IB DP Biology HL Evolution and speciation Study Notes

IB DP Biology HL Evolution and speciation Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of

What is the evidence for evolution?
How do analogous and homologous structures exemplify commonality and diversity?

IBDP Biology 2025 -Study Notes -All Topics

IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2

A4.1.1—Evolution as Change in the Heritable Characteristics of a Population

Definition of Evolution:

Evolution is defined as the change in the heritable characteristics (genetic traits) of a population over successive generations. This process results in the development of new species and adaptations, driven by natural selection and genetic variation.

Darwinian Evolution vs. Lamarckism:

Aspect	Darwinian Evolution	Lamarckism
Mechanism	Based on natural selection, where individuals with advantageous traits are more likely to survive and reproduce.	Based on the idea of inheritance of acquired characteristics gained during an organism’s lifetime.
Heritability	Traits are genetically inherited and passed on to offspring.	Acquired traits are not genetically inherited (e.g., muscle growth through exercise).
Evidence	Supported by extensive fossil records, genetic studies, and observed natural selection in real time.	Discredited by modern genetics as acquired traits do not alter DNA sequences.
Example	Giraffes with naturally longer necks are more likely to survive and reproduce, passing the trait to their offspring.	Giraffes stretch their necks to reach higher leaves, and this acquired longer neck is passed on to their offspring (disproven theory).

Nature of Science (NOS): The Theory of Evolution

The theory of evolution by natural selection, proposed by Charles Darwin and Alfred Russel Wallace, explains how evolution occurs through natural selection.
Natural selection acts on phenotypic variation within a population, favoring individuals with traits that increase survival and reproductive success.

Theory vs. Absolute Proof:

In the Nature of Science, a theory is a well-substantiated explanation of some aspect of the natural world.
Though extensively supported by evidence, the theory of evolution remains a theory because scientific inquiry does not deal in absolute proofs.
It is considered a pragmatic truth, providing practical explanations that fit known observations, but it cannot be formally proven like a mathematical theorem.

Misconceptions About Evolution

Before Darwin’s theory of natural selection, Lamarckism was a popular idea. It suggested that organisms could acquire traits during their lifetime and pass them on to their offspring. However, this theory has been disproven. Acquired characteristics are not heritable.

Supporting Evidence:

Fossil Record: Shows transitional forms and gradual changes in species over time.
Comparative Anatomy: Homologous structures in different species indicate common ancestry.

Molecular Biology: Genetic similarities among different organisms suggest shared evolutionary origins.
Observed Evolution: Cases like antibiotic resistance in bacteria demonstrate evolution in real-time.

A4.1.2—Evidence for evolution from base sequences in DNA or RNA and amino acid sequences in proteins

The Language of Life

DNA, the molecule of heredity, is the language in which the story of life is written. Each organism carries within its cells a unique genetic code, a blueprint that shapes its form, function, and behavior. By comparing these genetic sequences, scientists can reconstruct the evolutionary history of life on Earth.

A Shared Ancestry

When we examine the DNA of different species, we find striking similarities. This shared genetic heritage is a testament to our common ancestry. The more closely related two species are, the more similar their DNA sequences will be. For example, humans and chimpanzees share over 98% of their DNA, reflecting our recent evolutionary divergence.

The Hox Gene Family: Architects of Development

A fascinating example of evolutionary conservation lies in the Hox gene family. These genes play a crucial role in determining the body plan of animals, specifying the development of various body parts. Remarkably, Hox genes are found in a wide range of organisms, from insects to humans. The striking similarity in Hox gene sequences across diverse species provides compelling evidence for a common ancestor and the mechanisms of evolution.

A Visual Representation of Evolutionary Relationships

A phylogenetic tree is a diagram that illustrates the evolutionary relationships between different species. The branching pattern of the tree represents the divergence of species over time. The closer two species are on the tree, the more recently they shared a common ancestor.

Conclusion

The study of DNA and protein sequences has revolutionized our understanding of evolution. By analyzing the genetic code, we can trace the intricate pathways of life’s history, revealing the shared ancestry of all living things.

A4.1.3—Evidence for evolution from selective breeding of domesticated animals and crop plants

Selective Breeding: A Human-Driven Evolution

Humans have been actively shaping the course of evolution for thousands of years through the practice of selective breeding. By carefully selecting individuals with desirable traits and breeding them together, we have created a vast array of domesticated animals and crop plants.

Examples of Selective Breeding

Livestock: Modern breeds of livestock, such as cows, pigs, and chickens, bear little resemblance to their wild ancestors. Through selective breeding, we have increased their size, meat production, and milk yield.
Dogs: The incredible diversity of dog breeds, from tiny Chihuahuas to massive Great Danes, is a testament to the power of selective breeding. Humans have bred dogs for specific purposes, such as hunting, herding, and companionship.
Crop Plants: Modern crop plants, such as corn and wheat, are far removed from their wild progenitors. Selective breeding has led to increased yields, disease resistance, and improved nutritional value.

The Mechanism of Selective Breeding

Selective breeding works by increasing the frequency of desirable alleles in a population. Alleles are different versions of a gene. By selecting individuals with the desired alleles and breeding them together, we increase the likelihood that their offspring will inherit these alleles. Over many generations, this process can lead to significant changes in the appearance and behavior of a population.

Implications for Evolution

Selective breeding demonstrates that evolution can occur rapidly, even within a few generations. It also highlights the role of humans in shaping the course of evolution. By understanding the principles of selective breeding, we can gain insights into the natural processes of evolution and apply this knowledge to improve agriculture, conservation, and medicine.

Visual Representation

These images illustrate the dramatic changes that can be achieved through selective breeding. By understanding the power of artificial selection, we can appreciate the remarkable diversity of life on Earth and the role that humans play in shaping its future.

A4.1.4—Evidence for evolution from homologous structures

Darwin’s Curiosity

Charles Darwin was intrigued by the striking similarities in the skeletal structures of various animals, despite their diverse lifestyles and functions. He observed that the forelimbs of humans, moles, horses, porpoises, and bats, though adapted for different purposes, share a fundamental underlying pattern. This unity of type, as Darwin called it, is a testament to the shared ancestry of these organisms.

Pentadactyl Limbs

The pentadactyl limb is a classic example of a homologous structure, characterized by a five-digit pattern. This limb structure is found in a wide range of vertebrates, including amphibians, reptiles, birds, and mammals.

The Basic Structure

The pentadactyl limb typically consists of the following structures:

Single bone in the proximal part: humerus (forelimb) or femur (hindlimb)
Two bones in the distal part: radius and ulna (forelimb) or tibia and fibula (hindlimb)
Group of wrist or ankle bones: carpals or tarsals
Series of bones in each of five digits: metacarpals and phalanges (forelimb) or metatarsals and phalanges (hindlimb)

Diverse Functions, Common Structure

Despite their shared anatomical structure, pentadactyl limbs have evolved to perform a variety of functions. For example:

Crocodiles: Use their hindlimbs for walking and their forelimbs for swimming.
Penguins: Use their hindlimbs for walking and their forelimbs as flippers for swimming.
Echidnas: Use all four limbs for walking and their forelimbs for digging.
Frogs: Use all four limbs for walking and their hindlimbs for jumping.

Visual Representation

Evolutionary Significance

The presence of homologous structures, such as the pentadactyl limb, provides strong evidence for the evolutionary theory. These structures suggest a common ancestor from which diverse organisms have evolved. The modifications observed in different species reflect adaptations to specific environmental pressures, demonstrating the remarkable power of evolution to shape life’s diversity.

Rudimentary Organs

Another piece of evidence for evolution comes from the existence of rudimentary organs, also known as vestigial structures. These are reduced structures that no longer serve a function but are remnants of structures that were functional in ancestors. Examples include the appendix in humans, the pelvic bones in whales, and the wings of flightless birds. These structures provide further support for the idea of common ancestry and the gradual process of evolution.

A4.1.5—Convergent evolution as the origin of analogous structures

A Tale of Two Tails

Consider the tails of fishes and whales. At first glance, they appear quite similar. However, a closer examination reveals that these structures have evolved independently. They share a similar function—propulsion through water—but their underlying anatomy and developmental origins are distinct. Such structures, which share a similar function but have different evolutionary origins, are known as analogous structures.

The Wings of Flight

Another striking example of analogous structures is the wings of birds and insects. Both structures enable flight, but they have evolved independently. Bird wings are modified forelimbs, while insect wings are outgrowths of the body wall.

The Evolutionary Explanation

Convergent evolution occurs when unrelated organisms evolve similar traits as a result of adapting to similar environmental pressures. In the case of analogous structures, different organisms have independently evolved similar solutions to the same functional challenges.

Distinguishing Homology from Analogy

It can often be challenging to determine whether similar structures in different organisms are homologous or analogous. Homologous structures share a common evolutionary origin, while analogous structures have evolved independently. Cladistics, a method of classification based on evolutionary relationships, is a valuable tool for distinguishing between these two types of similarities.

The Central Nervous System: A Complex Case

The central nervous systems (CNS) of annelids, arthropods, and vertebrates offer a fascinating example of the complexity of evolutionary relationships. While these groups share some similarities in their CNS, such as the presence of a brain and nerve cord, the underlying structures and developmental mechanisms differ significantly. This suggests that the CNS in these groups has evolved independently, despite sharing a common function.

Visual Representation

The image of the central nervous system highlights the similarities in the basic structure of the nervous system in these three groups. However, the differences in the underlying developmental pathways suggest that these similarities are the result of convergent evolution, rather than common ancestry.

The comparison of the human and octopus eyes further illustrates the concept of convergent evolution. Both eyes have similar structures, such as a lens and retina, but they have evolved independently. The differences in the placement of the nerve fibers and the presence or absence of a blind spot indicate that these structures are analogous, not homologous.

A4.1.6—Speciation by splitting of pre-existing species

The Process of Speciation

Speciation is the evolutionary process by which new species arise from existing ones. This occurs when populations of a single species become isolated from each other, preventing interbreeding. Over time, the isolated populations accumulate genetic differences due to natural selection, genetic drift, or other evolutionary forces. These genetic differences can eventually lead to reproductive isolation, meaning that individuals from the two populations can no longer interbreed and produce viable offspring.

Key Mechanisms of Speciation

There are several mechanisms that can lead to speciation:

Geographic Isolation:
- Physical barriers like mountains, rivers, or bodies of water can separate populations.
- This geographic isolation prevents gene flow between the populations, allowing them to evolve independently.
Ecological Isolation:
- Even if populations inhabit the same geographic area, they may occupy different ecological niches.
- Differences in habitat, food preferences, or mating behaviors can prevent interbreeding.
Reproductive Isolation:
- Genetic incompatibilities can arise between populations, making it difficult or impossible for them to produce viable offspring.
- This can be due to differences in mating behaviors, incompatible gametes, or developmental incompatibilities.

The Outcome of Speciation

The outcome of speciation is the formation of two or more distinct species that can no longer interbreed. The new species may exhibit significant differences in their physical characteristics, behavior, or genetic makeup.

Speciation and Biodiversity

Speciation is a fundamental process in the evolution of biodiversity. It generates new species and contributes to the diversity of life on Earth. By understanding the mechanisms of speciation, we can gain insights into the origins of different species and the patterns of evolution.

Explosive Speciation and Adaptive Radiation

In some cases, speciation can occur rapidly, leading to a burst of diversification. This is known as explosive speciation or adaptive radiation. One example is the genus Zosterops, which includes over 100 species of white-eyes distributed across Africa, Asia, and Australia. These birds have diversified into a wide range of ecological niches, each with its own unique adaptations.

Visual Representation

The fractal tree shows a branching pattern that resembles the process of speciation. Each branch represents a new species that has diverged from a common ancestor. As the tree branches further, the diversity of species increases.

The Abyssinian white-eye, Zosterops abyssinicus, is one of many species that have evolved through speciation. This bird is part of a diverse group of white-eyes that have adapted to a variety of habitats across Africa, Asia, and Australia.

A4.1.7—Roles of reproductive isolation and differential selection in speciation

Speciation: A Two-Step Process

Speciation, the formation of new species, requires two key processes:

Reproductive Isolation: Populations must become isolated from each other, preventing interbreeding. This can be achieved through geographic barriers, ecological differences, or behavioral barriers.
Differential Selection: Once isolated, the populations must experience different selective pressures, leading to genetic divergence.

Reproductive Isolation

Reproductive isolation is the key to speciation. It prevents gene flow between populations, allowing them to evolve independently. Several mechanisms can lead to reproductive isolation:

Geographic Isolation:
- Physical barriers like mountains, rivers, or bodies of water can separate populations.
- This geographic isolation prevents gene flow between the populations, allowing them to evolve independently.
Ecological Isolation:
- Even if populations inhabit the same geographic area, they may occupy different ecological niches.
- Differences in habitat, food preferences, or mating behaviors can prevent interbreeding.
Temporal Isolation:
- Populations may have different breeding seasons or times of day, preventing interbreeding.
Behavioral Isolation:
- Differences in mating behaviors, such as courtship rituals or pheromone signals, can prevent individuals from recognizing or attracting potential mates from other populations.
Mechanical Isolation:
- Physical differences in reproductive organs can prevent successful mating.
Gametic Isolation:
- Even if mating occurs, genetic incompatibilities can prevent fertilization.

Differential Selection

Once populations are reproductively isolated, they can diverge genetically due to different selective pressures. These pressures can arise from differences in climate, predators, food sources, or other environmental factors. Over time, these differences can accumulate, leading to the evolution of distinct traits and eventually, the formation of new species.

The Role of the Congo River in Bonobo Speciation

The bonobo and chimpanzee are two closely related primate species. They are thought to have diverged from a common ancestor due to geographic isolation caused by the Congo River. The river served as a barrier, preventing gene flow between the two populations. Over time, the isolated populations experienced different selective pressures, leading to the evolution of distinct physical and behavioral traits.

Example : Analyzing Speciation in Flightless Steamer Ducks

Steamer ducks, a genus of waterfowl found in South America, provide a fascinating case study in speciation. There are four species:

Fuegian Flightless Steamer Duck (Tachyeres pteneres)
Chubut Flightless Steamer Duck (Tachyeres leucocephalus)
Falklands/Malvinas Flightless Steamer Duck (Tachyeres brachypterus)
Flying Steamer Duck (Tachyeres patachonicus)

Key Points and Questions

Geographic Isolation and Speciation:
- During periods of glaciation, sea levels dropped, creating land bridges and isolating populations of steamer ducks.
- These isolated populations could have diverged genetically over time, leading to the formation of distinct species.
Current Interbreeding:
- It’s unlikely that there is significant interbreeding between flightless and flying species due to behavioral and ecological differences.
- However, within flightless or flying species, some interbreeding might occur, especially if their ranges overlap.
Evolutionary History:
- The genetic data suggests that the flightless species on the mainland diverged from a common ancestor around 2.2-0.6 million years ago.
- The flying species diverged from this common ancestor more recently, about 15,000 years ago.
Potential for Future Speciation:
- The Falklands/Malvinas flightless steamer duck, T. brachypterus, could potentially diverge into flightless and flying species if populations become isolated and experience different selective pressures.

Further Analysis and Discussion

Genetic Evidence: A more detailed analysis of the genetic data could provide further insights into the timing and mechanisms of speciation.
Ecological Factors: Studying the ecological niches of different species can help understand the factors driving speciation.
Behavioral Differences: Investigating differences in mating behaviors, courtship rituals, and other social behaviors can provide clues about reproductive isolation.

By examining the factors that have influenced the evolution of steamer ducks, we can gain a better understanding of the complex processes involved in speciation.

A4.1.8—Differences and similarities between sympatric and allopatric speciation

Speciation is the evolutionary process by which new species arise from existing ones. It requires reproductive isolation, which prevents interbreeding between populations.

Allopatric Speciation

Geographic Separation: Populations become isolated due to physical barriers like mountains, rivers, or bodies of water.
Genetic Divergence: Over time, isolated populations accumulate genetic differences through natural selection and genetic drift.
Reproductive Isolation: If these differences become significant enough, the populations can no longer interbreed, even if the geographic barrier is removed.

Sympatric Speciation

Reproductive Isolation Without Geographic Separation: Populations living in the same area become reproductively isolated due to behavioral, ecological, or genetic factors.
Genetic Divergence: Isolated populations diverge genetically, leading to the formation of new species.

Example of Sympatric Speciation

Lake Malawi Cichlids: Different species of cichlid fish have evolved in Lake Malawi due to differences in habitat preference, mating behavior, and other factors. This is an example of sympatric speciation.

Note: Sympatric speciation is less common than allopatric speciation, and it can be challenging to definitively identify cases of sympatric speciation, as allopatric speciation followed by migration can sometimes mimic sympatric speciation.

Temporal Separation is a form of reproductive isolation where populations of the same species have different breeding times. This can lead to speciation over time.

Example: Winter Processionary Moth

The winter pine processionary moth (Thaumetopoea pityocampa) provides a fascinating example of temporal separation. In one region of Portugal, researchers have discovered two populations of this moth with different life cycles:

Winter Form: Adults emerge in summer or early autumn, and larvae feed during autumn and winter.
Summer Form: Adults emerge in May or June, and larvae feed during the summer.

How Temporal Separation Leads to Speciation

Reproductive Isolation: The two forms of the moth have different breeding seasons, preventing them from interbreeding.
Genetic Divergence: Over time, the two populations may accumulate genetic differences due to different selective pressures.
Potential for Speciation: If the genetic differences become significant enough, the two forms may eventually evolve into separate species.

Key Points:

Temporal separation is a prezygotic reproductive barrier.
It can lead to speciation by preventing gene flow between populations.
The winter processionary moth example demonstrates how temporal separation can drive the evolution of new species.

By understanding the mechanisms of temporal separation and other forms of reproductive isolation, we can gain insights into the complex process of speciation.

A4.1.9—Adaptive radiation as a source of biodiversity

Adaptive radiation is a rapid evolutionary process where a single ancestral species diversifies into multiple species, each adapted to a specific ecological niche. This burst of speciation occurs when a population encounters a new environment with abundant resources and few competitors.

Key Factors Driving Adaptive Radiation:

Ecological Opportunity: The availability of new ecological niches can trigger adaptive radiation. For example, the colonization of a new island or the extinction of a dominant species can create opportunities for surviving species to diversify.
Key Innovations: The evolution of novel traits, such as wings, fins, or specialized feeding structures, can allow organisms to exploit new resources and habitats.
Reduced Competition: When competition is low, populations can diversify into different niches without direct competition with other species.

Examples of Adaptive Radiation

Darwin’s Finches

The Galapagos finches are a classic example of adaptive radiation. A single ancestral finch species diversified into numerous species with different beak shapes, each adapted to a specific food source. This adaptation allowed them to coexist without direct competition.

Brocchinia Bromeliads

The Brocchinia genus of bromeliads, found in the Guiana Shield, has also undergone adaptive radiation. These plants have evolved various strategies to capture nutrients in nutrient-poor soils, including:

Root-based nutrient acquisition
Insectivorous traps that attract and digest insects

These examples illustrate the power of adaptive radiation to generate biodiversity. By understanding the factors that drive adaptive radiation, we can gain insights into the evolution of life on Earth.

A4.1.10—Barriers to hybridization and sterility of interspecific hybrids as mechanisms for of preventing the mixing of alleles between species

Interspecific hybridization occurs when individuals from different species interbreed. While this can sometimes produce viable offspring, it often results in hybrid sterility, where the offspring are unable to reproduce. This sterility serves as a crucial barrier to gene flow between species, maintaining reproductive isolation and preventing the merging of the two species.

Mechanisms of Hybrid Sterility

Several mechanisms can lead to hybrid sterility:

Chromosomal Incompatibility:
- Different species often have different numbers or structures of chromosomes.
- When individuals from different species mate, their offspring may inherit a mismatched set of chromosomes.
- This can disrupt meiosis, the process of cell division that produces gametes, leading to sterility.
Genetic Incompatibility:
- Genes from different species may not interact properly, leading to developmental abnormalities or other problems.
- This can result in reduced viability or fertility of hybrid offspring.
Hybrid Breakdown:
- Even if hybrid offspring are viable and fertile, their offspring may have reduced fitness or be sterile.
- This can lead to the breakdown of hybrid populations over time.

The Role of Behavior in Preventing Hybridization

Courtship Behavior: Many animals have elaborate courtship displays that help individuals recognize members of their own species. Differences in courtship behavior can prevent interspecific mating, even if the species have overlapping ranges.

Example: Clark’s Grebe

Clark’s grebe has a complex courtship display that helps individuals recognize potential mates. This display involves a series of coordinated movements and vocalizations. If a bird fails to perform the correct sequence of behaviors, it may be rejected by a potential mate. This helps to maintain reproductive isolation and prevent hybridization with other species.

In Conclusion

Hybrid sterility and behavioral barriers play crucial roles in preventing hybridization and maintaining species boundaries. By understanding these mechanisms, we can gain insights into the complex process of speciation and the diversity of life on Earth.

A4.1.11—Abrupt speciation in plants by hybridization and polyploidy

Polyploidy is a phenomenon where an organism has more than two sets of chromosomes. It occurs when whole sets of chromosomes are duplicated without subsequent cell division. Genome sequencing studies have revealed that polyploidy has played a significant role in plant evolution.

Autotetraploidy

When a diploid cell undergoes whole genome duplication, it becomes a tetraploid cell with four sets of chromosomes. This is known as autotetraploidy. Autotetraploidy can lead to reproductive isolation because the increased number of chromosomes can disrupt meiosis, the process of cell division that produces gametes.

Speciation Through Polyploidy

Autotetraploid individuals may be reproductively isolated from their diploid ancestors. This can lead to the formation of new species, even if the populations remain in the same geographic area. This process is known as sympatric speciation.

Example: Sand Rock-cress

Diploid Populations: Diploid sand rock-cress (Arabidopsis arenosa) is found in eastern and southeastern Europe.
Autotetraploid Populations: Autotetraploid populations have arisen in the Balkan Peninsula and Western Carpathian Mountains.
Reproductive Isolation: The increased chromosome number in autotetraploids can lead to reproductive isolation from diploid individuals.
Spread and Diversification: Autotetraploid populations have spread to Western Europe and Scandinavia, where they may further diversify into new species.

Allopolyploidy is a type of polyploidy that arises from the hybridization of two different species followed by a chromosome doubling event. It is a significant mechanism of speciation, particularly in plants.

The Process of Allopolyploidy

Interspecific Hybridization: Two individuals from different species mate, producing a hybrid offspring.
Sterility of Hybrid: Due to chromosomal differences between the parent species, the hybrid is often sterile. It cannot produce viable gametes through meiosis.
Chromosome Doubling: A rare event occurs where the hybrid’s chromosomes double. This results in an allotetraploid, which has four sets of chromosomes, two from each parent species.
Fertility Restored: With two complete sets of chromosomes from each parent species, the allotetraploid can undergo meiosis and produce fertile gametes.
Reproductive Isolation: The allotetraploid is reproductively isolated from both parent species. It can only interbreed with other allotetraploids, forming a new species.

Significance of Allopolyploidy in Speciation

Rapid Speciation: Allopolyploidy can lead to rapid speciation, as it can create new species in a single generation.
Increased Genetic Diversity: Allopolyploids can combine the genetic diversity of two different species, leading to novel traits and adaptations.
Ecological Opportunity: Allopolyploids may be able to colonize new habitats or exploit new resources, further promoting diversification.

Examples of Allopolyploidy

Allopolyploidy has played a significant role in the evolution of many plant species, including wheat, cotton, and tobacco.

Example: Persicaria maculosa

Persicaria maculosa is a species that originated through allopolyploidy. It arose from a hybridization event between Persicaria foliosa and Persicaria lapathifolia, followed by a doubling of the chromosome number. This process resulted in a new species with unique characteristics.

Significance of Hybridization and Polyploidy

Rapid Speciation: Hybridization and polyploidy can lead to rapid speciation, as new species can arise in a single generation.
Increased Genetic Diversity: Hybridization can introduce new genetic variation into a population, leading to increased genetic diversity.
Adaptation to New Environments: Polyploid species may be better adapted to new environments, as they have a larger genetic pool.

IB DP Biology HL Evolution and speciation Study Notes | 2025 Syllabus