Published on February 12, 2008
Lecture roadmap: Lecture roadmap Evolution: history and background Darwin’s Theory of Evolution Natural selection as a mechanism of adaptive evolution Evidence for evolution Fossil record Biogeography Comparative anatomy Comparative embryology Molecular biology Examples of natural selection in action Population genetics Hardy-Weinberg equilibrium Genetic drift Gene flow Variation and natural selection The blue footed booby: Large, blue, webbed feet Great flippers, propel bird’s body through water at high speeds. Clumsy on land, but gets food in the water. Streamlined body shape Torpedo-like body shape minimizes friction when diving into water from great heights. Large tail Allows birds to pull themselves back out of the high-speed dive once they hit the water. Nostrils that close when underwater To prevent water from entering their lungs during dive. Glands that secrete oil Keeps their bodies waterproof. Glands that secrete concentrated salt solution Prevents salt level in their bodies from reaching dangerous levels. The blue footed booby The blue footed booby: All of the features listed on the previous page are called evolutionary adaptations: Inherited traits that enhance an organism’s ability to survive and reproduce in a particular environment. In this chapter we’ll begin to learn about the process of evolution: the changes in organisms over time. The blue footed booby Pre-Darwinian ideas : Pre-Darwinian ideas 18th and 19th centuries: evidence for evolution accumulates. Georges Buffon (late 1700’s) Geologist who analyzed fossil data, suggested the world was much older than 6,000 years. Observed similarities between fossils and living organisms, suggesting that fossil forms might be ancient versions of similar living species. Jean Baptiste Lamarck (early 1800’s) French naturalist Believed best explanation for fossil evidence was that life evolved. Proposed that all organisms had been created in a simple state and were improved by gradual changes into more complex structures through a “drive for perfection.” Proposed incorrect mechanism for how evolution occurred, known as “inheritance of acquired characteristics.” Lamarck & the “inheritance of acquired characteristics” model: Lamarck & the “inheritance of acquired characteristics” model Inheritance of acquired characteristics model proposed that evolution occurs when an organism uses a body part in such a way that it is altered during its lifetime and this change is then inherited by its offspring. If this were true, working-out and building up big muscles would mean that your children will have big muscles . . . . not true. Lamarckian evolution Darwinian evolution (through natural selection) Pre-Darwinian ideas : Pre-Darwinian ideas 18th and 19th centuries: Geologic Timeframe begins to be more understood through fossil record and geology. Georges Cuvier (early 1800’s) French naturalist Tried to explain existence of fossil forms unearthed in the construction of canals and mines by a theory stating that great catastrophes had wiped out very ancient species. Still believed that species were fixed and could not evolve. Charles Lyell (early 1800’s) Scottish geologist, wrote Principles of Geology Geologic data provided evidence for a very old earth that has been subjected to natural forces that gradually change over time, such as erosion, earthquakes, glacial movements, volcanoes, etc. Darwin’s famous sea voyage: Darwin’s famous sea voyage Went around the world at age 22 on the HMS Beagle (in the 1830’s). Collected thousands of specimens of fossils and living plants and animals. Observed similarities between living and fossil organisms and the diversity of life on the Galápagos Islands. • these experiences helped him frame his ideas about evolution. Darwin proposed natural selection as the mechanism of evolution: Darwin proposed natural selection as the mechanism of evolution Natural selection: differential (unequal) success in reproduction by different phenotypes as a result of interactions with the environment. Evolution occurs when natural selection produces changes in the relative frequencies of alleles in a population’s gene pool. Darwin proposed natural selection as the mechanism of evolution: Darwin proposed natural selection as the mechanism of evolution Natural selection: 3 important observations led Darwin to natural selection as a mechanism of evolution: 1) overproduction of offspring in an environment where resources are limited. 2) extensive variability in characteristics among individuals of a population. 3) many of the varying traits are inherited. RESULT: traits that are favored are represented more and more and unfavored ones less and less in ensuing generations of organisms Artificial selection supported the idea of natural selection as a mechanism of evolution: Artificial selection supported the idea of natural selection as a mechanism of evolution Artifical selection: the selective breeding of domesticated plants and animals. Humans selected plants and animals with desired traits, playing the role of the environment. Resulted in modification of species from their wild ancestors. Mechanism is similar to natural selection, but sped up by human artificial intervention. broccoli, cauliflower, kale, cabbage, and brussels sprouts are all varieties of a single species of wild mustard Natural selection operates over vast spans of time to produce heritable changes that permit the evolution of new species: Natural selection operates over vast spans of time to produce heritable changes that permit the evolution of new species The 2 main ideas in Darwin’s theory of evolution:: The 2 main ideas in Darwin’s theory of evolution: Diverse forms of life arose by descent with modification from ancestral species. The mechanism of modification has been natural selection working over very long spans of time. Evidence for evolution of life: Evidence for evolution of life Fossil record: the ordered array in which fossils appear within layers of sedimentary rocks. Biogeography: the geographic distribution of species. Comparative anatomy: the comparison of body structures in different species. Comparative embryology: the comparison of early (embryonic) stages of development. Molecular biology: DNA sequence data provides a molecular history of evolution How are fossils made?: Most organic substances in a dead organism usually decay rapidly, but the hard, mineral-rich parts (bones, teeth, shells) may remain as fossils. Petrification of remains: when minerals in the groundwater seep into tissues of dead organisms and replace organic matter. Rocks can act like a mold or cast when a dead organism is captured in sediment and then decays. Footprints, burrows, etc. are remnants of ancient organisms’ behavior. Some organic material can be preserved if captured in a medium that keeps out decomposing bacteria and fungi How are fossils made? Petrified tree Skull of Homo erectus Ammonite casts Dinosaur tracks “Ice Man” Insect in amber Fossilized organic matter of a leaf The study of fossils provides strong evidence for evolution: The study of fossils provides strong evidence for evolution Changes in sea level, the drying and refilling of lakes, and the movement of glaciers affect sediment. Each rock layer therefore represents a specific “slice of time” that can be dated by geologists. This is the fossil record. Fossils are specific to certain layers of rock that provide information about when and where an organism lived. As you go from older to younger layers of rock, you progress through evolution from the oldest species to the youngest. Prokaryotes first, then eukaryotes. Within the vertebrate animals, fishlike fossils are oldest, then amphibians, then reptiles, then mammals and birds. 1 2 3 The study of fossils provides strong evidence for evolution: The study of fossils provides strong evidence for evolution Linkages between ancient extinct organisms and species alive today are also found in the fossil record. EX#1: fossils that document the change in skull shape as mammals evolved from reptiles. EX#2: fossils that link whales living today to the four-legged land mammals from which they evolved. Extinct whale whose hind legs link living whales with their land-dwelling ancestors. These were already aquatic animals--other fossil whales have longer limbs and spent only part of their time in the water. Evidence supporting evolution, cont.: Evidence supporting evolution, cont. Biogeography (geographic distribution of species) Supported the idea that organisms evolve from common ancestors. Patterns of distribution of species and level of similarities between species mirrors historical distribution of land. Island biogeography: Island inhabitants (like those found by Darwin on the Galápagos islands) are most similar to organisms on the nearest continent. The animals on the Galápagos resembled species of the South American mainland more than animals on similar but distant islands. As the organisms became isolated on an island, they adapted to the specific environment of the island. Evidence supporting evolution, cont.: Evidence supporting evolution, cont. Comparative anatomy (the comparison of body structures in different species) Homology: the similarity in characteristics that results from having common ancestry Homologous structures: features that often have different functions but are structurally similar because of common ancestry. Homologous structures in the tetrapod limb. The various bones have been modified in different ways to fit their functions for that organism. Evidence supporting evolution, cont.: Evidence supporting evolution, cont. Comparative embryology the comparison of early (embryonic) stages of development among different organisms. Vertebrate embryos have similar structures during development. The earlier the stage of development, the more similar the embryonic structure. Evidence supporting evolution, cont.: Evidence supporting evolution, cont. Molecular biology the comparison of DNA and amino acid sequences between different organisms. Reveals evolutionary relationships. The more similar two organisms DNA and amino acid sequences are, the more recently they diverged from a common ancestor. Examples of natural selection in action:pesticide resistance in insects: Examples of natural selection in action: pesticide resistance in insects When a pesticide is first used to kill pests, a small amount is sprayed on a crop, and this kills almost all the insects. The few surviving insects have genes that somehow enabled them to survive the poison. These surviving insects reproduce and pass their pesticide resistance genes on to future generations. Eventually, the proportion of pesticide-resistant insects increases and the pesticide is less and less effective. Figure 13.5B Populations are the units of evolution: Populations are the units of evolution Population: a group of individuals of the same species living in the same place at the same time. Species: a group of populations whose individuals can interbreed and produce fertile offspring. Populations, not individual organisms, evolve. Population genetics: studies how populations change genetically over time. Gene pool: the total collection of genes in a population at any one time. Includes all the alleles (different forms of genes) in all the individuals making up a population. Changes in the relative frequencies of alleles in a gene pool is evolution occurring on its smallest scale Slide23: Microevolution: evolution on its smallest scale; changes in the relative frequencies of alleles in a population over generations. Macroevolution: evolutionary change on a grand scale, including the origin of new species and large-scale evolutionary trends. To understand how evolution occurs in a population, it is useful to first examine a nonevolving population: To understand how evolution occurs in a population, it is useful to first examine a nonevolving population The shuffling of alleles that occurs as a result of sexual reproduction does not lead to evolution. The frequency of alleles in a population will remain constant unless acted on by other forces. This principle is called the Hardy-Weinberg equilibrium Understanding Hardy-Weinberg equilibrium: Understanding Hardy-Weinberg equilibrium Webbing No webbing Figure 13.7A Imaginary, nonevolving population of blue- footed boobies: Individuals differ in foot webbing Foot webbing is controlled by a single gene Allele for non-webbed feet (W) is completely dominant to allele for webbed feet (w). Understanding Hardy-Weinberg equilibrium: Understanding Hardy-Weinberg equilibrium Following two generations in the imaginary blue-footed booby population: 500 boobies, 320 are WW, 160 Ww, 20 ww. Genotype frequencies are calculated by the proportion of each genotype (divide by total # of boobies). Can calculate allele frequencies from genotype frequencies. Add total number of each allele found in each genotype, then divide by total number of alleles in all the boobies (1000). The frequency of the dominant allele is called “p.” The frequency of the recessive allele is called “q.” Understanding Hardy-Weinberg equilibrium: Understanding Hardy-Weinberg equilibrium We can follow the allele frequencies in a population over a generation to test for Hardy-Weinberg equilibrium. Punnett square uses gamete frequencies and the rule of multiplication to calculate the genotype frequencies in the next generation. Can see that the allele frequencies don’t change --they’re the same in the next generation as they were in the parent generation. This population is in Hardy-Weinberg equilibrium. For a population to be in Hardy-Weinberg equilibrium, it must satisfy 5 main conditions:: For a population to be in Hardy-Weinberg equilibrium, it must satisfy 5 main conditions: The population is very large. There is no gene flow. • meaning, there’s no migration of individuals or gametes into or out of the population. Mutations do not alter the gene pool. Mating is random. All individuals are equal in reproductive success • meaning, natural selection is not occurring in the population. When populations deviate from the conditions for Hardy-Weinberg equilibrium, the gene pool changes, and evolution occurs.: When populations deviate from the conditions for Hardy-Weinberg equilibrium, the gene pool changes, and evolution occurs. 4 main causes of evolutionary change are: Genetic drift Gene flow Natural selection of these, only natural selection contributes to adaptive evolution. (genetic drift and gene flow change the gene pool, but don’t lead to a population becoming better adapted to its environment.) Mutation (but, over a very long time . . . unlike the other 3 causes of evolutionary change) - Mutations are rare and change allele frequencies very little to from one generation to the next--but over time, they become important. Genetic drift: Genetic drift Genetic drift: a change in the gene pool of a population due to chance. Can alter allele frequencies in a population. The smaller the population, the more impact genetic drift is likely to have. 2 situations that can allow genetic drift to have a large impact by shrinking a population:: 2 situations that can allow genetic drift to have a large impact by shrinking a population: The bottleneck effect: when a population is drastically reduced in size. • Small surviving population has different allele frequency than original population. - Some alleles become more frequent, some less, and some are lost completely. • EX: elephant seals: were hunted down to only 20 seals in the 1890s; now, protected population is back up to 30,000, but when 24 gene loci were examined, researchers found no variation. (unlike the high genetic variation in the close relative southern elephant seal). The founder effect: results from the colonization of a new location by a small number of individuals. • Can be seen in divergence of finches that migrated to the Galapagos islands from South America. Gene flow: Gene flow The movement of individuals or gametes (such as plant pollen) between populations. Can alter allele frequencies in a population. Natural selection: Natural selection Leads to differential reproductive success in a population. Changes allele frequencies in a population. Result is the accumulation of traits that adapt a population to its environment. Natural selection can alter variation in a population in three ways : Natural selection can alter variation in a population in three ways Stabilizing selection Favors intermediate phenotypes. Directional selection Acts against individuals at one of the phenotypic extremes. Disruptive selection Favors individuals at both extremes of the phenotypic range. A patchy environmental condition can promote disruptive selection. Natural selection can alter variation in a population in three ways : Natural selection can alter variation in a population in three ways Sexual selection is a special type of natural selection: Sexual selection is a special type of natural selection Sexual selection leads to the evolution of secondary sexual characteristics These characteristics are not directly involved with reproduction itself, but: they may give individuals an advantage in mating. Two different types of sexual selection: One type of sexual selection involves a trait to attract the opposite sex EX: colorful feathers on the tails of male peacocks. Another type of sexual selection involves a trait that may be used to compete with members of the same sex for mates. EX: antlers on deer. Figure 13.17A Slide37: The evolution of antibiotic resistance in bacteria is a serious public health concern. The excessive use of antibiotics Is leading to the evolution of antibiotic-resistant bacteria Antibiotics added to animal feed Overprescription of antibiotics by doctors Misuse of antibiotics (not finishing prescriptions). 0 Colorized SEM 5,600 Figure 13.13 Some strains of the tuberculosis-causing bacterium are now resistant to all three of the antibiotics commonly used to treat TB. Genetic variation: Genetic variation Most populations exhibit genetic variation. Endangered species often have reduced variation. As a result of the bottlenecking effect, a population that has become severely reduced loses genetic variability. Even if the population increases in number again, it is still vulnerable because it has a reduced capacity to adapt to environmental changes. EX: cheetah numbers fell drastically during last ice age ~10,000 years ago (hunting, disease, drought). Studies of the African cheetah population shows that they have even less genetic variability than some highly inbred laboratory mice. Figure 13.10 Sources of genetic variation: Sources of genetic variation Mutations: changes in the nucleotide sequence of DNA. Can create new alleles. Mutations in alleles can sometimes be harmful in one environmental condition can be advantageous in a new environmental condition. EX: mutations that give flies resistance to the pesticide DDT also slow their growth rate. Before DDT was introduced, this mutation was a handicap to the fly’s growth, but once DDT was in the environment, now these mutant alleles were advantageous, and natural selection increased their frequency in the fly populations. Sources of genetic variation: Sources of genetic variation 2) Sexual reproduction Independent assortment of homologous chromosomes in meiosis I. Crossing over during meiosis I. Random fertilization (which sperm fertilizes which egg is random). Diploidy and balancing selection preserve variation: Diploidy and balancing selection preserve variation Natural selection tends to reduce the frequency of unfavorable alleles--so why aren’t these alleles completely eliminated? 1) Diploidy A recessive allele is only subject to natural selection when it influences the phenotype. Having two chromosomes allows an unfavorable recessive allele to be hidden, or protected from natural selection. These alleles may prove to be beneficial in later generations if the environment changes. 2) Balancing selection Natural selection generally reduces variability, but sometimes it acts to preserve it. With balancing selection, natural selection maintains two or more different phenotypes in a population. 2 types of balancing selection: 2 types of balancing selection 1) heterozygote advantage When heterozygous individuals have greater reproductive success than homozygous individuals. Result: natural selection will maintain 2 or more alleles for a characteristic. EX: Sickle-cell anemia allele protects against malaria in heterozygous individuals. The frequency of this allele in Africa is highest in areas where malaria is a major cause of death. Therefore, this environment favors heterozygotes. 2) frequency-dependent selection When the survival and reproduction of any one phenotype declines if that phenotype becomes too common in the population. EX: a species of butterfly with several color patterns: if bird predator develops a “search image” for one pattern that is more common, the butterflies with this pattern will be eaten more. Neutral variation will not be affected by natural selection: Neutral variation will not be affected by natural selection Neutral variation: genetic variation that provides no apparent selective advantage for some individuals over others Neutral alleles may randomly increase or decrease their frequency in the gene pool, but this is not due to natural selection. An variation that is neutral in one environment may not be neutral in a different environment, so this variation is still important. EX: human fingerprints, hair color. Figure 13.14 The perpetuation of genes defines evolutionary fitness: The perpetuation of genes defines evolutionary fitness Evolutionary definition of fitness: the contribution an individual makes to the gene pool of the next generation. The individuals with the greatest evolutionary fitness are those that produce the largest number of viable, fertile offspring. Natural selection won’t result in “perfect” organismsWHY IS THIS?: Natural selection won’t result in “perfect” organisms WHY IS THIS? 1) Organisms are limited by historical constraints-- selection can only edit existing variations. Evolution doesn’t build new organisms from scratch; it adapts existing structures to new environments. 2) Adaptations are often compromises EX: having flexible limbs and joints can increase athleticism, but also contribute to increased tendency toward sprains and torn ligaments. 3) Chance and natural selection interact. EX: if a windstorm blows insects over an ocean onto an island, the wind doesn’t necessarily transport the bugs that are best suited to the new environment--it’s random.