EVOLUTION 1A

1 EJC H2 Biology T4W9 Evolution IA Evolution IA 1. Introduction Charles Darwin’s ideas were published in "On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life" in 1859. Darwin used this concept of natural selection to explain the variation he observed wit hin and between species. It was clear to him that those adaptations which conferred survival advantages in a given environment would come to dominate a population, all else being equal. Darwin never knew or mentioned anything about genes, alleles or muta tions. Neo-Darwinism is the theory of evolution that represents a synthesis of Darwin’s theory in terms of natural selection and modern population genetics. It supports natural selection as the major process that would account for biological evolution. 2. Learning Outcomes (a) Explain why variation (as a result of mutation, meiosis and sexual reproduction) is important in natural selection (b) Explain, with examples, how environmental factors act as forces of natural selection (c) Explain the role of natural selection in evolution (d) Explain why the population is the smallest unit that can evolve (f) Define biological evolution as descent with modification and explain the link between micro-evolution and macro-evolution (h) Explain the various concepts of the species (biological, ecological, morphological, genetic and phylogenetic concepts) (j) Explain how new species are formed with respect to geographical isolation (allopatric speciation) and behavioural or physiological isolation within the same geographical location (sympatric speciation) 3. References Campbell, N.A. and Reece, J.B. (2008). Biology, 9th edition. Pearson. Contents 1. Introduction ............................................................................................................... 1 2. Learning Outcomes.................................................................................................... 1 3. References ................................................................................................................. 1 4. Microevolution & Macroevolution ............................................................................ 2 5. Five Agents of Evolutionary Change .......................................................................... 3 6. Populations Evolve, Not Individuals ........................................................................ 14

2 EJC H2 Biology T4W9 Evolution IA 4. Microevolution & Macroevolution Broadly, biological evolution is defined as descent with modification from a common ancestor: Modification: Evolution only occurs when there are changes in allele frequency within a population over time . (Allele frequency is a measure of the chances of finding a particular allele of a gene within the population.) Descent: Evolution involves passing on of these heritable genetic changes to the next generation. Common ancestor: Present day species are related through a common prehistoric ancestor that have changed and adapted over tim e. Therefore, all species come from pre-existing species. Microevolution Microevolution is the change in allele frequency within a population or species over time. This change is due to the “5 agents of evolutionary change” which we will discuss next. Macroevolution Gradual changes in allele frequencies over many thousands of years may eventually give rise to a new species. This is macroevolution. Microevolution and macroevolution are fundamentally identical processes but on a different time scale. Microevolution may lead eventually to macroevolution, given enough time. The defining feature of macroevolution is speciation. The scale of changes is also different between the two. Microevolution is concerned with changes in allele frequencies within a species or population while macroevolution focuses on changes that occur at or above the level of species. Fig. 1: Changes in allele frequencies over time. Time Generation #1 Allele frequencies G – Green pod color = 14/20 = 7/10 or 0.7 g – Yellow pod color = 6/20 = 3/10 or 0.3 Generation #2 Allele frequencies G – Green pod color = 10/20 = 5/10 or 0.5 g – Yellow pod color = 10/20 = 5/10 or 0.5 Notes to self Notes to self

3 EJC H2 Biology T4W9 Evolution IA Examples you will learn  microevolution: antibiotic resistance in a bacteria and pigmentation of the peppered moth  macroevolution at the level of the species: Darwin’s Galapagos finches where new species of finches arise from an ancestral finch from South America  macroevolution above the level of the species: mammals and birds diverged from reptiles, reptiles diverged from amphibians, and all tetrapods diverged from fish. 5. Five Agents of Evolutionary Change The 5 agents of evolutionary change are natural selection, disruption of gene flow, genetic drift, non-random mating and mutation. (a) Natural Selection Natural selectio n results in the survival of individuals with the most favourable adaptations to a particular environment . This results in differential reproductive success in a population and, over many generations, can change the basic attributes of the population. Principles of Natural Selection & How it Brings About Evolution: (1) Overproduction of offspring  All organisms produce large numbers of offspring. This can lead to an exponential increase in size of any population, if all offspring survive. (2) Constancy of numbers  Despite the tendency towards overpopulation, most populations remain relatively stable in size. This is because only a small fraction of the offspring population survives, matures and reproduces. Majority of offspring will die before reproductive age. (3) Struggle for survival  On the basis of points 1 and 2, individuals of a species constantly struggle to survive. Competition for finite resources such as food, soil nutrients, water, mates, habitats; and other factors such as disease and predators impose a limit on their number. (4) Variation within a population  Variation is the differences between individuals of the same species (due to the presence of different alleles).  It includes morphological, physiological, biochemical and genetic differences. Notes to self

4 EJC H2 Biology T4W9 Evolution IA  It is the raw material for natural selection to act on . If not for variation, selection would not be possible as there would be no differences between individuals of a population.  Variations arise spontaneously and are not dictated by the need of organisms to survive better in the environment. Some variants are less adapted (due to the presence of unfavourable alleles) to survive in an environment compared to others. Other variants are better adapted to survive.  Variation helps to ensure perpetuation of species and safeguard species from extinction when the environment changes.  We have learnt earlier that meiosis and sexual reproduction, with the random fusion of male and female gametes, is a significant source of variation. A mating pair can give rise to offspring that are extremely varied. This is in contrast to asexual reproduction where the offspring are genetically identical to the parent.  Meiosis and random fusion of gametes however do not add new alleles to the population gene pool . They merely reshuffle existing alleles among individuals, resulting in differences in phenotypes.  Mutations, however, introduce new alleles , and enrich the gene pool. Because new alleles are introduced, there will be new phenotypes and hence increased variation. (5) Survival of the fittest by natural selection  When environmental changes such as a change in climate or an introduction of new p