A. divine design and perfection modelII. Darwin’s theory: evolution occurs by natural selectionIII. The modern synthesis of evolutionary theory combines Darwin’s concept of natural selection with geneticsV. Genetic equilibrium in populations: the Hardy-Weinberg theoremVII. Genetic variation must exist for natural selection to occurBIOL 1020 – CHAPTER 23 LECTURE NOTESChapter 23: The Evolution of Population (Microevolution)I. Historical perspective – ideas on evolution and related topics up to DarwinA. divine design and perfection model1. espoused by Aristotle, the ancient Greek philosopher (284-322 B.C.) whose ideas on biology dominated most thinking on the subject until the renaissance2. species were viewed on a scale from simple to complex3. all organisms were seen as moving toward perfection (which was associated with complexity)4. based on divine intervention and design (thus supernatural, outside the true realm of science)5. now discredited in biology, but still part of the social consciousnessB. fossils1. fossils were known for centuries before Darwin2. fossils reveal organisms unlike any living today, and the idea that some fossils represent species that had become extinct was recognized even by Leonardo da Vinci (1452-1519)C. acquired traits1. mostly associated with Lamarck (1744-1829)2. still focused on a model of organisms driven toward complexity, but involved an explanation with natural causes3. postulated that changes or “acquired characteristics” during an organism’s life could be passed on to offspring4. famous example was Lamarck’s model for how giraffes developed long necks – he claimed that stretching of the neck in one generation would lead to offspring with longer necks5. understanding of genetic inheritance has led to rejection of acquired traits modelsD. birth of modern geology – Lyell, uniformitarianism, and the ancient Earth1. prior to the early 1800s, the world view of most was that the Earth is very young (around 6000 years old)2. in the early 1800s, geologists began to apply scientific reasoning to studies of geological processes, and quickly recognized that these processes require that the Earth be very old (billions of years) to occur naturally3. this “uniformitarian” model of geological processes was made famous by Lyell’s Principles of Geology, which influenced Charles Darwin4. the uniformitarian model is essentially the basis of geology today; confirming tests of this model include dating rocks using radioisotope ratios (more on that later)E. artificial selection 1. it was well known that domesticated animals and plants had been breed over centuries by humans to produce different varieties, indicating that the characteristics of a species can be modified by selection2. some examples are different breeds of dogs and the “wild cabbage” lineage of cabbage, broccoli, cauliflower, Brussels sprouts, collards, kale, etc.F. population limits that would allow selection to act naturally were recognized1. Malthus (1766-1834) wrote the most influential works on this subject2. mathematically, populations will grow geometrically if unchecked3. food supplies rarely can be expected to grow faster than arithmetically, thus putting a limit on population growthII. Darwin’s theory: evolution occurs by natural selectionA. Darwin proposed that species evolve by natural selection; his theory of evolution was based on four general observations:1. overproduction – each species produces more offspring than will survive to maturity2. variation – individuals in a population vary, and some of the variation is heritable (this was expanded by others later, as genetics came to be understood)3. competition – there is competition among the individuals of a population for limited resources (struggle for existence)4. differential reproductive success – individuals that possess more favorable characteristics (in the pool of variation) aremore likely to survive and reproduce; those with less favorable characteristics are less likely to survive and reproduceB. thus, natural selection will produce a population of individuals more suited to their environment through timeC. when populations are separated (such as the geographic separation of islands from each other and a nearby continent), natural selection on two separate populations can produce two distinct populations with different characteristics – resulting in two separate speciesD. note that for this theory to explain the current variety of species on Earth, there is a need for a long amount of time for natural selection to produce the variety observed; thus, the idea of an ancient Earth hundreds of millions to billions of years old is crucialE. note also that this theory really has two major branches: microevolution, or changes of a population over time, and macroevolution, or the formation of speciesIII. The modern synthesis of evolutionary theory combines Darwin’s concept of natural selection with geneticsA. although Mendel was a contemporary of Darwin, remember that his work was largely unrecognized until around 19001 of 4BIOL 1020 – CHAPTER 23 LECTURE NOTESB. how traits are inherited was central to Darwin’s theory of evolution, and thus Darwin (and others) were keenly interested in finding working models of inheritanceC. when genetic mechanisms came to be widely understood, they were quickly combined with Darwin’s model in the modern synthesis, also called Neo-Darwinism or the synthetic theory of evolution1. this model emphasizes the genetics of populations2. evolution is seen as working by natural selection on individuals to change the genetic makeup of populations over successive generationsD. mutations play a key role in providing a source of genetic variation1. without genetic variation, evolution cannot occur2. mutations are necessary to produce genetic variation3. while many mutations have no impact and many others are harmful, it is critical to recognize that some mutations are advantageousIV. Microevolution is a change in allele frequencies or genotype frequencies in a population over timeA. population – a group of individuals capable or interbreeding and producing fertile offspring, and that are more or less isolated from other such groupsB. gene pool – all alleles present in a population at a given timeC. phenotype frequency – proportion of a population with a given phenotypeD. genotype frequency – proportion of a population with a given genotypeE. allele