Evolution and Its Processes

Introduction to Evolution

• Evolution: Genetic change in a population over time, indicated by a change in allele frequencies.

• Misconception: Individuals evolve.

Fruit Fly Starvation Resistance Experiment

• Experiment Setup:

  • Start with 5,000 fruit flies.

  • Remove food and wait until 80% starve.

  • Record average starvation resistance time.

  • After surviving flies lay eggs, transfer to a new cage—continue for 60 generations.

• Results:

  • Initial average: 20 hours.

  • After 60 generations: nearly 1 week.

• Concepts:

  • Only the most starvation-resistant genotypes survive to reproduce.

Understanding Natural Selection

• Defined as differential survival and reproduction of individuals based on inherited traits.

• Key Principles:

  1. Variation: Individuals in a population vary in traits.

  2. Inheritance: Variations must be heritable.

  3. Increased fitness: Certain traits enhance reproductive success in a given environment.

Sources of Variation

• Mutation: A change in the nucleotide sequence of DNA that may affect fitness.

  • Can be beneficial, harmful, or neutral.

• Gene Flow: Movement of alleles between populations through migration.

• Genetic Drift: Random changes in allele frequencies, especially impactful in small populations.

Mechanisms of Evolution

• Natural Selection:

  • Individuals with traits suited for their environment reproduce more successfully.

  • Example: Green beetles are easier for predators to find; brown beetles thrive better against brown bark.

• Genetic Drift:

  • Random allele frequency changes are often caused by chance events (bottleneck or founder effects).

• Bottleneck Effect: Significant decrease in population size reduces genetic diversity.

• Founder Effect: A small group becomes isolated, leading to a new population with different allele frequencies.

Speciation

• Definition: The process through which one species splits into two or more new species.

• Phases of Speciation:

  1. Reproductive Isolation: No gene flow occurs between populations.

  2. Genetic Divergence: Mutations or natural selection affect each population independently.

• Types of Speciation:

  • Allopatric Speciation: Geographic isolation leads to the formation of new species.

  • Sympatric Speciation: New species form without geographical barriers; often due to polyploidy in plants.

Reproductive Barriers

• Mechanisms preventing species from producing fertile offspring include:

  1. Prezygotic barriers (before zygote forms):

  • Habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gamete isolation.

  1. Postzygotic barriers (after zygote forms):

  • Hybrid sterility, zygote mortality, reduced hybrid viability.

• Example: Mules (hybrids of horse and donkey) are typically sterile and cannot reproduce.

Taxonomy

• Taxon (plural: taxa): The classification system used to categorize organisms.

• Hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

• Naming Convention: Binomial nomenclature—Genus (capitalized) + specific epithet (not capitalized), e.g., Homo sapiens.

Conclusion: Evolution leads to changes in populations over generations, driven by mechanisms such as natural selection, mutation, gene flow, and genetic drift, contributing to the emergence of new species and biodiversity.

11.1 Discovering How Populations Change

1. Define evolution:
   Evolution is the change in the heritable traits of biological populations over successive generations. It is driven by processes such as natural selection, mutation, gene flow, and genetic drift.

2. Three conditions necessary for natural selection to occur:

   • Variation in traits: Individuals in a population must exhibit variations in their traits.

   • Differential survival and reproduction: Some traits provide individuals with advantages in survival or reproduction, leading to greater representation of those traits in subsequent generations.

   • Heritability: Traits must be heritable, meaning they can be passed from parents to offspring.

3. Natural vs. artificial selection:

   • Natural selection is the process where organisms better adapted to their environment tend to survive and produce more offspring naturally.

   • Artificial selection involves human intervention in breeding, selecting for desired traits in species.

4. Biological fitness measurement:
   Biological fitness is measured by an organism's reproductive success and the ability to pass on its genes to the next generation. Variations in traits can affect survival, competition for resources, and mate selection, all influencing fitness.

11.2 Mechanisms of Evolution

1. Four mechanisms that lead to evolution:

   • Natural selection: Adaptive traits increase in frequency over time.

   • Mutation: Random changes in DNA can introduce new genetic variation.

   • Genetic drift: Random fluctuations in allele frequencies due to chance events can lead to changes, especially in small populations.

   • Gene flow: The movement of alleles between populations can introduce new genetic material.

2. Impact of mutations on variation within a population:
   Mutations are a primary source of genetic variation, providing new alleles that can be acted upon by natural selection, impacting the overall diversity within a population.

3. Founder effect and bottleneck effect:

   • Founder effect occurs when a small group establishes a new population, potentially carrying only a fraction of the original population's genetic diversity.

   • Bottleneck effect happens when a significant reduction in population size leads to a loss of genetic diversity due to random sampling of alleles.

11.4 Speciation

1. Microevolution vs. macroevolution:

   • Microevolution refers to small-scale changes within a species or population over time.

   • Macroevolution involves large-scale changes that result in the formation of new species or groups.

2. Biological species concept:
   The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring under natural conditions.

3. Two phases of speciation:

   • Allopatric speciation involves geographic separation leading to reproductive isolation.

   • Sympatric speciation occurs without geographic isolation, often through polyploidy or behavioral changes.

4. Allopatric vs. sympatric speciation:

   • Allopatric speciation: Geographic barriers divide populations, restricting gene flow.

   • Sympatric speciation: New species evolve from a single population in the same geographic area due to reproductive isolation mechanisms.

5. Prezygotic and postzygotic isolating mechanisms:

   • Prezygotic mechanisms prevent mating or fertilization between species, such as temporal isolation (different mating times).

   • Postzygotic mechanisms occur after fertilization and include hybrid inviability (offspring do not survive) and hybrid sterility (offspring are sterile).

6. Hierarchical levels of taxonomic classification:

   • Domain

   • Kingdom

   • Phylum

   • Class

   • Order

   • Family

   • Genus

   • Species

7. Writing a scientific name in correct format:
   A scientific name is written in italicized format, with the genus name capitalized and the species name in lower case (e.g., Homo sapiens).