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Evolution What is evolution? • The process by which modern organisms have descended from ancient organisms. • In other words, species are not constant, they change over time. Paradoxophyla palmata, a narrow-headed frog native to Madagascar. The frog's brown and yellow coloring, as well as its rough texture, allow it to blend in with the mud and tree trunks in its environment. tartan hawkfish: The fish's striking coloration allows it to blend in with these bright gorgonian fans. cryptic frog - This species has developed a coloring, texture and form that are similar to the leaves found in its environment. Evidence for evolution • Fossils • Embryo similarities • Body structure similarities – – – – Homologous Structures Vestigial Structures Convergent Evolution Analogous Structures • Molecular evidence I. Fossils • Fossils are the preserved remains of ancient organisms. How are fossils formed? • • • • • Preservation in sap Mineral replacement Preservation in ice Traces e.g. footprints Molds • Fossils demonstrate the existence of intermediate forms of species, thus demonstrating evolution. • The fossil record demonstrates that more primitive species preceded more recent species • e.g. fish precede amphibians, which precede reptiles, which precede mammals. Dating Fossils • Relative dating is the method of dating fossil specimens relative to other fossils in in different rock layers. • Older fossil are in deeper rock strata. Radiometric Dating • Radiometric dating is the method of dating fossils based upon rates of the decay of radioactive elements. Radioactive Parent Stable Daughter Potassium 40 Argon 40 Rubidium 87 Strontium 87 Thorium 232 Lead 208 Uranium 235 Lead 207 Uranium 238 Lead 206 Carbon 14 Nitrogen 14 Half-Life’s • Half-life’s determine how long it takes for a radioactive element to decay into a stable daughter element. • A half-life is defined as the amount of time it takes for ½ of the atoms in a radioactive element to decay. • For example, the half-life of uranium238 is 4.5 BY. • The half-life of potassium40 is 1.3 BY • And the half-life of carbon14 is about 5’600 years. II. Embryo Similarities • Although certain adult organisms may be very different from each other, a comparison of the early stages of their embryonic development may show similarities that suggest a common ancestry. • For example, the early embryos of fish, birds, pigs and humans closely resemble one another. Note the presence of gill slits and a fish-like tail in all of the embryos. III. Body Structure Similarities • Anatomical similarities between different species suggest evolution from a common ancestor with modification. • If organisms in an area evolve to have structures that are best suited for survival in that environment, then it makes sense that different species in that area will have similar structures. Convergent Evolution: two unrelated species have similar form • • Example: a whale and a shark. Example : different types of trees in the rainforest will have similar shaped leaves (they come to a point) to allow water to flow off of them. Analogous structures: Structurally different, but used for similar functions. -Example: the wings of a fly and the wings of a bird Homologous structures: Body parts in different organisms that have similar bones and similar arrangements of muscles, but do not necessarily serve the same function. • Vestigial organs are remains of a structure that was functional in some ancestor but is no longer functional in the organism in question. • For example, humans have a tail bone (the coccyx) but no tail. What else do humans have that we don’t need? A vestigial structure in the whale is the pelvic bone Whale Evolution A reconstruction of an early close cousin of whales. IV. Molecular Evidence • DNA, RNA, amino acids and proteins have all been used to determine evolutionary relationships between organisms. • For example, differences between amino acid sequences in the protein hemoglobin in various species can be used to construct an “evolutionary tree”. • The greater the differences in amino acids, the more distant two species are. • The numbers represent the number of amino acid differences in the hemoglobin of humans and the hemoglobin’s of the other species. Human Hemoglobin (beta) Gorilla Gibbon 0 1 2 Rhesus monkey Dog Horse, cow 8 15 25 Mouse Gray kangaroo Chicken 27 38 45 Frog Lamprey 67 125 Sea slug (a mollusk) 127 Soybean (leghemoglobin) 124 A phylogenetic "tree of life" Theories of Evolution • The first comprehensive theory of evolution was proposed by Jean Lamarck in 1809. • Lamarck believed that evolution was driven by an innate tendency toward greater and greater complexity as organisms became perfectly Jean Lamarck (1744-1829) adapted to their environments. Lamarcks two part theory: • A. Theory of use and disuse, which stated that those parts of the body that were used extensively would become larger and stronger, whereas those parts not used would waste away. • B. The second part of Lamarck’s theory was called inheritance by acquired characteristics. • According to this concept, the modification an organism acquired in its lifetime could be passed on to its offspring. • The classic example of this is the giraffes long neck. • Can you think of any reasons why Lamarck's theory is flawed? Weismann’s experiment • In the 1880’s Weismann challenged Lamarck’s theory. • Weismann cut the tails off 22 consecutive generations of mice. • Even though 22 generations of mice could not use their tails, what was true about the 23rd generation? August Weissmann • Who was responsible for articulating a theory that explained how evolution actually worked? • Charles Darwin • In 1859, Darwin published a book entitled The Origin of Species by Means of Natural Selection. • Two themes in this famous book were: – A. Common descent: which meant that all species have descended from a single common ancestor. – B. Adaptation: which refers to modifications that enable organisms to be better suited to their environment, that is better able to survive and reproduce. The Darwinian Revolution • Charles Darwin, born in 1809 in Shrewsbury England, came from a family of doctors. Having studied medicine from 1825 to 1828 at Edinburgh University he abandoned his medical pursuit to study theology instead. A mediocre student, he was more interested in botany and Charles Darwin geology. (1809-1882) • One of the most important influences that helped shape Darwin’s view of life was his voyage aboard the HMS Beagle (1831-1836). One of the most important visits Darwin made on his voyage was to the Galapagos Islands. • Although, many of the animals on the Galapagos resembled mainland species, they were unique. • Darwin puzzled over how such species could colonize the islands and become different from the mainland species. Darwin’s Finches Among the thousands of specimens Darwin collected, were 13 species of finches. The finches were unique in their beak adaptations. • Darwin would only later propose that the Galapagos finches evolved on the islands from a single species of finch from mainland South America. Charles Lyell’s influence • While on board the Beagle, Darwin read Charles Lyell’s Principles of Geology. •In the book, Lyell made the argument that the Earth was very old and that it’s landscape changed slowly and gradually. • By acknowledging the Earth was indeed very old and constantly changing, Darwin recognized that organisms had to slowly adapt to these changes. That is organisms had to evolve. The influence of Thomas Malthus • In An Essay on the Principle of Population, written in 1798, Thomas Malthus predicted that the human demand for food would inevitably surpass its supply and many people would die of starvation. • This prediction was based on the idea that human population increased at a exponential rate ( 24-8-16…)while the food supply grew at the slower linear rate (1-23-4-5…). • Darwin applied Malthus’ idea to animals in nature and argued that since most animals overreproduce, limited food supplies causes a “struggle for existence”. • That is, there is competition for resources, so that only the best adapted survive to reproduce. The long delay… • Upon returning to Great Britain in 1836, Darwin began to ponder over how the many unique species adaptations he had observed could have originated. • By the early 1840’s, Darwin had worked out the major features of his theory of natural selection as the mechanism of evolution. • Chronic illness and perhaps fear of criticism from the clergy however, had prevented Darwin from publishing. Competition from another naturalist……. • Then in 1858, Darwin received a letter from a young naturalist called Alfred Wallace. The letter was accompanied by a theory of evolution. Alfred Russel Wallace (1823-1913) • This encouraged Darwin to publish his book, The Origin of Species. • Darwin published in 1859. • In his book, Darwin avoids using the word evolution until the last page. The only illustration in Darwin’s book • The theory outlined in Darwin’s book was called natural selection. • Natural selection occurs when there are differences in individuals abilities to survive and reproduce based upon inheritable traits. • • • • • How does evolution work according to Darwin? The Theory of Natural Selection: More offspring are produced than actually survive due to limited resources (Malthus). This causes a “struggle for existence”. Survival is not random, but depends on hereditary factors. Those individuals with favorable inheritable traits will survive and reproduce. Those with less favorable inheritable traits will be eliminated. This will lead to a gradual change in the population, with favorable hereditary variations accumulating over time i.e. the species will change. “I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection”. • • • • What have we learned since Darwin? The modern theory of evolution recognizes gene mutations (point and frame-shift mutations) and how they introduce inheritable variations into a population. Occasionally, a mutation is advantageous and will be favored by natural selection. An individual born with an advantageous mutation will survive and reproduce better than its competitors. In time, the new mutation will spread through the population and the species will evolve. Global warming threatens millions of species • A major international study has warned that global warming may drive 25% of land animals and plants to the edge of extinction by 2050 (NewScientist). • Why can’t species adapt to increased global temperatures? Observed cases of natural selection • • • • Peppered moth (Industrial Melanism) Insects resistance to insecticides Bacterial resistance to antibiotics Increased frequency of sickle cell anemia in Africans. Case Study I: Industrial Melanism • In England, there are two varieties of the peppered moth (Biston betularia), a dark variety and a light variety. • Prior to the industrial revolution, there was a much higher frequency of the light variety of the peppered moth, which, were adapted to the light colored lichen on tree bark. • However, industrial pollution in the 1800’s began to kill the lichen, turning the tree bark into a dark color. • Now, the number of dark variety of peppered moth increased i.e. were naturally selected. Case study II: Resistance to Insecticides Suppose that there is a crop infested by a large population of insects. Note the genetic variation that exists. Insecticides are sprayed in order to protect the crop. Note how some insects have the inherent ability to be resistant to the insecticide. Those insects with inheritable traits that enable them to be resistant to the insecticide survive and reproduce. In time, the insecticide is no longer effective. The insects have evolved by natural selection. Case study III: Bacterial resistance to antibiotics Staphylococcus aureus • In 1943, penicillin was introduced as an antibiotic to protect against Staphylococcus infections. • By 1946, a number of strains of Staphylococcus demonstrated resistance to penicillin. • Today, as many as 80% of Staphylococcus aureus are resistant to penicillin. a. Gene for antibiotic resistance in plasmid b. The antibiotic gene/plasmid is replicated and transferred to another bacterial thereby conferring antibiotic resistance to it. • In a population of bacteria, there may be a few individuals with mutant genes that provide antibiotic resistance, these bacteria will survive and reproduce. • Natural selection will favor these bacteria. Case study IV: Sickle Cell Anemia • Read: Recall that sickle cell anemia is a fatal disease resulting from homozygous recessive alleles which code for a part of hemoglobin, the oxygen transporting molecule in human blood • Despite the lethality of the allele, it occurs at frequencies as high as 40% in some parts of tropical Africa. (By contrast it occurs at less than 5% in African Americans and 0.1% in Caucasian Americans) • Since those people who are heterozygous for sickle cell are protected against malaria, natural selection has favored those who are carriers in areas where there is malaria. (like Africa) Speciation Speciation • How are new species formed? • In order for a new species to form, one population must break away from another and be reproductively isolated. • In other words, genes can’t be exchanged between both populations. • Over time, the populations will experience different mutations and be subject to different kinds of natural selection. • When this happens, the populations can no longer interbreed. A new species has formed. • How are two populations reproductively isolated? • Two populations may be isolated by the formation of mountains, rivers, or land masses separating (Pangea). Continental Drift • The idea was thought up by Alfred Wegener in the early 1900’s. • It was based upon the observation that continental coastlines often match up. 14 BIOL 1010 – Ch 25 Continental Drift and Evolution New World and Old World monkeys are • New World and Old classified into two World monkeys are distinct groups. taxonomically groups; The different ancestors to these • These lineages two groups separated afterevolved Pangeaseparately broke up. on different continents after Pangea broke up Fig 34.35 24 BIOL 1010 – Ch 25 • When the Grand Canyon formed, two related species of squirrels evolved. Can you explain how different species of Darwin’s Finches evolved now? Chance events can affect the evolution of species. 65 MYA a meteor five miles wide struck the earth 28 BIOL 1010 – Ch 25 • As a result of this impact, all the dinosaurs became extinct as well as many other species including 50% of all marine organisms. Would primates have evolved? • The mammals were a small, specialized group of organisms during the Cretaceous • The big mammal divergence occurred after the dinosaurs went extinct • Mammals probably would not have diverged, producing primates (and humans!) if the meteor had not struck the planet! 29 BIOL 1010 – Ch 25 Fig 25.19 The Evolution of Populations • • • • Populations evolve, individuals don’t! Although natural selection acts on individuals, by way of differential survival and reproduction, individuals do not evolve. Biological evolution is only evident by examining the impact of natural selection on a population of individuals. Technically, evolution results from the change in gene frequencies within a population overtime. Therefore, in order to understand evolution, it would be important to describe those events that change gene frequencies in a population. Darwin’s dilemma • Although Mendel had written to Darwin explaining his discoveries for heredity, Darwin apparently never read Mendel’s thesis. • Indeed, natural selection requires a hereditary process that Darwin could not explain. Mr. Darwin still hasn’t written back! A major flaw in my theory is explaining how adaptive inheritable traits are passed on. Population Genetics • When Mendel’s laws were rediscovered in the early 20th century, geneticists were perplexed about how Mendel’s discrete “either-or” traits could be used to explain the continuous variation within populations that natural selection acted on. • The birth of population genetics, which emphasizes extensive genetic variation within populations and recognizes the significance of quantitative traits, helped to improve this uncertainty. The modern synthesis: A comprehensive theory of evolution • By the early 1940’s, the discoveries and ideas from many scientific fields including paleontology, taxonomy, biogeography and population genetics were integrated into what became known as the modern synthesis. • The modern synthesis emphasized the importance of populations as the units of evolution, the central role of natural selection as the main mechanism of evolution and the idea that evolution is a slow and gradual process (as we will see later, some aspects of this model are now challenged). • • • • The genetic structure of a population. Some basic definitions: A population is a localized group of individuals belonging to the same species. A species is a group of populations whose individuals can potentially interbreed. Although members of a species may be geographically isolated from one another, the integrity of the species is maintained by interbreeding members of adjacent populations. A gene pool is all of the genes in a population at any one time. It consists of all alleles at all gene loci in all individuals of the population. Measuring gene frequencies in populations: the first step to understanding generation-to- generational change in a population’s allele and genotype frequencies • We will not get into the math… it’s complicated! What causes gene frequencies to change in a population i.e. evolution? • Ironically, two scientists (Hardy and Weinberg) addressed this question by first describing how gene frequencies could remain constant over time, essentially describing the circumstances necessary for a non-evolving population. • The Hardy-Weinberg theorem states that the frequencies of alleles and genotypes in a population’s gene pool remain constant over the generations unless acted upon by agents other than sexual recombination. • In other words, the sexual reshuffling of alleles due to meiosis and random fertilization has no effect on the overall genetic structure of a population. • • • • • • Microevolution What conditions are required for gene frequencies to remain constant and for the Hardy-Weinberg equilibrium to be maintained in a population? 1. Very large population size. 2. No gene flow (population isolation). 3. No mutations. 4. Mating must be random. 5. No natural selection. • Are each of these condition met? Each one of the conditions required to maintain Hardy-Weinberg equilibrium is violated! • Are all populations large? – Although many populations are, some are small enough for random events to change gene frequencies. • This microevolutionary phenomena is called genetic drift. • Bottleneck Effect: • In this case, disasters such as floods and fires can drastically reduce the size of the population, leaving by chance, individuals that are not necessarily representative of the original population. • Founder Effect • This occurs whenever a few individuals colonize a new habitat. • The founding population is usually small. • Their gene pool may not be representative of the entire gene pool they left. • In the Amish, in fact, Ellis- van Creveld syndrome has been traced back to one couple, Samuel King and his wife, who came to the area in 1744. The mutated gene that causes the syndrome was passed along from the Kings and their offspring, and today it is many times more common in the Amish population than in the American population at large. Polydactyly Gene flow occurs • Populations may gain or lose alleles by the migration or immigration of individuals, seeds, pollen etc. • For example, a wind storm may blow pollen from an aa population into a population consisting of just AA individuals. Mutations occur…… • A new mutation that is transmitted in gametes can immediately change a gene pool of a population. • In fact, rates of one mutation per locus per 105 to 106 gametes is typical for most species. Mating is nonrandom • Inbreeding, especially among plants (i.e. selfpollination) increases the frequency of homozygous genotypes at the expense of heterozygous genotypes. • Another type of non-random mating is assortative mating, in which individuals select mates that are phenotypically similar. • For example, tall women prefer tall men. • In fruit flies, studies have demonstrated although 4% of all females fail to mate successfully, 20% of all males fail to mate. Snow geese demonstrate assortative mating Differential reproductive success (natural selection) occurs in nature. • See the study of finch beak size. Three modes of natural selection • Stabilizing Selection • Directional Selection • Diversifying Selection • Stabilizing selection – Favors the intermediate phenotype out of a range of phenotypes. – The extremes in variation are selected against. – For example, infants weighing significantly less or more than 7.5 pounds have higher rates of infant mortality. – Selection works against both extremes. • Directional selection – Favors phenotypes at one extreme of the range of variation. – Insecticide resistance is an example. DDT was a widely used insecticide. After a few years of extensive use, DDT lost its effectiveness on insects. Resistance to DDT is a genetic trait that the presence of DDT in the environment made into a favored trait. Only those insects resistant to DDT survived, leading over time to populations largely resistant to DDT. • Diversifying selection – favors individuals at both extremes of variation: selection is against the middle of the curve. – This causes a discontinuity of the variations, causing two or more morphs or distinct phenotypes. – The African swallowtail butterfly (Papilo dardanus) produces two distinct morphs, both of which resemble brightly colored but distasteful butterflies of other species. Each morph gains protection from predation although it is in fact quite edible. Are Humans Exempt from Natural Selection? • It has been argued that advances in medicine, sanitation, etc. have removed humans from the rigors of natural selection. There is probably some truth to this, but consider: – Of all the human eggs that are fertilized, only one-third of so will ever reproduce themselves. The others are eliminated as follows: • Mortality selection – Some 30% of pregnancies end by spontaneous abortion of embryos and fetuses. – 5% by stillbirths and infant deaths. – 3% by childhood deaths. • Sexual selection – Another 20% will survive to adulthood but never marry. • Fecundity selection – Of those that do marry, 10% will have no children.