of the mutations that we think matter to evolution are "naturally-occurring." For example, when a cell divides, it makes a copy of its DNA and sometimes the copy is not quite perfect. That small difference from the original DNA sequence is a mutation. An image shows a DNA strand. The caption reads, "Original". An arrow points down to another DNA stand. This stand is the same as the original DNA stand except for a slight change in the last few nucleotides, where the caption reads, "Mutant copy". 2. **External influences can create mutations** Mutations can also be caused by exposure to specific chemicals or radiation. These agents cause the DNA to break down. This is not necessarily unnatural - even in the most isolated and pristine environments, DNA breaks down. Nevertheless, when the cell repairs the DNA, it might not do a perfect job of the repair. So the cell would end up with DNA slightly different than the original DNA and hence, a mutation. An image shows a warning sign for radioactive materials. ### Sex and genetic shuffling Sex can introduce new gene combinations into a population and is an important source of genetic variation. You probably know from experience that siblings are not genetically identical to their parents or to each other (except, of course, for identical twins). That's because when organisms reproduce sexually, some genetic "shuffling" occurs, bringing together new combinations of genes. For example, you might have bushy eyebrows and a big nose since your mom had genes associated with bushy eyebrows and your dad had genes associated with a big nose. These combinations can be good, bad, or neutral. If your spouse is wild about the bushy eyebrows/big nose combination, you were lucky and hit on a winning combination! An image shows a side-by-side comparison of a father with a big nose, a mother with bushy eyebrows, and a son with both a big nose and bushy eyebrows. This shuffling is important for evolution because it can introduce new combinations of genes every generation. However, it can also break up "good" combinations of genes. ### 2. Genetic Drift This can occur when a small group of individuals leaves a population and establishes a new one in a geographically isolated region. Fitness of a population is not considered in genetic drift, nor does genetic drift occur in a very large population. Imagine that in one generation, two brown beetles happened to have four offspring survive to reproduce. Several green beetles were killed when someone stepped on them and had no offspring. The next generation would have a few more brown beetles than the previous generation - but just by chance. These chance changes from generation to generation are known as genetic drift. In each generation, some individuals may, just by chance, leave behind a few more descendent (and genes, of course!) than other individuals. The genes of the next generation will be the genes of the "lucky" individuals, not necessarily the healthier or "better" individuals. That, in a nutshell, is genetic drift. It happens to ALL populations - there's no avoiding the vagaries of chance. ## **Random Drift** consists of random fluctuations in the frequency of appearance of a gene, usually, in a small population. The process may cause gene variants to disappear completely, thereby reducing genetic variability. In contrast to natural selection, environmental or adaptive pressures do not drive changes due to genetic drift. The effect of genetic drift is larger in small populations and smaller in large populations. ### **2 EXAMPLES OF RANDOM DRIFT** 1. **Bottleneck effect** occurs when there is a sudden sharp decline in a population's size typically due to environmental factors (natural disasters). It is a random event, in which some genes are extinguished from the population. This results in a drastic reduction of the total genetic diversity of the original gene pool. The small surviving population is considerably be farther from the original one in its genetic makeup. 2. **Founder effect** is the loss of genetic variation that occurs when a new population is established by a small number of individuals that are cleaved from a larger population. This new population does not have the genetic diversity of the previous one. Because the community is very small and also geographical or socially isolated, some genetic traits are becoming more prevalent in the population. This leads to the presence of certain genetic diseases in the next generations. In some cases, founder effect plays a fundamental role in the emergence of new species. ### 1. Bottleneck Effect An image shows a diagram of the Bottleneck Effect. The caption reads, "Large genetic diversity, generation 1" as a circle with brown and orange dots are in a larger group. The caption below the circle reads, "Original population." An arrow points to the right of the image. This arrow is labeled, "Bottleneck event", and shows a circle with a smaller number of brown and orange dots. An arrow points to the right. This arrow is labeled, "surviving population", and shows the same circle with the small number of brown and orange dots. An arrow points to the right. This arrow is labeled, "Final population (recovery)", and shows a larger circle with all orange dots. **• Generation 1: The frequency of alleles in the population is the same.** **• Generation 2: Randomly and due to a catastrophic natural or man-made event, most of individuals of the population died (there is no influence of adaptive pressures).** **• Generation 3: As a result, the original large population is reduced to a small population composed by few individuals. This new surviving population subset contains much less genetic variability than the previous population.** **• Generation 4: Later, the drastic reduction in the population size is followed by an expansion (population is recovered). The final population is no longer genetically representative of the original one. In this particular case, an allele is completely removed from the gene pool.** ### 2. Founder Effect An image shows a diagram of the Founder Effect. The caption reads, "Mother population" as a large circle with both red and orange dots. An arrow points to the right of the image. This arrow is labeled "Founder Effect", and shows a smaller circle with mostly orange dots and one red dot. The caption below the smaller circle reads, "New population". ### 3. Migration/Gene flow Is any movement of individuals, and/or the genetic material they carry, from one population to another. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. If gene versions are carried to a population where those gene versions previously did not exist, gene flow can be a very important source of genetic variation. In the graphic below, the gene version for brown coloration moves from one population to another. Gene flow is the movement of genes between populations. This may happen through the migration of organisms or the movement of gametes (such as pollen blown to a new location). ### 4. Natural Selection Another mechanism for evolution is natural selection, which occurs when populations of organisms are subjected to the environment. The fittest creatures are more likely to survive and pass their genes to their offspring, producing a population that is better adapted to the environment. The genes of less-fit individuals are less likely to be passed on to the next generation. The important selective force in natural selection is the environment. Imagine that green beetles are easier for birds to spot (and hence, eat). Brown beetles are a little more likely to survive to produce offspring. They pass their genes for brown coloration onto their offspring. So, in the next generation, brown beetles are more common than in the previous generation. All of these mechanisms can cause changes in the frequencies of genes in populations, and so all of them are mechanisms of evolutionary change. However, natural selection and genetic drift cannot operate unless there is genetic variation - that is, unless some individuals are genetically different from others. If the population of beetles were 100% green, selection and drift would not have any effect because their genetic mke-up could not change. Natural Selection leads to an evolutionary change when some individuals with certain traits in a population have a higher survival and reproductive rate than others and pass on these inheritable genetic features to their offspring. Evolution acts through natural selection whereby reproductive and genetic qualities that prove advantageous to survival prevail into future generations. The cumulative effects of natural selection process have giving rise to populations that have evolved to succeed in specific environments. Natural selection operates by differential reproductive success (fitness) of individuals. ## Charles Darwin and His Contribution to Evolution * Charles Darwin, in full Charles Robert Darwin, (born February 12, 1809, Shrewsbury, Shropshire, England - Died April 19, 1882, Downe, Kent). * Charles Darwin is an English naturalist best known for his theory of evolution by natural selection. Charles Darwin contributed a lot to pave the way for the study of modern evolution. An image of Charles Darwin According to him, humans and animals came from common ancestry which became controversial especially because in their time, when people's knowledge was still conservative and in a religious community that believed that humans were created by God in this form, it did not become acceptable. An image of a drawing of a pre-humanoid creature. His observations and theories began when he was able to sail aboard the HMS Beagle in 1835, when he was only 22 years old. During his exploration, he collected various specimens of animals which he used as the basis for his notes and his published books such as 'On the origin of species' which he published 2 decades after his exploration on the Galapagos Islands. An image shows a map of the world with a red, winding line showing the path of the voyage of the HMS Beagle. The map's key identifies the various locations visited by the HMS Beagle. His observations and theories began when he was able to sail aboard the HMS Beagle in 1835, when he was only 22 years old. During his exploration, he collected various specimens of animals which he used as the basis for his notes and his published books such as 'On the origin of species' which he published 2 decades after his exploration on the Galapagos Islands. An image shows a diagram of the Galapagos finches and their beaks. The diagram explains how the Galapagos finches adapted to their environment. **Adaptive radiation in Galapagos finches** * **Evolution acts through change in allele frequency at each generation** * **Darwin did not understand how genetic variation was passed on from generation to generation.** An image of Charles Darwin. * **Based on Darwin's observation, there is genetic variation or evolution in organisms, and it is passed from generation to generation. Evolution changes the allele frequency at each generation.** * **Allele frequency refers to how common an allele is in a population. It is expressed as a percentage or fraction that changes because of the microevolution that occurs in the population of organisms. What Darwin did not explain is how genetic variation occurs and how it is passed from generation to generation.** ## Gregor Mendel and his Contribution to Genetics * **"Father of Genetics** * **Mendel presented a mechanism for how traits got passed on "Individuals pass alleles on to their offspring intact".** * **The idea of particulate (genes) inheritance** An image shows a diagram of the alleles and phenotypes of a pink flamingo. ## Gregor Mendel Gregor Mendel was an Austrian monk born in 1822 in Austria. He is known as the father of genetics and he was the one who began the idea of particulate inheritance which he used to explain the inheritance of traits. According to him, each parent is able to transfer one allele to offspring. (Alleles are the alternative forms or versions of genes and the phenotype or actual appearance of the offspring depends on the dominance of the genes that have been passed on). ## Hardy-Weinberg Principle Testing for Hardy-Weinberg equilibrium can be used to assess whether a population is evolving. ## Hardy-Weinberg Equilibrium * A population's allele and genotype frequencies are constant, unless there is some type of evolutionary force acting upon them. An image shows a diagram of Godfrey Hardy and Wilhelm Weinberg. ## Hardy-Weinberg Principle The Hardy-Weinberg principle is used to determine if evolution is occurring in a population. This name comes from the names of a physician and mathematician, Godfrey Hardy and Wilhelm Weinberg. The Hardy-Weinberg principle shows the equilibrium or balance of allele frequency in a population, meaning that no evolution occurs even in several consecutive generations of a population. It will only change if there is a so-called evolutionary force that will cause genetic variation, and this is what is called the mechanism of evolution. ## The Hardy-Weinberg principle states that allele and genotype frequencies remain stable in a population over generations if certain conditions are met: ## ASSUMPTIONS OF HARDY-WEINBERG EQUILIBRIUM 1. **No Selection:** All alleles confer equal fitness (make organisms equally likely to survive and reproduce). 2. **No Mutation:** No new alleles are generated by mutation, nor are genes duplicated or deleted. 3. **No Migration (gene flow):** Neither individuals nor their gametes (e.g., windborne pollen) enter or exit the population. 4. **Very large population size:** The population should be effectively infinite in size. 5. **Random mating:** Organisms mate randomly with each other, with no preference for particular genotypes. An image shows a series of diagrams illustrating each of the assumptions. * **If a population is not in Hardy-Weinberg equilibrium, it can be concluded that the population is evolving.** ## Thank you for listening! An image shows various abstract shapes and designs.