Core Concepts in Evolution, Genetics, and Molecular Biology

Evolutionary Concepts and Theories

Pre-Evolutionary Ideas

Before modern evolutionary theory, several ideas attempted to explain the diversity of life:

• Fixism: This theory posited that species remain unchanged since their divine creation.
• Catastrophism: Proposed that Earth's history was shaped by sudden, short-lived, violent events, leading to the extinction of species, after which new species would appear.

Key Evolutionary Theories

Lamarckism: Inheritance of Acquired Characteristics

Jean-Baptiste Lamarck proposed that living organisms change throughout history. His theory was based on the following principles:

• Tendency Towards Perfection: Living things strive for perfection.
• Adaptation to Environment: Organisms adapt to their environment, and environmental changes drive the need for adaptation.
• Use and Disuse: The use or disuse of organs leads to their development or degeneration.
• Inheritance of Acquired Traits: Changes acquired during an organism's lifetime are inherited by its offspring.

Darwinism: Natural Selection

Charles Darwin's theory of evolution by natural selection is based on several observations:

• Overproduction: A breeding pair produces more offspring than can survive.
• Struggle for Survival: There is competition among individuals for limited resources.
• Variation: Individuals within a population exhibit variations in their characteristics.
• Natural Selection: The environment presents challenges, and individuals with advantageous traits are more likely to survive and reproduce.
• Inheritance: Surviving individuals transmit their beneficial characteristics to their offspring.

Neo-Darwinism: The Modern Synthesis

Neo-Darwinism integrates Darwin's theory with modern genetics. It states that:

• Evolutionary changes occur due to mutations in the genetic material.
• The formation and union of male and female gametes happen randomly.
• Evolution acts on the population, not the individual organism.

Pace of Evolutionary Change

Saltationism: Abrupt Changes

This theory suggests periods of stability are punctuated by sudden, rapid evolutionary changes, often driven by significant mutations.

Gradualism: Incremental Changes

In contrast, gradualism proposes that evolutionary changes occur slowly and steadily over long periods.

Evidence for Evolution

Numerous lines of evidence support the theory of evolution:

• Paleontological Evidence: The fossil record shows a progression of life forms over geological time.
• Anatomical Evidence: Includes homologous structures (similar structures with different functions), analogous structures (different structures with similar functions), and vestigial organs (reduced or non-functional structures).
• Embryonic Evidence: Similarities in the embryonic development of different species suggest common ancestry.
• Biochemical and Genetic Evidence: Similarities in DNA, RNA, and protein sequences across species.
• Biogeographical Evidence: The distribution of species across different geographical regions.

Speciation and Biodiversity

Speciation Mechanisms

The formation of new species (speciation) typically occurs through:

• Geographic Isolation: Physical barriers separate populations, preventing gene flow.
• Reproductive Isolation: Mechanisms that prevent interbreeding between different species.

Understanding Biodiversity

Biodiversity encompasses the variety of life on Earth and includes:

• Genetic Diversity: Variation within a species.
• Species Diversity: The variety of different species in an ecosystem.
• Ecological Diversity: The variety of ecosystems and habitats.

Defining a Species

A species is generally defined as a group of organisms with common characteristics that can interbreed and produce fertile offspring.

Fundamentals of Genetics and Molecular Biology

Mendelian Genetics: Principles of Inheritance

Mendel's Experiments with Pea Plants

Gregor Mendel's pioneering work established the basic laws of inheritance:

• First Law (Law of Uniformity): When crossing two pure-breeding parents for a single trait (e.g., yellow peas and green peas), the first filial (F1) generation will all display the dominant trait (e.g., all yellow peas).
• Second Law (Law of Segregation): When the F1 generation self-pollinates, the second filial (F2) generation will show a phenotypic ratio of 3:1 (e.g., 3 yellow peas to 1 green pea). This demonstrates that alleles segregate during gamete formation.
• Third Law (Law of Independent Assortment): When considering two different traits (e.g., yellow/green color and smooth/wrinkled texture), the alleles for each trait assort independently during gamete formation. A dihybrid cross (e.g., between yellow smooth and green wrinkled parents) in the F2 generation yields a phenotypic ratio of 9:3:3:1 (9 yellow smooth: 3 yellow wrinkled: 3 green smooth: 1 green wrinkled).

Key Genetic Terminology

• Gene: A segment of DNA that carries specific genetic information, typically coding for a protein or functional RNA.
• Allele: An alternative form or variant of a gene.
• Dominant Allele: An allele that is always expressed in the phenotype when present, even if only one copy is inherited.
• Recessive Allele: An allele that is only expressed in the phenotype when two copies are inherited (i.e., in a homozygous recessive state).
• Homozygous: An individual possessing two identical alleles for a particular gene (e.g., AA or aa).
• Heterozygous: An individual possessing two different alleles for a particular gene (e.g., Aa).
• Genotype: The complete set of genes or alleles an organism possesses for a particular character or its entire genetic makeup.
• Phenotype: The observable physical or biochemical characteristics of an organism, resulting from the interaction of its genotype with the environment.
• Lethal Gene: A gene whose expression results in the death of the individual, often before birth or maturity.
• Sex-linked Gene: A gene located on one of the sex chromosomes (e.g., X or Y in humans). Its expression and inheritance patterns are influenced by the individual's sex.

Molecular Biology: From DNA to Protein

Central Dogma Processes

The flow of genetic information in a cell typically follows these steps:

• DNA Replication: The process by which DNA makes a copy of itself.
• Transcription: The process of synthesizing RNA from a DNA template.
• Translation: The process by which ribosomes synthesize proteins using the information carried by messenger RNA (mRNA).

Nucleic Acid Components

Nucleic acids (DNA and RNA) are polymers made of nucleotide monomers. A nucleotide consists of three main components:

• A deoxyribose (in DNA) or ribose (in RNA) sugar.
• A phosphate group.
• A nitrogenous base.

The four nitrogenous bases in DNA are Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). In RNA, Thymine is replaced by Uracil (U).

DNA Structure: The Double Helix

DNA typically exists as a double helix, composed of two complementary and antiparallel strands. These strands are held together by hydrogen bonds between specific base pairs (A with T, C with G).