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A history of life and natural selection 1 Evidence for Evolution 2 Fossils A fossil is the remains or traces of an organism that died long ago Many of the oldest fossils we find are of extinct species Most fossils are found in sedimentary rocks that settle at the bottom of seas, lakes and marshes All the fossils together have created a geologic record of Earth’s history 3 4 Fossils Fossils show us different organisms lived at different times For example, rock strata from about 2-3 bya would show fossils of only single celled organism However, rock strata from 150 mya would show fossils of dinosaurs, the first birds, and a wide variety of plant life 5 Fossils If evolution has occurred, we would see different species throughout history Fossils show us that there has been!! 6 Fossils Fossils also give us clues to transitional species. Transitional species show how organisms gradually change over time For example, scientists believe the whale ancestor’s were once land dwelling We have found several transitional fossils to support this idea 7 Anatomy Anatomy is the study of the body structure If organisms have evolved from common ancestors, then they would have similar anatomical features…RIGHT?!?!?! Well, THEY DO! 8 Anatomy Some organisms have homologous structures, which are anatomical structures that occur in different species and that originated by heredity from a common ancestor The function of that structure may differ in related organisms Finding homologous structures in different species indicates they have a common ancestor Ex: Limb bones in mammals 9 Homologous Structures 10 Anatomy Some organisms have analogous structures, which are anatomical structures that have closely related functions, but were not derived from the same ancestor Analogous structures evolve independently, but have the same function EX: wings in bats, birds, and bugs 11 Anatomy Many organisms display vestigial structures, which are anatomical structures that seem to have no function but resemble structures used in ancestors Remember, that just because something becomes useless, DOES NOT mean it goes away…it must become detrimental to survival 12 Vestigial structures Eyes on a mole Hip bones on a whale or snake 13 Embryology Embryology studies the development of embryos The early stages of vertebrate development are incredibly similar The explanation for this is that vertebrates share a common ancestor and inherited common stages of development 14 Fish, reptile, bird, and mammal embryos all have a tail and gill slits. 15 16 Biological Molecules Biologists now have the technology to compare DNA, RNA, proteins, and other biological molecules in different organisms (molecular homology) Organisms that have the least amount of differences in these molecules are closely related by a common ancestor 17 Biogeography Closely related species live in the same geographic regions This results from having similar needs so that they need similar habitats Furthermore, in isolated areas (islands), you will find species that are unique in the world (endemic) Additionally, similar environments will give rise to different species that have similar traits Example: Flying squirrel (mammal)of North America is very similar to the Sugar glider (marsupial) of Australia Meiosis KM 18 Can we see evolution today? Organisms on Earth ARE currently evolving One familiar example is bacteria- we have to keep coming up with different antibiotics to fight the rapid evolution of bacteria. They evolve to become resistant to the antibiotics 19 Can we see evolution today? It is very hard to see evolution in higher organisms- as the process takes hundreds-thousands of years But we CAN observe natural selection in many species in a relatively short period of time (such as with the peppered moths) 20 The goal of population genetics is to understand the genetic composition of a population and the forces that determine and change that composition. 21 Variation in a Population Population genetics is the study of evolution from a genetic point of view Evolution at the genetic level is microevolution Population genetics looks at the alleles (variations) in a population and how they change over time (evolve) 22 Variation in a Population Population genetics also looks at populations as a whole, because population is the smallest unit in which evolution can occur A population is defined as a group of individuals of the same species that routinely interbreed 23 What can cause traits to vary in a population? Mutations-which are random changes in DNA sequences Recombination-genes are reshuffled in meiosis-due to independent assortment and crossing over Random pairing of gametesorganisms produce many gametesany one can be involved in fertilization The environment also influences the outcome of many traits 24 The Gene Pool The gene pool is the total genetic information available in a population So, ALL alleles for EVERY gene in a population 25 Genetic Equilibrium Hardy-Weinberg Genetic Equilibrium is a principle that states genotype frequencies tend to remain the same over generations unless acted upon by an outside force Hardy-Weinberg equation(s): p2 +2pq + q2 = 1 p+q=1 p = dominant allele q = recessive allele p2 = homozygous dominant pq = heterozygous q2 = homozygous recessive 26 Example A population of bunnies has the phenotype of 36% white bunnies(the recessive gene) and the rest are black. Based on this data, what are the frequency of each genotype? Things to consider: What are the possible genotypes? Do you know any variables? 2pq, p2, or q2 How do you solve for p or q? 27 Solution 36 % of the bunnies are ANSWER: white Genotype = bb p2 = 0.16 or 16% (BB) Therefore, 36% of the 2 pq = 0.48 or 48% (Bb) q2 bunnies = q2 = 0.36 q = 0.6 Since q = 0.6, then p+q=1 1–q=p p = 0.4 (40%) q2 = 0.36 or 36% (bb) 28 Genetic Equilibrium Hardy-Weinberg Genetic Equilibrium assumptions: No net mutations occur, so alleles remain the same Individuals neither leave nor come into the population (no gene flow) The population is large (ideally infinite) Natural Selection DOES NOT occur (so random mating and no environmental pressures) Random mating (no sexual selection) 29 Non-Equilibrium If the alleles and genes do not stay the same over generations, we know populations are out of equilibrium and evolution may occur Equilibrium is no change, evolution is change We can tell if frequencies stay the same by calculating them using the Hardy-Weinberg equation So, when genes and alleles change, natural selection and evolution are occurring according to Hardy-Weinberg 30 Non-Equilibrium Many things can disrupt genetic equilibrium and in fact populations RARELY stay at equilibrium for very long What disrupts equilibrium and cause evolution? Mutations Gene flow Genetic drift Founder effect Nonrandom mating Natural Selection 31 Mutations We have already discussed many avenues of genetic mutation Mutation rates tend to be low in animals and plants (about 1 mutation in every 100,000 genes per generation) In sexually reproducing organisms, sexual recombination is a more important vector for change Meiosis KM 32 Gene flow Immigration (individuals coming in) and emigration (individuals leaving) naturally occurs in many populations This causes gene flow- which is the movement of genes from one population to another 33 Genetic Drift In small populations, chance events can change allele frequencies in a population A change in allele frequencies is called Genetic Drift Frequency means how often something occurs 34 Bottleneck effect A disaster in the environment can lead to a drastic change in allele frequency Through random chance, certain alleles may be overrepresented in a population This gives us an avenue to alter the genotypic and phenotypic expression of the population This bottleneck effect would change the new population into something different from the original population Meiosis KM 35 Genetic Drift Example In a population of 25 trees, where there are two alleles for height- tall (T) and short (t) and the allele frequency is 50:50 A natural disaster- such as a fire, wipes out most if the population Let’s say 2 trees survive, but they are both homozygous tall (TT)- so now the allele frequency will be 100% for the tall allele In a population of more (say 1,000), this is less likely to happen- more trees and therefore more of the original alleles would survive 36 Genetic drift The random change of allelic frequency in a population 37 Founder effect When a few individuals from a population become isolated from the source population, they may change to fill new habitats (founder effect) Darwin observed this phenomena when he observed the finches on the Galapagos islands Meiosis KM 38 Nonrandom mating Many species do not mate randomly Some mate with individuals close to them Some mate with individuals that have similar traits to them Both of these result in increasing certain allele frequencies For example, very tall birds, may only mate with other tall birds (not medium or small), this would cause the tall allele to become more prevalent 39 Nonrandom mating Many species of birds, such as peacocks look for specific characteristics when they mate, like elaborate colors This is called sexual selection This leads to sexual dimorphism, a difference between the physical characteristics of males vs. females 40 Intersexual vs. Intrasexual Intrasexual selection: Intersexual selection: Occurs between the Occurs when one sex same sex Direct competition between individuals of the same sex to maintain the ability to pass on their traits Example: Male lions and control of the pride is able to select a mate Becomes a competition to attract a mate Examples: Birds – showiness of plumage, ability to build a nest, birdsong 41 Natural Selection Most significant factor in evolution of populations Nature selects against non-fit individuals Reduces harmful alleles Only acts on expressed phenotypes 42 Agents of evolutionary change 1. Mutation 2. Natural selection 3. Genetic drift 4. Gene flow 5. Nonrandom mating 43 44 What is a species? The biological species concept says a species is a population of organisms that can successfully interbreed The morphological species concept says a species is a population of organisms that have a similar appearance Modern Species Concept- biologists use both of these criteria to classify both living and extinct species today 45 Other definitions of species Paleontological species: focuses on the morphological characteristics of organisms in the fossil record. Ecological species: looks at defining species bases on their role in an ecosystem (niche) Phylogenetic species: how organisms develop from a common ancestor (we will be examining phylogenetic trees later on) Meiosis KM 46 Forming new species In order for new species to form, you must have genetic variation (remember from meiosis, mutation, etc) Yet not all variation is a result of genetics There are also numerous environmental factors that affect phenotype Lack of nutrition/too much nutrition Key thing to remember: environmental changes to phenotype are NOT heritable Meiosis KM 47 How does a species form? The process of species formation is called speciation Speciation begins with isolation In order to form a new species, you must begin with some sort of reproductive isolation 48 Scale of speciation When you talk about changes in a single gene pool, this is described as microevolution Example: Peppered moths of England When you discuss changes over vast tracts of time, this is referred to as macroevolution Example: Going from the age of reptiles to the age of mammals Meiosis KM 49 Fitness In order to determine which species will be successful (or what traits are passed on to offspring), we must examine adaptive advantages Fitness: contribution of an individual to the gene pool for the next generation Relative fitness: contribution of a particular genotype for the next generation Therefore, for speciation to occur, the new traits must have some sort of fitness “advantage” for them to be passed to a new generation Meiosis KM 50 How does a species form? Allopatric speciation is a result of geographic isolation (some physical barrier that separates populations) Allopatric means “different homelands” Once one species is separated into two (or more), gene flow between them stops As each experiences different environmental pressures, genetic drift occurs in different ways 51 How does a species form? Sometimes groups of organisms become reproductively isolated This may or may not be due to a physical barrier Reproductive isolation is when two individuals cannot successfully mate (this means mate and produce healthy FERTILE offspring) 52 How does a species form? Types of reproductive isolation Prezygotic isolation- occurs before fertilization Different species do not breed at the same time Different species have different mating rituals (such as a mating call or “dance”) Basically this type of isolation means the different species WILL JUST NOT MATE in nature 53 Types of Pre-zygotic Isolation 1. Geographic isolation 2. Ecological isolation 4. Behavioral isolation 3. Temporal isolation 5. Mechanical isolation 54 How does a species form? Postzygotic isolation- occurs after fertilization Gametes are not be compatible and do not produce healthy offspring If offspring is healthy it may be infertile 55 How does a species form? Sympatric Speciation- occurs when 2 subpopulation become reproductively isolated, but have no physical barriers between them. 56 Sympatric speciation Examples of sympatric speciation: Polyploidy is a mutation that often occurs in plants (can change chromosome number – go from 2n to 4n, autopolyploid) As a result the 4n plant can no longer breed with the 2n plant . . . different chromosome number Animals taking advantage of different aspects of the same resources (Darwin’s finches) An environment that repeatedly, and drastically, changes Lake or pond repeatedly drying out and refilling Meiosis KM 57 Rate of Speciation: Punctuated Equilibrium vs. Gradualism Gradualism Evolution occurs much more slowly (gradually) and consistently Speciation occurs at intervals further apart Punctuated Equilibrium Evolution that occurs at more intervals and less consistently Speciation occurs more frequently 58 59 Types of Evolution Convergent evolution- the process by which different species evolve similar traits This often occurs due to the different species living in similar types of environments Sugar Gliders and Flying squirrels both adapted to living in tall trees, but on different continents 60 Types of Evolution Divergent Evolution- a process in which the descendents of a single ancestor diversify into several different species that fit a variety of habitats A great example is Darwin’s finches One species of finch came from South America and evolved into 13 distinct species-each of which has a different habitat 61 Types of Evolution Adaptive Radiation– when a new population in a new environment undergoes divergent evolution until it fills many parts of the environment The finches evolved in almost every part of the Galapagos Islands 62 Types of Evolution Coevolution– when two or more species have evolved adaptations due to each other’s influence 63 Coevolution: Pollinators help plants reproduce and plants give food to pollinators 64 Steps of Speciation in Darwin’s finches 1. 2. 3. 4. 5. Founding Fathers & Mothers-finches made their way from South America Separation of Populations-finches crossed to different islands Changes in gene pool-Over time, populations adapted to the needs of their environment Reproductive isolation-birds prefer to mate with birds that have same beak as they do-2 species have evolved. Share same island-co-existance, extinction, or further evolution 65 66