What is a theory? • A widely accepted explanatory idea that is broad in scope and supported by a large body of evidence. • http://www.pbs.org/wgbh/evolution/library/11/2/e_s_1.html
What is evolution? • History of defining evolution: • About 2,500 years ago, the Greek philosopher Anaximander • promoted the idea that life arose in water and that simpler forms of life preceded more complex ones. • The Greek philosopher Aristotle • generally held that species are fixed, or permanent, and do not evolve. • Judeo-Christian culture • fortified this idea with a literal interpretation of the Book of Genesis, holding that all species were individually designed by a divine creator. • The idea that all living species are static in form and inhabit an Earth that is most about 6,000 years old dominated the intellectual and cultural climate of the Western world for centuries.
What is evolution? • History of defining evolution: • Carl Linneaus (18th century)– • The father of taxonomy. • Used binomial nomenclature, came up with the hierarchical classification theme, used visible characteristics to classify plants and animals. • Thomas Malthus (18th – 19th century): • Attempting to justify the conditions of the poor by stating that poverty and starvation were merely a consequence of overpopulation.
What is evolution? • History of defining evolution: • Lamarck (18th – 19th century) – • First to publish a reasoned theory of evolution: A) the idea of use and disuse B) inheritance of acquired characteristics • Lyell (19th century) – • natural processes form geological formations over a long period of time, erosion and other forces that shape rocks are very slow processes that take millions of years, so the earth must be older than previously believed. • Wallas (19th – 20th century): • theory of evolution by natural selection.
What is evolution? • History of evolution (continued): • In the early 1800s, French naturalist Jean Baptiste Lamarck suggested that the best explanation for this relationship of fossils to current organisms is that life evolves. • Today, we remember Lamarck mainly for his erroneous view of how species evolve. • He proposed that by using or not using its body parts, an individual may develop certain traits that it passes on to its offspring. • Larmarck’s idea is known as the inheritance of acquired characteristics. • He suggested, for instance, that the ancestors of the giraffe had lengthened their necks by stretching higher and higher into the trees to reach leaves. • Although his ideas were incorrect, he helped to set the stage for Darwin by strongly advocating evolution and by proposing that species evolve as a result of interactions with their environment.
What is evolution? Charles Darwin
What is evolution? • Charles Darwin: • Born in 1809, the same year that Lamarck published some of his ideas on evolution. • In December 1831, at the age of 22, Darwin began a round-the-world sea voyage that profoundly influenced his thinking and eventually the thinking of the entire world. • He accompanied the captain of the HMS Beagle, a surveying ship, on a mission to chart poorly known stretches of the South American coastline. • Darwin actually spent most of his time onshore, collecting thousands of specimens of fossils and living plants and animals. • He noted the unique adaptations of these South American organisms in places as different as the Brazilian jungle, the grasslands of the pampas, and the frigid Anarctica. • He asked himself why fossils of the South American continent were more similar to modern South American species than to fossils of other continents.
What is evolution? • Charles Darwin (continued): • Other questions arose from Darwin’s visit to the Galapagos Islands. • Darwin observed that these islands had many unique organisms, most of which were similar to, but different from, the plants and animals of the nearest mainland. • Even the individual islands had some species that differed from those on other islands. • Referring to the islands and their unique inhabitants, he later wrote, “Both in space and time, we seem to be brought somewhat near to that great fact—that mystery of mysteries—the first appearance of new beings on the earth.”
What is evolution? • Charles Darwin (continued): • While on the voyage, Darwin read and was strongly influenced by the recently published Principles of Geology, by Scottish geologist Charles Lyell. • Lyell’s work led Darwin to realize that natural forces gradually change Earth’s surface and that these forces are still operating in modern times. • Darwin had collected fossils of marine snails in the Andes. Having read Lyell’s book and witnessed an earthquake that raised part of the coastline of Chile almost a meter, he came to believe that slow, natural processes such as the growth of mountains as a result of earthquakes, could account for the presence of marine snails on mountaintops.
What is evolution? • Charles Darwin (continued): • Upon Darwin’s return to Great Britain after his voyage, his experiences and reading had led him to doubt seriously that Earth and living organisms were unchangeable and had been specially created only a few thousand years earlier. • He analyzed his collections, discussed them with colleagues, continued to read, and maintained extensive journals of his observations, studies, and thoughts. • By the early 1840s, he had composed a long essay describing the major features of his theory of evolution. He realized that his ideas would cause a social furor, however, and he delayed publishing his essay. • In the mid 1850s, Wallace conceived a theory almost identical to Darwin’s. He asked Darwin to evaluate the manuscript he had written about his theory to see if it merited publication. • In 1858, two of Darwin’s colleagues presented Wallace’s paper and excerpts of Darwin’s earlier essay together to the scientific community.
What is evolution? • Charles Darwin (continued): • He published his book On the Origin of Species by Means of Natural Selection in 1859. • He presented the world with an avalanche of evidence and a strong, logical argument for evolution. He also described his theory of natural selection, an explanation of how evolution occurs. • In the first edition of his book, Darwin did not actually use the word evolved until the very end, referring instead to descent with modification. • This phrase summarized Darwin’s view of life: he perceived a unity among species, with all organism related through descent from an ancestor that lived in the remote past. • As the descendants of that ancestor spread into various habitats over millions of years, they accumulated diverse modifications, or adaptations, that accommodated them to diverse ways of life. • The history of life seemed to resemble a tree, with multiple branchings from a common trunk to the tips of the twigs. • At each fork of the evolutionary tree is an ancestor common to all lines of descent branching from that fork. • Species that are closely related share many characteristics because their lineage of common descent extends to the smallest branches of the tree of life.
Natural Selection • Darwin’s Theory of Evolution is based on several key observations and inferences: • 1. Overproduction of offspring • Populations produce too many offspring, many must die. • 2. Struggle for existence • Food, water, and other resources are limited; organisms compete with one another for these resources • 3. Variation • Individuals exhibit variation in a population and have a unique set of traits, and these traits get passed on from one generation to the next. • Some of these traits will improve their chance for survival while others are less favorable. • 4. Differential reproduction • Within a varied population, individuals whose characteristics adapt them best to their environment are most likely to survive and reproduce; these individuals thus tend to leave more offspring than less fit individuals do. • Reproduction is central to what Darwin saw as the basic mechanism of evolution, the process he called natural selection. • Means by which the environment filters variations, favoring some over others. • Allows for a gradual change in characteristics of a population of organisms overtime
Artificial Selection • Darwin found convincing evidence for his ideas in the results of artificial selection, the selective breeding of domesticated plants and animals. • He saw that by selecting individuals with the desired traits as breeding stock, humans were playing the role of the environment and bringing about differential reproduction. • They were, in fact, modifying species. • Darwin reasoned that if so much change could be achieved in a relatively short period of time by artificial selection, then over hundreds or thousands of generations natural selection should be able to modify species considerably. • Such changes could account for the evolution of new species.
Evidence of Evolution • Darwin developed his theory of descent with modification mainly with evidence from the geographic distribution of species, examples of artificial selection, and the fossil record. • His careful documentation convinced many of the scientists of his day that organisms do indeed evolve. • 1. Fossil record • 2. Biogeography • 3. Comparative anatomy • 4. Comparative embryology • 5. Molecular biology
Evidence of Evolution • 1. Fossil record • Fossils document some of the drastic changes that life has undergone over time. • The organic substances of a dead organism usually decay rapidly, but hard parts of an animal that are rich in minerals, such as the bones and teeth of dinosaurs and the shells of clams and snails, may remain as fossils. • The fossil record—the ordered array in which fossils appear within layers of sedimentary rock—provides some of the strongest evidence of evolution. • Sedimentary rocks form from layers (strata) of sand and mud that settle to the bottom of seas, lakes, and marshes. • Younger strata are on top of older ones; thus, the relative ages of fossils can be determined by the layers in which they are found. • Fossil record shows that the oldest known fossils, dating from 3.5 billion years ago, are prokaryotes. • Fossils in younger layers reveal the evolution of various groups of eukaryotic organisms.
Evidence of Evolution http://www.pbs.org/wgbh/evolution/library/03/3/l_033_01.html
Evidence of Evolution • 2. Biogeography • It was the geographic distributing of species, known as biogeography, that first suggested to Darwin that organisms evolve from common ancestors. • Darwin noted that Galapagos animals resembled species of the South American mainland more than they resembled animals on similar but distant islands. • The logical explanation was that the Galapagos species evolved from South American immigrants. • http://www.ucmp.berkeley.edu/geology/anim1.html
Evidence of Evolution • 3. Comparative Anatomy • The comparison of body structures in different species. • Anatomical similarities between many species give signs of common descent. • Similarity in characteristics that results from common ancestry is known as homology. • Homologous structures are features that often have different functions but are structurally similar because of common ancestry. • For example, the same skeletal elements make up the forelimbs of humans, cats, whales, and bats, all of which are mammals. However, the functions of these forelimbs differ. • The logical explanation is that the arms, forelegs, flippers, and wings of different mammals are variations on a common anatomical plan that has become adapted to different functions.
Evidence of Evolution • 3. Comparative Anatomy (continued) • Some of the most interesting homologous structures are vestigial organs, structures of marginal, if any, importance to the organism. • Remnants of structures that served important functions in the organism’s ancestors. • For example, the small hind-leg and foot bones of modern whales, the skeletons of some snakes retain vestiges of the pelvis and leg bones of walking ancestors.
Evidence of Evolution • 4. Comparative Embryology • The comparison of early stages of development, called comparative embryology, is another major source of evidence fro the common descent of organisms. • One sign that vertebrates evolved from a common ancestor is that all of them have structures on the sides of the throat called pharyngeal (throat) pouches. • At this stage, the embryos of fishes, frogs, snakes, birds, and mammals look relatively alike. • They take on more and more distinctive features as development progresses. • For example, pharyngeal pouches develop into gills in fishes, but into parts of the ears and throats in humans.
Evidence of Evolution • 5. Molecular Biology • Recent advances in molecular biology have enabled biologists to read a molecular history of evolution in the DNA sequences of organisms. • If two species have genes with sequences that match closely, biologists conclude that these sequences must have been inherited from a relatively recent common ancestor. • In contrast, the greater the number of sequence differences between species, the less likely they share a close common ancestor. • Molecular comparisons between diverse organisms have allowed biologists to develop hypotheses about the evolutionary divergence of major branches on the tree of life. • Studies of the amino acid sequences of similar (homologous) proteins in different species have been a rich source of data about evolutionary relationships. • By comparing the amino acid sequence of hemoglobin, the following hypothesis was made about evolutionary relationships: Rhesus monkeys are much more closely related to humans that are lampreys; mice, chickens, and frogs fall in between. • This hypothesis agrees with conclusions from comparative anatomy and embryology, along with fossil evidence.
Evidence of Evolution • 5. Molecular Biology (continued) • There is also evidence showing that most (if not all) multicellular eukaryotes have similar genes regulating their early development. • The logical explanation is that these genes first arose in a common ancestor. • Darwin’s boldest hypothesis is that all life-forms are related. • Molecular biology has provided strong evidence for it: All forms of life use DNA and RNA, and the genetic code is essentially universal. • This genetic language has been passed along through all the branches of evolution ever since its beginnings in an early form of life. • http://www.pbs.org/wgbh/evolution/library/03/4/l_034_04.html
Natural Selection in Action • Many cases of natural selection in nature have been documented. • A classic example involves finches in the Galapagos Islands over a period of 20 years. • The research of scientists showed measurable changes in in beak size in a population of ground finches. • In dry years, when small seed are in short supply, birds must eat more large seeds. Birds with larger, stronger beaks have a feeding advantage and greater reproductive success, and the average beak depth for the population increases. • During wet years, smaller beaks are more efficient for eating the now abundant small seeds, and the average beak depth decreases.
Natural Selection in Action • Evolution in pesticide resistance in hundreds of insect species. • Pesticides are poisons used to kill insect pests in farmlands, swamps, backyards, and homes. • Whenever a new type of pesticide is used to control agricultural pests, a relatively small amount of poison dusted onto a crop may kill 99% of the insects, but subsequent sprayings are less and less effective. • The few survivors of the first pesticide wave are insects with genes that somehow enabled them to resist chemical attack. So, the poison kills most members of the population, leaving the resistant individuals to reproduce and pass the genes for pesticide resistance to their offspring. • The proportion of pesticide-resistant individuals increases in each generation. • Like the finches, the insect population has adapted to environmental change through natural selection.
Natural Selection in Action • Antibiotics Resistance • Antibiotics are drugs that disable or kill infectious microorganisms. • Most antibiotics are naturally occurring chemicals derived from other microorganisms. • Pencillin, for example, was originally isolated from a mold and has been widely prescribed since the 1940s. A revolution in human healthy rapidly followed its introduction, rendering many previously fatal diseases easily curable (such as strep throat and surgical infections). • During the 1950s, some doctors even predicted the end of human infectious disease. Unfortunately, evolution destroys the optimism of this prediction. • In the same way that pesticides select for resistant insects, antibiotics select for resistant bacteria. • The genes that confer such antibiotic resistance are often carried on R plasmids, which are passed on to bacterial offspring and may even be transferred to other bacteria. • For nearly every antibiotic that has been developed, a resistant strain of bacterium has appeared within a few decades. • For example, some strains of the tuberculosis-causing bacterium are now resistant to all three of the antibiotics commonly used to treat the disease.
Natural Selection in Action • Antibiotics Resistance (continued) • In some ways, we are contributing to the problem of antibiotic resistance: • Livestock producers add antibiotics to animal feed as a growth promoter. • As a result, much of the packaged meat for sale in supermarkets contains bacteria that are resistant to standard antibiotics. • Doctors contribute to the problem by overprescribing antibiotics, for example, to patients with viral infections, which do not respond to antibiotic treatment. • And patients contribute to this problem through misuse of prescribed antibiotics—for example, by prematurely stopping the medication before they feel better. This allows mutant bacteria that may be killed more slowly by the drug to survive and multiply. Subsequent mutations in such bacteria may lead to full-blown antibiotic resitance. • During the anthrax crisis of 2001, public health officials urged panicked citizens to avoid unnecessarily taking ciprofloxacin (Cipro), the drug used to treat the deadliest form of anthrax, because doing so could select for resistant bacteria.
Genetic Variation- Where does it come from? • Individual variation occurs in populations of all species that reproduce sexually. • Fresh assortments of existing alleles arise every generation from three random components of sexual recombination: crossing over, independent assortment of homologous chromosomes, and random fertilization. • Mutation is the ultimate source of the genetic variation that serves as raw material for evolution. • Mutations are changes in the nucleotide sequence of DNA that can create new alleles. • Most mutations occur in body cells and are lost when the individual dies. • Only mutations in cells that produce gametes can be passed to offspring and potentially affect a population’s gene pool.
How Does Evolution Work? • Populations (group of organisms that belong to the same species) change over time and evolve, not individual organisms (unlike Lamarck’s idea) • Mutations (small changes in the nucleotide sequence of DNA) result in new traits and increasing variation in the population • One mutation alone usually does not change the population, however, beneficial mutations can change the population
How Does Evolution Work? • Genetic drift – change in the allele frequency in a population based on chance • Founder effect – small group of organisms move away from the main population and give rise to a new population • Bottleneck effect – after a natural disaster, a small group of organisms with different characteristics survive
How Does Evolution Work? • Gene flow – movement of organisms from one population to another • Natural selection – (this is already familiar) • Nonrandom mating – “coolness”
Modern Evolutionary Theory • Several scientists improved on Darwin’s theory and this improvement is still going on. • Today we explain the reasons for evolution with mutations, changes in DNA and sexual reproduction. • Evolution is closely related to genetics. • Genetic change in a population or species over generations; all the changes that transform life on Earth; the heritable changes that have produced Earth’s diversity of organisms.