Pre-scientific Beliefs A Brief and Idiosyncratic History of the Concept of Evolution or why we think we know how we got here Understanding of the universe was based on the interpretation of supernatural concepts There were two long-standing notions that impeded the development of the concept of evolution: Fixity of species Relatively short duration since the creation of the earth Ptolemy (2nd century A.D.) Earth as center of universe Inorganic and organic world fixed by creation Great Scale of Being: simple to complex (from Aristotle) No new species since creation Endorsed by Christian church Archbishop James Ussher (1581-1656) 1656) A A scholar in the pre-scientific tradition, Ussher used a literal interpretation of the bible to estimate the time of creation By evaluating how many years individuals lived and how long from one begat to the next, Ussher s reading of Genesis led him to estimate that the earth was created in 4,004 B.C. I.e., the earth was less than 6,000 years old Scientific Revolution Science provided a framework to attempt to discover the Natural Laws or fundamental principles that govern the universe The procedure includes The formulation of testable hypotheses The application of empiricism, the collection of data to test the hypotheses Copernicus (1473-1543) 1543) An early proponent of the scientific method, Copernicus postulated that the sun (not the earth) was center of universe, and the earth rotated around the sun He espoused the removal of a supernatural interpretation of the cosmos His predictions were subject to observation The planets obeyed Natural Laws 1
Carolus Linnaeus (1707-1778) 1778) Proposed a hierarchical classification of organisms into four levels C, O, G, S Attempted to discover the grand design of the Scale of Being Accepted fixity of species, as created Binomial Nomenclature: Genus and species e.g., Homo sapiens for man Comte de Buffon, 1707-1788 While denying the existence of macroevolution, believing that all species continue as created, he espoused microevolution In his view, the differences between populations of a species were the result of their accommodation to local environmental conditions Erasmus Darwin (1731-1802) 1802) Charles Grandfather Discussed Natural Laws in his poetry Published several ideas that presaged evolution in his Zoonomia Competition for resources between members of a species He was aware that species change through time but did not have a mechanism Chevalier de Lamarck (1744-1829) 1829) Evolution of species according to fixed laws Species adapt to changing environment by use/disuse of organs Systems develop according to wants & needs of organism e.g., fish in drying lake develop lungs as needed for oxygen Proposed mechanism: Inheritance of acquired characteristics Georges Cuvier (1769-1832) 1832) Pope of Bones-- --Father of Paleontology Skills in comparative anatomy gained from the reassembling of disarticulated skeletons from slaughter houses made him an expert for the reconstruction of fossils Believed in the fixity of species Proposed Catastrophism from natural causes as a mechanism to produce fossils Charles Lyell (1797-1875) 1875) Amplified and popularized work of earlier scholars (e.g., Hutton) proposing uniformitarianism as an explanation for the fossils and geologic deposits Uniformitarianism Geological processes same in past as they are today Slow, gradual process of earth's formation 2
Thomas Malthus (1766-1834) 1834) In his 1798 An Essay on the Principle of Population as It Affects the Future Improvement of Society he noted: Population growth potentially exceeds increases in food resources In nature, overpopulation checked by struggle for existence Not all organisms secure adequate food every generation Most die, only a few survive to perpetuate the species Charles Darwin (1809-1882) 1882) Born in a well-to to-do family, father a physician Had an early interest in nature, collecting eggs, pebbles, and the like Did not do well in school Could not stomach the cruelty of medicine when he went to the University of Edinburgh to study Charles Darwin (1809-1882) 1882) Transferred to Cambridge for a degree in divinity Read Lamarck and his grandfather Erasmus writings at this time As he was about to become a clergyman he was invited to join the Honor Expedition as the ship s naturalist aboard the H.M.S. Beagle Charles Darwin During the 5 year cruise of the Beagle, Darwin collected specimens, read Lyel s Principles of Geology,, and became convinced of the transmutability of species After returning home, a reading of Malthus in 1838 finally triggered the notion of natural selection as the mechanism driving evolution Charles Darwin In 1838, after reading Malthus, Darwin realized that through the struggle for existence, favourable variations would tend to be preserved and unfavourable ones to be destroyed. The result of this would be the formation of new species. (Darwin 1892:43). Alfred R. Wallace (1823-1913) 1913) Came to the same conclusions about evolution by natural selection as a result of his work in the Malay Archipelago He is supposed to have arrived at his conclusions about natural selection while lying delirious from a malarial fever 3
Alfred Russell Wallace 1858 sent essay to Darwin On the Tendency of Varieties To Depart Indefinitely from the Original Type Wallace became a fervent selectionist, ending up not believing in human evolution because he could not envision the adaptive purpose for the human mind Publication Lyell helped to persuade Darwin to publish an extract of his book along with Walace s essay. Appeared in 1858 Journal of the Linnean Society 1859 On the Origin of Species by Means of Natural Selection or The Preservation of Favoured Races in the Struggle for Life Thomas Huxley (1825-1895) Acquainted with both Darwin and Wallace Upon reading Darwin s On the Origin of Species... is reported to have said, How obvious. How stupid of me not to have thought of it myself. Became the Darwin s Buldog, the fierce proponent of evolution by natural selection in the public forum Evolution by Natural Selection Offspring produced faster than food supply increases All living organisms vary (especially within species) Because more individuals are born than survive, there is a struggle for existence individuals with favorable adaptations are most likely to survive and leave offspring, sometimes known as survival of the fittest Evolution by Natural Selection As a result, individuals with favorable adaptations increase in relative number from generation to generation, so greater and greater numbers within the species have the adaptation Over long periods of time, such successful variations (favorable adaptations) produce differences that result in the formation of new species Darwin's Evidence Changes bred into domesticated organisms, especially pigeons Geographic distributions of species such as the turtles of the Galapagos Islands The geological and paleontological record Comparative anatomy (e.g. the resemblance between apes and man) and embryology (e.g., the possession of gills by mammalian embryos) 4
Darwin's Failures Never understood the source of the variation that is so key for the operation of natural selection Never understood the particulate nature of inheritance, in spite of having a reprint of Mendel s 1865 publication Darwin's view of how speciation occurs, circa 1870 Variation between individuals occurs in all species and is caused by the variability of the environment to which a population is exposed. Individual variation is inheritable since, as my provisional hypothesis of pangenesis explains, each part of the body emits invisible gemmules that pass through the bloodstream to collect in the germ cells, carrying with them information on the exact nature of the body part from which they came. This slight variation that benefits an individual in its struggle for existence enables it to survive longer and reproduce more successfully than others. Conversely, individuals with injurious variations perish. This principle I have called natural selection. Due to the inheritance of beneficial variations, natural selection also acts on further variation in successive generations, so that initially small beneficial traits may in time become major through continued descent with modification. Since the individuals whose favorable variations diverge most markedly from the parent stock receive the most benefit, those most divergent variations are preserved and accumulated by natural selection. After much variation has accumulated, fairly well marked varieties and then species may be noted, since species are only strongly marked and well defined varieties. In this way, transmutation of species occurs and new species originate. The process resulting in new species, repeated all over the earth and over great spans of time, has produced the present diversity of life, the clear relationships between species alive and extinct, and the descent of all life from some primordial species. 5
Evolution The Modern Synthesis Nothing in biology makes sense except in the light of evolution Theodosius Dobzhansky The alternative to thinking in evolutionary terms is not to think at al Sir Peter Medawar Simultaneous Rediscovery of Mendel (1900) Hugo de Vries,, Holland Erich von Tschermak,, Austria Carl Correns,, Germany Basis of population genetics-- equilibrium model William E. Castle, 1903, American animal breeder Godfrey H. Hardy, 1908, British mathematician Wilhelm Weinberg, 1908, German physician Development of Population Genetics from H-W H W Model R.A. Fisher, 1930, The Genetical Theory of Natural Selection J.B.S. Haldane,, 1932, The Causes of Evolution Sewall Wright, 1931, Evolution in Mendelian Populations 6
Synthesis of genetics and Darwinism Theodosius Dobzhansky, 1937, Genetics and the Origin of Species Julian S. Huxley, 1942, Evolution. The Modern Synthesis Ernst Mayr, 1942, Systematics and the Origin of Species George Gaylord Simpson, 1944, Tempo and Mode in Evolution Species Concepts Species is derived from the Latin specere,, to look at, refering to the outward appearance as the basis of species Aristotle conceived of species as unstable and highly changeable believed in spontaneous generation and all kinds of crosses between different species Species Concepts Typological species Discreteness of species Fixity of species One member of each species was all that was necessary to serve as a type specimen representing the morphology of the entire species Think of a dog show where the judge evaluates the dog against a notion of the perfect type for the breed Darwin s Species Concept On the view that species are only strongly marked and permanent varieties, and that each species first existed as a variety, we can see why it is that no line of demarcation can be drawn between species, commonly supposed to have been produced by special acts of creation, and varieties which are acknowledged to have been produced by secondary laws (Darwin, 1872:171 sixth edition of Origin of Species ). Biological Species Concept Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups (Mayr, 1963:19) Defines species in terms of a gene pool, isolated from other similar gene pools Limitations Restricted to sexually reproducing organisms, so does not apply to single cell organisms that reproduce by simple cell division (mitosis) Also does not fit many plants species No clear application to the fossil record, since reproductive isolation does not show up in fossilized materials 7
Where does evolution take place? Biological Hierarchy Biosphere Community Population Individual Organs, tissues, cells, molecules Where is the species? Our taxonomy doesn t reflect ecological reality Reproductive Isolation Pre-mating Mechanisms that prevent the transfer of gametes between members of different species Post-mating Mechanisms that reduce the viability or fertility of hybrid offspring Pre-mating Mechanisms Mechanisms causing a failure to meet potential mates in space or time Habitat isolation Species occupying different segments of the habitat, e.g., small-mouthed mouthed salamander, Ambystoma texanum,, breeds in ponds, but A. barbouri breeds in streams Seasonal isolation Plant species which release pollen at different times Pre-mating Mechanisms Mechanisms causing a failure to mate between individuals who meet Mechanical isolation Copulation is attempted but no transfer of sperm take place. Many insect species require a lock and key link for sperm transfer Behavioral or ethological isolation Potential mates meet but do not mate Coloration, movement, song, or odor may not elicit readiness for sperm transfer Post-mating Mechanisms Mechanisms reducing the viability of hybrid offspring Gametic mortality Sperm transfer takes place but the egg is not fertilized, e.g., Sonoran topminnow and nematodes Zygotic mortality Egg is fertilized but zygote dies. Goat by sheep cross results in early zygotic death, as do some leopard frog hybrids (Rana( pipiens) Post-mating Mechanisms Mechanisms reducing the fertility of hybrid offspring Hybrid inviability Zygote produces offspring of reduced viability, e.g., some frogs in the genus Pseudophryne Reduced fertility Horse by donkey cross produces a sterile mule Papio hamadryas male by Papio anubis female results in fertile female and infertile males 8
Paleospecies A A paleospecies or chronospecies is a morphologically or anatomically defined fossil species Since interbreeding is not determinable for extinct species, species boundaries are determined by similarity of form Speciation The process by which new species are produced from earlier ones. This is the central process of macroevolution, or evolution at the level of the species or above Microevolution involves evolutionary change at the level of the population, and is defined by changes in allele frequencies within the population Origin of Species Speciation occurs when populations of a species become reproductively isolated so that they can no longer interbreed with full fertility Geographic isolation of populations usually initiates the process Selection operates to produce isolating mechanisms of anatomy, behavior, genes 9
Synthetic Evolution, circa 1950s New variants of genes are continually being added to populations through mutation. In addition, as explained by Mendel's two laws, recombination of genes increases the variation initially produced by mutation, since segregation and independent assortment of genes result in almost endless novelty in gene combinations and thus an almost infinite variety of possible genotypes in a population. Darwin s Finches Because of the particulate nature of inheritance, no loss of variation occurs from parent to progeny. Thus, each population has an enormous store of variation on which natural selection can act. The fittest individuals, defined as those which leave the most viable progeny that mature and reproduce, contribute the most genes to the next generation, and thus their genes become most common in the gene pool of the population. New mutations and new combinations of genes continually produce some individuals that are better adapted than others; natural selection favors these and selects out less fit types. Gradually, the population evolves through the differential perpetuation of genes. If a population should become divided into two by a geographic barrier, evolution of each new population continues independently. Differences between the two, including differences in reproductive processes, gradually accumulate such that reproductive isolating mechanisms become more and more effective over prolonged periods of time. Ultimately, on secondary contact, the two populations are reproductively isolated and two new species have evolved by the process of geographic speciation. Therefore, species can be defined as groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups. 10
Speciation events lead to multiplication and diversification of species into higher taxa that retain clear phylogenetic links with parental stock, so that all species can be traced to the origin of life itself. Modes of Evolution Cladogenesis is branching evolution, the production of two or more daughter species from a single ancestral species Anagenesis is linear evolution, the production of a single daughter species from a single ancestral species Tempo of Evolution Gradualism views evolutionary change as occurring at a slow, steady rate over time Punctuated Equilibrium views the tempo of evolution as consisting of long periods of stability (equilibrium), followed by relatively short periods of rapid evolutionary change (punctuations) Tempo of Evolution a. Gradual change through time b. Increasing rate of change through time c. Punctuated equilibrium: stasis and bursts of change Sources Darwin, C. 1872 On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life. 6th Edition. London: Murray. Mayr, E. 1963 Animal Species and Evolution. Cambridge, MA: Belknap Press 11
Sources Price, P.W. 1996 Biological Evolution. Philadelphia: Saunders. Relethford, J.H. 1997 The Human Species: An Introduction to Biological Anthropology. Third Edition. Mountain View, CA: Mayfield Press. 12