Charles Darwin was not the first person to propose evolution, theory dates back to the Ancient Greeks

Transcription

1 Evolution Lecture 1: Evolution = biological change in the characteristics of a population over time Genetic change in a population over generations = Descent with modification Individuals do not evolve Charles Darwin was not the first person to propose evolution, theory dates back to the Ancient Greeks Example: Peppered Moth 2 forms - melanic/ peppered In Mid 19th C: 1% melanic seen near industrialised UK End 19th C (50 years later): almost 99% melanic Fact: Do organisms change over time? Course: What course did evolution take? Mechanism: What is the mechanism for evolutionary change? Charles Darwin & Alfred Russel Wallace Conditions: Intrinsic increase in no. individuals within species Competition for resources Survival of few Mechanism: Natural Selection Those individuals with more favourable features would, on average, fare better than competitors and survive, passing on to their offspring those advantageous characteristics Fact: Evidence - fossil records/ living species Course: Descent with modification Gradualism vs punctuated equilibrium Mosaic evolution Mechanism: Natural Selection Mutation Gene Flow Random Genetic Drift = Evolutionary forces

2 Pre Darwinian Evolutionary Ideas: Early Greek Philosophers Genesis "Transmutation of species" Fixity vs. change of species Carolus Linnaeus ( ) Argued that species were fixed and unchangeable - no change as we see them today Saw different species as evidence of higher power Jean Baptiste de Lamarck ( ) Published Philosophie Zoologique in 1809 (year of Darwin's birth) Proposed progressive scale (Scala Naturae) Simple forms arose spontaneously from inanimate matter and evolved into higher life forms Proposed that need itself caused evolution (Inheritance of Acquired Characteristics) Continued use = enhancement of characteristic, Disuse = characteristic dies out Darwin & Wallace - "Evolutionary change is caused by differential survival and reproduction among organisms differing in hereditary traits" Rev. Thomas Robert Malthus ( ) "An Essay on the Principle of Population, as it affects the further improvement of society" Competition for resources between humans - those who are the most able due to their characteristics will survive whereas those who aren't will die Sir Charles Lyell ( ) Principles of Uniformitarianism Laws of physics and chemistry have not changed Past geological events occurred by natural processes similar to those observed today Darwin ( ) Originally believed in fixity of species Galapagos finches - not just different varieties, completely separate species Descent from a common ancestor - Descent with Modification Alfred Russel Wallace ( ) Same theories of evolution Created a joint paper Origin of Species: the case for evolution by natural selection Published in 1859 "The preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection"

3 Darwin's model of Evolution by Natural Selection Observation 1 Organisms have great potential fertility permitting exponential population growth Observation 2 Natural populations generally don't increase exponentially - remain fairly constant in size Observation 3 Natural resources are limited - exponential growth would require unlimited resources Inference 1 A struggle for existence occurs within populations Observation 4 Variation occurs among organisms within populations Observation 5 Variation is heritable Inference 2 Varying organisms show differential survival and reproduction, favouring advantageous traits (natural selection) Inference 3 Natural selection acts over many generations Peppered Moths Increased popn. = change in selection pressure of environment Lichen on trees dies, soot replaces, therefore peppered moths experienced increased predation which led to increased survival of melanic moths Therefore increase in frequencies of melanic form Cliff Swallow Change in environment (decrease in temperature) - survivors had larger bodies and more symmetrical plumage These traits made them more energy efficient Darwin's Theories 1. Perpetual Change - living world is neither constant nor perpetually cycling, but is always changing 2. Common Descent - all forms of life descend from a common ancestor through a branching of lineages 3. Multiplication of Species - evolutionary process produces new species by splitting and transforming older species 4. Gradualism - Large differences in anatomical traits that characterise disparate species originate through the accumulation of many small incremental changes over very long periods of time 5. Natural Selection

4 To summarise Darwin and Wallace's Theories: Fact: Populations of species changed over time Course: Descent with modification Mechanism: Natural Selection These theories were challenged by: Fact: Biblical creation - genesis Fixity of species (Linnaeus) Course: Ladder of Nature (Lamarck) Mechanism: Inheritance of acquired characteristics (Lamarck) Time: Not enough time Inheritance: Where did variation come from? How are traits passed on? Lecture 2: Time (main issue with Natural Selection Theory): Charles Lyell: earth is millions of years old Archbishop James Usher/ Dr John Lightfoot: earth created in 4004BC Lord Kelvin: earth 100 million years old (revised to 20 million years old) Earth = 4.6 billion years old Heredity: "The laws governing inheritance are for the most art unknown. No one can say why the same peculiarity in different individuals of the same species, or in different species, is sometimes inherited and sometimes not so" - Origin of Species Nature of Inheritance How are traits passed on? Blending inheritance? Sources of variation How do new traits arrive? Mutation of genes Gregor Johann Mendel ( ) Inherited particles did not blend but were transmitted as discrete particles (Elemente - Genes) Each elemente has 2 possible expressions (alleles) in the individual - one dominant, one recessive

5 Mendel's Laws of Inheritance: 1. Inheritance is particulate, not blending 2. Each individual carries a pair of alleles for each trait 3. Alleles exhibit dominant or recessive expression 4. During formation of gametes, each allele of a pair travels into its own gamete (principle of segregation) 5. Alleles controlling a trait reach gametes independently of those controlling other traits (principle of independent assortment) Variation: How do traits arise? Mutation: A spontaneous error in DNA replication leafing to a heritable change in an individual's genome Some are harmful, some are helpful, some have no effects How are new genetic combinations created? Crossing over during meiosis Sexual reproduction Fitness: "Survival of the fittest" - Herbert Spencer: Principles of Biology (1864) Fitness = survival & reproductive success Selection: "The preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection." - Charles Darwin, Origin of Species Adaptation: Anatomical structure, physiological process, or behavioural trait that evolved by natural selection and improves an organism's ability to survive and leave descendants. Selection Pressures: Biotic Factors Competition Predation Disease Abiotic Factors Climate Topography Habitat Selection pressures Act on the phenotype Act on all stages of the life cycle May act for many generations

6 Types of Selection: Natural Selection Stabilising Selection: selects against extreme phenotypes Directional Selection: phenotypic character shifts in one direction Disruptive Selection: selects against average phenotypes (Polymorphism - two or more distinctive varieties within a species at different extremes) Artificial Selection Humans breed only the desired genes and therefore determine the fitness of the surviving generations Sexual Selection Selection of traits that give an individual an advantage in attracting mates, even if these traits are neutral or harmful for survival Sexual Dimorphism: difference between reproductive organs in males and females Species & Speciation: Species: Members descend from a common ancestral population Interbreeding occurs within a species but not amongst different species Genotype and phenotype within a species is similar Speciation: The evolutionary process by which a new species arises Speciation occurs due to: Isolation: Geographical: a geographical barrier separates two populations of a species, causing the two populations to differ within their own population, eventually resulting in two different species unable to interbreed Gradualism (Phyletic Evolution): Large differences in anatomical traits that characterise species originate through the accumulation of many small incremental changes over very long periods of time. The change occurs slowly over time, eventually forming morphologically quite different species - anagenesis Fossils will show many gradual 'in-between' species to show slow change Punctuated Equilibrium: Long periods of evolutionary stasis (no change) followed by relatively short periods of rapid evolutionary change. This punctuated moment is marked by speciation producing new lineages (clades) - cladogenesis

7 Lecture 3: Principles of Science: Nature of Science: Science is guided by natural law Science has to be explained by reference to natural law Science is testable The conclusions of science are tentative Science is falsifiable Scientific Method: Hypothetico-deductive Method: Scientific process of making a conjecture and then seeking empirical tests that potentially leads to its rejection Evidence for Evolution: The Fossil Record Evidence for fact and course of evolution Stratigraphy: Animals become embedded in rock as layers of sediment compact, forming sedimentary layers If perfectly preserved, age of fossils is directly proportionate to depth Radiometric Dating: Decay of isotopes over time at a known rate (half-life) Time can be calculated from amount of radioactive decay over time Good for dating igneous rocks, however fossils are found in sedimentary rocks - used to date layers above and below sedimentary layer Radiocarbon-14 Dating: Carbon-14 is produced in the upper atmosphere Half-life of C-14 is approximately 5730 years C-14 uptake stops at death - C-14 begins to decay Difference between atmosphericc-14 and fossilised C-14 is proportional to the age of the fossil This form of dating dates the fossil rather than the rock These dating methods allow us to place species along a timeline, producing a dated sequence of morphological change The fossil record provides evidence for common ancestry

8 Evolutionary Trends: Anagenesis - changes within a lineage over time Cladogenesis - splitting of a lineage (divergent evolution) Stasis - long periods of no change (evolutionary stability) Eg. Lingula (unchanged since the Silurian), Horseshoe Crab (unchanged since the Triassic period) Intermediate Form: Archaeopteryx is an example of an intermediate form - a species between reptile and bird Evolutionary Failure (Extinction) - body forms which did not survive selective pressures of their time Comparative Anatomy: Homology - "the same organ in different organisms under every variety of form and function" - Richard Owen Homologous features are present in multiple animals which point to common ancestry eg. Vertebrate forelimb, however this feature may not be used for the same function Homologous features often represent phylogenetic constraints - features unable to be completely modified Comparative Embryology: Homologies between embryos suggest common ancestry, once again representative of phylogenetic constraint Some of the embryologic features have no function in the adult form. Vestigial Features: Anatomical features which have decreased in size (and therefore functionality) throughout evolved species due to there being no need for these features in the evolved species Phylogenetic constraints can also prevent these vestigial features from decreasing in size or disappearing completely Atavistic Features: The return of a feature present in past evolutions due to remnants of the ancestral genome