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Chapter 24

Unit 5 - Chapter 24 Bio 1M03 .docx
Unit 5 - Chapter 24 Bio 1M03 .docx

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McMaster University
James S Quinn

Bio 1AA3 1 Unit Five: Evolutionary Processes and Patterns Chapter 24: Evolution by Natural Selection Key Concepts  Evidence of evolution: o Change through time o Extinction o Vestigial traits o Inter-relatedness  Evidence of Natural Selection: o Industrial melanism o Antibiotic resistance o Beak size in Galapagos finches  Selection on individuals, but evolution of populations over time  Evolution by natural selection is not progressive, and it does not change the characteristics of the individuals that are selected – it changes only the characteristics of the population  Animals do not do things for the good of the species (but might behave altruistically under particular conditions), and not all traits are adaptive  Adaptations are constrained by trade-offs as well as genetic and historical factors 24.1 The Evolution of Evolutionary Thought  Plato claimed that every organism was an example of a perfect essence or type created by God and that these types were unchanging  Aristotle proposed that species were organized into a sequence based on increased size and complexity with humans at the top  Typological Thinking – based on the idea that species are unchanging types and that variations within species are unimportant or even misleading  In 1809 Jean-Baptiste de Lamarck was the first to propose a formal theory of evolution  Lamarck proposed that characteristics that are not used were lost; characteristics that are used are passed on, including in a state acquired by the parent o For example, a blacksmith acquires strong arms from working and will thus have sons with strong arms o Initially based on the great chain of being – simple organisms originate at the base of the chain by spontaneous generation and then evolve by moving up the chain over time  Lamarckism has been generally refuted with experiments  However, there is evidence that characteristics acquired during a parents lifetime could be passed on to offspring via mechanisms that do not involve changes in the nucleotide sequence of DNA (i.e. DNA methylation)  Darwin and Wallace went a major step farther and proposed that change in species over time is based on natural selection acting on variation among individuals  Darwin described evolution as descent with modification, meaning that change over time produced modern species from ancestral species  Population – consists of individuals of the same species that are living in the same area at the same time Bio 1AA3 2 24.2 The Pattern of Evolution  The pattern component of the theory of evolution by natural selection makes two claims about the nature of species o They change through time o They are related by common ancestry Evidence for Evolution and Change Through Time  Fossils – any trace of an organism that lived in the past; range from bones and branches, to shells, tracks or impressions and dung o Found in sedimentary rocks, which form from sand or mud or other materials deposited at locations such as beaches or river mouths, and because sedimentary rocks are known to form in layers o Fossil Record – consists of all the fossils that have been found and described in the scientific literature o Fossils found in deeper layers were judged to be older than the shallower layers o Geologic Time Scale – different periods or geologic time, creating the sequence of eons, epochs and periods o Began to use radiometric dating  Extinction – a species that no longer exists o Many fossils provide evidence for extinct species, those that are no longer living  Prehistoric turds and ancient DNA  Speciation in real time o Artificial selection o Speciation by hybridization o Speciation of fruit flies by divergence of host plant usage (eg/ the apple maggot fly Rhagoletic o Ring species o Speciation by polyploidization  Eg/ In African clawed frogs  Transitional Forms – a fossil species with traits that are intermediate between those of older and younger species o As the fossil record has become more complete, many transitional forms have been discovered with traits that are intermediate between earlier and later species  Vestigial Traits – a reduced or incompletely developed structure that has no function or reduced function, but it is clearly similar to functioning organs or structures in closely related species Bio 1AA3 3  Geographic Continuity o Early scientists observed that extinct fossil species are typically succeeded, in the same region, by similar living species o Darwin and Wallace interpreted this as evidence that extinct forms and living forms are related, that they represent ancestors and descendants  Geographic Relationships (Species Are Related) o Darwin discovered distinct species of mockingbirds from the Galapagos Islands o Proposed that the mockingbirds were similar because they had descended from the same common ancestor o The mockingbird populations that colonized in different islands had changed through time and formed new species Evidence That Species are Related  Homology – similarity that exists in species descended from a common ancestor 1. Genetic Homology – is a similarity among species in DNA sequences, gene content, or other genetic attributes o An identical or almost identical system is used by all known organisms to store the sequence of amino acids in proteins in nucleotides o Variation – mitochondrial DNA, substitution of single codons in other species (eg/ Mycoplasma, Candida) or substituted use of start or stop codons o Chimps and humans differ by only ~1.5% of shared nucleotides  If you incorporate insertions and deletions, they differ by ~6% o Eyeless and Aniridia influence where eyes are located in fruit flies and humans respectively; presumably both are derived from an ancestral gene with the same function 2. Developmental Homology– is a similarity among species in embryonic traits Bio 1AA3 4 3. Structural Homology – similarities in adult morphology  Traits are similar in different species because the species in question are related to each other by common descent – if species were created independently of one another, these types of similarities would not occur Table 24.1 Evidence For Evolution (page 513) Prediction 1: Species Are Not Static, but Change Prediction 2: Species are Related, Not Independent Through Time  Most species have gone extinct  Closely related species often live in the same  Fossil (extinct) species frequently resemble geographic area living species found in the same area  Homologous traits are common and exist at  Transitional form document change in traits three levels: through time 1. Genetic (gene structure and the genetic  Earth is ancient code)  Vestigial Traits are common 2. Developmental (embryonic structures and processes)  Populations and species can be observed changing today 3. Structural (morphological traits) 24.3 How Does Natural Selection Work?  Four simple postulates of evolution by natural selection 1. Individual organisms in a population vary in their traits (variation) 2. Some of these traits differences are heritable, meaning they are passed on to offspring (heritability) 3. In each generation, not all individuals reproduce the same amount and/or not all offspring survive (fitness effects and competition) 4. Individuals with certain heritable traits are more likely to survive and reproduce  Natural selection is one of the processes that determines whether individuals with certain traits produce more offspring than do individuals without those traits  Heritable selected traits will increase in frequency in the population from one generation to the next, causing evolution – a change in the genetic characteristics of a population over time  Evolution – a change in the genetic characteristics of a population over time o Evolution is a logical outcome of these steps  You can also think of evolution as a change in allele frequencies in a population over time  Example of Evolution by Natural Selection – Industrial Melanism  Moths with A A 1 1otype have light wings, and moths with A A or1 2A hav2 2ark wings o Depending on the environment, different moths have a different chance at survival Bio 1AA3 5
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