Evolution. Objectives. Darwin s contribution to science

Transcription

1 Evolution Dr. Mario A. Fares Evolutionary Genetics and Bioinformmatics Laboratory Department of Genetics, Smurfit Institute of Genetics, TCD Objectives Darwin s contribution to science Significance of Darwin s theory of evolution Darwin and society Phone number: faresm@tcd.ie Web page: Evolution before Darwin Evolution before Darwin Platon: All the variable phenomena on the world have a fixed essence Plato ruled out evolutionary thinking because everything remains essentially unaltered under his philosophical view Aristotle also supported that the essential properties of things are fixed Categorised all living things in the world into gradated classes. These classes were sorted in increasing complexity. His view summarised the concept of Scala Naturae (Great Chain of Being) Under this view species are fixed natural entities (and therefore do not evolve 1

2 Evolution before Darwin Christian thought was very much based on the Platonic and Aristotelian philosophy and defended the immutability and perfection of design of living things This design was perfect because it came from God and was therefore unchangeable Implicit in this idea was the idea of purpose, only known by God Evolution was then negatively stigmatised Evolution before Darwin By the early 19 s century the basis over which to build the idea of organic evolution were present Geologists conceived the age of earth in millions of years The discovered fossil record was a witness of previously extinct species Systematists and comparative anatomists realised about the similarities among species The idea of inheritance was accepted by most if not all of the scientists The mechanism underlying this change among organisms was still a mistery Lamarck and the Transformism Postulates of Lamarck s Theory Evolution was driven by perfection which Lamarck expressed as the tendency towards greater organism complexity Lamarck assumed that the closer organisms from perfection the more adapted they were to the environmental conditions Those part of the body most used to overcome the environmental challenges became larger and stronger, while those not used deteriorated (Use and Disuse) Acquired characteristics are heritable 2

3 Charles Darwin ( ) Darwin presented his theories in the Origin of Species and these can be summarised into two main points: -Descent with Modification: All species descend from previous ones with no interruption - Darwin discovered and explained the mechanism of evolution by natural selection. In summary this theory has three main points: - Changes observed over time in a population of organisms are the consequence of an unbalance in the number of offspring left by individuals with different heritable traits - Survival depends on the heredity contribution of individuals - Survival of the fittest against of the flattest Lamarck s view versus Darwin s 3

4 HERITABLE VARIATION IS SUFFICIENT TO LEAD TO EVOLUTION section 3.3 (F&H) -set up experiment so that we know we have heritable variation (in this case, flower colour= variation, and we know that it is controlled by genes, so it is heritable) - let bees pollenate naturally (bees are performing the natural selection) - after several generations, look again at the population of flowers - - observe that the frequency of yellow and white flowers has changed white flowers increased in number, so were apparently selected more often by bees Darwin and the Galápagos Darwin s observations During Charles Darwin s five-year voyage ( ) on the HMS Beagle, he visited the Galápagos Islands where he made important observations that changed his ideas about the then popular concept called the fixity of species an idea holding that all present-day species had been created in their present form and had changed little or not at all Fossil mammals in South America are similar yet different from present-day llamas, sloths, and armadillos that the finches and giant tortoises living on the Galápagos Islands vary from island to island and still resemble ones from South America, even though they differ in subtle ways He concluded that organisms descended with modification from ancestors that lived during the past the central claim of the theory of evolution 4

5 Galápagos Finches Darwin s finches from the Galápagos Islands arranged to show evolutionary relationships Darwin and society Theologists claimed to have been given a new theological insight Scientists did not support the idea of change, descent with modification and considered Darwin s theory incomplete or ambiguous not explaining concepts such as, complexity or the role of design Insect eaters Insect eaters Berry eater Seed eaters Cactus eaters In fact Darwin s theory could not explain: - How is variation generated - How is this variation passed to the next generations His ideas were based on blending of parental characteristics This however would not work because blending leads to loss of variation Despite the limitations, Darwin s theory was very successful, because: a) The synthesis of the inheritance theory of Mendel with the Darwinian evolution b) New research directions c) The Darwinian s theory can explain a lot: - Patterns of biological diversity - Geographical distribution of organisms - Anatomical anomalies - Design or chance? 5

6 Anatomical convergence Absence of design Anatomical anomalies Respiratory and digestive systems for fishes and mammals 6

7 Mendel s Peas Around the same time as Darwin was doing his work, Gregor Mendel was doing some very careful and insightful experiments with plants (published 1866) Monk in Brno in present day Czech Republic Mendel used Peas for his inheritance experiments because: - True breeding commercial strains were available - easy to cultivate - easy to observe traits: Seed colour and seed shape Pod colour and Pod shape Flower color and Flower position Plant size Note that each of seed is a new individual of a different generation seeds are not of the same generation as the plant that bears them. Two Alternative Traits of the Seed Shape Character Mendel s Monohybrid Cross P to F 1 Progeny were all from one parental type. The idea of Dominance: one allele may conceal the presence of another in a heterozygote Yellow is dominant over green 7

8 Genes, Alleles, and Chromosomes Staying the Course Mendel Continued Crosses to the F 2 (the grandchildren) What was learned? The green trait was not lost or altered, even though it disappeared in the F 1. One trait is dominant to the other in its expression. The reappearance of the recessive trait in ¼ of the F 2, suggests genes come in pairs that separate in the formation of sex cells. Principle of Segregation Independent assortment A cross between individuals differing in single character is a monohybrid cross. The analysis of monohybrid crosses allowed Mendel to deduce the Principle of Segregation... Mendel performed dihybrid crosses to find out. Mendel s conclusion: Different characters are inherited independently. Genes come in pairs that separate in the formation of sex cells (and these sex cells unite randomly at fertilization). 8

9 Why Did Mendel Conclude That The Inheritance of one Trait is Independent of Another? The alternative and incorrect hypothesis: dependent inheritance. Because it s the only way to explain the pattern of inheritance? Evolutionary Theories The Punnett Square for a Dihybrid Cross Note that we re simultaneously applying the Principles of Segregations and Independent Assortment. The Modern Synthesis August Weissman Weissman s major breakthrough was in distinguishing somatic and germ line cells Distinguished somatic cells (all around your body) from germ line cells (cells that go on to form gametes [sperm and egg]) Argued that only changes in the germ line were heritable Changes in the rest of the body (phenotype) were not heritable Proposed that spontaneous and random changes can occur in genes during reproduction Offspring will have a changed genotype and phenotype Even though evolution by common descent was commonly accepted, the mechanism of natural selection was not. For example, some people believed that mutation was the only mechanism of evolution (mutationists). Reconcillation of Darwin s theory with facts of Genetics by Fisher, Haldane and Wright. Developped a mathematical theory of population genetics which showed that mutation and natural selection together cause evolution Mutation, recombination and natural selection operating within species account for the origin of new species and long-term features of evolution. 9

10 Key ideas: Genes are the key to evolution 1. Gradual evolution results from small genetic changes that are acted upon by natural selection 2. The origin of species (macroevolution) can be explained in terms of natural selection acting on individuals (microevolution) Genes are both the material of evolution, and the markers of evolution [material] They are the heritable characters that change and result in evolutionary changes [markers] They are also our major source of information for understanding the process of evolution Mutations occur in DNA sequences giving rise to variation = source of heritable variation Without mutation there is no evolution Mutations are mistakes in the copying of DNA. When a mutation occurs in the DNA sequence of a gene it may have one of several possible effects: 1. Deleterious e.g., breast cancer gene BRCA2 2. Advantageous e.g., herbicide resistance nightshade chloroplast gene 3. Neutral (or, we don t really know!) e.g., ABO blood group alleles A, B, and O alleles all persist One is not obviously more successful than the others Mutations generate new versions of the gene = alleles Allele = variant form of a gene. 10