1 Darwin’s Delay Vestiges published 1844, anonymously by Robert ChambersElaborated the idea that all matter, inorganic and organic, evolved out of inorganic dust by the accumulation of accidental mutations caused somehow by changes in nutrition or environment. Debate and ridicule of Vestiges dissuaded Darwin from publishing ideas on speciation and to work to accumulate as much evidence as possible before publishing. Spent next decade studying and documenting variation in nature.
2 Alfred Russel Wallace Wallace the much greater naturalist. Emphasizes importance of biogeography and geographical isolation as mechanism of speciation over natural selection. Also highly influenced by Malthus. Sends paper of ideas, including role of natural selection to Darwin… “oh crap” Co-published at society meeting… virtually ignored—Darwin’s book makes splash a year later.
3 Natural Selection Darwin elaborated an evolutionary mechanism — natural selection — but was not the first to think about it. Species mutable and randomly assembled-nature has no purpose or design: Anixmander—simpler forms give rise to more complex Empedocles (c. 490–430 bc) suggested that life assembled from free-floating parts assembled and those that adapted to “some purpose” survived—process is random and directionless—not really natural selection in a true sense. Lucretius—near identical concepts to Darwin—random variations acted on by natural selection (survival of fittest). Life arose spontaneously from water &heat (first primordial soup if you will).
4 Natural Selection Aristotle—Scale of Nature-immutability of species: goal-directed process (telology) that arises through a fixed progression of steps from lowest to highest stages. There cannot, therefore, be anything arbitrary or random in this progression that would require selection. Buffon (18th century) recognized natural/artificial selection as the possible agent of extinction. Did not see natural selection as responsible for the generation of new species. Buffon believed that new species arose by spontaneous generation (BOX 6.1 ) and that differences in the conditions under which spontaneous generation occurred caused the differences between species.
5 Natural Selection Lamarck (18th century) materialist who sought only material explanations for natural phenomena (naturalism)—except his notion of vital force. Held that organisms adapted to environment through inheritance of acquired characteristics. No extinction of defective or imperfect species because organisms could always adapt themselves to changing environments by inheriting acquired characteristics. To Lamarck, variations were not separate from evolution, and therefore could not be random. Thus, selection was not needed for adaptive traits to appear. William Wells, Patrick Matthew, and others (early-mid 1800s), separated the origin of variations from the forces responsible for preserving them and used the principle of natural selection to explain changes within species. Work lacked evidence and unknown due to publication in obscure places.
6 Variation and Initiating ChangeSmall continuous variations Variation central to reality “Nature makes no leaps” but evolves by small increments and gradually (1859)-in 1836 he held the reverse position Darwin does not know how small changes could be selected (genetics now thought to rescue this-debatable as we will see later) and does not recognize that populations evolve, not individuals
7 Variation and Initiating ChangeGetting selection started “Half a wing” argument (non-flying wing serves other purposes) Evolution of the eye (Box 6.3) Dr. Strohmeyer see these as ad hoc (just-so-story) accounts A just-so story, also called the ad hoc fallacy, is a term used in academic anthropology, biological sciences, social sciences, and philosophy.
8 Variation and Initiating ChangePre-Adaptation (aka co-option and exaptation)—Evolving from something (an attempt to get around the information problem) New traits arise from pre-existing traits—a new organ (gene, protein) need not arise de novo but may be present in an organism but being used for a purpose other than that for which it is later selected. Reptilian limb to bird wing Cementing of barnacles to substrate arose from cementing mechanism to attach egg to substrate Lungs from swim bladders in fish—floatation to respiration Loss of sight and eye structures in cave organisms and brain pathways optimized for other sensory functions Evolution of the eye from simple light patch (a popular story for illustrating gradual evolutionary progression by intermediate structures)—eyes held to have arisen convergently >40X Fails to explain where initial trait comes from (information dilemma)—analogous to origin of earth life seeded by aliens) and still an ad hoc explanation
9 Evolution of the Eye: A Molecular Perspective
10 Evolving the Eye What sort of questions should does a skeptic ask?Why do all examples exist in extant organisms? Are all levels of eye complexity present at the Cambrian explosion? What other changes at any level of eye complexity are required to co-develop with eye in order for selection to create increasingly complex eyes?
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12 The evolutionary principle expressed by Darwin and Wallace is outlined in FIGURE 6.5 and 6.6. We can summarize the process proposed as a twofold cycle. Excess reproduction + limited resources → competition (struggle for existence—survival of the fittest), which, because of natural variation and natural selection allows those best adapted (those that survived selection—reproductive sieving) to pass their existing characters to the next generation.
13 Changing environments + hereditary variation + natural selection results in the modification of existing characters or the origin of new characters that become established, and spread throughout a population/species.
14 A phylogenetic tree shown as continuous branches undergoing evolutionary changes through time.Some populations have become extinct. Others have merged or diverged to produce new and different forms. The X and Y axes represent different genetic traits or phenotypes. The differences between A and H may be sufficient to warrant separate taxonomic designations. “Phylogeny happened… classification didn’t”
15 Aspects of the Natural World Explained by the Theory of Evolution by Natural SelectionBiological Diversity Natural Selection— is it the central mechanism or even a mechanism at all?
16 Acceptance of evolution as a fact came because the theory conceived by Wallace and Darwin explained much that had puzzled naturalists about the natural world. Intermediate forms observed by taxonomists could be explained as transitions between species (fossil or living). Evolutionary theory explained why organisms with similar characteristics occur geographically close to each other, while groups separated by geographical barriers have fewer characteristics in common. Structures found in different organisms but having an underlying similarity of plan (homology) could be explained by their relationship through a common ancestor. Vestigial structures such as the rudiments of limb bones in snakes and whales would be the remnants of organs that were present, fully formed, in ancestors. Indeed, whale embryos produce hind limb buds that regress early in development (Chapter 16). The similarities among vertebrate embryos during early developmental stages, even when organs were vestigial, also suggested a common evolutionary past (Chapter 16).
17 Wider Impacts of On the Origin of SpeciesMade concept of evolution of species acceptable by proposing a completely naturalistic explanation of biological diversity. Thomas Huxley (Darwin’s Bulldog)—increasingly defended Darwin’s ideas but had areas of disagreement on natural selection and species concepts. Expanded the role of biology to include the study of relationships among all living creatures, including humans (made animals of humans in a sense). Darwinian ideas have come to be applied in many fields of science and even other areas such as law, politics, sociology, psychology, religion, etc.
18 Arguments Raised Against the Theory of Evolution by Natural SelectionOthers contemporary with Darwin also criticized Darwin for not explaining how new species would arise in same local as parent population. Interbreeding maintains populations and destroys newly formed individual varieties. In essence, a population has plasticity to adapt to changing environment but speciation does not occur. Speciation requires some sort of barrier (isolating mechanism).
19 5 replies/responses/rebuttals by Darwin (think about how you would critique these) Isolation of the individuals with the new trait (adaptation) as a means to maintain beneficial trait. Some characters dominant and appear undiluted. Variation is common and arises often and so does not dilute out as it would if it was rare. Some forms that carry a particular variation pass on to future generations a propensity for increased occurrence of the same variation. Dilution of beneficial traits reduced by natural selection because enhances reproductive success of favorable while diminishing unfavorable traits, thus increasing favorable trait frequency.
20 With respect to heredity and inheritance, the rediscovery in 1900 of Mendel’s work produced a generation of Mendelians, many of whom were opposed to Darwin, especially to his mechanisms of natural selection. Evolution meant something very different to them. It would be the 1930s and 1940s before many of the difficulties Darwin faced were resolved (PopGen and Quantitative Genetics). This is when neo-Darwinism really gains a foothold in science and comes to dominate.
21 Crash course in cladistics…Plesiomorphy Apomorphy Synapomorphy Monophyletic Polyphyletic Paraphyletic Taxon Clade Sister taxa Outgroup Founder Effect