Adaptation via natural selection is a fundamental concept in evolutionary biology, explaining how species evolve over time to better survive and reproduce in their environments. This topic is pivotal for the Cambridge IGCSE Biology curriculum, particularly within the unit on Variation and Selection. Understanding adaptation through natural selection provides students with insights into the mechanisms driving biodiversity and the intricate relationships between organisms and their habitats.

Key Concepts

Understanding Natural Selection

Natural selection is the process by which heritable traits become more or less common in a population due to the differential reproductive success of individuals exhibiting those traits. Introduced by Charles Darwin and Alfred Russel Wallace, natural selection is a cornerstone of the theory of evolution. It operates on the variation within a population, where certain traits confer a survival or reproductive advantage.

Variation in Populations

For natural selection to occur, there must be variation within a population. This variation arises from mutations, gene flow, sexual reproduction, and genetic recombination. These genetic differences can affect an organism's morphology, physiology, and behavior, enabling some individuals to better adapt to their environment.

Heritability of Traits

Traits that provide a survival or reproductive advantage must be heritable for natural selection to influence them. Heritability ensures that advantageous traits are passed down from one generation to the next. For example, the peppered moth in England exhibited color variations that affected their camouflage against predators. The darker-colored moths had a higher survival rate during the Industrial Revolution when soot darkened tree trunks.

Survival of the Fittest

The phrase "survival of the fittest" encapsulates the essence of natural selection. Fitness, in this context, refers to an organism's ability to survive and reproduce in its environment. Individuals with higher fitness are more likely to pass their genes to the next generation, leading to an increase in the frequency of advantageous traits within the population.

Types of Natural Selection

Directional Selection: Favors one extreme phenotype over others, causing a shift in the population’s trait distribution.
Stabilizing Selection: Favors the average phenotype, reducing variation and maintaining the status quo.
Disruptive Selection: Favors extreme phenotypes on both ends, potentially leading to speciation.

Examples of Natural Selection

Numerous examples illustrate natural selection in action:

Galápagos Finches: Different species have evolved distinct beak shapes to exploit various food sources, demonstrating adaptation to specific environmental niches.
Antibiotic Resistance in Bacteria: Bacteria with mutations that confer resistance survive antibiotic treatments, leading to the prevalence of resistant strains.
Camel Adaptations: Camels have evolved thick fur, humps for fat storage, and specialized kidneys to thrive in arid desert environments.

Mechanisms Facilitating Natural Selection

Several mechanisms facilitate the process of natural selection:

Molecular Genetics: Understanding DNA and gene expression helps elucidate how traits are inherited and how genetic variations arise.
Genetic Drift: Random changes in allele frequencies can influence the genetic makeup of populations, especially in small populations.
Gene Flow: The transfer of genes between populations can introduce new genetic variations, impacting the course of natural selection.

Environmental Pressures

Environmental factors exert pressure on populations, influencing which traits are advantageous. These pressures include:

Predation: Predators can influence prey populations by favoring traits that enhance escape or camouflage.
Climate: Temperature and weather patterns can select for traits that improve thermal regulation and survival under specific climatic conditions.
Resource Availability: Scarcity or abundance of resources like food and shelter can shape the traits that are favored within a population.

Fitness Landscapes

A fitness landscape is a conceptual model that visualizes the relationship between an organism's genotype or phenotype and its reproductive success. Peaks represent high fitness, while valleys indicate low fitness. Natural selection drives populations toward peaks on the fitness landscape, optimizing their adaptation to the environment.

Balancing Selection

Balancing selection maintains genetic diversity within a population by favoring multiple alleles. This can occur through mechanisms such as heterozygote advantage, where individuals with two different alleles have higher fitness than those with identical alleles.

Co-evolution

Co-evolution refers to the reciprocal evolutionary changes that occur between interacting species. For example, predators and their prey may evolve in tandem, each adapting traits that improve their chances of survival and reproduction in response to the other.

Speciation as a Result of Natural Selection

Over time, natural selection can lead to speciation, the formation of new and distinct species. When populations become reproductively isolated and evolve independently under different selective pressures, genetic differences accumulate, eventually resulting in distinct species.

The Role of Sexual Selection

Sexual selection is a subset of natural selection where traits are favored not solely for survival but also for attracting mates. Traits such as bright plumage in birds or elaborate mating dances can enhance an individual's reproductive success, influencing the direction of natural selection.

Genetic Variation and Population Genetics

Genetic variation within a population is the raw material for evolution through natural selection. Population genetics studies the distribution and changes in allele frequencies under the influence of factors like selection, mutation, and gene flow. Understanding population genetics is crucial for comprehending how natural selection shapes populations over generations.

Adaptive Radiation

Adaptive radiation occurs when a single ancestral species diversifies into multiple species, each adapted to different niches. This process often follows events that create new habitats or remove existing competitors, allowing for rapid diversification driven by natural selection.

Examples of Adaptive Radiation

Darwin’s Finches: On the Galápagos Islands, finches evolved various beak shapes to exploit different food sources, exemplifying adaptive radiation.
Cichlid Fishes: In African Great Lakes, cichlid fishes have diversified into hundreds of species with distinct ecological roles and morphological traits.

Challenges and Limitations of Natural Selection

While natural selection is a powerful mechanism of evolution, it has limitations:

Genetic Drift: Random changes can override the effects of natural selection, especially in small populations.
Mutation Rates: High mutation rates can introduce deleterious traits faster than they can be eliminated by natural selection.
Gene Flow: Migration of individuals between populations can introduce maladaptive genes, complicating the selection process.

Human Influence on Natural Selection

Human activities have significantly impacted natural selection processes:

Environmental Change: Habitat destruction and climate change alter selective pressures, leading to shifts in population traits.
Artificial Selection: Through selective breeding, humans have shaped the traits of domesticated species, demonstrating the principles of natural selection in a controlled environment.
Pollution and Pesticides: The introduction of pollutants and pesticides can create new selective pressures, resulting in the evolution of resistance in certain species.

Case Study: The Evolution of the Peppered Moth

The peppered moth (Biston betularia) is a classic example of natural selection observed in response to environmental changes. Originally, the majority of peppered moths had light coloring, which provided camouflage against lichen-covered trees. During the Industrial Revolution, soot and pollution darkened the trees, favoring moths with darker pigmentation. Consequently, the frequency of the dark-colored moths increased in polluted areas. With the reduction of pollution in later years, lighter-colored moths became more prevalent again. This case study illustrates how environmental changes can directly influence the direction of natural selection.

Mathematical Models of Natural Selection

Population genetics employs mathematical models to predict how allele frequencies change under natural selection. One fundamental model is the Hardy-Weinberg equilibrium, which provides a baseline to measure evolutionary changes. The equation is:

$$ p^2 + 2pq + q^2 = 1 $$

Where:

p: Frequency of the dominant allele.
q: Frequency of the recessive allele.

Natural selection disrupts this equilibrium by favoring certain alleles, thereby altering genotype frequencies in subsequent generations.

Genetic Bottlenecks and Founder Effects

Genetic bottlenecks occur when populations are drastically reduced in size due to events like natural disasters, leading to a loss of genetic variation. The founder effect is a related phenomenon where a new population is established by a small number of individuals from a larger population. Both situations can result in limited genetic diversity, making populations more susceptible to the effects of natural selection and genetic drift.

Fitness and Relative Fitness

Fitness, in evolutionary biology, refers to an organism's ability to survive and reproduce. Relative fitness compares the fitness of different genotypes or phenotypes within a population. It is a measure of how successful a trait is compared to others, influencing the direction and strength of natural selection.

Sexual Dimorphism and Natural Selection

Sexual dimorphism, where males and females of a species exhibit different traits, often results from sexual selection. For instance, the elaborate plumage of male peacocks serves to attract females, enhancing reproductive success. These traits, while potentially costly in terms of survival, are favored because they increase mating opportunities.

Epigenetics and Natural Selection

Epigenetics involves changes in gene expression without alterations to the DNA sequence. While traditional natural selection focuses on genetic variations, epigenetic factors can also influence an organism's traits and adaptability. Environmental factors can induce epigenetic modifications that may be heritable, adding another layer of complexity to the process of natural selection.

Role of Sexual Reproduction in Maintaining Genetic Variation

Sexual reproduction promotes genetic variation through the recombination of alleles during meiosis and fertilization. This variation is crucial for natural selection, as it provides a diverse pool of traits for selection to act upon, enhancing the adaptability of populations to changing environments.

Climate Change and Adaptation

Climate change poses significant selective pressures on species, driving rapid evolutionary changes. Organisms must adapt to shifts in temperature, precipitation, and habitat conditions. Those that cannot adapt may face increased mortality or extinction, while others may evolve traits that enhance their resilience to new climatic conditions.

Human-Induced Selective Pressures

Humans impose selective pressures on other species through activities such as agriculture, urbanization, and pollution. For example, the widespread use of pesticides has led to the evolution of resistant insect populations. Similarly, urban environments select for traits that facilitate survival in altered landscapes, such as altered foraging behaviors or tolerance to human presence.

Conservation Biology and Natural Selection

Conservation biology leverages the principles of natural selection to preserve biodiversity. Understanding how species adapt to their environments enables the development of strategies to protect endangered species, manage habitats, and maintain genetic diversity. Conservation efforts often focus on mitigating human-induced selective pressures and promoting environments that support natural selection processes.

Advanced Concepts

In-depth Theoretical Explanations

At the core of natural selection lies the understanding of heredity and genetic variation. The modern synthesis, integrating Darwinian evolution and Mendelian genetics, provides a comprehensive framework for explaining the mechanisms of natural selection. Central to this is the concept of allele frequencies within populations, governed by factors such as selection coefficients and dominance relationships.

Mathematically, the change in allele frequency ($\Delta p$) can be described using the selection coefficient (s) and the initial allele frequency (p): $$ \Delta p = \frac{spq^2}{1 - s q^2} $$ Where:

p: Frequency of the dominant allele.
q: Frequency of the recessive allele.
s: Selection coefficient against the recessive allele.

This equation models how advantageous alleles increase in frequency over generations, underlining the predictive power of population genetics in natural selection.

Complex Problem-Solving

Consider a population of 1,000 moths with two color morphs: light (L) and dark (D). The initial frequency of the light allele (p) is 0.6, and the dark allele (q) is 0.4. Suppose that dark moths have a selective advantage, increasing their survival rate by 20% compared to light moths. Using the Hardy-Weinberg principle and selection models, we can predict the allele frequencies in the next generation.

First, calculate the fitness of each genotype:

LL: $1.0$
LD: $1.0$
DD: $1.2$

Next, determine the average fitness ($\overline{w}$) of the population:

$$ \overline{w} = p^2 \cdot w_{LL} + 2pq \cdot w_{LD} + q^2 \cdot w_{DD} $$ $$ \overline{w} = (0.6)^2 \cdot 1.0 + 2 \cdot 0.6 \cdot 0.4 \cdot 1.0 + (0.4)^2 \cdot 1.2 $$ $$ \overline{w} = 0.36 + 0.48 + 0.192 = 1.032 $$

Finally, calculate the new allele frequency for the dark allele (q'):

$$ q' = \frac{q^2 \cdot w_{DD} + pq \cdot w_{LD}}{\overline{w}} $$ $$ q' = \frac{(0.4)^2 \cdot 1.2 + 0.6 \cdot 0.4 \cdot 1.0}{1.032} $$ $$ q' = \frac{0.192 + 0.24}{1.032} \approx 0.414 $$

Thus, the frequency of the dark allele increases from 0.4 to approximately 0.414 in the next generation, demonstrating the impact of selective advantage on allele frequencies.

Interdisciplinary Connections

Natural selection intersects with various scientific disciplines, enhancing our understanding of complex biological systems:

Genetics: The study of heredity and gene function is fundamental to natural selection, providing insights into how traits are inherited and expressed.
Ecology: Understanding interactions among organisms and their environments informs the selective pressures driving adaptation.
Biochemistry: Molecular mechanisms underpinning trait development and cellular functions are essential for comprehending how genetic variations influence phenotypes.
Mathematics: Mathematical models and statistical methods are employed to predict evolutionary trends and analyze genetic data.
Geography: The distribution of species and environmental factors across different regions impacts the pathways of natural selection.

Evolutionary Developmental Biology (Evo-Devo)

Evo-Devo explores the relationship between the development of an organism and its evolutionary history. It investigates how changes in developmental processes lead to morphological and functional diversity. Understanding the developmental basis of traits provides deeper insights into how natural selection acts on specific developmental pathways, facilitating the emergence of novel adaptations.

Quantitative Genetics and Natural Selection

Quantitative genetics examines the inheritance of traits controlled by multiple genes, often influenced by environmental factors. It uses statistical methods to assess the genetic basis of phenotypic variation and predict responses to selection. This field bridges the gap between Mendelian genetics and population genetics, providing tools to analyze complex traits subject to natural selection.

Phylogenetics and Natural Selection

Phylogenetics, the study of evolutionary relationships among species, utilizes natural selection to trace lineage divergence and speciation events. By reconstructing phylogenetic trees, scientists can infer the selective pressures that shaped the evolution of different taxa, shedding light on the adaptive pathways that led to current biodiversity.

Behavioral Ecology and Natural Selection

Behavioral ecology examines how animal behaviors evolve through natural selection to enhance survival and reproductive success. Traits such as mating rituals, foraging strategies, and social structures are shaped by selective pressures, illustrating the interplay between behavior and evolutionary fitness.

Convergence and Divergence in Evolution

Convergent evolution occurs when unrelated species independently evolve similar traits in response to similar environmental challenges, while divergent evolution leads to the accumulation of differences among related species. Both processes are driven by natural selection, highlighting how selective pressures can lead to both similarity and diversity in the natural world.

Genomic Evolution and Natural Selection

Advances in genomics have revolutionized our understanding of natural selection by enabling the exploration of entire genomes. Genomic studies identify specific genes and regulatory elements under selection, revealing the molecular basis of adaptation. This holistic approach provides a comprehensive view of how natural selection shapes genetic architecture and phenotypic diversity.

Climate Modeling and Predicting Natural Selection

Climate models project future environmental changes, allowing scientists to predict how natural selection may drive evolutionary responses in various species. By anticipating shifts in habitats, temperature ranges, and resource availability, researchers can forecast potential adaptations and assess the resilience of populations to climate-induced changes.

Biotechnology and Natural Selection

Biotechnological advances harness the principles of natural selection for applications such as genetic engineering, selective breeding, and artificial selection. Techniques like CRISPR-Cas9 enable precise modifications of genetic material, facilitating the development of organisms with desired traits. Understanding natural selection informs these technologies, ensuring ethical and effective implementations.

Ethical Considerations in Studying Natural Selection

Research on natural selection and evolutionary biology raises ethical questions, particularly in areas like genetic modification and conservation. Balancing scientific advancement with ethical responsibility is crucial to ensure that interventions promote biodiversity and ecosystem health without causing unintended harm.

Future Directions in Natural Selection Research

Emerging fields and technologies continue to expand our knowledge of natural selection:

Metagenomics: Studying genetic material recovered directly from environmental samples provides insights into microbial adaptation and ecosystem dynamics.
Artificial Intelligence: AI and machine learning algorithms analyze complex evolutionary data, identifying patterns and predicting evolutionary trends.
Synthetic Biology: Designing and constructing new biological parts and systems offer opportunities to explore evolutionary principles in novel contexts.

Continued interdisciplinary collaboration and technological innovation will drive future discoveries, deepening our understanding of natural selection and its role in shaping life on Earth.

Mathematical Models: Fisher’s Fundamental Theorem of Natural Selection

Fisher's Fundamental Theorem of Natural Selection states that the rate of increase in mean fitness of a population is equal to the genetic variance in fitness at that time. Mathematically, it is expressed as: $$ \frac{d\overline{w}}{dt} = Var(w) $$ Where:

$\overline{w}$: Average fitness of the population.
Var(w): Genetic variance in fitness.

This theorem underscores the importance of genetic diversity in enabling populations to adapt and evolve. It highlights that natural selection can only act on existing genetic variation, driving the population towards higher fitness as long as such variation exists.

Game Theory and Evolutionary Strategies

Game theory, traditionally used in economics and social sciences, has applications in understanding evolutionary strategies. Concepts such as the Prisoner's Dilemma and the Hawk-Dove game model the strategic interactions between individuals, predicting behaviors that can evolve through natural selection. These models illustrate how cooperation, competition, and conflict arise and stabilize within populations.

Mathematical Derivation: Hardy-Weinberg Principle

The Hardy-Weinberg principle provides a mathematical framework to study genetic variation in populations, serving as a null model for evolution. The principle is based on five assumptions: large population size, random mating, no mutation, no migration, and no selection. The equilibrium state is described by the equation: $$ p^2 + 2pq + q^2 = 1 $$

Where:

p: Frequency of the dominant allele.
q: Frequency of the recessive allele.

Under Hardy-Weinberg equilibrium, allele and genotype frequencies remain constant across generations, allowing scientists to detect deviations caused by evolutionary forces like natural selection.

Quantifying Selection: Selection Coefficient

The selection coefficient (s) quantifies the relative fitness of a genotype compared to others. It is defined as: $$ s = \frac{w_{max} - w_i}{w_{max}} $$ Where:

w_max: Fitness of the most advantageous genotype.
w_i: Fitness of genotype i.

This coefficient measures the strength of selection acting on a particular genotype, influencing the rate at which advantageous alleles increase in frequency within a population.

Migration and Its Impact on Natural Selection

Migration, or gene flow, involves the movement of individuals and their genetic material between populations. It can introduce new alleles, increase genetic diversity, and counteract the effects of genetic drift and local selection pressures. However, excessive gene flow can homogenize populations, reducing local adaptations and potentially hindering speciation.

Phenotypic Plasticity and Natural Selection

Phenotypic plasticity refers to an organism's ability to alter its phenotype in response to environmental changes without genetic modification. While plasticity can enhance survival and reproductive success in variable environments, it also interacts with natural selection. Plastic responses can buffer populations against selective pressures, potentially slowing the rate of genetic adaptation.

Speciation Mechanisms Driven by Natural Selection

Natural selection drives speciation through mechanisms such as:

Allopatric Speciation: Geographic isolation leads to independent evolution, with natural selection favoring distinct adaptations in separated populations.
Sympatric Speciation: Speciation occurs within a single geographic area, often driven by niche differentiation and divergent natural selection.
Parapatric Speciation: Adjacent populations evolve distinct traits due to varying selective pressures across a gradient, facilitating reproductive isolation.

Behavioral Adaptations and Natural Selection

Behavioral traits, such as foraging strategies, mating rituals, and social hierarchies, are subject to natural selection. These behaviors can significantly impact an organism's fitness by influencing resource acquisition, reproductive success, and survival rates. For instance, cooperative hunting strategies in wolves enhance their ability to capture prey, increasing their fitness compared to solitary hunters.

Genomic Tools in Studying Natural Selection

Modern genomic tools, such as genome-wide association studies (GWAS) and next-generation sequencing, enable the identification of specific genes and genetic variants under selection. These tools facilitate the exploration of the genetic basis of adaptation, allowing researchers to link phenotypic traits to underlying genetic changes influenced by natural selection.

Adaptive Genetic Variation and Population Resilience

Adaptive genetic variation enhances population resilience by providing a pool of traits that can respond to environmental changes. Populations with high genetic diversity are better equipped to adapt to shifting conditions, reducing the risk of extinction and promoting long-term survival.

Impact of Horizontal Gene Transfer on Natural Selection

Horizontal gene transfer (HGT) involves the movement of genetic material between organisms outside of traditional reproduction. HGT can introduce new genes and functions into populations, offering novel adaptive potential. In bacteria, HGT contributes to rapid adaptation, such as the acquisition of antibiotic resistance genes.

Evo-Paleo Studies: Natural Selection Through Time

Evo-paleo studies combine evolutionary biology with paleontology to examine how natural selection has shaped life over geological time scales. Fossil records provide evidence of trait changes and diversification events, allowing scientists to trace the evolutionary history and understand the selective pressures that influenced major transitions in life on Earth.

Multivariate Selection and Complex Traits

Multivariate selection involves multiple traits being subject to selection simultaneously, often leading to complex evolutionary dynamics. Traits can be correlated, and the selection on one trait may indirectly affect others. Understanding multivariate selection is essential for comprehending the evolution of integrated physiological and morphological characteristics.

Balancing Selection and Maintenance of Genetic Diversity

Balancing selection maintains multiple alleles in a population, preserving genetic diversity. Mechanisms such as heterozygote advantage, frequency-dependent selection, and varying selection pressures across environments contribute to the persistence of diverse genetic variants, enhancing the population's capacity to adapt to changing conditions.

Comparison Table

Aspect	Natural Selection	Genetic Drift
Definition	The process by which heritable traits become more or less common in a population due to differential reproductive success.	Random fluctuations in allele frequencies occurring in small populations.
Cause	Selective pressures such as predation, climate, and competition.	Chance events, such as natural disasters or random mating patterns.
Effect on Genetic Variation	Increases the frequency of advantageous alleles, potentially reducing variation.	Can reduce genetic variation regardless of allele advantage.
Population Size Impact	Operates effectively in large populations.	More pronounced in small populations.
Predictability	Predictable based on environmental pressures and trait advantages.	Unpredictable due to its random nature.
Role in Evolution	Primary driver of adaptive evolution and speciation.	Introduces random changes that can influence evolutionary paths.

Summary and Key Takeaways

Adaptation via natural selection explains how traits become prevalent based on survival and reproduction advantages.
Genetic variation, heritability, and environmental pressures are essential components driving natural selection.
Advanced concepts include mathematical models, interdisciplinary connections, and the impact of human activities on natural selection.
Understanding natural selection aids in comprehending biodiversity, evolutionary mechanisms, and conservation strategies.

Examiner Tip

Tips

Use Mnemonics: Remember the types of natural selection with the mnemonic "DSSD": Directional, Stabilizing, Disruptive, and Sexual selection.

Relate to Real-World Examples: Connect theoretical concepts to real-life scenarios like antibiotic resistance or Darwin’s finches to better understand and remember how natural selection operates.

Practice Diagram Drawing: Draw and label fitness landscapes and population genetics models to visualize how allele frequencies change over generations, aiding in comprehension and retention.

Teach What You Learn: Explain the concepts of natural selection to a study partner or even to yourself. Teaching reinforces your understanding and highlights areas that need more focus.

Did You Know

1. The Galápagos Finches: The diverse beak shapes of the Galápagos finches, famously studied by Charles Darwin, are a prime example of adaptation through natural selection. Each finch species has evolved a beak shape that allows it to exploit different food sources, from seeds to insects. This specialization minimizes competition and maximizes survival in varied environments.

2. Antibiotic Resistance: Bacteria can rapidly adapt to antibiotics through natural selection. When exposed to antibiotics, resistant bacteria survive and reproduce, leading to the emergence of antibiotic-resistant strains. This real-world scenario underscores the importance of understanding natural selection to combat public health challenges.

3. The Survival of Tasmanian Devils: Tasmanian devils are facing a contagious cancer known as Devil Facial Tumour Disease (DFTD). Some populations are showing signs of natural selection, with individuals developing resistance to the disease. This ongoing evolutionary battle highlights natural selection's role in species survival.

Common Mistakes

Mistake 1: Confusing natural selection with artificial selection.
Incorrect: Believing that all selection is guided by human intervention.
Correct: Understanding that natural selection is an unguided process where environmental pressures determine which traits are favored.

Mistake 2: Overlooking the importance of genetic variation.
Incorrect: Thinking that any trait can become prevalent without genetic diversity.
Correct: Recognizing that genetic variation is essential for natural selection to act upon, enabling populations to adapt to changing environments.

Mistake 3: Misunderstanding "fitness" as physical strength.
Incorrect: Equating fitness solely with an organism's ability to fight or compete.
Correct: Defining fitness in terms of an organism's reproductive success and its ability to pass on advantageous traits to the next generation.

FAQ

What is natural selection?

Natural selection is the process by which heritable traits that enhance survival and reproduction become more common in successive generations of a population.

How does genetic variation contribute to natural selection?

Genetic variation provides the diverse traits upon which natural selection can act. Without variation, populations cannot adapt to changing environments, reducing their chances of survival.

What is the difference between natural selection and genetic drift?

Natural selection is a non-random process where traits that confer advantages become more common, while genetic drift is a random change in allele frequencies, especially in small populations.

Can natural selection lead to the development of new species?

Yes, over long periods, natural selection can lead to speciation when populations become reproductively isolated and evolve distinct traits that prevent interbreeding.

How do environmental changes affect natural selection?

Environmental changes alter the selective pressures on a population, favoring different traits that enhance survival and reproduction under the new conditions.

What role does mutation play in natural selection?

Mutations introduce new genetic variations into a population, providing additional traits that natural selection can act upon to promote adaptability and evolution.

1. Transport in Animals

1.1 Blood

1.1.1 Identify lymphocytes and phagocytes in diagrams

1.1.2 Lymphocytes produce antibodies

1.1.3 Phagocytes engulf pathogens by phagocytosis

1.1.4 Clotting: fibrinogen converts to fibrin to form mesh

1.2 Circulatory Systems

1.2.1 Single circulation in fish

1.2.2 Double circulation in mammals

1.2.3 Advantages of double circulation

1.3 Heart

1.3.1 Identify atrioventricular and semilunar valves