Class 12 Biology Chapter 6 Evolution

Key Concept: Comparative Analysis

b) Gradualism

[Solution Description] Darwin’s theory includes the concepts of overproduction, struggle for existence, variation and inheritance, and survival of the fittest. Overproduction means organisms produce more offspring than can survive. Struggle for existence indicates that individuals compete for limited resources. Variation exists among individuals, affecting their chances of survival. Inheritance allows these beneficial traits to be passed on. Thus, gradualism, which refers to slow accumulation of changes, is not exclusive to Darwin’s theory but rather an overarching theme in evolutionary thought.

Key Concept: Basic Concept

d) Natural Selection
[Solution Description] The primary mechanism that drives evolution according to Darwin’s theory is natural selection. It involves the differential survival and reproduction of individuals due to differences in phenotype.

Key Concept: Complex Evolutionary Change, Genetic Recombination

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

The assertion states that genetic recombination is a major contributor to rapid evolutionary change. Recombination indeed results in new allele combinations, contributing to genetic diversity but not directly accelerating evolutionary changes unless coupled with forces such as selection or mutation.

However, the reason states that mutations add new genetic material leading to variations, while recombination shuffles existing alleles. This statement about mutations is true and highlights an important distinction: recombination does not create new alleles but reshuffles existing ones.

In summary, both statements are individually true; however, reason does not explain why recombination alone would cause rapid evolution without new mutations. Thus:

The correct answer is (b): Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

Key Concept: Interdisciplinary Connections, Theory Integration

d) By linking volcanic eruptions to the atmospheric changes needed for life.
[Solution Description] Chemical evolution provides insights into how organic molecules could form spontaneously under early Earth conditions. Geological studies offer evidence about these conditions, such as the presence of hydrothermal vents or mineral surfaces that may have catalyzed organic reactions. Together, they create a comprehensive narrative where geological settings provided the necessary environment for chemical processes leading to life.

Key Concept: Origin Environment

b) Water
[Solution Description] The syllabus mentions that early life forms were likely single-celled organisms existing in a water environment.

Key Concept: Evolutionary Process

a) Formation of giant molecules
[Solution Description] The transition from non-cellular to cellular life forms is marked by the organization and assembly of biomolecules into structures capable of self-replication and metabolism. Initially, life may have consisted of large molecules such as RNA and proteins, which possessed catalytic properties. Over time, these molecules likely formed more complex assemblies, leading to the first protocells—a key step towards fully functional cellular life. This process underscores the gradual complexity increase in molecular structures, facilitating the emergence of the first true cells.

Key Concept: Definition Recall

b) Evolution of different species from a common ancestor in a new environment
[Solution Description] Adaptive radiation is the process by which different species evolve from a common ancestor when they encounter new environments, allowing them to exploit various ecological niches.

Key Concept: Gene Migration

d) Allele frequencies change in both populations
[Solution Description] Gene migration causes allele frequencies to change in both the original and new populations as genes are exchanged between them. This leads to genetic diversity.

Key Concept: Tool Usage

c) Homo habilis
[Solution Description] Homo habilis is recognized for its use of basic stone tools, marking an important step in human technological evolution.

Key Concept: Evolutionary Implications, Evolutionary Innovations

c) Continuous evolution and increased biodiversity
[Solution Description]
The evolution of multi-celled organisms had profound effects on Earth’s biodiversity and ecological interactions. It enabled the development of complex life forms and ecosystems, significantly increasing species diversity. As multi-celled organisms developed varied structures and functions, they occupied new niches, leading to intricate food webs and interdependencies. Continuous evolution allows for adaptive radiation and evolution into myriad forms suited to diverse environments.

Key Concept: Evolutionary Processes, Genetic Drift and Speciation

c) A new species could potentially emerge from this population.

[Solution Description]

In a small population, genetic drift can lead to significant changes in allele frequencies. When a rare allele becomes common due to random shifts, it may lead to reproductive isolation if that allele affects traits important for mating. Over time, these changes could result in the formation of a new species, especially if combined with environmental pressures or selective forces.

Key Concept: Historical Context

c) 2500 million years ago
[Solution Description] The syllabus states that the first life forms appeared over 2000 million years ago. Therefore, the answer must be greater than this threshold.

Key Concept: Miller’s Experiment

c) It provided evidence that organic compounds necessary for life could form under prebiotic conditions.

[Solution Description]

Miller’s experiment demonstrated that organic molecules, such as amino acids, could be formed from inorganic precursors under conditions thought to simulate those of early Earth. This provided experimental support for the theory of chemical evolution, suggesting that life’s building blocks could naturally arise in prebiotic conditions. By applying electric sparks to a mixture of methane ($CH_4$), ammonia ($NH_3$), hydrogen ($H_2$), and water vapor ($H_2O$), Miller successfully synthesized several amino acids.

Key Concept: Radioactive Dating

d) It helps determine the ages of fossils, aiding in understanding the timeline of evolution.

[Solution Description] Radioactive dating plays a crucial role in evolutionary biology by providing accurate ages for fossils. By measuring the decay of certain isotopes within fossilized materials, scientists can determine when these organisms lived, thereby constructing a timeline of evolutionary history.

Key Concept: Ancestral Links

c) It provided evidence for fish ancestry related to amphibians.
[Solution Description]
The Coelacanth’s discovery provided a living link to an ancient lineage of lobe-finned fishes thought to be extinct, offering insights into the evolution of fish to tetrapods. This living fossil helps understand the evolutionary transitions between aquatic and terrestrial life forms.

Key Concept: Comparative Analysis

b) Adaptive radiation results in many new species from a common ancestor, whereas convergent evolution leads to similar features in unrelated species

[Solution Description] Adaptive radiation involves the evolution of diverse species from a common ancestor when they adapt to different environments. In contrast, convergent evolution refers to the development of similar traits in unrelated lineages due to similar environmental pressures. Thus, adaptive radiation focuses on diversification from a single lineage while convergent evolution involves similarities between different lineages.

Key Concept: Evolutionary Significance

c) They provided the basis for eukaryotic cell evolution.
[Solution Description] Prokaryotic cells are considered significant because they were among the first cellular forms of life, providing a foundation for more complex organisms to evolve. Their simple structure and ability to thrive in various environments allowed them to play a crucial role in shaping early life on Earth.

Key Concept: Founder Effect

b) A small group colonizes an isolated island, leading to different allele frequencies than the mainland.
[Solution Description] The founder effect occurs when a small group of individuals establishes a new population separate from the original population. As a result, the new population’s allele frequencies differ significantly from the original due to the limited genetic variation carried by the founders. Among the given options, the scenario where a small group moves to a new island and establishes a population that differs genetically from the mainland best demonstrates this concept. This has nothing to do with large populations or specific environmental adaptations but relates specifically to the isolation and founding of the population by a small number of individuals.

Key Concept: Paleontological Evidence

d) Paleontological evidence
[Solution Description] The study of fossils in sedimentary layers is called paleontological evidence. This field involves analyzing fossil records to interpret the history of life on Earth.

Key Concept: Equation Application

c) 0.09

[Solution Description]

To find the frequency of genotype $aa$, we need to calculate $q^2$. Given that $p = 0.7$, we can use the equation $p + q = 1$ to find $q$.

Calculate $q$:

$$q = 1 – p = 1 – 0.7 = 0.3$$

Now, calculate $q^2$:

$q^2 = (0.3)^2 = 0.09$

Thus, the expected frequency of genotype $aa$ is 0.09.

Key Concept: Basic Definition

d) Mutation
[Solution Description] According to Hugo deVries, mutations are responsible for large differences that appear suddenly within a population. This concept challenges the gradual changes described by traditional Darwinian evolution.

Key Concept: Comparative Analysis

d) Finches evolved due to food variety, while marsupials adapted due to isolation.
[Solution Description] Darwin’s finches are an example of adaptive radiation where species evolved due to variations in food resources on different islands. In contrast, Australian marsupials developed diversity primarily due to geographical isolation from other land masses, allowing them to occupy various ecological niches.

Key Concept: Geological Periods

c) Devonian
[Solution Description] Jawless fish primarily evolved during the Devonian period, also known as the “Age of Fishes.”

Key Concept: Factors Affecting Equilibrium

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Gene migration, or gene flow, involves the movement of individuals and their genetic material between different populations. This process can introduce new alleles to the population, thereby changing the overall allele frequencies. Since the Hardy-Weinberg principle relies on constant allele frequencies for equilibrium, any change caused by gene flow will disrupt this balance. Thus, both the assertion and reason are correct, and the reason accurately explains why gene migration affects Hardy-Weinberg equilibrium.

Key Concept: Allele Frequency

c) Frequency of recessive allele
[Solution Description] The symbol ‘q’ represents the frequency of the recessive allele in the population. When added to ‘p’, which is the frequency of the dominant allele, they sum up to 1, indicating all alleles in the population.

Key Concept: Evolutionary Evidence, Evolutionary Theories

b) Fossils of early multicellular organisms found in ocean sedimentary rocks
[Solution Description] Fossil evidence shows that some of the earliest known multicellular organisms were found in sedimentary rock layers formed in ancient oceans. These fossils display characteristics consistent with marine life forms, suggesting that multicellular life likely began in marine environments before adapting to terrestrial habitats over time.

Key Concept: Timeline Recall

c) 400,000 years ago
[Solution Description] The Neanderthal man primarily existed around 40,000 to 400,000 years ago. This timeline is known for their development and adaptation before they became extinct.

Key Concept: Basic Definition

b) Large-scale evolutionary changes that result in the formation of new species
[Solution Description] Macroevolution refers to evolutionary changes that occur at or above the level of species, leading to the formation of new species. It involves large-scale changes over long periods of time, as opposed to microevolution, which involves small-scale changes within populations.

Key Concept: Comparative Anatomy

d) Wings of insects and birds
[Solution Description] Analogous structures are those that have a similar function but do not share a common ancestry. They have evolved independently in different species. For this question, the wings of insects and birds are considered analogous because they serve the same purpose (flight) but evolved separately.

Key Concept: Analogous Structures

b) Wings of a butterfly and a bird
[Solution Description] Analogous structures perform similar functions but do not have a common evolutionary origin. This results from convergent evolution, where different species adapt similarly to comparable environmental challenges. The wings of butterflies and birds serve a similar function in flight but evolved independently, making them analogous.

Key Concept: Role of Mutations

c) Mutations are random and directionless
[Solution Description] Hugo de Vries proposed that mutations are random and directionless changes in an organism’s genetic material. Unlike Darwinian variations, which were thought to be gradual, de Vries emphasized that these mutations occur suddenly and without any specific purpose or direction.

Key Concept: Embryological Evidence

b) Pharyngeal pouches
[Solution Description] During early development, all vertebrate embryos exhibit pharyngeal pouches which suggest a shared evolutionary ancestor. These structures may develop into different features in adults, such as gills in fish or parts of the ear in mammals.

Key Concept: Fitness Concept

c) Reproductive success and adaptability
[Solution Description] Fitness, in terms of natural selection, refers to an organism’s ability to survive and reproduce in its environment. It does not necessarily relate to physical strength or size but rather to reproductive success and how well an organism’s traits allow it to adapt to its surroundings.

Key Concept: Key Characteristics

c) Results in species with identical traits
[Solution Description] Adaptive radiation involves species evolving from a common ancestor in isolated geographical areas, adapting to different habitats and filling various ecological niches. It does not result in species with identical traits; instead, it leads to diversity among species.

Key Concept: Mechanisms

d) Genetic drift

[Solution Description] Genetic drift is a mechanism of evolution that results in random changes in allele frequencies, particularly in small populations. It occurs due to chance events causing some alleles to be passed on more frequently than others, without any selective pressure.

Key Concept: Process Understanding

b) Formation of inorganic molecules, formation of organic molecules, formation of life
[Solution Description] Chemical evolution started with the formation of inorganic molecules, which then progressed to organic molecules, and ultimately led to the formation of life. Initially, simple inorganic compounds present in Earth’s early atmosphere reacted under specific conditions, leading to more complex organic molecules. Eventually, these molecules underwent further transformations to give rise to primitive cellular forms.

Key Concept: Ancestral Link

b) Frogs and salamanders.
[Solution Description] The first amphibians gave rise to modern frogs and salamanders, which share morphological and genetic traits inherited from their lobe-fin ancestors.

Key Concept: Concept Application

b) Adaptive radiation

[Solution Description] This scenario describes adaptive radiation. It involves a single ancestral species resulting in multiple descendant species, each adapted to different ecological niches on the newly discovered island.

Key Concept: Feature Identification

a) Beak shape
[Solution Description] In Darwin’s finches, adaptive radiation led to the evolution of various beak shapes, each specialized for different feeding strategies. This allows the birds to exploit different food sources on the Galapagos Islands.

Key Concept: Historical Figures

d) Thomas Malthus
[Solution Description] Thomas Malthus was an economist who proposed that populations grow exponentially while resources increase arithmetically, leading to competition for limited resources. This concept greatly influenced Charles Darwin’s formulation of the theory of natural selection, as organisms compete for resources, leading to survival of the fittest.

Key Concept: Theory Application

a) An antelope developing stronger legs because it runs often
[Solution Description] According to Lamarck, if an organism frequently uses a particular organ, that organ will become more developed over time, while organs that are not used will diminish in size.

Key Concept: Migration Patterns

c) Europe

[Solution Description]

The Neanderthal man lived between 100,000 to 40,000 years ago and primarily existed in present-day Europe and parts of western Asia. They adapted to colder climates during this period but eventually disappeared as Homo sapiens spread across these regions.