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Transcript
UNIT 7 NOTES CHAPTER 22 SECTIONS 2-3 I’m sure by now you have all heard of Charles Darwin and his theory of evolution. Most teachers of this subject start out with a discussion of Darwin’s life, his voyage on the Beagle, and how he finally published his theory. None of that will be on your test. The brief coverage of this in the first seven slides will be plenty. For 100+ years, Darwin was the only one given credit for the theory. Lately, though, Alfred Russell Wallace has also been given credit since he had the same idea at the same time. He had contacted Darwin, who was older and better known at the time, to ask him what he thought. When Darwin realized that Wallace had come up with basically the same theory, he panicked. The two talked and agreed that Darwin would publish first and get credit. Today you might hear it called the Darwin-Wallace theory of evolution. I mention this just so you will know it is the same theory. In Darwin’s words, how can evolution be defined? What does that mean? What is a fossil? Where are older fossils found? Darwin was greatly influenced by a geologist named Lyell. Why? What did Lyell believe that Darwin thought must also be true of living organisms? Compare the finches in the slide (#11, Fig. 22.6 pg.456). Each has a slightly different type of beak that is suited to the particular food each species eats. This is the trait that helps each species survive. Each one has a different allele for beak type. Over time mutations in the beak gene would produce the alleles that make the different beak types. These alleles are the adaptations or genetic variations that are favored by natural selection and provide some type of advantage to the individual’s survival. 1 UNIT 7 NOTES Darwin’s two main ideas were: 1) Descent with modification explains life’s unity and diversity. This means that organisms that come from the same ancestors (descendants) are a lot alike (unity) but different (diversity) at the same time due to changes (modification) that occur along the way. Now we know that is because of genes and mutations and alleles, but none of that was known in Darwin’s time. Recent ancestors are also descendants of more ancient ancestors, so it goes back a long way. 2) Natural selection is a cause of adaptive evolution. This means that the individuals that have the best adaptations to survive in a particular environment (nature) are more likely to survive and reproduce and pass on their genes. Over time the best adaptations will increase in number and adaptations that aren’t such a good fit will become less numerous. Thus, over great periods of time, species change (evolve). Darwin also noticed that people modified other species by selecting breeding pairs. He called this artificial selection because people were actively involved and interfering with the natural process. Just look at all the things we got from wild mustard over the years (Fig. 22.9 pg. 458, slide 15). And they are good for you too, so eat’em up! The main reason Darwin was able to come up with the theory of evolution was that he was a very observant guy. And very patient. He did a lot of looking and thinking. Darwin made four important observations and from these drew two inferences to explain his theory. Define each of them: 1. Variation 2. Heritability 3. Overproduction 4. Competition 2 UNIT 7 NOTES 1. Fitness 2. Adaptation According to Darwin’s theory of natural selection, competition for limited resources results in differential survival. Individuals with more favorable phenotypes are more likely to survive and produce more offspring, thus passing traits to subsequent generations. • Individuals with certain heritable characteristics survive and reproduce at a higher rate than other individuals • Natural selection increases the adaptation of organisms to their environment over time • If an environment changes over time, natural selection may result in adaptation to these new conditions and may give rise to new species INDIVIDUALS DO NOT EVOLVE. POPULATIONS EVOLVE OVER TIME. Scientific evidence supports the idea that evolution has occurred in all species. The fossil record, comparison of proteins and DNA, phylogeny Scientific evidence supports the idea that evolution continues to occur. These are all examples that we can look at and study today and we can almost watch the changes as they happen. • Chemical resistance (mutations for resistance to antibiotics, pesticides, herbicides or chemotherapy drugs occur in the absence of the chemical) • Emergent diseases – HIV, H1N1, West Nile, Lyme disease New diseases are the result of changes in the disease organism (evolution) that allow it to infect new hosts. • Observed directional phenotypic change in a population (Grants’ observations of Darwin’s finches in the Galapagos pg. 468) • A eukaryotic example that describes evolution of a structure or process such as heart chambers, limbs, the brain and the immune system. The panda’s thumb is one example (you can google it) and there is a comparison of heart chambers in different organisms on pg. 902. Evolution is supported by an overwhelming amount of scientific evidence. Your book provides two examples of direct observations of evolutionary change. One, the evolution of drug-resistant HIV, is not only relevant but is demonstrative of all bacteria and viruses due to their rapid rate of reproduction. 3 UNIT 7 NOTES Does natural selection produce new traits? Explain. What determines which traits are selected? How does the fossil record provide evidence of evolution? Fig. 22.16 pg. 462, slide 33, shows a great example of what can be found in the fossil record that demonstrates change. If you just had the first and last ones you might not make the connection. But having the two in between shows how whales evolved from terrestrial organisms. IT WAS NOT DE-EVOLUTION!! There is no such thing. Yes, terrestrial life came from the oceans, but when it got crowded on land, some took to the sea for food and shelter, and eventually evolved enough to move back to the sea and fill a niche there. Today whales are very successful. Their only predator is us. The fossil record is very important because it allows us to look at organisms that are no longer living. Some people believe that they aren’t real and are just a hoax to discredit religion. The theory of evolution does not in any way say that there was no creator. The theory states that organisms have changed over time – descent with modification. Maybe the creator, if there was one, created the very first life form and then evolution took over. This is why evolution is still a theory and not a law. We don’t know, and probably never will know, for surescientifically. You are free to believe whatever you want. Maybe Adam and Eve were two Archaebacteria? Who knows? The following is an important concept and you will have to keep the three types of structures straight. The prefix homo- means same, so homo- anything means the same something, like homosexuals prefer people of the same sex. I’m sorry if that makes you uncomfortable (it shouldn’t) but that is the best way to remember it because everyone knows what that means. This should not be confused with the homo in Homo sapiens because here, as a genus name in Latin, it means man. What is homology? 4 UNIT 7 NOTES Homologous structures represent features shared by common ancestry. These structures are built in the same way, but the parts may have changed slightly in shape and/or size. Examples of homologous structures are: Take a good look at Fig. 22.17 pg. 463, slide 35, and compare the bones that are the same color. Notice how they have changed in shape or size, but the overall structure is the same in each one. We are all animals and much more alike than most people realize. More anatomical similarities can be found by studying embryos – all vertebrate embryos have tails at some point in their development. Vestigial structures are remnants of functional structures, which can be compared to fossils and provide evidence for evolution. Examples of vestigial structures are: (Hint: things you have but don’t use) Biochemical and genetic similarities, in particular DNA nucleotide and protein sequences, provide evidence for evolution and ancestry. The more similar any of these sequences are between species, the more closely related those species are. The genetic code, that use of A’s, C’s, G’s, and T’s, is universal- every organism on Earth uses it. This is what allows us to manipulate it by genetic engineering and cross breeding. There is no better evidence of evolution than that. Since everything does use this code we can put all organisms somewhere on an evolutionary tree. Darwin envisioned all forms of life being related through ancestry and descent such that they could all be put on branches of a tree showing their relatedness, with the root being the first life form, and the major branches the six kingdoms. We are still working on a ‘complete’ one, but many smaller ones for thousands of species exist. It is difficult to complete because only 1 percent of all species that ever lived are still alive today to provide DNA. Many trees are made using anatomical data because DNA is not available, but fossils are. Fig. 22.19 pg. 464, slide 40, is an example. Convergent evolution produces analogous structures. This occurs when organisms that are not related develop the same or similar features because they live in the same environment. Remember, natural selection chooses the design that works best. If it works best in one group of organisms, then chances are pretty good it will work as well in another group. Your book gives a stupid example so I’ll give you some that are easier to remember. Don’t get me wrong, 5 UNIT 7 NOTES I love flying squirrels and sugar gliders, but that’s just not what comes to mind first on this subject. Birds and butterflies are a good example. Both live much of their lives in the air and both can fly because both have wings. The bird wings has bones and feathers, the butterfly wing has some sort of support (I really don’t know what) and is covered in scales. They are not made in the same way at all, and, thus, are not homologous. They evolved separately in these species as a result of natural selection in the environment in which they live. Another good example is penguins and dolphins or sharks. Both have similar shapes to help them glide through the water. Again, it is a result of the environment in which they live and NOT common ancestry. CHAPTER 23 SECTIONS 1-4 Before we talk about the evolution of populations, you should know what a population is. A population is a group of organisms of the same species occupying a certain area. The gene pool is all of the alleles of all the genes that all the individuals in the population have. A population is the smallest unit of evolution. POPULATIONS EVOLVE NOT INDIVIDUALS!! Natural selection, however, does act on individuals, which, over time, produces the changes in the populations that are evolution. For example, Individual A with characteristic Q, may be better suited to the environment than Individual B with characteristic F. There are not many individuals with Q, but those that have it are highly sought after. Therefore, Individual A will find a better mate, possibly multiple mates, have more offspring, and pass on the gene for Q to many individuals. Individual B is not so lucky. F is not considered as desirable so he will either find an inferior mate, fewer mates, or none at all. Either way fewer genes for F are passed to the next generation. As this continues over time, a GREEEEAAAAT deal of time, what will happen to the occurrences, or frequencies, of these alleles? Correct! A million years later almost every individual in this species has Q!! Very few, if any, will have F. The frequencies of the alleles have changed. This is evolution. Define microevolution – What two processes produce the variation in gene pools that contributes to differences among individuals? 6 UNIT 7 NOTES Where does a mutation have to occur for it to be inherited? Genetic variation and mutation play roles in natural selection. A diverse gene pool is important for the survival of a species in a changing environment. The more variation there is in a population the better the chances are that the species will continue to survive should there be environmental changes. Diversity provides many choices for natural selection. Mutations are the primary source of genetic variation. Changes in genotype can result in changes in phenotype. Whether or not a mutation is detrimental, beneficial or neutral depends on the environmental context. Genetic changes that enhance survival and reproduction can be selected by environmental conditions, for example antibiotic resistance mutations, pesticide resistance mutations, and sickle cell disorder and heterozygote advantage. Review what you learned about mutations, mRNA processing, and transposons in Units 5 and 6. Generally, smaller mutations cause fewer problems, but it all depends on exactly where the mutation has occurred. Point mutations often go unnoticed due to wobble effect. Sickle cell disorder is an exception. Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful. Duplicated genes can take on new functions by further mutation. Changes in chromosome number often result in human disorders with developmental limitations, including Trisomy 21 (Down syndrome) and XO (Turner syndrome). Since these changes are detrimental to the individual they would be selected against. These individuals are less fit in the evolutionary sense. Animals born with conditions such as these usually do not survive at all, and, therefore, do not reproduce and do not pass on these genetic changes. Mutations are not always bad and they are the mechanism for change. Without them we would not exist. Selection (of mutations) results in evolutionary change. How does sexual reproduction contribute to genetic variation? How did the different alleles originate? 7 UNIT 7 NOTES THE HARDY-WEINBERG PRINCIPLE Define: Population – Gene pool – Fixed locus – Hardy-Weinberg is always used for populations of diploid organisms. I have never seen an example that was not diploid. If you are confused here, I’m sure you are not alone. If you are not confused by this, congratulations, but you probably are alone. We will go over this part of the notes in class and many example problems. The only way to truly learn this is by doing problems, so yes, you will have homework. Because there is that grid-in section of the AP exam that is mathematical in nature, I’m pretty sure you will see this on your exam, and I know it is on your unit test, so mind your p’s and q’s. (Pun intended) The frequency of an allele in a population can be calculated: The total number of alleles at a locus = The total number of dominant (or recessive) alleles = By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies. The frequency of all alleles in a population will add up to 1. Write this equation: The Hardy-Weinberg principle describes a population that is not evolving (allele frequencies do not change). 8 UNIT 7 NOTES If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving (allele frequencies will change). In a given population where gametes contribute to the next generation randomly, allele frequencies will not change. Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool. If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then: p2 + 2pq + q2 = 1 If the alleles are represented as B and b then: p2 = q2 = 2pq = Conditions for a population or an allele to be in Hardy-Weinberg equilibrium are: (1) a large population size, (2) no gene flow - no migration (3) no mutations, (4) random mating and (5) no natural selection. These conditions are seldom met. Organisms can be evolving at some loci but not at others. Not every gene an organism has undergoes change at the same time. Mutations are actually few and far between so even when there seems to be drastic changes in an organism over time, the actual genetic change is probably very small. Your book gives an example of how to use the H-W equation to estimate the percentage of a population carrying the allele for an inherited disease (pg. 474, slide 67). It can also be used to determine whether or not a population is evolving. Try slide 68, Concept check question 3 pg. 475: 9 UNIT 7 NOTES Previously you were given 5 conditions for a population to be in Hardy-Weinberg equilibrium. Any violation of any one of those conditions and the population is not in H-W but is evolving. Any migration into or out of the population causes a change in allele frequency because as an individual comes into or leaves so do the alleles it has. If mating is not random, but choices are being made by individuals based on some characteristic, then that is selection. Mutations introduce new alleles so frequencies must change. Small populations are subject to other conditions that can bring about drastic change almost overnight since one individual represents a higher percentage than it would in a larger population. There are three major factors that alter allele frequencies and bring about most evolutionary change: – – – 1. 2. 3. In natural selection choices are being made based on some phenotypic characteristic. Selection is made FOR a specific characteristic so its allele frequency would go up over time, like we saw with characteristic Q at the beginning of this chapter. Genetic drift describes the loss of alleles in a small population that can happen randomly and just by chance. Slide 72, Fig. 23.8, pg. 476, shows an example, which is the best way to explain this concept. If you have a small population and only some of the individuals reproduce, alleles are lost from the gene pool. Again using the flowers as an example, someone could pick all the red ones, before they reproduce, because she likes those best, and that would remove all those alleles from that population so that the following year they would be no more red ones. This is something that happened to the flowers by chance. The flowers made no choices or selections. Natural disasters are often the causes of genetic drift and the population is left with whoever happened to survive. The founder effect is another example of genetic drift. This time instead of losing most of a population and being left with a few random individuals, the few random individuals somehow become isolated from the original population (that could be very large) and start their own new population. Since they were selected at random, chances are that their gene frequencies are different from those in the original population. Think of the pioneers that settled the old west in the 1800s. They were founders of new towns with just a few alleles to begin with. If you want a really great example, google HMS Bounty and Pitcairn Island. This is a story about a sailing ship that went looking for breadfruit in Tahiti, stayed too long so the men got attached to some women, they left, there was a 10 UNIT 7 NOTES mutiny on the ship and they through out the captain, they went back to Tahiti to get their women, and then went looking for somewhere to hide- Pitcairn Island. Today it is still relatively isolated and almost all the people there are descended from the mutineers of the Bounty. It is a very small gene pool with little genetic diversity. Other examples of founders include the Amish and the Mormons. The bottleneck effect is a sudden reduction in population size due to a change in the environment. A sudden change in the environment may cause selection for a specific group leaving only certain individuals that may not be reflective of the original diversity. Again, this could be caused by natural disaster, overhunting, or habitat destruction by us. Understanding the bottleneck effect can increase understanding of how human activity affects other species. (Case study of the greater prairie chicken pg. 477) Reduction of genetic variation within a given population can increase the differences between populations of the same species. (Greater prairie chicken) Genetic drift can cause harmful alleles to become fixed. If the selected individuals have a high frequency of a harmful allele then there is the chance of it becoming fixed. This would mean that all individuals would be homozygous for the harmful gene which could lead to extinction of the population. Gene flow is the movement of alleles among populations. This would tend to reduce the differences between populations rather than increase them the way genetic drift does. This can be caused by migration of individuals or gametes such as pollen that is carried by the wind. Gene flow is not necessarily a good thing. Individuals that migrate into a population can bring bad alleles as well as good ones. There are examples of each in the PowerPoint, slides 79-80. While genetic drift and gene flow can cause evolutionary changes that are good OR bad, natural selection always chooses the phenotypic traits that are the best match for the environment. By choosing phenotypic traits, the alleles that produce these traits are also selected for. Traits that are selected for will increase within the population and traits selected against will decrease. Natural selection acts on phenotypic variations in populations. The level of variation in a population affects population dynamics. A population’s ability to respond to changes in the environment is affected by genetic diversity. Species and populations with little genetic diversity are at risk for extinction because there are fewer alleles to choose from: • California condors • Black-footed ferrets 11 UNIT 7 NOTES • • • • Prairie chickens Potato blight causing the potato famine Corn rust affects on agricultural crops Tasmanian devils and infectious cancer How, though, does selection actually happen? Is there a little gnome out there in the woods pointing at different individuals and saying “yeah” or “nay”? A fairy that sprinkles pixie dust? A Red Queen who offs heads? Of course not. That would be too easy. Selection is caused by the fact that individuals that have traits that are better suited to a particular environment will get more food, hide from predators easier, live longer, and have more offspring. The more offspring you have the more genes you pass on. The more genes you pass on the more you contribute to the gene pool of the next generation. If you pass on more than the next guy, you are more fit. I just described what concept? Environments can be more or less stable or fluctuating, and this affects evolutionary rate and direction; different genetic variations can be selected in each generation. Genetic diversity allows individuals in a population to respond differently to the same changes in environmental conditions. • Not all animals in a population stampede. • Not all individuals in a population in a disease outbreak are equally affected; some may not show symptoms, some may have mild symptoms, or some may be naturally immune and resistant to the disease. Some phenotypic variations significantly increase or decrease fitness of the organism and the population. This may cause directional, disruptive, or stabilizing selection. See Fig. 23.13 pg. 480, slide 85. • Sickle cell anemia – heterozygote advantage • Peppered moth • DDT resistance in insects – insects with this mutation could survive when others could not and thus, the frequency of this allele has increased in the fruit fly population. Define and sketch an example of each: Directional selection – 12 UNIT 7 NOTES Disruptive selection – Stabilizing selection – When does adaptive evolution occur? Environments change and act as a selective mechanism on populations. The example of the peppered moth is a good one- the environment got darker due to pollution and the darker moths became more prevalent. It is also a good example of how humans can impact variation in other species, albeit indirectly. Here are several other examples: • Artificial selection • Loss of genetic diversity within a crop species • Overuse of antibiotics – leads to strains of drug resistant pathogens Phenotypic variations are not directed by the environment but occur through random changes in the DNA and through new gene combinations. These changes can then be selected for or against by the environment. Nature can only select from the choices it is given – it does NOT create the choices. Chapter 24 Sections 1-4 This chapter deals with the origin of species, as you can tell, obviously, from the title. Darwin considered this to be the “mystery of mysteries” because, at the time, there was no scientific explanation as to where different types of organisms came from. Speciation is the process by which one species splits into two or more species, or the transformation of one species into a new species over time. This concept explains both the unity and diversity of life on Earth. The diversity is obvious because when speciation occurs you have new species, or different kinds of organisms, which is the definition of diversity- lots of different kinds of things. The unity part of it is more difficult for people to understand. Unity means to be 13 UNIT 7 NOTES the same or have consistency between groups. Organisms that have evolved from previous organisms share many similarities which is where the unity comes in. You share more than 98% of your DNA with a chimpanzee. There are obvious differences yet many similarities as well since you both share a common ancestor. Exactly how much difference in DNA is required to make a new species is still up for debate, and it may also depend on the species involved. There may be more changes required to produce a new species in some types of organisms than in others. Define these terms: Microevolution – Macroevolution – Biologists compare many things when determining groups, or species, of organisms. What are they? According to the biological species concept, what is a species? So, basically, a species is a bunch of organisms that are all the same and are able to mate and make more of the same NATURALLY, and those offspring CAN ALSO REPRODUCE. I emphasize this because somebody is going to say “What about the ligers and tigons? There’s an example of two species (lions and tigers) that mated and produced offspring!” Yup, they are, BUT: 1) ligers and tigons are not capable of reproduction, they are sterile, and 2) lions live in Africa and tigers live in Asia. They are not even on the same continent, let alone in the same country, so they would not mate NATURALLY. The more we learn, the more we are able to do in a lab and mess with stuff. Just because it happens in a lab doesn’t mean that’s how it really works, and this is certainly NOT an example of natural selection in any way. This is more of the ‘just because we can doesn’t mean we should’ category. 14 UNIT 7 NOTES This definition also mentions populations. Let’s talk more about lions. They live in groups called prides. You’ve all seen The Lion King, so you know how it works. These prides are actually different populations that live in an area. They are mainly females with young and the males kinda wander around on their own. Which males mate with which females over the years can change as new males wander in or grow up and challenge the older males. This creates gene flow, and that is what keeps these different populations from becoming different species. All the lions in Africa are free to roam about and mate with whomever they want. Some males will go great distances and spend their lives far from where they were born so lion genes are spread around amongst all the lions and not just in one area, thus maintaining the lion species and not creating new ones. That’s how we maintain species. So, how do we get new ones? Pretend you are Spock for a moment and think logically. If members of populations are free to move about and migrate from one population to another and mate, and that maintains species, what would happen if those same members were NOT allowed to migrate and mate? What if the populations were isolated from each other and there was no gene flow? Logically, one would have to assume that such a situation would have the opposite effect and rather than maintain species, new species would eventually emerge due to mutations, sexual reproduction, and natural selection. Thank you, Mr. Spock. New species arise from reproductive isolation over time, which can involve scales of hundreds of thousands or even millions of years, or speciation can occur rapidly through mechanisms such as polyploidy in plants. (We will discuss this later.) Speciation may occur when two populations become reproductively isolated from each other. Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring. See Fig. 24.4 pgs. 490-491, slides 96-101. Reproductive isolation can be classified by whether factors act before or after fertilization. Pre-zygotic barriers do what? List them here with an example: (there’s more room on the next page) 15 UNIT 7 NOTES What do post-zygotic barriers do? List them here with an example: There are many other definitions of “species” and some are more useful than others for different circumstances. We will only be using the biological species concept, as does your text, with its focus on reproductive barriers. Speciation results in diversity of life forms. Species can be physically separated by a geographic barrier such as an ocean or a mountain range, or various preand post-zygotic mechanisms can maintain reproductive isolation and prevent gene flow. Reproductive isolation keeps species apart. How do separate species form in the first place? There are two main ways: Allopatric speciation involves geographic isolation. Gene flow is interrupted because a population is physically divided by some geographical feature such as mountains or a highway. Over time reproductive barriers may develop. What is the evidence of this? Sympatric speciation takes place in geographically overlapping populations. It is less common than allopatric speciation due the continued gene flow in the 16 UNIT 7 NOTES overlapping populations. It usually occurs due to polyploidy, habitat differentiation, or sexual selection. Polyploidy is Draw an example: See Fig. 24.10 pg. 295, slide 111. Changes in chromosome number often result in new phenotypes, including sterility caused by triploidy and increased vigor of other polyploids. Polyploidy is much more common in plants than in animals. Many important crop species such as wheat, cotton, and tobacco are polyploids. Habitat differentiation happens when a population becomes isolated due to a switch in habitat or food source (a new niche). If some members of a species start to eat a different food such as a different species of tree, they could, over a long period of time, become a new species based on their new food source which gives them a different niche than the rest of the original population. Sexual selection is the result of some members choosing and mating with a specific subset of the original population (a color preference, for example). This is thought to be how the many species of cichlid fish in Lake Victoria evolved. Here, it happens quite often apparently, because they are always finding new species of those fish there. In summary: • • • • • In allopatric speciation, geographic isolation restricts gene flow between populations Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations Even if contact is restored between populations, interbreeding is prevented In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species Sympatric speciation can result from polyploidy, natural selection, or sexual selection 17 UNIT 7 NOTES As we saw with the ligers, sometimes it is possible for members of different species to mate. This example doesn’t happen naturally, but mules do, and so do others. Animals, or plants, such as these are called hybrids. What is a hybrid zone? Keep in mind that hybrids are only going to happen between closely related species. Horses and donkeys are pretty close (same family), as are lions and tigers. Frogs and rabbits are not so you won’t see a frabbit anytime soon. These hybrid zones do give scientists an opportunity to study evolution in action because they can study the factors that cause reproductive isolation. In a hybrid zone reproductive barriers can be (1) strengthened, leading to reinforcement of the two species and a decrease in hybrids, (2) weakened, leading to fusion of the two species into one, or (3) continued formation of hybrids and stabilization of the two species. See Fig. 24.14 pg. 499, slides 122-125. There are still questions concerning how long it takes new species to form and how many genes need to be different for organisms to be considered different species. Scientists study the fossil record every day and many scientists have many opinions as to what it says. Basically, there are two schools of thought as to how fast evolution occurs. The first theory is called gradualism. This theory states that change is very slow and that species make many small changes over a long period of time to become new species. See Fig. 24.17 (b) pg. 502, slide 129. This can be seen in the fossil record for some species. The second theory is called punctuated equilibrium and was first published by Stephen J. Gould. He wrote many books on this and other subjects if you are interested. This theory says that species stay the same for long periods of time and then undergo rapid change to new species. (Fig. 24.17 (a))This can also be seen in the fossil record for some species. It is hard to distinguish between the two, though, because what looks like rapid change in the fossil record could still be tens of thousands of years. Sediment build up to cover remains and make fossils takes a very long time. There may be changes that occur that never produce fossils making it seem rapid when it isn’t. All species are different and change in different ways. Species with shorter life cycles and that produce more offspring have the potential to evolve at a faster 18 UNIT 7 NOTES rate than species with longer life cycles that produce less offspring. This doesn’t mean they do just that they could. It seems likely though, since the dinosaurs are gone and mammals are still here. Just throwing that out there for you to consider. There is also the question of how much change is needed for a new species. For some species it can be as little as one allele or it could be very many. Genomics is a new field of study involving the identification and study of gene sequences in different organisms and comparing them. By comparing the same genes in different organisms letter by letter, the amount of change that has actually occurred can be determined. From this type of data we can find out how much change is needed to become a different species. There will be different answers for different species depending on the gene that changes, I think. Some changes will be subtle and not produce great change, while others may be more dramatic. Right now, no one knows for sure. Chapter 25 Sections 1-4 There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence. You do not have to learn about each hypothesis, just be aware that we are still just making a guess as to what happened since there wasn’t anybody there with their cell phone at the time to put it on Facebook and Twitter. There have been experiments done to recreate the conditions of early Earth to see if it was possible for life to form on its own from the mixture of inorganic compounds thought to be present at the time. This may have produced very simple cells through a series of stages: 1. 2. 3. 4. Abiotic synthesis of small organic molecules Joining of these small molecules into macromolecules Packaging of molecules into “protobionts” Origin of self-replicating molecules Put simply, this is one version of how they think life began: First there was this nasty atmosphere with lots of inorganic chemicals in it like nitrogen (N2), hydrogen (H2), carbon dioxide (CO2), carbon monoxide (CO), and water vapor (H2O). It was really, really hot too, like a sauna. This made these chemicals all mix together and start to make new stuff like ammonia (NH3) and methane (CH4). This was step 1. Now there are small organic molecules that can start to get together to make bigger organic molecules (macromolecules) like amino acids and lipids. (Step2.) Since lipids are hydrophobic they want to stay with each other and keep out the water, so they start to form small round 19 UNIT 7 NOTES structures called protobionts, (Step 3.) a sort of type of cell without organelles, or really much of anything in it. More like fat droplets that just might catch some other stuff inside by accident when they form. (Remember when we made oil drops in water in the very first lab we did?) As time went on, larger, more complicated macromolecules like proteins and nucleic acids may have formed. Once there was RNA and/or DNA with the ability to replicate (Step 4.), you now have a crude form of reproduction, and the start of life. Keep in mind that this may have taken over a billion years to complete, and, yes, it was all random and by chance. That is the true miracle. Abiotic synthesis of small organic molecules (Step 1.) has several possibilities: • Evidence is provided by experiments of Miller and Urey. See Fig. 4.2 on pg. 59. Be familiar with this experiment! This is what I outlined above and is often referred to as the heterotroph hypothesis. • It is also possible that life began underwater near hydrothermal vents rather than in the atmosphere. These vents provide inorganic nutrients that present day organisms use. • Amino acids have also been found in meteorites. The joining of these monomers produced polymers (macromolecules) with the ability to replicate, store and transfer information. These complex reaction sets could have occurred in solution (organic soup model) or as reactions on solid reactive surfaces. Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock. Whether on land or in the sea, these polymers could then spontaneously form protobionts. What is a protobiont? What are the two key properties of life that protobionts exhibit? The RNA World hypothesis proposes that RNA could have been the earliest genetic material. What advantage did protobionts with self-replicating, catalyzing RNA have? 20 UNIT 7 NOTES What is the evolutionary significance of this? The fossil record documents the history of life. By looking at fossils scientists can see patterns of change. Using dating methods to put the fossils in order chronologically the patterns of evolution emerge. Keep in mind that not every organism that ever existed left behind a fossil. What types of organisms tended to form fossils? **You are NOT required to know how any of the dating methods work. See Fig. 25.6 pg. 513, slide 145, for an example of how fossils can be used to show change. Key events in life’s history include the origins of single-celled and multi-celled organisms and the colonization of land. **You are NOT required to learn the names of the eons, eras, or epochs or any of the dates. The Earth formed approximately 4.6 billion years ago (bya), and the environment was too hostile for life until 3.9 bya, while the earliest fossil evidence for life dates to 3.5 bya (stromatolites). Taken together, this evidence provides a plausible range of dates when the origin of life could have occurred. What types of organisms were the only life forms for over 1 billion years and still exist today? As oxygen began to accumulate in the early atmosphere things began to change. What is the likely source of this oxygen? What opportunity did this “oxygen revolution” provide? By what process? 21 UNIT 7 NOTES The hypothesis of endosymbiosis proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells. What is an endosymbiont? Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events. This just means that the same host cell (or, most likely, its ancestor) had an endosymbiont that became a mitochondrion and then had a different endosymbiont that became a chloroplast. Remember that these are evolutionary events that took place over LARGE amounts of time so it is not the same host cell continually, but these endosymbionts are being passed down to the next generation and eventually replicating and forming new ones for the next generation. See Fig. 25-9 pg. 517, slide 156. What is some evidence that supports this theory? After these events unicellular organisms could now be different. Some were still prokaryotic. Some were now eukaryotic, and, of these, some had mitochondria only, and some had plastids as well. All that was left now was for some of them to get together and start living as an interdependent group, or what is commonly called today, a multicellular organism. Further diversification then led to plants, animals, and fungi. The Cambrian explosion refers to the sudden appearance of fossils resembling modern phyla in the Cambrian period around 530 million years ago. Colonization of land began to occur around 500 million years ago. Arthropods and tetrapods are the most widespread and diverse land animals. What are they? Arthropods are 22 UNIT 7 NOTES Tetrapods are **The following section provides examples of evolution previously discussed and that you should be familiar with. Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. a. Structural and functional evidence supports the relatedness of all domains. 1. DNA and RNA are carriers of genetic information through transcription, translation and replication. 2. Major features of the genetic code are shared by all modern living systems. All organisms use the same nucleotides to make DNA and RNA – A, T, C, G, U. This can be evidenced by studying viruses which must use other types of organisms in order to reproduce, and genetic engineering technology where genes from one organism can be put into another and protein products are produced. 3. Metabolic pathways are conserved across all currently recognized domains. Glycolysis (splitting glucose as the first step in cellular respiration) occurs in most organisms. Cell communication processes such as G-protein coupled receptors; cAMP, etc. are common among many organisms. b. Structural evidence supports the relatedness of all eukaryotes. These features are all common among all eukaryotes and similar in form and function thus providing evidence of common ancestry: • Cytoskeleton (a network of structural proteins that facilitate cell movement, morphological integrity and organelle transport) • Membrane-bound organelles (mitochondria and/or chloroplasts) • Linear chromosomes • Endomembrane systems, including the nuclear envelope The rise and fall of dominant groups reflect continental drift, mass extinctions, and adaptive radiations. Describe continental drift: What kind of speciation occurred after Pangea broke up? 23 UNIT 7 NOTES How do they know that the continents were once attached? Species extinction rates are rapid at times of ecological stress. Ecological stress can be caused by many things: climate change, extreme volcanism, or meteorite impacts for instance. All of these have been known to have happened. Nothing like a meteorite crashing to Earth to break up the monotony! • The fossil record shows that most species that have ever lived are now extinct • At times, the rate of extinction has increased dramatically and caused a mass extinction What is a mass extinction? How many were there? It is possible that we are currently undergoing another mass extinction. If we are, this time we are the cause. By massive burning of fossil fuels we have let loose tremendous amounts of CO2 and methane which are greenhouse gases. This has caused an increase in the global temperature which in turn may have dire consequences. The details of this will be further discussed in Unit 9. Due to our tremendous ability to reproduce and need for space we have also destroyed most of the habitats on Earth that other species need to survive. In evolutionary time, these changes are extremely rapid so organisms do not have time to adapt. This is what leads to extinction. Will we wake up and realize what we are doing in time to change and stop a mass extinction? I doubt it. It would not be economically feasible. What most people do not realize is that we can not live here on Earth alone. We need the other species that share this world with us. We need biodiversity. Without it we, as a species, will not survive. Then the cockroaches will take over the world because they are indestructible. ¡¡Viva La Cucaracha!! What are some of the consequences of a mass extinction? 24 UNIT 7 NOTES What is adaptive radiation? Give an example. Chapter 26 Sections 1-3, 6 Define: Phylogeny – Systematics – Taxonomy – What are the two parts of Linnaeus’ system that we still use today? 1. 2. Make sure you know what the binomial, or two-part scientific name of a species is made up of, and how it is written. Yours, for example, is Homo sapiens. Write the taxonomic groups in order from largest to smallest. Circle the ones that make up the two-part scientific name of a species. Phylogenetic trees and cladograms are graphical representations (models) of evolutionary history that can be tested. Systematists depict evolutionary relationships in branching phylogenetic trees. Basically, taxonomy is how we group and name species based on their phylogeny- who is related to or descended from whom based on certain characteristics they have and DNA. A phylogenetic tree represents a hypothesis about evolutionary relationships. As new information becomes available phylogenetic trees are redrawn to reflect that. Here is an example: 25 UNIT 7 NOTES Fig. 26-5 Branch point (node) Taxon A Taxon B Taxon C ANCESTRAL LINEAGE Sister taxa Taxon D Taxon E Taxon F Common ancestor of taxa A–F Polytomy On the model above, define the circled terms. Phylogenetic trees show only patterns of descent. They DO NOT indicate when species evolved or how much genetic change there was. The lengths of the lines do not indicate any specific length of time. (unless they have been specially constructed or a timeline added. See pg. 544 Figs. 26.12 and 26.13) Phylogenetic trees and cladograms can be constructed from morphological (shape) similarities of living or fossil species, and from DNA and protein sequence similarities, by employing computer programs that have sophisticated ways of measuring and representing relatedness among organisms. (BLAST lab) Phylogeny provides important information about similar characteristics in closely related species. For examples of how scientists use and apply phylogenetic data in the real world read “Applying Phylogenies” on pg. 539 and look at the inquiry box. In both cases DNA data was used to construct trees that helped identify organisms involved in criminal action. How scientists infer (make) phylogenetic trees: The term “infer” is used because these trees are always changing whenever new data is available from new technologies. Infer implies that this is the best guess they can make at this time. First scientists gather data. They will look at organisms’ structures (morphology), DNA, and proteins. Organisms with similar morphologies or DNA sequences are likely to be more closely related than organisms with different structures or 26 UNIT 7 NOTES sequences. Scientists then have to then decide whether similarities are due to homology or analogy. What is the difference? Analogous structures are not helpful in determining phylogenies. These homoplasies (analogous structures or molecular sequences that evolved independently) can be identified by computer programs. So once scientists have decided that a characteristic is homologous, that characteristic can be used to infer phylogeny and scientists can construct a tree. Shared characters are used to construct phylogenetic trees. Cladistics is grouping organisms by common descent. What is a clade? Fig. 26-10 Define each term under the picture: A A A B B C C C D D E E B Group I D Group II E F F F G G G (a) Monophyletic group (clade) (b) Paraphyletic group Group III (c) Polyphyletic group As scientists look at organism’s characteristics they determine which characteristics appeared first in ancestors and then which ones came later in descendents. They often work backwards as well. Grouping them together in this way leads to the formation of phylogenetic trees. What is a: Shared ancestral character – 27 UNIT 7 NOTES Shared derived character – When looking at several characteristics at a time and several organisms, scientists often use a character table like the one shown in Fig. 26.11 pg. 543, slide 210. By checking off who has what characters a pattern of relatedness forms from which a phylogenetic tree can be made. Phylogenetic trees and cladograms can represent traits that are either derived or lost due to evolution. For example: • Number of heart chambers in animals – evidence shows evolution in higher organisms from two chambered hearts to three and then four like ours. See pgs. 901-902. • Opposable thumbs – a characteristic only of monkeys, apes, and humans. See pg. 723. • Absence of legs in some sea mammals – whales and dolphins show evidence in bone structure of having had legs at one time. These bones have mostly disappeared leaving only remnants. Dolphin embryos show hind limbs during development, but these disappear before birth. An outgroup is a species or group of species closely related to the ingroup (species being studied) to which scientists can compare to determine whether characters are shared or derived. A character common to both the ingroup and outgroup would be ancestral because everyone has it. As mentioned scientists can use these trees to work backwards and predict features of ancestors like egg-laying in dinosaurs. New information continues to revise our understanding of the tree of life. Now that scientists can look at DNA letter by letter, systematics has changed how organisms are classified. Early taxonomists classified all species as either plants or animals. Later, five kingdoms were recognized: Monera (prokaryotes), Protista, Plantae, Fungi, and Animalia. We now have a three-domain system: 1. 2. 3. The three-domain system is supported by data from many sequenced genomes, largely rRNA genes that evolve slowly. Horizontal gene transfer is Why is this a problem? 28