Chapter 6. Organic Evolution. Origins of Darwinian Evolutionary Theory. Lamarckism: The First Scientific Explanation of Evolution. Larmarck s Concept

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1 Chapter 6 Organic Evolution Origins of Darwinian Evolutionary Theory Pre-Darwinian Evolutionary Ideas Before 18 th century species origins based on mythology and superstition, not science Most creation myths did not include the idea of change in species after creation Some Greek philosophers developed early ideas of evolutionary change No major changes in ideas of creation until 18 th century Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lamarckism: The First Scientific Explanation of Evolution Lamarck was the first to provide a complete explanation of evolution (1809) Proposed Inheritance of Acquired Characteristics as the mechanism for evolutionary change. Larmarck s Concept Termed transformational because in claims that individuals transform their characteristics to produce evolution Lamarck s theory has been rejected due to genetic evidence of chromosomal inheritance through gametes rather than acquired characteristics. Darwin s theory is termed variational based on the distribution of genetic variation in populations Evolutionary change is caused by differential survival and reproduction among organisms that differ in hereditary traits. Darwin s Great Voyage of Discovery 5 year voyage that began in 1831, Darwin was almost 23 years old. Darwin was hired as the ships naturalist. Observations made during the trip provided the underlying evidence for his theory of evolution by natural selection Travels on the Beagle Most famous stop on the voyage was a 5 week visit to the Galapagos islands Darwin considered his observations taken there as the origin of all my views

2 After the Beagle Darwin returned to England in 1836 Published the journal of his voyage 3 years later The Voyage of the Beagle was a great success. Darwin did not publish his theory of evolution for more than 20 years. Influences on Darwin s Theory Charles Lyell Suggested the principle of Uniformitarianism Laws of physics and chemistry remain the same throughout earth s history Past geological events occurred by natural processes similar to those seen today Influences on Darwin s Theory Thomas Malthus Wrote an essay discussing principles of population growth Major point was that populations tend to increase beyond the capacity of the environment to support them. The Push to Publish Developed idea over time and presented it in 1844 as an unpublished essay Began to assemble data into a planned 4- volume book as perfect as I can make it Plans were interrupted Getting Scooped In 1858 received a manuscript from a colleague Alfred Russel Wallace Wallace had summarized the major points of the theory Darwin had been working on for 20 years Finally Published Persuaded to publish one page statement with Wallace s paper in the Journal of the Linnean Society Prepared abstract of his planned 4 volume set the next year (1859) titled On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life Produced many other books on evolution and other topics over the next 23 years

3 The Evidence for Evolution Fig. 6.9 Perpetual Change Main premise of Darwinian Evolution is the idea that the living world is always changing Most direct evidence is from the fossil record Fossil record is biased because preservation is selective Most organisms leave no fossils; the record is always incomplete and requires interpretation. Geological Time Law of Stratigraphy Method of relative dating with the oldest layers at the bottom and the youngest at the top of the sequence. Time divided into Eons, Eras, Periods, and Epochs (see back inside cover of your text) Radiometric methods more precise Use radioactive decay of naturally occurring elements to determine absolute age in years of rock formations Evolutionary Trends Fossil record used to view change across broadest scale of time. Most animal species survive ~1-10 million years on average-but this is highly variable Trends in fossil diversity produced by different rates of species formation vs extinction through time. Trends in the fossil record Horse Evolution Shows Clear Trend (Figure 6.11) From Eocene to Recent periods, genera and species of horses were replaced. Earlier horses had smaller sized and fewer grinding teeth, and more toes. Reduction in toes and increase in size and numbers of grinding teeth correlate with environmental changes. Change occurred in both features of horses and numbers of species Common Descent Darwin proposed that all plants and animals descended from a common ancestor. A history of life forms a branching tree called a phylogeny. This theory allows us to trace backward to determine converging lineages. All forms of life, including extinct branches, connect to this tree somewhere. Phylogenetic research is successful at reconstructing this history of life.

4 Homology and Phylogenetic Reconstruction Darwin saw homology as major evidence for common descent. Richard Owen described homology as the same organ in different organisms under every variety of form and function. Vertebrate limbs show the same basic structures modified for different functions. (Figure 6.14) Darwin s central idea that apes and humans have a common ancestor was explained by anatomical homologies Ground-dwelling birds illustrate homologies A new skeletal homology arises on each lineage shown. Different groups located at tips of branches contain homologies that reflect ancestry. Branches of the tree combine species into nested hierarchies of groups within groups. Analysis of the living species alone can reconstruct the branching pattern. The pattern of nested hierarchies forms the basis for classification of all forms of life. Structural, molecular, and chromosomal homologies are all combined to reconstruct evolutionary trees. Older theories that life arose many times forming unbranched lineages fails to predict the nested hierarchies of lineages; creationism fails to provide testable predictions; all fail as scientific hypotheses. Ontogeny, Phylogeny and Recapitulation Ontogeny is the history of development of an organism throughout its lifetime. Evolutionary alteration of developmental timing generates new traits allowing divergence among lineages. German zoologist Ernst Haeckel stated stages of development represented adult forms from evolutionary history. Ontogeny recapitulates phylogeny also became known as recapitulation or the biogenetic law. Haeckel, Darwin s contemporary, thought that this change was caused by adding new features onto the end of ancestral ontogeny; but this idea is Lamarckian. Embryonic similarities Embryologist K.E. von Baer showed early developmental features were simply more widely shared among different animals groups. (Figure 6.16) However, early development can undergo divergence among lineages too. Evolutionary change in timing of development is called heterochrony. Characteristics can be added late in development and features are then moved to an earlier stage. Fig Other factors Ontogeny can be shortened in evolution; terminal stages may be deleted causing adults of descendants to resemble youthful ancestors. Paedomorphosis is the retention of ancestral juvenile characteristics in descendent adults. (Figure 6.17) Organisms are a mosaic of both; ontogeny rarely completely recapitulates phylogeny. Multiplication of Species Evolution as a Branching Process A branch point occurs where an ancestral species splits into two different species. Darwin s theory is based on genetic variation. Total number of species increases in time; most species eventually become extinct. Much evolutionary research centers on mechanisms causing branching.

5 Speciation Definition of species varies and may include several criteria. Members descend from a common ancestral population. Interbreeding occurs within a species but not among different species. Genotype and phenotype within a species is similar; abrupt differences occur between species. Reproductive Barriers Reproductive barriers are central to forming new species. If diverging populations reunite, before they are isolated, interbreeding maintains one species. Evolution of diverging populations requires they be kept physically separate a long time. Geographical isolation with gradual divergence provides chance for reproductive barriers to form. Allopatric Speciation Allopatric populations occupy separate geographical areas. They cannot interbreed because they are separated, but could do so if barriers were removed. Separated populations evolve independently and adapt to different environments. Eventually they are distinct enough they cannot interbreed when reunited. Allopatric speciation occurs in two ways: 1) Vicariant speciation occurs when climate or geology causes populations to fragment; this may affect many populations at one time but does itself not induce genetic change. 2) Founder effect occurs when a small number of individuals disperse to a distant place; this has occurred with fruit flies in Hawaii Hybridization Hybridization is mating between divergent populations; offspring are hybrids. (Figure 6.19) Reproductive Barriers Premating barriers impair fertilization Members may not recognize each other. Male and female genitalia may not be compatible. Behavior may be inappropriate to elicit reproduction. Sibling species are indistinguishable in appearance but cannot mate. Postmating barriers impair growth and development or survival.

6 Nonallopatric Speciation Fig When there is no evidence of physical barriers, it is difficult to explain diversity of close species by allopatric speciation. The huge variety of cichlid fishes in African lakes are found nowhere else; yet lakes are evolutionarily young and without barriers. Sympatric speciation is the term for the hypothesis that individuals can speciate while living in different components of the environment. African cichlid fishes are very different in feeding specialization. Parasites may evolve with their host species. It is difficult to observe formation of distinct evolutionary lineages in allopatric speciation. From one-third to one-half of plant species show sympatric evolution using polyploidy; in animals polyploidy is rare. Adaptive Radiation Adaptive radiation produces diverse species from common ancestral stock. New lakes and islands provide new opportunities for organisms to evolve. Founders who were under heavy competition are now free to colonize the new habitat. The Galápagos Islands provided excellent isolation from mainland and each other. Darwin s finches are example of adaptive radiation from ancestral finch; finches varied to assume characteristics of missing warblers, woodpeckers, etc. Fig Gradualism Darwin s theory of gradualism is based on accumulation of small changes over time. He agreed with Lyell; past changes do not depend on catastrophes not seen today. We observe small, continuous changes; major differences therefore require thousands of years. 4Accumulation of quantitative changes leads to qualitative change. Ernst Mayr distinguishes between populational gradualism and phenotypic gradualism. Darwin s Concept Populational gradualism occurs when a new trait becomes more common; this is well established. Phenotypic Gradualism This theory states that strikingly different traits are produced in a series of small steps. It remains controversial ever since Darwin proposed it. Mutations that cause substantial phenotypic change are called sports. Opponents of phenotypic gradualism contend such mutations would be selected against. Recent work in evolutionary developmental genetics illustrates the continuing controversy surrounding phenotypical gradualism.

7 Punctuated Equilibrium Phyletic gradualism predicts that fossils would show a long series of intermediate forms. Fossil record does not show the predicted continuous series of fossils. Some Darwinists contend that fossilization is haphazard and slow compared to speciation. Niles Eldridge and Stephen Jay Gould proposed punctuated equilibrium. This theory contends phenotypic evolution is concentrated in brief events of speciation followed by long intervals of evolutionary stasis. Evidence for Punctuated Equilibrium Speciation is episodic with a duration of 10,000 to 100,000 years. Species survive for 5-10 million years; speciation may be less than 1% of species life span. Small fraction of evolutionary history contributes most morphological evolutionary change. Allopatric speciation provides a possible explanation. 1)A small founder population has little chance of leaving fossils that will every be found. 2)After a new genetic equilibrium forms and stabilizes, the larger but different population is more likely to be preserved. 3)However, punctuated equilibrium occurs in groups where founder events are unlikely. Natural Selection Natural selection gives a natural explanation for origins of adaptation. It applies to developmental, behavioral, anatomical and physiological traits. Darwin s theory of natural selection consists of five observations and three inferences. Observation 1 Organisms have great potential fertility. 1) If all individuals produced would survive, populations would explode exponentially (Malthus). 2) Darwin calculated that a single pair of elephants could produce 19 million offspring in 750 years. Observation 2 Natural populations normally remain constant in size with minor fluctuations. 1) Natural populations fluctuate in size across generations, sometimes going extinct. 2) No natural populations can sustain exponential growth. Source: Darwin and many others Observation 3 Natural resources are limited (Malthus). These three observations lead to Inference 1: Struggle for food, shelter, and space becomes increasingly severe with overpopulation. Survivors represent only a small part of those produced each generation.

8 Observations 4 and 5 All organisms show variation. Some variation is heritable. 1) Darwin only noted the resemblance of parents and offspring. 2) Gregor Mendel s mechanisms of heredity were applied to evolution many years later. Sources: Animal breeding and systematics Inferences 2 and 3 Inference: There is differential survival and reproduction among varying organisms in a population. Inference: Over many generations, differential survival and reproduction generates new adaptations and new species. Source: Darwin Natural Selection can be viewed as a two-part process: random and non-random Production of variation among organisms is random; mutation does not generate traits preferentially. The nonrandom component is the survival of different traits. 1) Differential survival and reproduction is called sorting; random processes may sort. 2) Natural selection is sorting that occurs because certain traits give their possessors advantages relative to others. Criticisms of Natural Selection Some critics contend natural selection cannot generate new structures, only modify old ones. (Irreducible complexity) 1) Many structures could not perform their function in early evolutionary stages. 2) However, many structures evolved initially for purposes different from the present. 3) Early feathers functioned in thermoregulation; they later became useful for flight. Revisions of Darwin s Theory Neo-Darwinism Darwin did not know the mechanism of inheritance. Darwin saw inheritance as a blending of parental traits. He also considered an organism could alter its heredity through use and disuse of parts. August Weismann s experiments showed an organism could not modify its heredity. Neo-Darwinism is Darwin s theory as revised by Weismann. Mendel s work provided linkage through inheritance that Darwin s theory required. Ironically, early geneticists thought mutations could cause speciation in a single large step; selection was merely an eliminator. Emergence of Modern Darwinism: The Synthetic Theory In 1930s, a synthesis occurred that tied together population genetics, paleontology, biogeography, embryology, systematics and animal behavior. Population genetics studies evolution as change in gene frequencies in populations. Microevolution is change of gene frequency over a short time. Macroevolution is evolution on a grand scale, originating new structures and designs, trends, mass extinctions, etc. The synthesis combines micro- and macroevolution and expands Darwinian theory.

9 Microevolution: Genetic Variation and Change Within Species: The Gene Pool Different allelic forms of a gene constitute polymorphism. All alleles of all genes that exist in a population are collectively the gene pool. Allelic frequency is the frequency of a particular allelic form in a population. Fig. 6.27: Frequency of type B allele in Europe Genetic Equilibrium Whether a gene is dominant or recessive does not affect its frequency; dominant genes do not supplant recessive genes. In large two-parent populations, genotypic ratios remain in balance unless disturbed. This is called the Hardy-Weinberg equilibrium. It accounts for the persistence of rare traits such as albinism and cystic fibrosis caused by recessive alleles. Calculating H-W equilibrium Genotype frequency can be calculated by expanding the binomial (p - q)2 where p and q are allele frequencies. For example, an albino is homozygous recessive and the trait is represented by q2 in the formula: p2 + 2pq + q2 = 1. Albinos (homozygous recessive) occur in one in 20,000; therefore q2 = 1/20,000 and q = 1/141. Non-albino (p) is 1 - q = 140/141. Carriers would be 2pq or 2 x 140/141 x 1/141 = 1/70; one person in 70 is a carrier. Eliminating a disadvantageous recessive allele is nearly impossible. Selection can only act when it is expressed; it will continue through heterozygous carriers See the box on page 122 How Genetic Equilibrium is Upset In natural populations, Hardy-Weinberg equilibrium is disturbed by one or more of five factors. 1. Genetic Drift (Figure 6.28) a. A small population does not contain much genetic variation. b. Each individual contains at most two alleles at a single locus; a mating pair has a maximum of four alleles to contribute for a trait. c. By chance alone, one or two of the alleles may not be passed on. d. Chance fluctuation from generation to generation, including loss of alleles, is genetic drift. e. There is no force causing perfect constancy in allelic frequencies. f.the smaller the population, the greater the effect of drift. g. If a population is small for a long time, alleles are lost and response to change is restricted. How Genetic Equilibrium is Upset 2. Nonrandom Mating a. If two alleles are equally frequent, one half of the population will be heterozygous and one quarter will be homozygous for each allele. b. In positive assortative mating, individuals mate with others of the same genotype. 1) This increases homozygous and decreases heterozygous genotypes. 2) It does not change allelic frequencies. c. Inbreeding is preferential mating among close relatives. 1) Inbreeding increases homozygosity. 2) While positive assortative mating affects one or a few traits, inbreeding affects all variable traits. 3) Inbreeding increases the chance that recessive alleles will become homozygous and express. 4) Inbreeding cannot change gene frequencies; genetic drift does and both are common in small populations.

10 How Genetic Equilibrium is Upset 3. Migration a. Migration prevents different populations from diverging. b. Continued migration between Russia and France keeps the ABO allele frequencies from becoming completely distinct How Genetic Equilibrium is Upset 4. Natural Selection a. Natural selection changes both allelic frequencies and genotypic frequencies. b. An organism that possesses a superior combination of traits has a higher relative fitness. c. Some traits are advantageous for certain aspects of survival or reproduction and disadvantageous for others. d. Sexual selection is selection for traits that obtain a mate but may be harmful for survival. e. Changes in environment alter selective value of traits making fitness a complex problem. How Genetic Equilibrium is Upset Quantitative Variation 5. Interactions of Selection, Drift and Migration a. Subdivision of a species into small populations that exchange migrants promotes rapid evolution. b. Genetic drift and selection allow many combinations of many genes to be tested. c. Migration allows favorable new combinations to spread. d. Interactions of all factors produce change different from what would result from one alone. e. Perpetual stability almost never occurs across any significant amount of evolutionary time. Quantitative traits show continuous variation with no Mendelian segregation pattern. Such traits are influenced by variation at many genes. Such traits show a bell-shaped frequency distribution. Stabilizing selection favors the average and trims the extreme. Directional selection favors an extreme value to one side. Disruptive selection favors the extremes to both sides and disfavors the average Fig Macroevolution: Major Evolutionary Events Speciation links macroevolution to microevolution. The timescale of population genetics processes is from tens to thousands of years. Rates of speciation and extinction are measured in millions of years. Periodic mass extinctions occur in tens to hundreds of millions of years. Five mass extinctions have been dramatic. Study of long-term changes in animal diversity focuses on this longest timescale Fig. 6.32

11 Speciation and Extinction Through Geological Time A species has two possible fates: become extinct or give rise to new species. Rates of speciation and extinction vary among species. Lineages with high speciation and low extinction produce the greatest diversity. Lineages whose characteristics increase probability of speciation and confer resistance to extinction should come to dominate. Species selection is differential survival and multiplication of species based on variation among lineages. Species-level properties include mating rituals, social structuring, migration patterns, geographic distribution, etc. Effect macroevolution Effect macroevolution is similar but differential speciation and extinction is caused by variation in organismal-level properties rather than species-level properties. Food specialists would therefore be more likely to be geographically isolated. A lineage of specialized grazers and browsers has high speciation and extinction rates. A lineage of generalist grazers and browsers shows neither branching speciation nor extinction during the same time. Interestingly, the two lineages have similar numbers of individual animals alive today. (see figure 6-33) Mass Extinction Periodic events where huge numbers of taxa go extinct simultaneously are mass extinctions. The Permian extinction occurred 225 million years ago; half of the families of shallow water invertebrates and 90% of marine invertebrates disappeared. The Cretaceous extinction occurred 65 million years ago and marked the end of the dinosaurs and many other taxa. Mass extinctions appear to occur at intervals of 26 million years.