YIJC [H2] CI4 Evolution (N)(S)

2025 JC2 BIOLOGY LECTURE NOTES CORE IDEA 4: BIOLOGICAL EVOLUTION TOPIC: BIOLOGICAL EVOLUTION 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. (e) Explain how genetic variation (including harmful recessive alleles) may be preserved in a natural population. (f) define biological evolution as descent with modification and explain the link between micro- evolution and macro-evolution (g) explain how evidence based on homologies identified in biochemical data (molecular homologies) and the fossil record (anatomical homologies), together with biogeography, supports Darwin’s theory of evolution (h) explain the various concepts of the species (biological, ecological, morphological, genetic and phylogenetic concepts) (i) define biological classification as the organisation of species according to shared characteristics and describe how evolutionary relationship is established (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) (k) define phylogeny as the organisation of species to show their evolutionary relationships (l) explain the importance of the use of genome sequences in reconstructing phylogenetic relationships and state the advantages of molecular methods, including multiple sequence alignment (nucleotide and amino acid), in classifying organisms. References: Campbell and Reece. Biology. (11th edition). Chapter 23: Microevolution, P. 540 – 559, Chapter 21: How Evolution Works, P. 504 – 521, Chapter 22: Phylogenetic Reconstruction, P. 523 – 542, Chapter 24: Species and Speciation, P. 564 – 580, Chapter 25: Macroevolution, P. 583 – 607 (or any correspponding chapters in other editions) A Bridge to O Level Biology (6093 syllabus) You should have learnt to: (j) describe the difference between continuous and discontinuous variation and give examples of each (k) state that variation and competition lead to differential survival of, and reproduction by, those organisms best fitted to the environment (l) give examples of environmental factors that act as forces of natural selection (m) explain the role of natural selection as a possible mechanism for evolution (n) give examples of artificial selection such as in the production of economically important plants and animals H2

2 1 INTRODUCTION  Evolution refers to a process of descent with modification from a common ancestor. It involves a change in allele frequencies within a population over generations.  Allele frequency refers to the relative proportion of the alleles of a gene present in a population. For example, in China, 85% of the adult population are lactose intolerant, carrying the recessive allele for β-galactosidase.  Evolution is a scientific theory that explains the great biodiversity present on Earth. Biodiversity refers to number and variety of organisms (flora and fauna) found within a specified geographical region. It can also refer to the variability within and between species and within and between ecosystems. Fig. 1.1: Descent with modification.

3 PRACTICES OF SCIENCE (POS 1.1 and 1.3, pg. 4 syllabus) The Evolution of the Theory of Evolution Jean-Baptiste Lamarck was one of the bigger proponents for evolution before Darwin’s time. He suggested that species change through time and proposed a progressive change along a scale of improving characteristics (exemplified in Fig. 1.2). However, his suggested mechanism for evolution was inaccurate and was not well accepted by the scientific community of that time. Fig. 1.2: Lamarck’s proposed theory of evolution. In the 1980s, Charles Darwin, a naturalist, toured a large part of the world on a famous voyage on HMS Beagle collecting data on geology, as well as biology. He travelled for five years and his keen observations on both geology and biology led him to formulate the theory of natural selection. He recorded his ideas in numerous journals and for almost twenty years gathered enough evidence and wrote out his theory carefully before he published the famous book of On the Origin of Species by Means of Natural Selection. Before Darwin published his book, Alfred Wallace, another naturalist (also known as the father of modern biogeography), came up with the same idea of evolution by natural selection. The two co-founders of the theory of evolution corresponded during this time, providing Darwin greater assurance of his proposed mechanism. Darwin’s theories were also influenced by economist Thomas Malthus, who argued that human population is kept constant despite overproduction of offspring, due to limited resources. Darwin proposed his mechanism for evolution without any knowledge in DNA and genetics. Six years after Darwin’s proposal, Gregor Mendel published his paper in 1865 on the discrete particles of inheritance we now define as genes. Modern evolution now incorporates the principles of Mendelian genetics and knowledge of molecular biology (e.g. DNA, mutations and allele frequency), synthesising Darwin’s theory of evolution into Neo-Darwinian theory of evolution.

4 2 VARIATION  Variation in a population refers to genotypic and / or phenotypic differences between individuals in a population.  Variations in populations are due to mutation and sexual reproduction. (a) Mutation – Mutation results in the production of new alleles, increasing the gene pool of a population. A gene pool refers to the sum of all the alleles present at all gene loci of all individuals in a population at any one time. – Somatic mutations are lost with the death of the individual while germline mutations in gametes can be passed on to the next generation. – Types of mutation include: (i) gene mutation: a change in the sequence of nucleotide bases of DNA in a gene, and (ii) chromosomal aberration: a change in chromosome number or chromosome structure. (Reference to CI 2.6 DNA Mutations and CI 2.7 Chromosomal Aberrations) (b) Sexual reproduction – Sexual reproduction generates new combinations of alleles (but does not create new alleles). – Variation in sexual reproduction is due to: (i) crossing over and exchange of segments between non-sister chromatids of homologous chromosomes, (ii) independent assortment, (iii) random segregation, and (iv) random fertilisation of gametes. (Reference to CI 2.3 Cell and Nuclear Division: Meiosis) Fig. 2.1.1: Independent assortment in meiosis.

5 3 MICROEVOLUTION  Microevolution involves a change in allele frequencies within the gene pool of a population over generations.  Microevolution can occur through the following mechanisms: (i) natural selection, (ii) sexual selection, (iii) artificial selection, (iv) genetic drift, and (v) gene flow. 3.1 NATURAL SELECTION  Natural selection refers to the process by which biotic and abiotic factors in the environment act as selection pressures and select for individuals with inherited traits that are best suited to the given environment.  Individuals in a population exhibit variation in their heritable traits, and those with traits that are better suited to the environment are more likely to experience higher survival rate and reproductive success (i.e. fitness). Therefore, they are more likely to survive till reproductive age and pass down favourable alleles to their offspring. 3.1.1 Features of Natural Selection (a) Overproduction of offspring – All organisms can produce large number of offspring. If all offspring reproduce successfully, there would