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How Populations Evolve Diversity is one of the key aspects of biology The diversity of living organisms is called biodiversity Change is normal in biology This makes sense since the planet is constantly changing The Diversity of Life For all of human history, people have named, described, and classified the inhabitants of our natural world Taxonomy is the branch of biology concerned with identifying, naming, and classifying species The Linnaean system includes a method of naming species and a hierarchical classification of species into broader groups of organisms In the Linnaean system, each species is given a two-part Latinized name, a binomial The first part of a binomial is the genus (plural, genera), a group of closely related species The second part of a binomial is used to distinguish species within a genus Linnaeus also introduced a system for grouping species into a hierarchy of categories Beyond the grouping of species within genera, taxonomy extends to progressively broader categories of classification - family, orders, classes, phyla (singular, phylum), kingdoms, and domains Grouping organisms into broader categories is a way to provide structure for our understanding of the world However, the criteria used to define more inclusive groups such as families, orders, and classes are ultimately arbitrary Explaining the Diversity of Life Before the 1800s, it was thought that the earth was only about 6000 years old All of life must have come into being relatively recently and in the current forms that were seen However, the discoveries of fossils of creatures no longer alive confused this line of thought Lamarck and Evolutionary Adaptations Naturalists compared fossil forms with living species and noted patterns of similarities and differences In the early 1800s, French naturalist Jean-Baptiste de Lamarck suggested that the best explanation for these observations is that life changes, that it evolves Lamarck explained evolution as the refinement of traits that equip organisms to perform successfully in their environments He proposed that by using or not using its body parts, an individual may develop certain traits that it passes on to its offspring Of course, Lamarck was wrong in his ideas but his progressive thinking set the stage for other naturalists to follow Charles Darwin In 1831, Charles Darwin set sail on the H.M.S. Beagle, employed as the ship’s naturalist and captain’s companion Darwin’s thoughts were shaped by the ideas of many of his contemporaries Charles Lyell noted the dynamic geological nature of Earth Jean-Baptiste de Lamark explored the possibility of descent with modification Cuvier noted the extinction of some species Earth and the appearance of others within different time-frames Darwin spent many years thinking about descent with modification before he concluded that the driving force was natural selection The key piece was an essay by Thomas Malthus on the limits of human population growth Alfred Russel Wallace Another English naturalist, Alfred Russel Wallace also concluded that natural selection is the principal process underlying evolution In 1859, Darwin finally published On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life He pulled together ideas about: the vast diversity of organisms their similarities and differences their origins and relationships their geographical distribution their adaptations to the surrounding environments Descent with modification Darwin argued that there was clear evidence that modern species are descended from earlier ancestral species Used the term “descent with modification”, not “survival of the fittest” He proposed that the descendants of the earliest organisms spread into various ecological habitats over millions of years They then accumulated different modifications or adaptations appropriate to these diverse ways of life In the Darwinian view, the history of life is like a tree At each fork of this evolutionary tree is an ancestor common to all of the evolutionary branches that extend from that fork There are successful (surviving) and unsuccessful (extinct) branches of the tree Once a branch ends, there are no more descendants of that lineage But “surviving” refers to an instant in time Evidence of evolution Evolution has left, and continues to leave observable signs There is extensive evidence in the fossil record There are also historical vestiges seen in modern life Radiometric dating has provided accurate data for the age of the planet as well as the times of various major geological events Fossil record Fossils are preserved remnants or impressions left behind by organisms Most fossils are found in sedimentary rocks Because of this, each rock strata contains a unique set of fossils that represents a local sampling of organisms that lived when the sediment was deposited Fossils from the same evolutionary periods are consistently found together in the same geologic strata We have also found a consistency between the relative ages of fossils as assigned by evolutionary theory and the absolute ages determined by radiometric dating Comparative anatomy The comparison of body structures between different species Similarities indicate common ancestry Forelimbs of mammals They have different functions They have similar structures Particularly the skeletal elements The structural similarity due to common ancestry is called homology This confirms that evolution is a constant remodeling process Old structures are modified to provide new functions Unfortunately, this descent with modification also leads to anatomical imperfections Consider the human spine and knee joint The retina of the eye is essentially installed backwards Goose bumps attempt to warm us by fluffing up our long-gone fur Descent with modification also leads to vestigial characters - structures which used to serve a purpose but no longer do so Comparative embryology The comparison of structures that are present during the early development of different organisms Related organisms often have similar stages of embryonic development All vertebrate embryos have a stage in which gill pouches are present Many organisms show embryonic structures that are gone by birth Darwin considered embryology “... the strongest single class of facts in favor of change of forms.” Biogeography This is the study of the geographical distribution of species, both surviving and extinct Discoveries in the field of biogeography were crucial to the development of Darwin’s ideas The animals and plants of the Galápagos Islands resembled species found on the South American mainland more than those found on similar but distant islands (Cape Verde Islands) Biogeography can explain the prevalence of certain types of organisms in certain places Australia There are a wide variety of marsupials but relatively few placental mammals This is much different from anywhere else in the world Introduced placental mammals have thrived in Australia The early Australian marsupials evolved in isolation from other regions where early placental mammals diversified In Australia, the marsupials were just more successful Madagascar Lemurs are only found in Madagascar Their closest relatives are in Africa Biogeography can also explain the unique distribution of certain organisms Like the lungfish The past and current distribution of organisms indicates that life on Earth has been shaped by major geological forces over vast periods of time Molecular biology DNA, RNA, proteins, macromolecules The more closely related two species are, the more similar the genes and gene products The more distantly related, the more different the genes and gene products Based on the degree of similarity, can determine how related two species are Remember that the same genetic code is used for all living organisms Evolutionary Trees Homologous structures, both anatomical and molecular, can be used to determine the branching sequence of an evolutionary tree Some homologous characters, such as the genetic code, are shared by all species because they date to the deep ancestral past In contrast, traits that evolved more recently are shared by smaller groups of organisms Individuals can adapt to their environment but they cannot evolve Although natural selection acts on individuals, it is the population that evolves over time as adaptive traits become more common in the group and other traits change or disappear Thus, evolution refers to generation-to-generation changes in populations Natural selection is more an editing process than a creative mechanism Natural selection can only amplify or diminish heritable traits Natural selection is not goal-directed; it does not lead to perfectly adapted organisms … because things change A trait that is favorable in one situation may be useless - or even detrimental - in different circumstances Darwin did not know about Mendel’s findings He did not know how the variations that are the raw material for natural selection arise in a population or how they are passed from parents to offspring Mutation New alleles originate by mutation providing the genetic variation that serves as raw material for evolution Sometimes, duplication of a gene or small pieces of DNA through errors in meiosis can provide an important source of genetic variation and eventually lead to new genes with novel functions Populations Evolve At the population level, biologists focus on the gene pool, which consists of all the alleles, for all the genes, for all the members of the population Gene Pools Consider a wildflower population with two varieties of different colored blooms The allele for red flowers (R) is dominant to the allele for white flowers (r) If these are the only two alleles affecting flower color and this population is stable (not evolving) for flower color, then genotype frequencies can be calculated from the allele frequencies Let p = ƒ (R) and q = ƒ (r) Then p + q = 1 The Hardy-Weinberg formula, p2 + 2pq + q2 = 1, can be used to calculate the frequencies of genotypes in a gene pool from the frequencies of the alleles Public health scientists use the Hardy-Weinberg equation to calculate the carrier frequency for certain inherited diseases PKU is a recessive disorder that prevents the breakdown of the amino acid phenylalanine and occurs in about one out of every 10,000 babies born in the United States Microevolution A nonevolving population is in genetic equilibrium, also known as Hardy-Weinberg equilibrium, in which the gene pool remains constant from generation to generation and the frequencies of alleles (p and q) and genotypes (p2, 2pq, and q2) are unchanged Evolution requires a change in allele frequency within a population That change in allele frequency is evolution at its smallest scale microevolution So processes that affect the allele frequencies in a gene pool are agents of microevolution Genetic drift Genetic drift is the chance alteration of allele frequencies in a population It has its greatest effects on small populations Bottleneck effect Certain catastrophic events can reduce the size of a population drastically The small surviving population may not be representative of the original population’s gene pool The reduced gene pool is now the starting material as the population once again increases The bottleneck effect will usually reduce the genetic variability in a population The reduction in individual variation will reduce adaptability and can threaten the viability of the species The founder effect Genetic drift in a new colony can lead to allele frequencies that are drastically different from the population that the founders left Gene flow Gene flow occurs from the movement of genes (alleles) from one population to another This is due to migration The Hawaiian islands were formed through volcanic activity Yet we find amazing examples of biodiversity Like silverswords Natural selection Only natural selection consistently leads to adaptive evolution Those individuals in a population that are better suited to the environment will be more likely to survive and reproduce Their contributions to the gene pool in subsequent generations will be greatly than those that are less well suited Evolutionary fitness is determined by the number of offspring an individual produces The more offspring surviving and reproducing, the greater the impact on the gene pool Fitness only applies to a specific environment at a specific time If the environmental conditions change, the relative fitness of an individual can change as well So “survival of the fittest” is misleading It should really be “survival of the fit-enough” So changes in allele frequencies can occur from generation to generation Can we see phenotypic changes as well? Peter and Rosemary Grant have studied the finches on the tiny Galapagos island of Daphne Major Every year, they capture each bird, take various physical measurements, and then release the bird In 1976, there were many more birds than there were in 1978 There were also some profound differences between the birds in 1976 and the ones in 1978 That small difference allowed the finches to eat the tougher seeds that were present after the drought Natural selection does not create advantageous traits It can only act upon the existing genetic variation But it’s all about timing Three General Outcomes of Natural Selection When natural selection occurs, what are the possible results? Results of natural selection Directional selection Disruptive selection Stabilizing selection Sexual selection Sexual selection is a special form of natural selection It occurs when differences in reproductive success are tied to differential success in mating Sexual dimorphism is a distinction in appearance between males and females not directly associated with reproduction or survival Does sexual selection always select for those that are the most fit?