Grade 10 Chapter 11: Heredity and Evolution

Introduction

Heredity and evolution are fundamental concepts in biology that explain how traits are passed from one generation to the next and how species change over time. From the inheritance of eye color to the development of antibiotic resistance in bacteria, these processes shape the diversity of life on Earth. This chapter explores the mechanisms of heredity, the evidence for evolution, and the theories that explain how these processes occur.

Heredity and Hereditary Changes

Heredity is the transfer of biological characters from one generation to another via genes. The study of heredity, known as genetics, has evolved significantly over time through the contributions of various scientists.

Historical Development of Genetics

• Gregor Johann Mendel (1822-1884): Pioneer of modern genetics who conducted experiments with pea plants and established the basic principles of inheritance.
• Hugo de Vries (1901): Proposed the mutation theory, explaining sudden changes in hereditary characteristics.
• Walter Sutton (1902): Observed paired chromosomes in grasshopper cells, linking chromosomes to heredity.
• Oswald Avery, Mclyn McCarty, and Colin McLeod (1944): Proved that DNA is the genetic material in all living organisms except viruses.
• Francois Jacob and Jack Monod (1961): Proposed a model for protein synthesis in bacterial cells, helping to uncover genetic codes in DNA.

Importance of Genetics

The science of heredity has numerous applications:

1. Diagnosis, treatment, and prevention of hereditary disorders
2. Production of hybrid varieties of plants and animals
3. Industrial processes using microbes
4. Genetic engineering and recombinant DNA technology

DNA and Protein Synthesis

DNA (deoxyribonucleic acid) is the genetic material that carries hereditary information in most organisms. The process of using this information to synthesize proteins involves several steps, collectively known as the Central Dogma of molecular biology.

Transcription

Transcription is the process of RNA synthesis from a DNA template:

1. Only one strand of the DNA double helix is used as a template.
2. RNA polymerase enzyme binds to the DNA and begins synthesizing RNA.
3. The nucleotide sequence in the resulting mRNA (messenger RNA) is complementary to the DNA template strand.
4. RNA contains uracil (U) instead of thymine (T) found in DNA.

Translation

Translation is the process by which the genetic code in mRNA is read to produce a specific sequence of amino acids:

1. mRNA formed in the nucleus moves to the cytoplasm.
2. The genetic code on mRNA consists of triplet codons, where each codon (three nucleotides) codes for a specific amino acid.
3. Dr. Har Govind Khorana, a scientist of Indian origin, made significant contributions to discovering these triplet codons, earning him the Nobel Prize in 1968.
4. Transfer RNA (tRNA) molecules, with their anticodons, bring the appropriate amino acids as specified by the mRNA codons.

Translocation

Translocation refers to the movement of the ribosome along the mRNA during protein synthesis:

1. Ribosomal RNA (rRNA) helps form peptide bonds between amino acids brought by tRNA.
2. The ribosome moves along the mRNA from one end to the other, one triplet codon at a time.
3. This process continues until a stop codon is reached.
4. Multiple amino acid chains may come together to form complex proteins.

Mutations

Mutations are changes in the nucleotide sequence of DNA:

1. Most genes are transmitted to the next generation without changes.
2. Sometimes, sudden changes occur in genes when nucleotides change position or are altered.
3. Mutations can be minor or significant.
4. Some mutations cause genetic disorders, such as sickle cell anemia.

Evolution

Evolution is the process by which species change over time. It explains the diversity of life on Earth and how organisms adapt to their environments.

Origin of Life

The origin of life on Earth is believed to have occurred approximately 3.5 billion years ago:

1. Initially, simple elements existed in Earth's oceans.
2. Simple organic and inorganic compounds formed from these elements.
3. Over a long period, complex compounds like proteins and nucleic acids developed.
4. Primitive cells formed from mixtures of organic and inorganic compounds.
5. These cells multiplied using surrounding chemicals as resources.
6. Natural selection favored cells that could grow and reproduce efficiently.

Evidences of Evolution

Several types of evidence support the theory of evolution:

Paleontological Evidence

• Fossils: Preserved remains or traces of ancient organisms that provide a record of species that existed in the past.
• Fossil Record: Shows a progression from simple to complex organisms over geological time.
• Transitional Fossils: Organisms with characteristics of both ancestral and descendant groups, such as Archaeopteryx (between reptiles and birds).

Morphological and Anatomical Evidence

• Homologous Organs: Structures with similar basic design and embryonic origin but different functions, such as the forelimbs of humans, whales, bats, and horses.
• Analogous Organs: Structures with similar functions but different basic designs and embryonic origins, such as the wings of birds and insects.
• Vestigial Organs: Reduced, functionless structures that were functional in ancestors, such as the appendix and wisdom teeth in humans.

Embryological Evidence

• Embryos of different vertebrates show remarkable similarities in early stages of development.
• All vertebrate embryos develop gill slits, notochord, and tail during early development, suggesting a common ancestry.

Biochemical Evidence

• All living organisms use the same genetic code (DNA/RNA).
• Similar proteins and enzymes are found across different species.
• The degree of biochemical similarity correlates with evolutionary relationships.

Theories of Evolution

Lamarckism

Jean-Baptiste Lamarck proposed one of the first theories of evolution in the early 19th century:

1. Use and Disuse: Organs that are used become more developed, while unused organs degenerate.
2. Inheritance of Acquired Characteristics: Changes acquired during an organism's lifetime can be passed to offspring.
3. Example: Lamarck suggested that giraffes developed long necks by stretching to reach high leaves, and this acquired trait was passed to their offspring.
4. Limitations: Modern genetics has shown that acquired characteristics are not inherited in the way Lamarck proposed.

Darwin's Theory of Natural Selection

Charles Darwin's theory, published in "On the Origin of Species" (1859), is the foundation of modern evolutionary theory:

1. Overproduction: Organisms produce more offspring than can survive.
2. Variation: Individuals within a species show variations in traits.
3. Competition: Limited resources lead to a struggle for existence.
4. Natural Selection: Individuals with favorable variations are more likely to survive and reproduce.
5. Inheritance: Favorable traits are passed to offspring, becoming more common over generations.
6. Example: In a population of beetles, those with coloration that blends with their environment are less likely to be eaten by predators, leading to an increase in camouflaged beetles over time.

Speciation

Speciation is the process by which new species form:

1. Definition: A species is a group of organisms that can interbreed and produce fertile offspring.
2. Mechanisms of Speciation:
3. Geographic Isolation: Physical barriers separate populations, preventing gene flow.
4. Reproductive Isolation: Mechanisms that prevent interbreeding between populations.
5. Genetic Drift: Random changes in gene frequencies in small populations.
6. Natural Selection: Different selective pressures in different environments.

Human Evolution

Human evolution is the evolutionary process leading to the emergence of modern humans:

Timeline of Human Evolution

1. Earliest Hominids (6-7 million years ago): Sahelanthropus tchadensis, Orrorin tugenensis
2. Australopithecines (4-2 million years ago): Australopithecus afarensis (including "Lucy")
3. Early Homo Species (2.5-1.5 million years ago): Homo habilis, first to use stone tools
4. Homo erectus (1.8 million-300,000 years ago): First to migrate out of Africa, used fire
5. Archaic Homo sapiens (500,000-200,000 years ago): Includes Neanderthals and Denisovans
6. Modern Homo sapiens (200,000 years ago to present): Emerged in Africa and spread worldwide

Key Features in Human Evolution

• Bipedalism: Walking upright on two legs
• Increased Brain Size: From about 400 cc in early hominids to 1,400 cc in modern humans
• Tool Use and Technology: Increasingly sophisticated tools and technology
• Language Development: Complex communication systems
• Social Organization: Complex social structures and cooperation

Applications and Implications

Genetic Engineering

Genetic engineering involves manipulating an organism's genes using biotechnology:

1. Recombinant DNA Technology: Combining DNA from different sources
2. Gene Therapy: Treating genetic disorders by replacing defective genes
3. Genetically Modified Organisms (GMOs): Organisms with altered DNA to achieve desired traits
4. Applications: Medicine, agriculture, industry, research

Evolutionary Medicine

Evolutionary medicine applies evolutionary principles to understanding health and disease:

1. Antibiotic Resistance: Evolution of bacteria resistant to antibiotics
2. Cancer: Understanding cancer as an evolutionary process within the body
3. Mismatch Diseases: Conditions arising from modern environments differing from those in which humans evolved

Conservation Biology

Conservation biology uses evolutionary principles to preserve biodiversity:

1. Genetic Diversity: Maintaining genetic variation within species
2. Endangered Species: Protecting species at risk of extinction
3. Habitat Preservation: Conserving natural habitats

Conclusion

Heredity and evolution are interconnected processes that explain both the continuity of life and the changes that occur over time. Heredity, through DNA and the mechanisms of protein synthesis, ensures that traits are passed from one generation to the next. Evolution, through natural selection and other mechanisms, explains how species change and adapt to their environments over many generations. Together, these processes have shaped the incredible diversity of life on Earth, from the simplest bacteria to complex organisms like humans.

Summary

• Heredity is the transmission of traits from parents to offspring through genes carried on DNA.
• Protein synthesis occurs through transcription (DNA to RNA), translation (RNA to protein), and translocation (movement of ribosomes along mRNA).
• Mutations are changes in DNA that can lead to new traits or genetic disorders.
• Evolution is the process by which species change over time due to changes in heritable traits.
• Evidence for evolution comes from fossils, comparative anatomy, embryology, and biochemistry.
• Lamarck proposed that acquired characteristics could be inherited, a theory now largely rejected.
• Darwin's theory of natural selection explains how favorable traits become more common in populations over time.
• Speciation is the process by which new species form, often due to geographic or reproductive isolation.
• Human evolution has involved bipedalism, increased brain size, tool use, language, and complex social organization.
• Applications of heredity and evolution include genetic engineering, evolutionary medicine, and conservation biology.