Evolution and Species

Evolution

Inheritance Over Time

• Inheritance occurs when offspring inherit genes from their parents.

  • Reproduction methods: May be via asexual or sexual reproduction.

  • Impact of inheritance: Inheritance can lead to new combinations of parental traits.

• Over time, sexual reproduction and accumulated mutations lead to increased diversity.

  • Diversity Elements:

  • New alleles

  • New combinations of alleles

• Accumulated changes can make offspring notably distinct from their ancestors.

  • This process is known as Evolution.

Questions Addressed

• How do new species arise?

• How do the populations change?

• Are we so different from other species?

Lamarckian Evolution

• Jean-Baptiste Lamarck was the first to propose the concept of evolution in 1809, 50 years before Mendel studied inheritance.

• Second Law of Lamarck: "All the acquisitions and losses wrought by nature in individuals….are preserved by reproduction to the new individuals which arise."

• Proposed that:

  • Organisms change over time by modifying their traits during their lifetime to become better adapted to their environment.

  • These Acquired Traits are passed on to their offspring.

  • Offspring continue to develop these beneficial traits further.

Darwinian Evolution

• Charles Darwin was inspired by Lamarck's ideas of evolution, born around the time Lamarck published his theories.

• Proposed a method of evolution called Descent with Modification:

  • Agrees that all organisms descend from ancestral ones.

  • Descend with distinctions, or ‘modifications’ in traits.

  • Natural selection drives the development of desirable traits.

Comparison: Lamarckian vs Darwinian Evolution

• Similarities:

  • Both agree that species change over time.

  • Changes are passed from parent to offspring.

• Differences:

  • Lamarck: Acquired Traits; individuals change traits during their life.

  • Darwin: Descent with Modification; species do not change, but offspring will have differences.

  • Directionalism: Lamarck suggests changes are to adapt to an environment.

  • Natural Selection: Darwin suggests changes are random, yet the best-fitting traits are selected for.

Darwinian Evolution: Descent with Modification

• Variations in genes can arise from Mutations.

• Variations in genotypes can also come from Sexual Reproduction:

  • Includes the Law of Independent Assortment and Crossing Over.

• Both mutations and sexual reproduction lead to variations in Phenotypes, which is what Natural Selection acts on.

Evolution: Not the Individual

• Evolution does not work on an individual scale: individuals do not change their genes during their lifetime.

• Evolution operates on a population scale:

  • Individuals remain the same, while offspring may differ from their parents.

• Gene Pool: A gene pool is the set of all alleles of all genes in a population, and the frequencies of each allele change over time.

Evolution: The Gene Pool

• Evolution is defined as changes in the gene pool.

  • Key Events:

  • Offspring Born

  • Offspring Who Survive

  • Offspring Who Mate

  • Reproduction process impacting gene pool.

Mechanisms of Evolution

• Evolution involves changes in the gene pool due to:

  • Mutations

  • Genetic Drift

  • Natural Selection

  • Gene Flow

  • Sexual Selection

Mutations

• Mutations lead to new genes in a population:

  • Most mutations are harmful and do not pass on because the offspring do not survive.

  • Some mutations are neutral and persist in the population.

  • Some may be beneficial in certain environments.

  • Example: Sickle Cell Anemia is generally harmful but confers resistance to Malaria.

Gene Flow

• Gene Flow occurs when individuals from one population breed with another population:

  • It allows for the mixing of gene pools and can lead to either gain or loss of alleles.

  • It reduces genetic differences between distinct populations.

Genetic Drift

• Genetic Drift occurs when a gene pool varies by chance:

  • No related reason driving the change in allele frequency.

  • It particularly occurs with neutral alleles.

• Bottleneck Effect:

  • Occurs when a large part of a population is lost at once, leading to a loss of alleles and decreased genetic variation.

  • Example: Cheetahs are believed to have experienced at least two genetic bottleneck events.

Founder Effect

• Founder Effects occur when just a few individuals colonize a habitat:

  • It results in low starting genetic diversity.

  • Can lead to higher prevalence of genetic disorders.

Natural Selection

• Natural Selection occurs under selective pressure from the environment:

  • Selection pressure influences survival.

  • Traits favored are better for survival, while others are less favorable.

  • Natural selection plays a role in adaptive evolution:

  • Not random; drives towards better-suited characteristics for environment.

  • Natural selection leads to adaptive evolution, producing specialized variations in traits.

Types of Natural Selection

1. Directional Selection: Selected traits favor one extreme phenotype.

2. Disruptive Selection: Favors extreme traits over intermediate traits.

3. Stabilizing Selection: Favors intermediate variants and acts against extreme phenotypes.

Sexual Selection

• Sexual Selection involves mate choice as the selective pressure:

  • Similar to natural selection but focuses on traits that improve mating chances, not necessarily survival.

Themes in Evolution

• Evolution is not goal-oriented and must work with existing structures.

• Major changes cannot happen without pre-existing structures or traits.

• Existing structures are often adapted for new functions.

• Complex structures evolve in stages.

Evidence of Evolution

• Numerous studies support the theory of evolution:

  1. Fossil Record: Shows historic progression and transitional forms between species.

  2. Biogeography: Distribution of related animals corresponds with tectonic history.

  3. Comparative Anatomy: Homologies and vestigial structures indicate evolution.

  4. Comparative Molecular Biology: Analysis of DNA and proteins reveals genetic relationships.

Speciation

Definition of a Species

• A species is defined as a group of populations capable of interbreeding and producing viable, fertile offspring.

• Species identification is based on reproductive capability, not just physical traits.

Speciation Process

• Speciation occurs through:

  • Geographical Isolation: Populations separated by barriers evolve differently, preventing interbreeding.

  • Allopatric Speciation: Isolation prevents interbreeding; populations evolve in different directions.

  • Sympatric Speciation: Population splits into different directions despite being in the same area, often driven by disruptive selection.

Mechanisms Keeping Species Separate

1. Prezygotic Barriers: Prevent attempts at mating before the zygote forms:

   • Ecological Isolation (different habitats)

   • Temporal Isolation (different mating times)

   • Behavioral Isolation (different mating behaviors)

   • Mechanical Isolation (anatomically incompatible)

   • Gametic Isolation (incompatible egg and sperm).

2. Postzygotic Barriers: Occur after zygote formation to prevent hybrid generations:

   • Producing hybrids that are typically not viable or fertile.