Solved Exercise, Bio-12, Ch-24


(i) Archaebacteria can tolerate high temperature up to _______. (120oC)

(ii) The first eukaryote appeared about _______ years ago. (1.5 billion)

(iii) _______ presented the theory of the origin of species by means of Natural Selection. (Charles Darwin)

(iv) _______ developed a theory of natural selection essentially identical to Darwin’s. (Wallace)

(v) _______ are considered to be the ancestors of all life. (prokaryotes)

(vi) A respiratory protein called _______ is found in all aerobic organisms. (cytochrome c)

(vii) Total aggregate of genes in a population at any time is called its _______. (gene pool)

(viii) Hardy Weinberg theorem describes a _______ population. (non-evolving)

(ix) _______ is a series of changes in the genetic composition of a population over time. (Evolution)

(x) Level of classification between species and family is called _______. (genus)

(xi) Hardy-Weinberg equation is binomial expansion of _______. (p+q)2

(xii) An _______ species is in imminent danger of extinction throughout its range. (endangered)

(xiii) A _______ is a localized group of individuals belonging to the same species. (population)

(xiv) The first photosynthetic organisms used _______ as source of hydrogen for reducing carbon dioxide to sugars. (H2S)

(xv) _______ published an essay on The Principle of Population’. (Malthus)


(i) The gill of mammals and birds’ embryos are:

(a)  Support for ontogeny recapitulates phylogeny

(b)  Used by the embryos to breathe

(c)  Homologous structure

(d)  Evidence for the degeneration of unused body parts

EXPLANATION: The gill structures in mammals and birds’ embryos are homologous, indicating a shared evolutionary origin despite different functions in adult organisms.

(ii) Darwin s theory as presented in The Origin of Species mainly:

(a)  How new species arise

(b)  How adaptations evolve

(c)  The origin of life

(d)  How extinction occur

(e)  The genetics of evolution

EXPLANATION: Darwin’s theory, as presented in “The Origin of Species,” mainly focuses on how adaptations evolve. It explains how species adapt to their environment over time, leading to the evolution of new traits and, ultimately, the emergence of new species.

(iii) The smallest biological unit that can evolve over time is:

(a)  A particular cell

(b)  A population

(c)  An individual organism

(d)  A species

(e)  An ecosystem

EXPLANATION: Species, defined by a group of interbreeding individuals, represent the smallest biological units capable of evolutionary change. Evolutionary processes, such as genetic variations and adaptations, primarily occur within populations of species.

(iv) A gene pool consists of:

(a)  All the alleles exposed to natural selection

(b)  The entire genome of a reproducing individual

(c)  The total of all alleles present in a population

(d)  The frequencies of the alleles for a gene locus within a population

(e)  All the gametes in a population

EXPLANATION: A gene pool refers to the complete set of alleles for all genes present in a population.

(v) In a population with two alleles for a particular locus, B and b, the allelic frequency of B is 0.7. What would be the frequency of heterozygote if the population is in Hardy-Weinberg equilibrium?

(a)  0.7

(b)  0.49

(c)  0.42

(d)  0.09

(e)  0.21

EXPLANATION: The Hardy-Weinberg principle states that in a population in genetic equilibrium, the genotype frequencies can be predicted based on the allele frequencies. We can use the following formula to calculate the frequency of heterozygotes (Bb): 2 ✕ p ✕ q. Where, p is the frequency of allele B (0.7 in this case) and q is the frequency of allele b (1 – p = 0.3). Plugging in the values: 2 ✕ 0.7 ✕ 0.3 = 0.42. Therefore, the frequency of heterozygotes in this population is 0.42

(vi) In a population that is in Hardy-Weinberg equilibrium, 16% of the individuals show the recessive trait. What is the frequency of the dominant alleles in the population?

(a)  0.84

(b)  0.6

(c)  0.36

(d)  0.4

(e)  0.48

EXPLANATION: In a population in Hardy-Weinberg equilibrium, the frequency of the recessive phenotype (q²) is given as 16%, or 0.16. Since q represents the frequency of the recessive allele, the square root of 0.16 gives us the frequency of q, which is 0.4. Then, subtracting q from 1 gives us the frequency of the dominant allele (p), which is 0.6. Therefore, the correct answer is (b) 0.6.

(vii) Selection acts directly on:

(a)  Phenotype

(b)  The entire genome

(c)  Genotype

(d)  Each allele

(e)  The entire gene pool

EXPLANATION: Selection, whether artificial or natural, affects the proportions of gene in a population.


Hydrothermal Vents:

“According to a hypothesis, life may have begun deep in the oceans, in underwater hot springs called hydrothermal vents.” These vents could have supplied the energy and raw materials for the origin and survival of early life forms.

Endosymbiont Hypothesis:

This is a hypothesis was first proposed by Lynn Margulis. According to this hypothesis:

“The first eukaryotic cell might have evolved when a large anaerobic amoeboid prokaryote ingested small aerobic bacteria and stabilized them instead of digesting them.” The aerobic bacteria developed into mitochondria, which are the sites of aerobic respiration in eukaryotes.

Population Genetics:

“Population genetics is the study of the distribution and change of genetic variation within populations. It explores how genetic traits and frequencies change over time within a group of interbreeding individuals, focusing on factors such as mutation, natural selection, genetic drift, and gene flow.”

Evidence of Evolution from Fossil Record:

The succession of fossil forms is a strong evidence in favour of evolution. It provides a visual record in a complete series showing the evolution of an organism.

For instance, evidence from biochemistry, molecular biology, and cell biology places prokaryotes as the ancestors of all life, and predicts that bacteria should precede all eukaryotic life in the fossil record. Indeed, the oldest known fossils are prokaryotes.

Another example is the chronological appearance of the different classes of vertebrate animals in the fossil record. Fossil fishes, the earliest vertebrates, with amphibians next, followed by reptiles, then mammals and birds. This sequence is consistent with the history of vertebrate descent. The evolution of horse provides an example of such a history


“Similarity in characteristics resulting from common ancestry is known as homology, and such anatomical signs of evolution are called homologous structures.”

Homologous organs are functionally different but structurally alike. e.g.


Fore limbs of man, bat, horse, whale etc., are examples homologous structures supporting divergent evolution.

Vestigial Organs:

“Vestigial organs are the historical remnants of structures that had important functions in ancestors but are no longer essential presently.”


1) The skeletons of whales and some snakes retain vestiges of the pelvis and leg bones of walking ancestors.

2) Vermiform appendix in carnivores.

3) Ear muscles in man.

Evolution from DNA and Proteins:

Evolutionary relationships among species are reflected in their DNA and proteins—in their genes and gene products. If two species have genes and proteins with sequences of monomers that match closely, the sequences must have been copied from a common ancestor. For example, a common genetic code brings evidence that all life is related.

Molecular biology has thus provided strong evidence in support of evolution as the basis for the unity and diversity of life. Similarly, taxonomically remote organisms, such as humans and bacteria, have some proteins in common. For instance, cytochrome c, a respiratory protein is found in all aerobic species.

Hardy Weinberg Theorem: This theorem was derived independently by two scientists in 1908. It states that:

“The frequencies of alleles and genotypes in a population’s gene pool remain constant over the generations unless acted upon by agents other than sexual recombination.”

So, shuffling of alleles due to meiosis and random fertilization has no effect on the overall genetic structure of a population.

Endangered Species:

An endangered species is in imminent danger of extinction throughout its range (where it lives).

Examples: Indus River Dolphin and Snow Leopard are among endangered species of animals in Pakistan.

Threatened Species:

A threatened species is likely to become endangered in the near future.

Examples: Himalayan brown bear and green sea turtle are among threatened species in Pakistan.

Extinct Species in Pakistan:

In Pakistan, Cheetah, Tiger, Asian lion, Indian rhino, Cheer pheasant, Crocodile and Gavial have been declared extinct.


Endangered Species:

An endangered species is in imminent danger of extinction throughout its range (where it lives).

Examples: Indus River Dolphin and Snow Leopard are among endangered species of animals in Pakistan.

Protective Measures:

Preservation of endangered species depends on a multifaceted conservation plan that includes the following components:

1) A global system of national parks to protect large tracts of land and wildlife corridors that allow movement between natural areas.

2) Protected landscapes and multiple-use areas that allow controlled private activity but also retain value as a wildlife habitat.

3) Zoos and botanical gardens to save species whose extinction is imminent.

Consult textbook at page 230 — 231.

Consult textbook at page 227 — 229.

Consult textbook at page 223 — 226.

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