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Biology Chapter 24.docx

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McMaster University
Ben Evans

Chapter 24: Evolution By Natural Selection The Evolution of Evolutionary Thought  Plato claimed that every organism was an example of a perfect essence created by God and that these types were unchanging  Aristotle ordered the types of organisms into a linear scheme called the great chain of being  Aristotle proposed that species were organized into a sequence based on increased size and complexity, with humans on top  Jean-Baptiste de Lamarck proposed that species are not static, but have changed throughout time  Lamarck was the first to propose a formal theory of evolution, the idea that species change through time  He claimed that simple organisms originate at the base of the chain by spontaneous generation and then evolve by moving up the chain over time  He also contended that species change through time via the inheritance of acquired characteristics  The idea here is that individuals change as they develop in response to challenges posed by the environment and they pass on these phenotypic changes to offspring  In contrast, Darwin and Wallace proposed that evolution does not follow this linear, progressive pattern  They emphasized that the process responsible for change through time is based on variation among individuals in populations  Darwin claimed that instead of being unimportant or an illusion, variation among individuals in a population was the key to understanding the nature of species  A population consists of individuals of the same species that are living in the same area at the same time  They proposed that evolution occurs because traits vary among the individuals in a population and because individuals with certain traits leave more offspring than others do The Pattern of Evolution: Have Species Changed through Time  Darwin described evolution as descent with modification, meaning that change over time produced modern species from ancestral species  Darwin emphasized that species existing today have descended from other, pre-existing species and that species are modified or change through time  Evidence for Change through Time o A fossil is any trance of an organism that lived in the past o The fossil record consists of all the fossils that have been found and described in the scientific literature o Most fossils are found in sedimentary rocks, which from sand or mud or other materials deposited at locations such as beaches or river mouths o Layers of sedimentary rock are associated with different intervals in the geological time scale, a relative time scale based upon fossil content o Extinction  Researchers began discovering fossil bones, leaves and shells that were unlike structures from any known animal or plant  At first, many scientists insisted that living examples of these species would be found in unexplored regions of the globe  As researchers continued and the number of diversity of fossil collections grew, the argument became much less plausible  After one scientists published an analysis of an extinct species, most scientists accepted extinction as a reality  Darwin interpreted the fossil species as evidence that species are not static, immutable entities, unchanged since the moment of special creation  He reasoned that if a species has gone extinct, then the array of species living on Earth has changed through time o Transitional Form  Researchers reported similarities between the fossils found in the rocks underlying certain regions and the living species found in the same geographic areas  Became known as the “law of succession”  General observation was that extinct species in the fossil record were succeeded, in the same region, by similar species  Darwin pointed out that it provided strong evidence in favour of the hypothesis that species had changed through time  His idea was that extinct forms and living forms were related, that they represented ancestors and descendants  A transitional form is a fossil species with traits that are intermediate between those of older and younger species  Ex. Intensive work over the past few decades has yielded fossils that document a gradual change over time from land-dwelling mammals that had limbs to ocean-dwelling mammals that had reduced limbs or no limbs  All of the species in this sequence have distinctive types of ear bones that identifies them as whales  Over many years, the fossil record shows that the limbs of whale species became more reduced  These observations support the hypothesis that whales gradually became more strictly aquatic and more like today’s whales in appearance and lifestyle o Vestigial Traits  A vestigial trait is a reduced or incompletely developed structure that has no function or reduced function, but is clearly similar to functioning organs or structures in closely related species  Genomes of humans and other organisms contain hundreds of pseudogenes, functionless DNA sequences  Monkeys and many other primates have long tails, but our coccyx is too tiny to help us maintain balance  Many mammals, including primates, are able to erect their hair when they are cold or excited  Our sparse fur does little to keep us warm and goose bumps are largely ineffective in signalling our emotional state  Existence of vestigial traits is inconsistent with theory of special creation, which maintains that species were perfectly designed by a supernatural being and that characteristics of species are static  Instead, vestigial traits are evidence that the characteristics of species have changed over time  Evidence That Species are Related o Geographic Relationships  Darwin began to realize that species are related by common ancestry while on the Galapagos Islands  He gathered extensive collections of plants and animals and among the birds he collected where Galapagos mockingbirds  One of his friends pointed out that the mockingbirds were distinct species based on colouration and beak size and shape  Darwin asked, why would species that inhabit neighbouring islands be so similar, yet clearly distinct  He proposed that mockingbirds were similar because they had descended from the same common ancestor  If so, then the mockingbird species were a part of a phylogeny, a family tree of populations or species  Further, the mockingbirds can be placed on a phylogenetic tree, a branching diagram that describes the ancestor-descendant relationships among species  Darwin’s hypothesis was that instead of being created independently, mockingbird populations that colonized different islands had changed through time and formed new species o Homology  Means “the study of likeness”  Today, biologists recognize that homology is a similarity that exists in species descended from a common ancestor  Human hair and dog fur are homologous  Homology can be recognized and studied at three interacting levels  Genetic homology o A similarity in DNA sequences of different species o Ex. The genetic code o Except for one or two codons, the same 64 mRNA codons specify the same amino acids in all organisms o To explain the existence of the universal genetic code, biologists hypothesize that today’s code also existed in the common ancestor of all organisms alive today o Similarly, all organisms living today have a plasma membrane consisting of a phospholipid bilayer with interspersed proteins, transcribe information coded in DNA to RNA via RNA polymerase, use ribosomes to synthesize proteins, employ ATP as an energy currency and make copies of their genome via DNA polymerase  Developmental homology o A similarity in embryonic traits o Are routinely observed in the overall morphology or form of embryos o Ex. Early in development, structures called gill pouches and tails that extend past the anus form in chicks, humans and cats o Later in development, gill pouches are lost in all three species and tails are lost in humans o In fish however, gill pouches stay intact and give rise to functioning gills in adults o To explain this observation, biologists hypothesized that gill pouches and tails exist in all three because they existed in the fishlike species that was the common ancestor of today’s fish, birds and mammals o Embryonic gill pouches are a vestigial trait in chicks, humans and cats, embryonic tails are a vestigial trait in humans  Structural homology o Similarities in adult morphology o Ex. The common structural plan observed in the limbs of vertebrates o Turtles, humans, horses, birds, bats and seals all have the same number of same arrangement of bones o General point is that in many cases, traits are similar in different species because the species in question are related to each other by common descent o If species were created independently of one another, these types of similarities would not occur The Process of Evolution: How Natural Selection Works  Darwin’s crucial insight lay in recognizing a process, called natural selection, which could explain the pattern of descent with modification  In his original formulation, Darwin broke the process of evolution by natural selection into four simple postulates, or steps in a logical sequence o The individual organisms that make up a population vary in the traits they possess, such as their size and shape o Some of the trait differences are heritable, meaning that they are passed on to offspring genetically  Ex. Tall parents may tend to have tall offspring o In each generation, many more offspring are produced than can possibly survive  Thus, only some individuals in the population survive long enough to produce offspring and among the individuals that produce offspring, some will produce more than others o The subset of individuals that survive best and produce the most offspring is not a random sample of the population  Instead, individuals with certain heritable traits are more likely to survive and reproduce  Natural selection occurs when individuals with certain characteristics produce more offspring than do individuals without those characteristics  Because the selected traits are passed on to offspring, the frequency of the selected traits increases from one generation to the next  Evolution is simply an outcome of these four steps  Variation among individuals is essential if evolution is to occur  Biologists today condense Darwin’s four postulates into two statements o Evolution by natural selection occurs when (1) heritable variation leads to (2) differential success in survival and reproduction  Ex. Peppered moths o Differences in wing colouration are due to two alleles of a single gene, A 1nd A 2 o Individuals with A A have a light grey colouration, while individuals with A A or 1 1 1 2 A 2 2enotype are black o Because the A an1 A all2les have such different effects on wing colour, o These moths are active at night and spend the day resting on tree trunks and branches, where they are hunted by birds o In an environment where trees with light-coloured barks are common, birds can find and eat dark-winged individuals much more readily than they can find and eat light-winged individuals o Because predation by birds causes natural selection on wing colour, there is differential success, in this environment, light-winged individuals survive better than dark-winged individuals o Evolution is defined as a change in allele frequencies in a population over time o In this case, the frequency of the A 1llele increases in population over time  To explain the process of natural selection, Darwin referred to successful individuals as “more fit” than other individuals  Darwinian fitness is the ability of an individual to produce offspring, relative to that ability in other individuals in the population  In biology, an adaptation is a heritable trait that increases the fitness of an individual in a particular environment relative to individuals lacking the trait  Adaptations increase fitness, the ability to produce offspring  Light-coloured wings are an adaptation in environments where most trees have grey bark Evolution in Action: Recent Research on Natural Selection  How Mycobacterium tuberculosis became resistant to antibiotics o The bacterium, Mycobacterium tuberculosis causes TB, a disease that killed up to a third of adults in big cities and still kills more adults than any other viral or bacterial disease o Sanitation, nutrition and general living conditions began to improve dramatically in the early twentieth century o When people are healthy and well nourished, their immune system works well enough to stop most M. tuberculosis infections quickly, before the infection can harm the individual and before the bacteria can be transmitted to a new host o In addition, antibiotics such as rifampin started to become available o In the late 1980s, rates of M. tuberculosis infections surged in many countries o Physicians noted that the strains of M. tuberculosis responsible for the increase were largely or completely resistant to other antibiotics that were once effective o DNA from rifampin-resistant bacteria was found to have a single point mutation in a gene called rpoB o This gene codes for a component of the enzyme RNA polymerase o Because RNA p
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