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Transcript
Biol 202: Lecture 4 Ecology & Evolution II History of Evolution • On November 24, 1859, Charles Darwin published On the Origin of Species by Means of Natural Selection. • Darwin made two points in The Origin of Species: – (1) Today’s organisms descended from ancestral species. – (2)Natural selection provided a mechanism for evolutionary change in populations. History of Evolution • Idea of evolution DID NOT originate with Charles Darwin • Ancient Greeks: Empedocles (495-435 BC), Aristotle (384-322 BC) described “concepts” of change in living organisms over time • Georges-Louis Buffon (1707-1788) spent years studying comparative anatomy. “Change must have occurred during the history of life on earth” 1 History of Evolution • Erasmus Darwin (1731-1802) - physician and grandfather of Charles Darwin was interested in questions of origin and change and believed in the common ancestry of all organisms • Jean Baptiste Lamarck (1744 - 1829) -French zoologist who studied animal classification believed that species are not constant and that existing species were derived from preexisting species. Lamarck and Evolution • In 1809, Jean Baptiste Lamarck published a theory of evolution based on his observations of fossil invertebrates • Central to Lamarck’s mechanism of evolution were the concepts of use and disuse of parts and of inheritance of acquired characteristics. – Species could not go extinct • Lamarck’s Mechanism of Evolution – Body parts used extensively to cope with the environment became larger and stronger, while those not used deteriorated. – Modifications acquired during the life of an organism could be passed to offspring. – A classic example of these is the long neck of the giraffe in which individuals could acquire longer necks by reaching for leaves on higher branches and would pass this characteristic to their offspring. 2 Darwin • Charles Darwin (1809-1882) • HMS Beagle (1831-1836) 5 weeks on Galapagos Islands • On voyage Darwin began formulating his ideas about evolution of species • By the early 1840’s Darwin had developed the major features of his theory of natural selection as the mechanism for evolution. • In 1844, he wrote a long essay on the origin of species and natural selection, but he was reluctant to publish his theory and continued to compile evidence to support his theory. • In June 1858, Alfred Wallace sent Darwin a manuscript containing a theory of natural selection essentially identical to Darwin’s. • Later that year, both Wallace’s paper and extracts of Darwin’s essay were published. • Darwin quickly finished The Origin of Species and published it the next year (1859). • While both Darwin and Wallace developed similar ideas independently, the essence of evolution by natural selection is attributed to Darwin because he developed and supported the theory of natural selection so much more extensively and earlier. 3 • Central to Darwin’s view of the evolution of life is descent with modification. • All present day organisms are related through descent from unknown ancestors. • Descendents of these ancestors accumulated diverse modifications/adaptations that fit them to specific ways of life and habitats. Darwin’s Observations & Inferences • Observation #1: All species have such great potential fertility that their population size would increase exponentially if all individuals that are born reproduced successfully. • Observation #2: Populations tend to remain stable in size, except for seasonal fluctuations. • Observation #3: Environmental resources are limited. • INFERENCE #1: Production of more individuals than the environment can support leads to a struggle for existence among the individuals of a population, with only a fraction of the offspring surviving each generation. • Observation #4: Individuals of a population vary extensively in their characteristics; no two individuals are exactly alike. • Observation #5: Much of this variation is heritable. 4 • INFERENCE #2: Survival in the struggle for existence is not random. – Those individuals whose inherited characteristics best fit them to their environment are likely to leave more offspring than less fit individuals. • INFERENCE #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over the generations. Darwin’s Main Ideas: A Summary 1. Natural selection is differential success in reproduction (unequal ability of individuals to survive and reproduce). 2. Natural selection occurs through an interaction between the environment and the variability inherent among the individual organisms making up a population. 3. The product of natural selection is the adaptation of populations of organisms to their environment. Some Definitions • Evolution - change of population over time • Microevolution - change in the gene pool of a population across generations (species level) • Macroevolution - formation of new species and new designs; evolutionary change above the level of species. 5 Microevolution Battling malaria: natural selection is fast! • DDT developed in 1942, used in India in late 1940’s • 95% effective initially but 16 months later only 20% effective 6 Industrial Melanism • Classic example of natural selection • Kettlewell (1959) conducted experiments to measure the survival of different genotypes • Calculated change in frequency of dark and light individuals Industrial Melanism Increase of melanic forms from 1848 to 1948 A population’s gene pool is defined by its allele frequencies • A population is a localized group of individuals that belong to the same species. – One definition of a species (among others) is a group of populations whose individuals have the potential to interbreed and produce fertile offspring in a nature. • The total aggregate of genes in a population at any one time is called the population’s gene pool. – It consists of all alleles at all gene loci in all individuals of a population. 7 The Hardy-Weinberg Theorem describes a non-evolving population • This theorem states that the frequencies of alleles and genotypes in a population’s gene pool will remain constant over generations in the absence of certain “forces” or “events” • Populations at Hardy-Weinberg equilibrium must satisfy five conditions. (1) Very large population size. In small populations, chance fluctuations in the gene pool can cause genotype frequencies to change over time. (2) No migrations. Gene flow, the transfer of alleles due to the movement of individuals or gametes into or out of our target population can change the proportions of alleles. (3) No new mutations. If one allele can mutate into another, the gene pool will be altered. (4) Random mating. If individuals pick mates with certain genotypes, then the mixing of gametes will not be random and the Hardy-Weinberg equilibrium does not occur. (5) No natural selection. If there is differential survival or mating success among genotypes, then the frequencies of alleles in the next variation will deviate from the frequencies predicted by the HardyWeinberg equation. • Evolution usually results when any of these five conditions are not met. 8 Hardy-Weinberg Calculations • If frequency of the dominant allele (A) in the population is p, and the frequency of the recessive allele (a) is q, then p + q = 1 • Frequency of heterozygotes (Aa), dominant homozygotes (AA) and recessive homozygotes (aa) is: • p 2 + 2pq + q2 = 1 • AA Aa aa The effect of selection on a variable characteristic/trait • Natural selection can affect the frequency of a heritable trait in a population, leading to (1) directional selection (2) diversifying selection (3) stabilizing selection 9 Speciation: the link between microevolution and macroevolution • The fossil record chronicles two patterns of speciation: anagenesis and cladogenesis. • Anagenesis is the accumulation of changes associated with the transformation of one species into another. • Cladogenesis, branching evolution, is the budding of one or more new species from a parent species. – Cladogenesis promotes biological diversity by increasing the number of species. 10 The biological species concept emphasizes reproductive isolation • In 1942 Ernst Mayr enunciated the biological species concept to divide biological diversity. – A species is a population or group of populations whose members have the potential to interbreed with each other in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species. • Species are based on ability to reproduce, not physical similarity. • Meadowlarks vs humans. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Reproductive Isolation: Mechanisms 11 Modes of Speciation • Two general modes of speciation are distinguished by the mechanism by which gene flow among populations becomes interrupted. • allopatric speciation vs sympatric speciation Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 12 Speed of Evolution 13