Evolution B-LS-4-2 PDF

Evolution B-LS-4-2 What is Evolution? Biological evolution is a scientific framework that analyzes how heritable traits change in frequency within a population over time. These traits include physical characteristics (morphology), molecular sequences (genetic and proteomic), and behavioral tr...

Evolution B-LS-4-2 What is Evolution? Biological evolution is a scientific framework that analyzes how heritable traits change in frequency within a population over time. These traits include physical characteristics (morphology), molecular sequences (genetic and proteomic), and behavioral traits to describe changes that have transformed life on Earth from the earliest beginnings to the diversity of organisms in the world today. Biological evolution is a unifying theme of biology and may occur on a small time and spatial scale affecting the gene pool of a single population (microevolution) or when those small changes accumulate over vast lengths of time producing noticeable changes in species (macroevolution). Natural Selection One mechanism that produces biological evolution is natural selection. Other mechanisms include nonrandom mating, genetic drift, mutation and gene flow. Natural selection results in changes in frequency in the inherited traits of a population over time and occurs when different traits of the individual members of a population result in those organisms dealing either more or less effectively with the current environment. ○ In other words…the organism is better adapted to live in the environment it is in. In comparison, artificial selection is when humans select which traits are preferred and intentionally breed organisms for a particular set of characteristics (e.g. how modern dog breeds such as the Great Dane or the Chihuahua were developed from their wolf ancestors). Natural Selection In the case of natural selection, if the environment remains stable for multiple generations, a population’s fitness (the ability of organisms to survive and reproduce) will increase over time as those advantageous traits become more and more common and honed. If the environment changes however, then different traits are likely to be advantageous. There are four prerequisites that must be in place in order for natural selection to occur… Necessary for Natural Selection to Occur 1. Overproduction of Offspring ○ Most species produce more offspring than the environment can support, so some individuals will not be able to reach their full potential for reproduction. ○ The ability of a population to produce many offspring raises the chance that some will survive but also increases the competition for resources. 2. Variation ○ Fundamental to the process of natural selection is genetic variation upon which selective forces can act in order for evolution to occur. ○ Within every population, there are inherited traits that show variability among individuals. Ex: Bacteria who are resistance to some antibiotics and not others ○ This variation is seen in the different phenotypes (body structures and characteristics) of the individuals within a population. Necessary for Natural Selection to Occur 2. Variation ○ An organism’s phenotype may influence its ability to find, obtain, or utilize its resources (food, water, shelter, and oxygen) and also might affect the organism’s ability to reproduce. ○ Phenotypic variation is determined by the organism’s genotype and by the environment. Those individuals with phenotypes that do not interact well with the environment are more likely to either die or produce fewer offspring than those that can interact well with the Environment. Necessary for Natural Selection to Occur 3. Adaptation The process of adaptation leads to the increase in frequency of a particular structure, physiological process, or behavior in a population of organisms that makes the organisms better able to survive and reproduce. ○ Individuals with inherited traits that are beneficial in that environment become more common. ○ As each generation progresses, those organisms that carry genes that hinder their ability to meet day to day needs become less and less common in the population. Organisms that have a harder time finding, obtaining, or utilizing, food, water, shelter, or oxygen will be less healthy and more likely to die before they reproduce or produce less viable or fewer offspring. In this manner, the gene pool of a population can change over time. ○ The concept of fitness is used to measure how a particular trait contributes to reproductive success in a given environment and results from adaptations. Natural selection has sometimes been popularized under the term survival of the fittest. Necessary for Natural Selection to Occur 4. Descent with modification ○ As the environment of a population changes, the entire process of natural selection can yield populations with new phenotypes adapted to new conditions. Natural selection can produce populations that have different structures and therefore, live in different niches or habitats from their ancestors. Each successive living species will have descended, with adaptations or other modifications, from previous generations. More individuals will have the successful traits in successive generations, as long as those traits are beneficial to the environmental conditions of the organism. Genetic Sharing The continuity of lifeforms on Earth is based on an organism’s success in passing genes to the next generation. Many organisms that lived long ago resemble those still alive today because the same genetic processes have passed along the genetic material of life. The continuity of life forms over time is due to the genetic processes that all organisms share. ○ Changes in DNA sequence or mutation help to drive evolutionary changes by creating new genes that can have advantage over old versions. This allows the new species to evolve. Genetic Sharing All living things that have ever existed on Earth, share at least two structures: ○ (1) Nucleic acids (RNA or DNA) that carry the genetic code for the synthesis of the organism’s proteins ○ (2) Proteins (composed of the same twenty amino acids in all life forms on Earth) The process by which nucleic acids code for proteins (transcription and translation) is the same in all life forms on Earth. In general, the same sequences of nucleotides code for the same specific amino acids. All organisms have a reliable means of passing genetic information to offspring through reproduction. The reproductive processes of organisms, whether sexual or asexual, result in offspring receiving genetic information from the parent or parents, though there may be some genetic variability. Sexual Reproduction In sexual reproduction, two parents contribute genetic information to produce unique offspring. Sexual reproduction uses the processes of meiosis (to create gametes) and fertilization to produce offspring that have new combinations of alleles that are different from those of the parents. Sexual reproduction is an important source of genetic variation among individuals within a population. The inheritance of allele combinations that result in traits that improve an individual’s chance of survival or reproduction ensures the continuity of that life form over time. Asexual Reproduction Asexual reproduction generates offspring that are genetically identical to a single parent (like bacteria). Examples of asexual reproduction are budding, fragmentation, binary fission, and vegetative propagation. The asexual reproduction rate is much higher than sexual reproduction and produces many individual offspring that are suited to continuing life in the present environment. Asexual reproduction may have a disadvantage in changing conditions because genetically identical offspring respond to the environment in the same way. If a population lacks traits that enable them to survive and reproduce, the entire population could become extinct. The genetic view of evolution includes the transfer of the genetic material through these processes of reproduction. The continuity of a species is contingent upon these genetic processes. Diversity A species is a population or group of populations whose members have the potential to interbreed and produce fertile offspring in nature. Because of interbreeding among individuals, species share a common gene pool (all genes, including all the different alleles, possessed by all of the individuals in a population). Because of the shared gene pool, a genetic change that occurs in one individual can spread through the population as that individual and its offspring mate with other individuals. If the genetic change increases fitness, it will eventually be found in many individuals in the population. Diversity Within a species, variability of phenotypic traits leads to diversity among individuals of the species. The greater the diversity within a population or species, the greater the chances are for that population or species to survive environmental changes. If an environment changes, organisms that have phenotypes which are well-suited to the new environment will be able to survive and reproduce at higher rates than those with less favorable phenotypes. Therefore, the alleles associated with favorable phenotypes increase in frequency and become more common and increase the chances of survival of the species. Diversity Favorable traits (such as coloration or odors in plants and animals, competitive strength, courting behaviors) in male and female organisms will enhance their reproductive success. Organisms with inherited traits that are beneficial to survival in its environment become more prevalent. For example, resistance of the organism to diseases or ability of the organism to obtain nutrients from a wide variety of foods or from new foods. Organisms with inherited traits that are detrimental to survival in its environment become less prevalent. Genetic Variation Genetic variation is random and ensures that each new generation results in individuals with unique genotypes and phenotypes. This genetic variability is a prerequisite to biological evolution. Factors that influence genetic variability within a population may be: Genetic drift is the random change in the frequency of alleles of a population over time. Due to chance, rare alleles in a population will decrease in frequency and become eliminated; other alleles will increase in frequency and become fixed. The phenotypic changes may be more apparent in smaller populations than in larger ones. Gene flow is the movement of genes into or out of a population. This occurs during the movement of individuals between populations (such as migration) thus increasing the genetic variability of the receiving population. Genetic Variation Non-random mating limits the frequency of the expression of certain alleles. Mutations increase the frequencies and types of allele changes within the population. Natural selection allows for the most favorable phenotypes to survive and thus be passed onto future generations. Genetic isolation is when organisms are limited to a variety of genetics usually because of location or resources. Genetic Variation When there is no change in the allele frequencies within a species, the population is said to be in genetic equilibrium. This concept is known as the Hardy-Weinberg principle. Five conditions that are required to maintain genetic equilibrium are: The population must be very large, no genetic drift occurs. There must be no movement into or out of a population. There must be random mating. There must be no mutations within the gene pool. There must be no natural selection Speciation Speciation is the process of forming of a new species by biological evolution from a preexisting species. New species may form when organisms in the population are isolated or separated so that the new population is prevented from reproducing with the original population, and its gene pools cease to blend. Once isolation (reproductive or temporal, behavioral, geographic) occurs, genetic variation and natural selection increase the differences between the separated populations. As different traits are favored in the two populations (original and new) because of isolation, the gene pools gradually become so different that they are no longer able to reproduce fertile offspring. At this point the two groups are by definition different species. Macroevolution Adaptive radiation/Divergent evolution. ○ In adaptive radiation (divergent evolution), a number of different species diverge (split off) from a common ancestor. This occurs when, over many generations, organisms (whose ancestors were all of the same species) evolve a variety of characteristics which allow them to survive in different niches. Coevolution ○ With coevolution, when two or more species living in close proximity change in response to each other. The evolution of one species may affect the evolution of the other. Macroevolution Extinction is the elimination of a species often occurring when a species as a whole cannot adapt to a change in its environment. This elimination can be gradual or rapid. Gradual extinction usually occurs at a slow rate and may be due to other organisms, changes in climate, or natural disasters. Speciation and gradual extinction occur at approximately the same rate. Mass extinction usually occurs when a catastrophic event changes the environment suddenly (such as a massive volcanic eruption, or a meteor hitting the earth causing climatic changes). It is often impossible for a species to adapt to rapid and extreme environmental changes.