BIO 302 — Population Biology and Evolution: Lecture Note

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BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Course code: BIO 302 Course title: Population Biology and Evolution Course credit load: 2 Units Series 1: Biological Properties of Species • Part 1.1: The Species Concept o Sub Part 1.1.1: Definitions and criteria for species o Sub Part 1.1.2: The Biological Species Concept o Sub Part 1.1.3: Alternative species concepts • Part 1.2: Morphological and Genetic Properties o Sub Part 1.2.1: Morphological characters and their variation o Sub Part 1.2.2: Genetic structure of species o Sub Part 1.2.3: Hardy-Weinberg equilibrium • Part 1.3: Ecological and Reproductive Properties o Sub Part 1.3.1: Ecological niche and habitat specificity o Sub Part 1.3.2: Reproductive strategies and mating systems o Sub Part 1.3.3: Life history traits • Part 1.4: Taxonomy and Nomenclature o Sub Part 1.4.1: Principles of biological classification o Sub Part 1.4.2: Binomial nomenclature and taxonomic hierarchy o Sub Part 1.4.3: Modern phylogenetic approaches

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Introduction A species is the fundamental unit of biological classification and the basic unit on which natural selection acts. Defining what constitutes a species, describing its morphological and genetic properties, understanding its ecological and reproductive characteristics, and placing it correctly in a taxonomic framework are the foundational skills of population biology and evolutionary biology. This Series introduces the major species concepts, the morphological and genetic properties used to characterise species, Hardy-Weinberg equilibrium as the null model of population genetics, the ecological and reproductive properties of species, and the principles of taxonomy and phylogenetics. These concepts underpin all subsequent Series in this course. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1 define a species and compare the major species concepts (Linked to Part 1.1), • SLO 1.2 describe the morphological and genetic properties used to characterise species (Linked to Part 1.2), • SLO 1.3 state the Hardy-Weinberg principle, apply it to calculate allele and genotype frequencies, and list the conditions required for equilibrium (Linked to Part 1.2), • SLO 1.4 describe the ecological and reproductive properties of species (Linked to Part 1.3), • SLO 1.5 describe the major life history strategies (Linked to Part 1.3), and • SLO 1.6 explain the principles of biological classification, binomial nomenclature, and modern phylogenetics (Linked to Part 1.4).

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS 1.1 The Species Concept 1.1.1 Definitions and criteria for species A species is a group of organisms that share common characteristics distinguishing them from all other groups. The species is the lowest universally recognised taxonomic rank. Defining a species is not straightforward: over 25 species concepts have been proposed. Each concept emphasises different criteria: morphological similarity, reproductive compatibility, ecological distinctness, or evolutionary history. Practical criteria used to recognise species: morphological distinctness (differences in size, shape, colour, or anatomy visible to an observer); reproductive isolation (inability to interbreed freely and produce fertile offspring with members of other groups); ecological distinctness (occupation of a distinct ecological niche); and genetic distinctness (degree of DNA sequence divergence). No single criterion works for all groups; palaeontologists rely on morphology; microbiologists rely on DNA divergence. Fill in the blank: The ________ is the lowest universally recognised taxonomic rank; over 25 species concepts have been proposed, each emphasising different criteria such as morphology, reproductive compatibility, or evolutionary history. 1.1.2 The Biological Species Concept The Biological Species Concept (BSC), proposed by Ernst Mayr in 1942, defines a species as a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. It emphasises reproductive compatibility as the defining criterion. The BSC is the most widely used concept in animal biology and has driven much of the theory of speciation. Strengths of the BSC: it provides a clear, testable criterion (can two populations interbreed and produce fertile offspring?); it explains the discreteness of species in nature. Limitations: it cannot be applied to asexual organisms (bacteria, many protists), fossils, or purely allopatric populations that never meet; hybridisation between recognised species is common in plants, challenging the

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS strict isolation criterion. Despite limitations, the BSC remains the dominant conceptual framework for animal speciation. Fill in the blank: Ernst Mayr's Biological Species Concept defines a species as a group of interbreeding natural populations that are reproductively ________ from other such groups; it is the most widely used concept in animal biology. 1.1.3 Alternative species concepts Morphological (typological) Species Concept: species defined by diagnostic morphological characters; practical and applicable to fossils and asexual organisms; but subjective (experts disagree on which characters are diagnostic) and ignores cryptic species (morphologically identical but genetically and reproductively distinct). Phylogenetic Species Concept (PSC): the smallest monophyletic group of organisms diagnosable by unique shared derived characters (synapomorphies); widely used in modern systematics. Ecological Species Concept: a species is a lineage that occupies an adaptive zone different from related lineages; emphasises ecological roles. Cohesion Species Concept: a species is the most inclusive population sharing genetic and demographic cohesion. Recognition Species Concept: species defined by shared fertilisation systems (specific mate recognition system). In practice, most biologists apply a pluralistic approach, using whichever concept best fits the organism and question being addressed. Fill in the blank: The ________ Species Concept defines a species as the smallest monophyletic group diagnosable by unique shared derived characters; it is widely used in modern phylogenetics and molecular systematics.

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Figure 1.1: Diagram comparing four major species concepts Pilot questions 1.1 1. Define a species and state four criteria used to recognise species. 2. State the Biological Species Concept and give two strengths and two limitations. 3. Distinguish between the Morphological and Phylogenetic Species Concepts. 4. Explain why no single species concept is universally applicable. 5. Explain what a cryptic species is and why it challenges the Morphological Species Concept. 1.2 Morphological and Genetic Properties 1.2.1 Morphological characters and their variation Morphological characters are the observable physical features of an organism: external (size, shape, colour, pattern, external anatomy) and internal (skeletal structure, organ morphology, cell ultrastructure). Morphological variation within a species is normal; it results from genetic variation, developmental plasticity, and environmental effects. Continuous variation (e.g. height,

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS body mass) is controlled by many genes; discontinuous variation (e.g. blood groups, flower colour) is controlled by one or few genes. Polymorphism is the existence of two or more genetically distinct forms (morphs) within a population, each at a frequency too high to be maintained solely by mutation. Classic examples: ABO blood groups in humans; sickle-cell trait in malaria-endemic populations; colour morphs in the African swallowtail butterfly (Papilio dardanus). Morphological characters used in systematics must be heritable, discrete, and informative about evolutionary relationships rather than simply reflecting convergent adaptation to similar environments. Fill in the blank: ________ is the existence of two or more genetically distinct forms within a population, each at a frequency too high to be maintained by mutation alone; ABO blood groups and sickle-cell trait are classic examples. 1.2.2 Genetic structure of species The genetic structure of a species describes how genetic variation is distributed within and among populations. Allele frequencies measure the proportion of each allelic variant at a locus in the population. Heterozygosity (H) measures the probability that two randomly drawn alleles at a locus are different; it is a standard measure of genetic diversity. A genetically diverse species has higher evolutionary potential to respond to selection and environmental change. Gene flow: the movement of alleles between populations through migration; it homogenises allele frequencies and reduces genetic differentiation. Genetic drift: random changes in allele frequencies, more pronounced in small populations; it reduces heterozygosity over time. Inbreeding: mating between close relatives; increases homozygosity and can expose recessive deleterious alleles. FST (fixation index) measures the degree of genetic differentiation among subpopulations: FST = 0 means no differentiation; FST = 1 means complete differentiation. Fill in the blank: ________ (F_ST) measures genetic differentiation among subpopulations; a value of 0 means all populations share identical allele frequencies; a value of 1 means populations are completely differentiated.

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS 1.2.3 Hardy-Weinberg equilibrium The Hardy-Weinberg principle states that in a large, randomly mating population free from mutation, migration, and selection, allele and genotype frequencies remain constant from generation to generation. For a locus with two alleles A (frequency p) and a (frequency q), where p + q = 1: genotype frequencies are AA = p2, Aa = 2pq, aa = q2, and p2 + 2pq + q2 = 1. This is the null model of population genetics: departures from equilibrium indicate evolutionary forces at work. Five conditions required for Hardy-Weinberg equilibrium: (1) large population size (no genetic drift); (2) random mating; (3) no

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