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Transcript
Genetics and Evolution IB 201 Natural Selection and Adaptation TERMS transmutationism transformationism Darwinian Natural Selection Darwinian fitness chance variation artificial selection adaptation preadaptation phenotypic plasticity genotype by environment interaction HISTORY Essentialism vs Populationism Darwin encouraged a change from typological or “essentialist” thinking about species to that of species as variable, changing populations of organisms. To the essentialist, a species is a class, which is completely constant—any variation within it was accidental noise and considered irrelevant. A species was defined by its essence, as a class. These typological or essentialist views were held by many scientists and philosophers and countered Darwin’s population thinking well into the 20th century, long after Darwin published the On the Origin of Species by Natural Selection (1859). The 3 essentialist perspectives differed in terms of the mechanism by which the species comes into being: 1. Transmutationism views evolution as occurring in jumps (saltation) via mutations 2. Transformationism: evolution occurs through the gradual transformation of the “type” into a new one by: • environmental influences or the use or disuse of parts and inheritance of acquired characters • an intrinsic drive toward a “goal”, toward greater perfection Darwin broke with the essentialist views by introducing the idea that a species is a set of local populations, each of which is variable, and within each population every individual is unique to some degree. Species-specific traits show variability. Thus, Darwin introduced “population thinking”. Population thinking infers evolutionary change to be a slow, gradual process within and between populations. The change of populations of organisms is what Darwin considered to be evolution. But what is the means by which a population changes from generation to generation. Natural Selection The populational view of evolution led Darwin (and, independently, Alfred Russel Wallace) to the theory of natural selection, which is the mechanism by which he viewed change to occur. In 1859 Darwin published the expanded treatise, On the Origin of Species, in which he developed his ideas. Darwin amassed evidence to demonstrate that organisms evolve from common ancestors. He included fossil evidence and evidence from living organisms to support the theory of common descent. The pattern of evolution became clear—thus the theory of common 1 descent was mostly accepted by the scientific community during Darwin’s lifetime. However, Darwin was not satisfied with simply pointing out that evolution occurred—he wanted to know how! What is the mechanism or the process that causes evolution? How do organisms change or speciate? This was the controversial part of his theory—evolution by natural selection. It was not until well into the 20th century that the mechanism of natural selection as an explanation of how organisms evolve over time was finally accepted (Mayr 2001). Let’s begin with the complete title of Darwin’s famous book: On the Origin of Species by Means of Natural Selection, Or the Preservation of the Favoured Races in the Struggle for Life. Most of the book is a long argument in support of the mechanism of evolution by natural selection. Becoming intimately familiar with the methods of artificial selection and by reading literature outside of biology, including economics theory, Darwin recognized the following postulates: 1. All traits show phenotypic variation in a population 2. The variation can be passed on from parent to offspring—it is heritable (that portion of phenotypic variation that is due to variation in the genes) 3. Some individuals in a population are better able to survive and reproduce than others. 4. Survival and reproduction are not random, but are tied to the variation among individuals. Thus, Darwin predicted that the individuals with the favored trait, who are better able to survive and reproduce, are naturally selected. This, in essence, is Darwinian Natural Selection See the examples from the text which provide evidence of Darwinian natural selection: • Snapdragons • Galapagos Finches Natural selection (N. S.) is an elimination process: individuals compete for food, shelter, mates; if they have particular traits that enable them to better survive and reproduce in a given environment, they will be the ones to pass on their heritable traits in greater numbers than those less able to survive and reproduce. The process of N. S. includes aspects of chance variation in a population plus nonrandom survival and reproduction. Chance variation arises via mutation and recombination during meiosis. **Read the caveats concerning natural selection on pp. 87-92 Adaptation How do organisms become so well adapted to a given environment? It appears that all organisms are perfectly adapted to the environment in which they live— those that move through air have wings, those in water have fins and streamlined bodies and gills for breathing. All organisms seem to be perfectly adapted in structure and behavior to the inanimate and animate environment. The evolution of the eye of vertebrates, molluscs and insects is an example of seeming perfection; the migration of animals to the appropriate 2 environments for over-wintering and for breeding (birds, insects, etc.); the cooperative societies of social insects, the correlation between tongue length of pollinating insects and the corolla length of flowers they visit—all examples of seeming perfect adaptations. Are all organisms perfectly adapted to their environments? Historically (17th and 18th centuries), such apparently perfect adaptations were presented as proof of a creator—the field of “natural theology”. However, as more and more information about the natural world was discovered and considered, it became clear that there were many features of existence that were not “perfect” (apparent cruelty, waste, parasitism, etc.), which did not fit with a creator’s design. Darwin’s explanation for adaptation was the process of adaptation based on population thinking A character is an adaptation when it has not been eliminated among the variable populations of its ancestors. The elimination of less well adapted organisms with a given trait results in the differential survival of those organisms with characters that are better fit to a given environment. Survival enables reproduction. Thus differential survival and reproduction are the key advantages that allow organisms to pass heritable adaptations along from generation to generation. Definition A general definition of an adaptation is any trait (structural, behavioral) that contributes to the survival and reproductive success of the individual or social group that has it. A trait improving the survival and reproduction (i.e., the fitness) of its possessor is thought to be an adaptive trait. It is thought that many such traits arise through the process of natural selection. If occasionally they arise by chance, then they are maintained by natural selection. Also, a given environment can change such that any given adaptation will become ill adapted after the change. There is no absolute adaptation; an adaptation is relative to the environment in which it occurs. Adaptations must be proven with observations and experiments See examples in F&H on how adaptations are determined 1. Need to demonstrate that a trait has been selected for a particular function 2. The possession of the trait must result in increased survival and reproduction Examples of Adaptations: • Neck length in giraffes • Thermoregulation in garter snakes • Mimicry in tephritid flies • MANY OTHERS—come up with some of these yourself Phenotypic Plasticity Variation in phenotype due to environment Behavior and morphology can often exhibit phenotypic plasticity This usually involves the interplay between genotype and environment (i.e., gene by environment interaction 3 References Freeman, S. and Herron, J. C. Evolutionary Analysis, 3rd Ed. Prentice Hall, New Jersey. (pp. 69-86; 331-344; 350-351. Mayr, E. 2001. What Evolution Is. Basic Books (Perseus Books Group), NY. 4