Mechanism of Evolution Questions in English

(C) Statement $I$ is correct because disruptive selection favors individuals at both extremes of the phenotypic range,leading to a bimodal distribution curve.
Statement $II$ is incorrect because industrial melanism is a classic example of directional selection,where one extreme phenotype is favored over the other due to environmental changes (e.g.,soot-covered trees favoring dark-colored moths).
Therefore,statement $I$ is correct and statement $II$ is incorrect.

(C) Ethological isolation,also known as behavioral isolation,occurs when two species or populations do not interbreed because they have different mating rituals,courtship displays,or specific mating behaviors.
Since the question specifies that mating does not occur due to specific mating behavior,the correct answer is ethological isolation.
$A$. Ecological isolation involves differences in habitat.
$B$. Temporal isolation involves differences in the timing of breeding.
$D$. Mechanical isolation involves physical incompatibility of reproductive organs.

(A) Statement $I$ is correct: Charles Darwin proposed that evolution occurs through natural selection acting on small,gradual,and inheritable variations that accumulate over a long period,eventually leading to the formation of new species (speciation).
Statement $II$ is correct: Hugo de Vries proposed the Mutation Theory,suggesting that large,sudden,and inheritable changes (mutations) are the primary cause of variations and evolution,rather than small,gradual changes.

(C) The phrase 'survival of the fittest' was coined by the British philosopher and scientist $H. Spencer$ (Herbert Spencer).
Although Charles Darwin proposed the theory of natural selection,he adopted the term 'survival of the fittest' in later editions of his book 'On the Origin of Species' to describe the mechanism of natural selection,acknowledging Spencer's contribution.

(D) Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region.
Unlike allopatric speciation,which is driven by physical barriers like mountains or glaciers,sympatric speciation occurs due to reproductive isolation.
This isolation can arise from mechanisms such as polyploidy (common in plants),behavioral changes,or temporal differences in mating,which prevent gene flow between populations despite living in the same area.
Therefore,reproductive isolation is the fundamental cause of sympatric speciation.

(C) In disruptive natural selection,extreme phenotypes are selected,and intermediate forms are eliminated.
Stabilizing natural selection is the type that favours intermediate forms,while disruptive selection favours both extremes.

(D) Directional natural selection acts to eliminate one of the extremes of the phenotypic range and favour the other,e.g.,systematic shift in population traits due to environmental changes.
Stabilizing natural selection tends to favour the intermediate forms and eliminate both phenotypic extremes,e.g.,birth weight in human infants.
Statement $I$ is incorrect because it describes stabilizing selection,not directional selection.
Statement $II$ is correct because directional selection shifts the population mean towards one of the extreme values.

(D) Genetic drift refers to the change in allele frequency of a population due to random sampling of organisms.
It occurs by pure chance and is directionless.
Genetic drift has a significant impact on small populations because random fluctuations in allele frequencies can lead to the fixation or loss of alleles more rapidly than in large populations.
Therefore,the statement that it has no significance in small populations is incorrect.

(B) Allopatric speciation occurs when a biological population becomes geographically isolated from the parent population,leading to the evolution of new species.
$1$. Geographical isolation is a prerequisite for allopatric speciation.
$2$. Mutations,along with natural selection and genetic drift,play a crucial role in the divergence of the isolated populations.
$3$. While reproductive isolation eventually occurs,it is the result of the geographical barrier preventing gene flow.
$4$. Adaptive radiation is a specific type of evolution where many species evolve from a single ancestor in a short time,often associated with allopatric speciation,but the most direct statement regarding the mechanism of allopatric speciation is that mutations contribute to the genetic divergence of these separated groups.

(C) Darwinism,or the theory of natural selection,is based on several key postulates:
$1$. Overproduction (Prodigality): Organisms produce more offspring than can survive.
$2$. Struggle for existence: Due to limited resources,individuals compete for survival.
$3$. Variations: Individuals within a population exhibit variations.
$4$. Survival of the fittest: Individuals with advantageous variations are more likely to survive and reproduce.
The term 'Arrival of the fittest' is not a postulate of Darwinism. Darwin explained the 'Survival of the fittest' but did not explain the origin or 'arrival' of these variations,which was later addressed by Hugo de Vries through the mutation theory. Therefore,option $C$ is the correct answer.

(D) The germplasm theory was proposed by $August \text{ } Weismann$ in $1892$.
According to this theory, the body of an organism is divided into two types of protoplasm: $somatoplasm$ and $germplasm$.
$Somatoplasm$ constitutes the body cells and is not inherited by the offspring.
$Germplasm$ constitutes the reproductive cells (gametes) and is passed from one generation to the next, serving as the physical basis of inheritance.

(A) Statement-$I$ is correct. Natural selection acts on phenotypes, favoring traits that provide a survival or reproductive advantage in a changing environment.
Statement-$II$ is correct. The core principle of natural selection is the differential survival and reproduction of individuals based on their fitness, which maximizes the adaptive efficiency of a population within its specific environment.
Therefore, both statements are scientifically accurate descriptions of the mechanism of evolution by natural selection.

(B) $Statement-I$ is correct because isolation restricts gene flow between populations, preventing interbreeding.
$Statement-II$ is incorrect because isolating mechanisms are actually the primary drivers of divergence and speciation. By preventing gene flow, populations accumulate independent genetic variations, which eventually leads to the formation of new species.

(A) Statement-$I$ is correct: Genetic drift refers to the change in allele frequency in a population due to chance events,such as natural disasters (bottleneck effect),which can lead to the elimination of specific alleles.
Statement-$II$ is incorrect: $A$ point mutation is a change in a single base pair of $DNA$. It is not defined as 'temporary'; mutations are generally permanent changes in the genetic material that can be inherited.

(A) $Statement-I$ is correct because mutation-selection balance occurs when the rate at which a harmful mutation is introduced by mutation is equal to the rate at which it is removed by natural selection.
$Statement-II$ is incorrect because, at mutation-selection balance, the allele frequency of harmful recessive alleles reaches an equilibrium state and does not change significantly from one generation to the next.

(C) The Sewall Wright effect is another name for $Genetic \ Drift$.
Genetic drift refers to the random fluctuations in allele frequencies in a population over generations due to chance events,rather than natural selection.
It is particularly significant in small populations where random sampling errors can lead to the loss of certain alleles or the fixation of others.
Therefore,it is defined as any alteration in allele frequency of a natural population by pure chance.

(A) Prodigality refers to the excessive production of offspring. In the given scenario,if an elephant produces six young ones in its lifespan of $100$ years,it implies a theoretical reproductive potential. However,this is not sustainable in reality due to limited resources,environmental constraints,and competition for resources among individuals. The concept of 'prodigality of reproduction' is a key observation in Darwinian evolution,which suggests that while organisms have the potential to produce many offspring,environmental resistance (including competition) keeps the population in check.

(A) Seasonal isolation,also known as temporal isolation,occurs when two populations of the same species reproduce at different times of the year or day.
Because their breeding seasons do not overlap,they cannot interbreed,which acts as a pre-zygotic reproductive barrier.
Mechanical isolation refers to physical incompatibility of reproductive organs.
Habitat isolation occurs when species live in the same area but occupy different niches or habitats.
Ethological isolation (behavioral isolation) occurs due to differences in mating rituals or signals.

(A) The provided diagram shows a population where individuals with extreme phenotypes are favored over those with intermediate phenotypes,leading to the formation of two peaks. This process is known as Disruptive selection. In this type of natural selection,the intermediate trait is selected against,while both extreme traits are favored,resulting in a bimodal distribution.

(C) The concept of saltation was proposed by Hugo de Vries. He defined saltation as a single-step,large mutation that leads to speciation. Unlike Darwin's theory of gradual evolution through small variations,de Vries emphasized that evolution occurs through sudden,discontinuous variations or mutations.

(C) The correct answer is $C$.
Natural selection can lead to three different outcomes based on the distribution of traits in a population:
$1$. Stabilising selection: The mean value of the trait is favoured,resulting in a single,narrower peak.
$2$. Directional selection: One extreme phenotype is favoured,shifting the peak in one direction.
$3$. Disruptive selection: Both extreme phenotypes are favoured over the intermediate phenotype,resulting in the formation of two peaks in the distribution graph.
Therefore,the formation of two peaks indicates the effect of disruptive selection.

(D) is the mismatch: Genetic drift - Constant gene.
Genetic drift refers to the random fluctuation in the frequencies of alleles in the gene pool of a finite population due to chance events.
It occurs over a few generations.
If the change in allele frequencies is very drastic,it can lead to the formation of new species.
Therefore,genetic drift does not maintain a constant gene frequency; rather,it causes unpredictable changes in it.

(D) is the correct answer.
By the statement "Survival of the Fittest," Darwin meant that the individuals of a species that are best adapted to their environment, through favorable variations and advantageous traits, are more likely to survive and reproduce in the struggle for existence.

(A) The correct answer is $A$ (disruptive).
Natural selection can lead to three main outcomes:
$1$. Stabilising selection: In this,more individuals acquire the mean character value,reducing variation.
$2$. Directional selection: In this,more individuals acquire values other than the mean character value,shifting the population towards one extreme.
$3$. Disruptive selection: In this,more individuals acquire peripheral character values at both ends of the distribution curve,while the intermediate (mean) individuals are selected against. Since the population shows only extreme tall and extreme dwarf plants with no intermediate forms,it is a clear case of disruptive selection.

(B) Genetic drift is defined as the change in the frequency of an existing gene variant (allele) in a population due to random sampling of organisms.
It occurs by chance and is more pronounced in small populations.
Unlike natural selection,which is a non-random process,genetic drift is a stochastic (random) process.
Therefore,the correct answer is Genetic drift.

(D) Organic evolution is the process of gradual change in the inherited characteristics of biological populations over successive generations.
$1$. $Abundant$ $genotypic$ $variations$ provide the raw material for evolution.
$2$. Natural selection acts upon these variations, favoring those that are better adapted to the environment.
$3$. While reproductive isolation is crucial for speciation, the presence of abundant genotypic variations is the fundamental requirement that accelerates the rate at which evolution can occur by providing more options for natural selection to act upon.
$4$. Therefore, abundant genotypic variations are the primary driver that hastens the process of organic evolution.

(A) Natural selection outcomes are classified as follows:
$(1)$ Stabilizing selection: More individuals acquire the mean character value.
$(2)$ Directional selection: More individuals acquire a value other than the mean character value in one specific direction.
$(3)$ Disruptive selection: More individuals acquire peripheral character values at both ends of the distribution curve.
Therefore,when more individuals acquire a value other than the mean character value,it indicates Directional selection.