Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Evolution and Natural Selection Review Presentation for Biology Keystone Assessment Created by Joshua Collins 2011 Table of Contents 1) What is Evolution? 2) Natural Selection 3) Evidence for Evolution 4) Evolution in Action 1) What is Evolution? 2) Natural Selection 3) Evidence for Evolution 4) Evolution in Action Section 3 of 4: Evidence for Evolution 3) Evidence for Evolution I. Geographic Distribution IV. The Fossil Record II. Homologous Anatomy V. Embryology III. Vestigial Structures VI. DNA and Genetics Evidence For Evolution: I. Geographic Distribution Geographic Distribution Related native species are found only where they could have migrated on their own. The distribution of ancient fossils, such as Mesosaurus, across the continents is a perfect example of this, although Darwin had no way of explaining how they got there, since plate tectonics had not yet been discovered! Evidence For Evolution: I. Geographic Distribution AFRICA The island of Madagascar is another great example of a set of species that naturally occur nowhere else in the world, for example, about 100 different species of lemur, a group of primates that branched off from earlier monkey-like primates when Madagascar split off from the African mainland about 160 million years ago. Madagascar • There are thousands of other species completely unique to this island. • They are all most closely related to the species found along the east coast of Africa, because they share common ancestors. Evidence For Evolution: I. Geographic Distribution While Darwin was putting together his theory of evolution by natural selection, another British naturalist named Alfred Wallace was doing his own work in the East Indies, where he made some new discoveries about the species there. Pacific Ocean Borneo Java New Guinea Alfred Russel Wallace Australia Evidence For Evolution: I. Geographic Distribution • Wallace saw that the islands of the East Indies were home to two completely different sets of mammals: monkeys and other placental mammals on the islands to the northwest, and marsupials to the southeast. • It was as if these two groups were divided by some invisible barrier that they could not cross even though the distance was less than 40 km across. • Other species including many birds were clearly cut off by the line as well. Borneo Pacific Ocean Java New Guinea Placental mammals Australia Marsupials Evidence For Evolution: I. Geographic Distribution The islands on either side of the line made up different geologic formations divided by a deep channel. The two groups, therefore, had never been connected. Even though the climates were almost identical, the flora and fauna were drastically different on either side of the line because the species had evolved in isolation from each other, just as the species on Madagascar had been isolated from the mainland of Africa. Borneo Pacific Ocean Java New Guinea Australia Evidence For Evolution: I. Geographic Distribution Flightless birds are another great example of evidence based on geographic distribution, because in most cases, their ancestry can be traced back to ancient birds who flew to the island from the mainland. • When birds colonize an island with no predators, they often lose the ability to fly over time because they don’t need to fly to survive. • Examples: kiwi, penguin, & the dodo bird. Evidence For Evolution: II. Homologous Anatomy Homologous Anatomy Traits that are similar between species because they originate with a common ancestor who had the same trait. • The example above compares the internal structure of a human arm with the forelimb of a whale. • Although the shape of each bone is different, each bone in the human arm has a corresponding match in the whale. Evidence For Evolution: II. Homologous Anatomy • In fact, every one of these animals has the same basic forelimb structure: a shoulder blade bone, a single humerus, two forearm bones, followed by many wrist bones and digits. • They all trace back to a common ancestor, the earliest tetrapods, which had this basic layout, almost as a template. It’s just been gradually modified for different uses over millions of years. Evidence For Evolution: II. Homologous Anatomy Looking at these feet up close gives you an idea of how similar they really are, bone for bone. Human (hand) Dog Pig Cow Tapir Horse r — Radius, u — Ulna, a — Scaphoid, b — Lunare, c — Triquetrum, d — Trapezium, e — Trapezoid, f — Capitatum, g — Hamatum, p — Pisiforme Evidence For Evolution: II. Homologous Anatomy In the case of the horse, the entire middle digit which has been modified into the lower part of the leg, while the other digits became smaller and smaller until they are completely gone. Human Tapir Horse A horse’s hoof corresponds to our middle fingernail while the knee actually matches up to our wrist bones. Evidence For Evolution: II. Homologous Anatomy Remember, homologous traits are adaptations that are similar between species because both species share an ancestor with the adaptation. For example, the following species all have wings, because they evolved from an earlier bird ancestor with wings. In this case, the three species have diverged from each other for long enough that their wings have gradually taken on different uses. Evidence For Evolution: II. Homologous Anatomy Bird wing • Not all similarities between species are homologous, however, because not all traits are inherited from a shared ancestor. • Although both birds and bats use wings to fly, when you look at their anatomy, it’s obvious that their wings are completely different. Bat wing • The similarity is not due to common ancestry. Rather, birds and bats have undergone convergent evolution. Evidence For Evolution: II. Homologous Anatomy Bird wing Convergent Evolution • When two species evolve a similar adaptation not because of common ancestry but because they have become adapted to a similar environment, or use the trait for a similar function. • In other words, they have both “converged” on the same adaptation in their own way. Bat wing Birds and bats have both evolved wings because they both use them to fly. Rather than “homology”, this is called an “analogous trait” or simply, an analogy. Evidence For Evolution: II. Homologous Anatomy Diagram 1 below illustrates how the eagle and the penguin evolved their wings from earlier birds. Diagram 2 shows how birds and bats evolved their wings separately. species A species B species A species B adaptation Homologous traits adaptation Diagram 1 adaptation Analogous traits Diagram 2 Evidence For Evolution: II. Homologous Anatomy Four legs: Homology or Analogy? Homology: This is a homologous trait because all of these species evolved from the earliest tetrapod, which also had four legs. Evidence For Evolution: II. Homologous Anatomy Two eyes: Homology or Analogy? Homology: This is a homologous trait because all vertebrates have two eyes. This allows some animals to see almost 360°, and it allows humans to see in 3D. Evidence For Evolution: II. Homologous Anatomy Protruding eyes: Homology or Analogy? Analogy: This is an example of convergent evolution because not all reptiles and amphibians have this trait. Frogs and alligators have evolved the same adaptation independently, but for similar use: to see above the water’s surface. Evidence For Evolution: II. Homologous Anatomy Analogy: Many placental mammal species have corresponding marsupial species, which occupy the same ecological niche on other continents. They are not related, however, and their similarities are due to similar environments and ways of life. This is, once again, convergent evolution at work. Marsupials Placentals All of the placental mammals are more closely related to each other than to any marsupial, and vice versa. Evidence For Evolution: II. Homologous Anatomy Evidence for evolution: Homologous traits Both homologous and analogous traits are excellent evidence for evolution, because they show how different species are related and how they evolved their adaptations for survival. Analogous traits Evidence For Evolution: III. Vestigial Structures Vestigial Structures Physical remnants of organs that once helped a species’ ancestors survive, but are no longer useful. • Vestigial structures tend to be very small, because they are not needed for survival. Hence, they are only “vestiges” of what they once were. • If a species has an anatomical structure that appears to be out of place, many times it is vestigial. • The human appendix is a good example of a “vestige” of an earlier digestive organ that helped our ancestors process plant material. Evidence For Evolution: III. Vestigial Structures Other human examples include the tailbone and wisdom teeth. Remnant of earlier primate ancestors with tails Leftover from our ancestors with larger jaws This is why wisdom teeth are so commonly removed. Modern humans’ jaws tend to be too small, so the teeth get impacted. Some people never even develop wisdom teeth. Evidence For Evolution: III. Vestigial Structures The wings on flightless birds are vestigial structures because they have shrunken over thousands of generations to the point where they can no longer be used for flight. Why did the wings “shrink” over time? Often, flightless birds are found on islands where their ancestors landed millions of years before. On an island with no predators, wings are not really needed for survival. In some cases, e.g. penguins, their smaller wings have since become useful in other ways. Evidence For Evolution: III. Vestigial Structures Though whales spend their entire lives underwater and have no hind limbs, they still have tiny leg bones and remains of a pelvis, which have shrunken to the point where they’re not even attached to the rest of the skeleton. In other words, whales have leg bones. What possible explanation makes sense unless they gradually evolved from earlier ancestors who had legs and lived on land? Evidence For Evolution: III. Vestigial Structures Rather than physical structures, some vestigial traits come in the form of reactions or behaviors. When you get goosebumps because you are startled or get a chill, your body is reacting in the same way as this cat. This reaction no longer serves any purpose in human beings but earlier mammal ancestors, as well as those still around, use it to puff up their fur for warmth, or to intimidate would-be predators. Evidence For Evolution: III. Vestigial Structures Here are a couple more examples: Similar to whales, pythons and boa constrictors have remnants of pelvic bones and legs. In this case, however, they are actually visible externally. Blind cave fish Pelvic spurs on a python Certain fish species which have lived in caves for thousands of years have evolved to have no eyes, since they are useless in the dark. Evidence For Evolution: III. Vestigial Structures • Even more tellingly, most species, including humans have huge sections of so-called “junk DNA” which seem to be completely non-functional. • Much of this “useless” genetic material is vestigial. In other words, it used to code for important traits in our ancestors, but is no longer needed. • This vestigial DNA has essentially become “deactivated” through genetic mutations over the course of millions of years, and is now just taking up space on the genome. Evidence For Evolution: IV. The Fossil Record The Fossil Record • A physical history of the life on Earth in the form of fossils buried within chronologically oriented layers of sedimentary rock. • Shows gradual progression, including transitional species. Tiktaalik Discovered in 2004, Tiktaalik is a fossil specimen that shows the transition between ancient fish and the first land-dwelling amphibians, around 380 million years ago. Evidence For Evolution: IV. The Fossil Record The fossil record of the modern horse is a great example of the continuous progression that shows gradual change over millions of years. Notice how the toes gradually get smaller and smaller, leaving only the middle toe, which becomes the foot and hoof of the modern horse. Evidence For Evolution: IV. The Fossil Record • The evolution of fourlimbed organisms, or tetrapods, shows a similar progression. • There are no giant leaps; only tiny, gradual incremental changes over millions of years. • New transitional fossils, sometimes called “missing links,” are continually being discovered. Evidence For Evolution: IV. The Fossil Record Ambulocetus (50 million years ago) Fossil evidence has demonstrated how whales have evolved from land mammals who walked on four legs. Rodhocetus (47 million years ago) Evidence For Evolution: IV. The Fossil Record Ancient land-dwelling ancestors • Once again, there is gradual progression in the fossil record, and new fossils are being discovered all the time. • The gradual change can even be seen in the evolution of the blowhole. Modern baleen whales Evidence For Evolution: IV. The Fossil Record Eupodophis Archaeopteryx (150 million years ago) Transition between dinosaurs and birds (92 mill.) Transition between lizards and the earliest snakes Evidence For Evolution: IV. The Fossil Record Australopithecus afarensis Dimetrodon (265 mill.) Transition between reptiles and early mammals (3.2 mill.) Transition between primitive ape-like hominids and more modern humans Evidence For Evolution: IV. The Fossil Record Thousands of hominid specimens have been discovered in the past century, allowing scientists to piece together the hominid family tree, extending back about 6 million years to our ape-like ancestors. Every newly discovered species is a transition between its ancestors and descendants on the tree of life. Evidence For Evolution: IV. The Fossil Record Duck-billed platypus Some modern species, such as the platypus still retain certain primitive traits, that link them to these early transitional “missing links.” The platypus belongs to a group of primitive mammals called monotremes. This group branched off from reptiles very early, and so these species still lay eggs like their reptile ancestors. Evidence For Evolution: IV. The Fossil Record • Similarly, mudskippers are amphibious fish which are able to gulp air and walk on land using their strong pectoral fins. • Not really a “living fossil” because the species evolved relatively recently Mudskipper • Still represents a modern look at the transition between sea and land, when the earliest amphibians evolved Evidence For Evolution: V. Embryology Embryology Very close similarities in early developmental stages of related species 1 cm Human embryo 5 weeks after conception Evidence For Evolution: V. Embryology Can you guess what type of animal this is? Evidence For Evolution: V. Embryology Can you guess what type of animal this is? Turtle embryo See the developing shell? Evidence For Evolution: V. Embryology Newly forming vertebrate embryos all look very similar so it’s difficult to tell what animal you’re looking at in the early stages of development. All vertebrates start out almost the same because they all share a common ancestor which was much more primitive. Turtle embryo • All vertebrates embryos start out with a tail and tiny neck slits resembling gills. This includes humans! • In land-dwelling groups such as mammals and reptiles, these slits disappear as the embryo develops, eventually becoming parts of the throat and mouth. • In humans, the tail is absorbed into the growing body and remains as the tailbone. Evidence For Evolution: V. Embryology early development (all very similar) middle stage embryo (starting to differentiate) fetal stage (identifiable as separate species) Evidence For Evolution: V. Embryology Notice how more closely related species tend to resemble each other further along in development. Click here to play “Which Embryo is Human?” Evidence For Evolution: V. Embryology Bottle-nosed dolphin Human Both embryos at about 5 weeks gestation! Evidence For Evolution: V. Embryology Bottle-nosed dolphin Notice the beginnings of hind leg buds which form early on and disappear later on in development. Evidence For Evolution: VI. DNA and Genetics DNA • Genetic code shared by all living things on Earth • Allows us to look at different species at the molecular level and compare their genes. • Shows conclusively that all living things are related; it’s just a matter of degree. Evidence For Evolution: VI. DNA and Genetics • Comparing our DNA allows us to determine which species are our closest relatives. • Humans and chimpanzees’ DNA are roughly 96% identical. human This evidence indicates that both chimps and humans received their identical genetic material from a common ancestor who lived about 6 million years ago. Evidence For Evolution: VI. DNA and Genetics apes “Great Apes” human hominids These primate groups are determined by how much homologous DNA they have in common. Evidence For Evolution: VI. DNA and Genetics • DNA evidence is some of the strongest evidence for evolution and the common ancestry of all living things on Earth. • It allows us to confirm at the molecular level the extent to which species are related AND estimate how long ago two species probably diverged from their common ancestor. • Molecular DNA evidence shows that humans and chimpanzees diverged from one another between 5 and 7 million years ago. • That common ancestor diverged from the gorilla family line even earlier, and so on down the family tree. Primatologist, Jane Goodall and one of her chimp friends Evidence For Evolution: VI. DNA and Genetics • All of the Great Apes have their DNA organized into 24 chromosome pairs, whereas humans have only have 23 pairs. • If humans share a common ancestor with all the other Great Apes, we should all have the same number of chromosome pairs because no one could survive if a whole section of DNA just disappeared! How is this possible? (24 pairs) (23 pairs) Evidence For Evolution: VI. DNA and Genetics If one chromosome pair wasn’t just lost, then what could have happened to it? What if two of the earlier ape chromosomes somehow got “stuck”, or fused, together during cell division at some point in the distant past? (24 pairs) (23 pairs) Is there any way to verify this? Evidence For Evolution: VI. DNA and Genetics Actually, it has been verified. • By examining the genes found on the ape chromosomes, scientists have found two ape chromosomes that contain the same genes that appear on ONE of our human chromosomes. • So it’s been conclusively demonstrated that they did get “stuck” together, and all modern humans have inherited that fused chromosome. (24 pairs) (23 pairs) Evidence For Evolution: VI. DNA and Genetics • This image shows the genes that match up between human and chimp DNA. • Note the two chimp chromosomes that match up to our own chromosome #2. This chart also shows how different sections of DNA have inverted themselves at multiples times in the past. This is one type of mutation. Evidence For Evolution: Click the image above to see biology professor Kenneth Miller explain the “Case of the Missing Chromosome”. Orangutan Gorilla Chimp This is extremely conclusive evidence of our common ancestry with all the other apes. Human VI. DNA and Genetics 1) What is Evolution? 2) Natural Selection 3) Evidence for Evolution Section 4 of 4: Evolution in Action 4) Evolution in Action 4) Evolution in Action Usually, the process of evolution takes place so slowly that we can’t see it happening. In selective breeding of livestock and hybridization of crops, humans have sped up and exaggerated this basic process so that we can actually observe the allele frequencies changing over shorter times. There are numerous examples, however, in which a species evolves rapidly enough on their own that we can see evolution in action. This usually happens because of one or both of the following: 1) A species reproduces very quickly so that numerous generations pass in a short time. 2) Selection pressure is very great. Bacteria reproduce extremely quickly, and many can double their population in under an hour. Bacteria can quickly evolve to be resistant to antibiotics if the drugs are not administered properly or for the entire duration of the infection. If any bacteria are not fully wiped out by the antibiotics, the more resistant ones that remain will continue to reproduce. An example of rapid evolution due to very high selection pressure is taking place among African elephants. As prized sources of ivory, they are being hunted closer and closer to extinction mainly for their tusks. • Since the elephants with the largest tusks are being hunted most, there is a heightened selection pressure (in this case, humans) that is wiping out large-tusked elephants. • The remaining population has higher frequency of smallertusked elephants, and some with no tusks at all! • So overall, it appears that elephant tusks are shrinking. Human evolution in action • Sickle-cell anemia is a genetic disorder in which the blood cells take on a shriveled sickle shape. • The disease can be life-threatening and generally tends to reduce life expectancy significantly. • The disease is treatable. In America, it affects about 90,000 people, while there are over 2 million genetic carriers who have one recessive sickle-cell allele, but do not have the actual disease. It has long been known that the frequency of the disease widely varies between certain ethnicities, and occurs at a much higher ratio among those of African descent. It appears that the frequency tends to correspond to people originating in more tropical climates. What might explain this curious connection? Incidence of the sickle-cell trait in Africa Distribution of Malaria Incidence of the sickle-cell trait in Africa In 1954, a scientist named Tony Allison found the clue when he realized how closely the frequency of sickle-cell anemia matched the distribution of malaria in Africa. • To explain the extremely high frequencies of the sickle-cell allele, Dr. Allison figured that the genotypes leading to the disease must be giving some sort of survival advantage to those who have the allele (including carriers of the disease). • Since the sickle-cell allele seemed to be geographically linked to the distribution of malaria, he hypothesized that the sickle-cell gene must somehow increase the carrier’s resistance to malaria. With Watson and Crick’s recent discovery of the structure of DNA, new molecular evidence soon confirmed Dr. Allison’s hypothesis. • Since malaria is a much bigger threat to survival than sickle-cell anemia, the sickle-cell trait had become prevalent in the areas where its benefits outweighed the negative effects. • Dr. Allison was able to show that the same mutation that led to the sickle-cell trait had occurred at several points in the past in different locations. But it only became prevalent in areas where malaria was widespread. This was an excellent confirmation of the process of evolution occurring in real-time in ways that directly affect humans. Table of Contents 1) What is Evolution? 2) Natural Selection 3) Evidence for Evolution 4) Evolution in Action