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Chapter 6: Evolution

Origin of Life

Evolutionary biology is the study of the history of life forms on earth. To understand changes in flora and fauna, we must understand the origin of life itself.

  • Big Bang Theory: Explains the origin of the universe as a singular huge explosion unimaginable in physical terms. The earth is believed to have been formed about 4.5 billion years back.
  • Oparin-Haldane Theory: Proposed that the first form of life could have come from pre-existing non-living organic molecules (e.g., RNA, protein, etc.) and that formation of life was preceded by chemical evolution.
  • Miller-Urey Experiment (1953): Simulated conditions of early earth (high temperature, volcanic storms, reducing atmosphere containing \(CH_4\), \(NH_3\), \(H_2\), \(H_2O\)) and observed the formation of amino acids, supporting chemical evolution.

Evidences for Evolution

  1. Paleontological Evidence: Fossils are remains of hard parts of life-forms found in rocks. Different-aged rock sediments contain fossils of different life forms, indicating life forms varied over time.
  2. Comparative Anatomy and Morphology:
    • Homologous Organs: Organs having fundamentally the same structure and origin but different functions (e.g., forelimbs of whales, bats, Cheetah, and human). Indicates divergent evolution.
    • Analogous Organs: Organs having a similar function but different structure and origin (e.g., wings of butterfly and birds). Indicates convergent evolution.
  3. Biochemical Evidence: Similarities in proteins and genes among diverse organisms give clues to common ancestry.

Adaptive Radiation

The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation.

  • Darwin’s Finches: On passing through the Galapagos Islands, Darwin observed many varieties of finches. From the original seed-eating features, beak shapes adapted for insectivorous and vegetarian diets evolved.
  • Australian Marsupials: A number of marsupials, each different from the other evolved from an ancestral stock, all within the Australian island continent.

Biological Evolution

Evolution by natural selection would have started when cellular forms of life with differences in metabolic capability originated on earth. Lamarck’s Theory: French naturalist Lamarck suggested that evolution of life forms occurred but driven by the use and disuse of organs (e.g., giraffe’s long neck). Darwinian Theory: The two key concepts of Darwinian theory of evolution are completely branching descent and natural selection. Natural selection is based on variations, overproduction, struggle for existence, and survival of the fittest.

Mechanism of Evolution

Hugo de Vries based on his work on evening primrose brought forth the idea of mutations – large differences arising suddenly in a population. He believed that mutation causes evolution and not the minor variations that Darwin talked about. Darwinian variations are small and directional. Mutations are random and directionless. De Vries termed single step large mutation as saltation.

Hardy-Weinberg Principle

This principle states that allele frequencies in a population are stable and is constant from generation to generation (Genetic equilibrium). In a diploid, if \(p\) and \(q\) represent the frequency of allele A and allele a, respectively, the frequency of \(AA\) individuals in a population is simply given by \(p^2\), \(aa\) by \(q^2\), and \(Aa\) by \(2pq\). $$ p^2 + 2pq + q^2 = 1 $$ When frequency measured differs from expected values, the difference indicates the extent of evolutionary change. Factors affecting Hardy-Weinberg equilibrium: Gene migration, Genetic drift, Mutation, Genetic recombination, Natural selection.

A Brief Account of Evolution

  • About 2000 million years ago (mya), initial cellular forms of life appeared.
  • First amphibians evolved from stout, strong finned lobe-finned fish (coelacanths) around 350 mya.
  • Reptiles evolved from amphibians, laying thick-shelled eggs.
  • Mammals evolved around the time dinosaurs went extinct (about 65 mya). Mammals were small, shrew-like.

Origin and Evolution of Man

  • Dryopithecus and Ramapithecus: Primates existing around 15 mya.
  • Australopithecines: Existed 2 mya in East African grasslands. Hunted with stone weapons but essentially ate fruit.
  • Homo habilis: The first human-like being the hominid, brain capacity 650-800cc. Did not eat meat.
  • Homo erectus: Fossils found in Java (1891). Lived 1.5 mya. Brain size 900cc. Ate meat.
  • Neanderthal man: Brain size 1400cc. Lived near East and Central Asia between 100,000-40,000 years back. Buried their dead.
  • Homo sapiens: Arose in Africa and moved across continents. Agriculture emerged around 10,000 years back.

Competency Based Questions

Q1. In a randomly mating population at Hardy-Weinberg equilibrium, the frequency of an autosomal recessive disease is 4 in 10,000. What is the frequency of the carrier state (heterozygotes) in this population?

(A) 0.0196 (approx. 2%)
(B) 0.0392 (approx. 4%)
(C) 0.0004 (approx. 0.04%)
(D) 0.9800 (approx. 98%)

Answer and Explanation Answer: (B) 0.0392 (approx. 4%)

Explanation:
According to the Hardy-Weinberg principle, the frequency of homozygous recessive individuals is \(q^2\).
Given \(q^2 = \frac{4}{10000} = 0.0004\)
Therefore, the frequency of the recessive allele \(q\) is:
$$ q = \sqrt{0.0004} = 0.02 $$
The frequency of the dominant allele \(p\) is:
$$ p = 1 - q = 1 - 0.02 = 0.98 $$
The frequency of carriers (heterozygotes) is \(2pq\):
$$ 2pq = 2 \times 0.98 \times 0.02 = 0.0392 $$
or approximately 3.92% (\(\approx 4\%\)).

Q2. During industrial melanism in England, the population of dark-winged moths increased significantly over the light-winged moths after industrialization. This is an example of:

(A) Disruptive selection
(B) Directional selection
(C) Stabilising selection
(D) Artificial selection

Answer and Explanation Answer: (B) Directional selection

Explanation:
In directional selection, one extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. Before industrialization, light coloured moths were favoured. After industrialization, trees became covered in dark soot, favouring the dark melanic phenotype for camouflage against predators. The population distribution shifted continuously towards the dark-winged extreme.

Q3. If the theory of punctuated equilibrium is correct for a specific lineage, how would the fossil record reflect this?

Answer and Explanation Answer: Long periods of stasis abruptly interrupted by sudden morphological changes.

Explanation:
Punctuated equilibrium proposes that most species will exhibit little net evolutionary change for most of their geological history, remaining in an extended state of stasis. When significant evolutionary change occurs, it is generally restricted to rare and geologically rapid events of branching speciation. Thus, the fossil record would show long sequences of identical fossils separated by few transitional forms and sudden appearances of new species.

Q4. Explain how the bottleneck effect could accelerate the extinction of an endangered species even if a conservation program successfully protects the remaining individuals from predators and habitat loss.

Answer and Explanation Answer: Severe loss of genetic diversity prevents adaptation and leads to inbreeding depression.

Explanation:
A bottleneck event occurs when a population’s size is drastically reduced for at least one generation. Even if the surviving individuals are perfectly protected and they breed successfully, they represent only a tiny fraction of the original gene pool.
This massive loss of genetic variation (alleles) means the new population has little adaptive capacity to deal with new diseases, changing climates, or novel environmental challenges. Additionally, the small population size forces inbreeding, increasing the frequency of deleterious recessive alleles expressing homozygous traits (inbreeding depression), threatening long-term survival.

Q5. Contrast Darwinian “variations” with de Vriesian “mutations” using specific evolutionary context terminology.

Answer and Explanation Answer: Darwinian variations are continuous and directional; Mutations are discrete and random.

Explanation:
Darwinian Variations: Darwin believed that phenotypic variation within a population was continuous, small, and directional. Evolution, according to Darwin, was a slow, gradual process where natural selection selectively favored minor beneficial variations over millions of years (Gradualism).
De Vriesian Mutations: Hugo de Vries argued that variation was based on mutations—large, discrete, and inherently random (directionless) genetic changes. He believed evolution occurred through these macroscopic, sudden leaps of speciation, which he termed saltation (single-step large mutations), completely disrupting the gradualist view.