Hardy Weinberg Principle & Evolution of Man | Biology Class 12 - NEET PDF Download

Hardy-Weinberg Principle

Frequency of Alleles

• The Hardy-Weinberg principle helps determine the frequency of alleles in a population, assuming these frequencies remain constant over generations.
• Allele frequencies in a population are considered stable and unchanging from one generation to the next.
• The total gene pool, which includes all the genes and their alleles in a population, is also assumed to be constant, a state known as genetic equilibrium.
• The sum of all allelic frequencies equals 1. For example, in a diploid organism, p and q represent the frequencies of alleles A and a, respectively.

Calculating Genotype Frequencies

• The frequency of homozygous dominant individuals (AA) in a population is represented by p².
• This is based on the principle that the probability of an allele A, with a frequency of p, appearing on both chromosomes of a diploid individual is p × p = p².
• Similarly, the frequencies of other genotypes are calculated as follows:
• Homozygous recessive (aa): q²
• Heterozygous (Aa): 2pq

Binomial Expansion and Equilibrium

• The equation p² + 2pq + q² = 1 is a binomial expansion of (p + q)².
• When the measured frequencies of alleles differ from the expected values, the direction of this difference indicates the extent of evolutionary change.
• A disturbance in genetic equilibrium, or Hardy-Weinberg equilibrium, signifies a change in allele frequencies within a population, which is interpreted as a sign of evolution.

Factors Affecting Hardy-Weinberg Equilibrium

• Gene Migration (Gene Flow): When a section of a population migrates to a new location, gene frequencies change in both the original and the new population. New genes or alleles are added to the new population while being lost from the old one. Repeated gene migration leads to gene flow.
• Genetic Drift: If allele frequency changes occur by chance, it is called genetic drift. In some cases, the change in allele frequency in a new population sample is so significant that it leads to the formation of a new species. The original population that drifted becomes the founders, and this phenomenon is known as the founder effect.
• Mutation: Experiments with microorganisms demonstrate that advantageous mutations, when selected, result in the emergence of new phenotypes. Over several generations, this process can lead to speciation.
• Natural Selection: Natural selection is the process by which heritable variations that enhance survival and reproductive success are favored, leading to greater numbers of offspring. Variations due to mutation, recombination during gamete formation, gene flow, or genetic drift alter gene and allele frequencies in future generations. When coupled with increased reproductive success, natural selection can make populations appear different.

Effects of Natural Selection

Natural selection can result in:

Diagrammatic representation of the operation of natural selection on different traits : (a) Stabilising (b) Directional and (c) Disruptive

• Stabilization: More individuals exhibit the mean character value.
• Directional Change: More individuals exhibit a character value different from the mean.
• Disruption: More individuals exhibit extreme character values at both ends of the distribution curve.

A Brief Account of Evolution

About 2000 million years ago (mya), the first cellular forms of life appeared on Earth. The process by which non-cellular aggregates of giant macromolecules evolved into cells with membranous envelopes is not well understood. Some of these cells had the ability to release O₂, possibly through a process similar to the light reaction in photosynthesis, where water is split using solar energy captured by light-harvesting pigments.

• First cellular forms of life appeared around 2000 million years ago (mya).
• Transition from non-cellular aggregates of giant macromolecules to cells with membranous envelopes is unclear.
• Some early cells could release oxygen, potentially through a process similar to photosynthesis.

A sketch of the evolution of plant forms through geological periods

• Evolution of Life Forms: Single-celled organisms gradually evolved into multi-cellular life forms.
• Invertebrates and Early Vertebrates: By around 500 mya, invertebrates had emerged and were active.Jawless fish likely evolved around 350 mya.
• Plants and Animals on Land: Seaweeds and some plants existed around 320 mya.Plants were among the first organisms to invade land, followed by animals. Fish with strong fins began to move onto land and then back into water around 350 mya.
• Amphibians: Fish like the Coelacanth, thought to be extinct, were caught in 1938, showing the evolutionary link. Lobefins evolved into the first amphibians, ancestors of modern frogs and salamanders.
• Reptiles: Amphibians evolved into reptiles, which laid thick-shelled eggs that did not dry out. Modern descendants include turtles, tortoises, and crocodiles.
• Diversity of Dinosaurs: Reptiles of various shapes and sizes, including dinosaurs, dominated the Earth.Giant ferns (pteridophytes) were present and eventually formed coal deposits.
• Return to Water and Fish-like Reptiles: Some land reptiles returned to the water, evolving into fish-like reptiles around 200 mya, like Ichthyosaurs.
• Dinosaurs: Dinosaurs, including the massive Tyrannosaurus rex, were among the largest reptiles. The exact reason for their extinction around 65 mya is unknown, with theories ranging from climatic changes to evolution into birds.

Representative evolutionary history of vertebrates through geological periods

• The First Mammals: The earliest mammals were small, shrew-like creatures. Mammals were viviparous, protecting their unborn young inside the mother’s body. They were more intelligent and better at sensing and avoiding danger.
• Continental Drift and Mammal Evolution: As continents drifted, South America joined North America, leading to competition between species. Pouched mammals in Australia survived due to lack of competition.
• Marine Mammals: Some mammals adapted to living entirely in water, such as whales, dolphins, seals, and sea cows.
• Evolutionary Stories: The evolution of specific animals like horses, elephants, and dogs will be covered in higher classes.
• Evolution of Humans: Humans are noted for their language skills and self-consciousness, marking a significant evolutionary success.

Origin and Evolution of Man

About 15 million years ago, primates like Dryopithecus and Ramapithecus existed. They were hairy and walked similarly to gorillas and chimpanzees. While Ramapithecus had more human-like features, Dryopithecus resembled apes more closely.

Fossils of human-like bones found in Ethiopia and Tanzania suggest that around 3 to 4 million years ago, human-like primates walked upright in eastern Africa. These early humans were likely under 4 feet tall but walked on two legs.

Austalopithecines

• Around 2 million years ago, a group known as Austalopithecines lived in the grasslands of East Africa.
• They are believed to have used stone tools for hunting but primarily ate fruits.

Homo habilis

• Among the discovered bones, some were different, leading to the identification of the first human-like species called Homo habilis. These early humans had brain sizes ranging from 650 to 800 cubic centimeters and likely did not eat meat.

Homo Erectus

• Fossils found in Java in 1891 marked the next stage of human evolution: Homo erectus, which lived about 1.5 million years ago.
• Homo erectus had a larger brain, around 900 cubic centimeters, and likely included meat in their diet.

Neanderthal Man

• The Neanderthal man, with a brain size of 1400 cubic centimeters, lived in the Near East and Central Asia between 100,000 and 40,000 years ago.
• They used animal hides for clothing and practiced burial of their dead.

A comparison of the skulls of adult modern human being, baby chimpanzee and adult chimpanzee. The skull of baby chimpanzee is more like adult human skull than adult chimpanzee skull

Homo Sapiens

Homo sapiens originated in Africa and spread across continents, developing into distinct races. During the Ice Age, between 75,000 and 10,000 years ago, modern Homo sapiens emerged.

Prehistoric cave art began around 18,000 years ago, with notable examples like the cave paintings at Bhimbetka rock shelter in Madhya Pradesh. Agriculture started about 10,000 years ago, leading to the establishment of human settlements. The subsequent events are part of human history, marked by the rise and fall of civilizations.