CZ

Study Guide Unit 1: Natural Selection and the Theory of Evolution

1. Natural Selection

Natural selection is a primary driver of evolution. Organisms with traits that increase their fitness are more likely to survive and reproduce, passing these traits to future generations.

Mechanisms of Natural Selection:

  • Variation: Genetic differences within a population.

  • Heritability: Traits must be inherited.

  • Differential Reproductive Success: Organisms with favorable traits survive longer and reproduce more.

  • Adaptation: Traits that enhance survival become more common over time.

Types of Natural Selection:

  • Directional Selection: Favors one extreme phenotype, shifting the population curve in one direction.

  • Stabilizing Selection: Favors the average phenotype, reducing variation.

  • Disruptive Selection: Favors both extremes of a phenotype, leading to greater diversity.

Example:

  • Peppered Moths: Before the industrial revolution, light-colored moths were favored in lichen-covered forests. After pollution darkened the environment, darker moths became more common due to camouflage advantages.

2. Evolution and Genetic Variation

Evolution occurs when genetic changes accumulate over generations, often influenced by the environment and random factors like genetic drift.

Sources of Genetic Variation:

  • Mutations: Random changes in DNA that can introduce new traits.

  • Sexual Reproduction: Genetic recombination during meiosis creates variation.

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

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

Genetic Drift:

  • Bottleneck Effect: Drastic reduction in population size reduces genetic diversity.

  • Founder Effect: A small group colonizes a new area, leading to limited genetic variation in the new population.

3. Population Genetics and Hardy-Weinberg Equilibrium

Population genetics studies allele frequencies within a population over time. The Hardy-Weinberg Equilibrium describes populations that are not evolving, given certain conditions.

Hardy-Weinberg Conditions:

  1. No mutations

  2. No gene flow

  3. Random mating

  4. No natural selection

  5. Large population size

Hardy-Weinberg Equations:

  • p² + 2pq + q² = 1: Genotype frequencies (p² for homozygous dominant, 2pq for heterozygous, q² for homozygous recessive).

  • p + q = 1: Allele frequencies (p for dominant allele, q for recessive allele).

Example:

If 16% of a population shows the recessive trait (q² = 0.16), then q = 0.4 and p = 0.6. Use these values to calculate genotype frequencies.

4. Speciation and Isolation Mechanisms

Speciation is the formation of new species from existing populations. It occurs when populations become reproductively isolated and diverge genetically over time.

Types of Speciation:

  • Allopatric Speciation: Geographic isolation (e.g., river formation or mountain range) separates populations, leading to divergence and the formation of new species.

  • Sympatric Speciation: Speciation occurs within the same geographic region, often due to behavioral, temporal, or ecological isolation.

Reproductive Isolation Mechanisms:

  • Prezygotic Barriers (before fertilization):

    • Temporal Isolation: Species breed at different times.

    • Behavioral Isolation: Different mating rituals prevent interbreeding.

    • Mechanical Isolation: Incompatible reproductive organs.

    • Gametic Isolation: Sperm cannot fertilize eggs.

  • Postzygotic Barriers (after fertilization):

    • Hybrid Inviability: Hybrid offspring do not develop properly.

    • Hybrid Sterility: Hybrids are sterile (e.g., mule, a horse-donkey hybrid).

    • Hybrid Breakdown: Hybrids are fertile but their offspring are inviable or sterile.

Example:

The Galápagos finches developed different beak shapes due to geographic and ecological isolation, adapting to different food sources on the islands.

5. Patterns of Evolution

Evolution can occur in several ways depending on environmental pressures and relationships between species.

Types of Evolution:

  • Divergent Evolution: Species with a common ancestor evolve to have distinct traits, often due to different environments.

  • Convergent Evolution: Unrelated species evolve similar traits due to similar environmental pressures (e.g., wings in bats and birds).

  • Parallel Evolution: Related species evolve in similar ways for a long period of time.

  • Coevolution: Two or more species influence each other’s evolution (e.g., predator-prey dynamics).

Adaptive Radiation:

Occurs when a single ancestral species evolves into many different species, each adapted to a unique environment. Example: Darwin’s finches on the Galápagos Islands.

6. Evidence for Evolution

Multiple lines of evidence support the theory of evolution:

Fossil Record:

  • Shows transitional forms between ancient species and modern species, documenting changes over time.

Comparative Anatomy:

  • Homologous Structures: Traits inherited from a common ancestor, such as the limb bones in humans and whales.

  • Analogous Structures: Traits that serve similar functions but evolved independently (e.g., wings of bats and insects).

  • Vestigial Structures: Non-functional or reduced structures that had functions in ancestors (e.g., human tailbone).

Molecular Biology:

  • DNA comparisons show evolutionary relationships. Similarities in genetic code across species point to common ancestry.

Biogeography:

  • Geographic distribution of species supports evolution. For instance, species on islands resemble species on nearby continents more than species from other distant locations.

Embryology:

  • Similar embryonic development across species provides evidence for common ancestry.

7. Origin of Life

The origin of life on Earth is a major topic in evolutionary biology. Various hypotheses attempt to explain how life arose from non-living matter.

Chemical Evolution:

Early Earth’s conditions (volcanic activity, lightning, and the presence of gases like methane, ammonia, and hydrogen) could have facilitated the formation of organic molecules.

Miller-Urey Experiment:

In 1953, Stanley Miller and Harold Urey simulated early Earth conditions in a laboratory. Their experiment produced organic molecules, including amino acids, suggesting that life’s building blocks could have formed naturally.

Steps in the Origin of Life:

  1. Abiotic Synthesis of Organic Molecules: Formation of simple organic compounds, such as amino acids and nucleotides, from inorganic materials.

  2. Formation of Polymers: Small molecules (monomers) link together to form complex molecules (polymers) like proteins and nucleic acids.

  3. Formation of Protobionts: Aggregates of organic molecules surrounded by a membrane-like structure that could have exhibited some properties of life (e.g., metabolism, simple reproduction).

  4. Development of Self-Replicating Molecules: RNA is believed to have been the first self-replicating molecule due to its ability to store genetic information and catalyze chemical reactions (RNA world hypothesis).

8. Cladograms and Phylogenetic Trees

Cladograms and phylogenetic trees are visual tools used to represent evolutionary relationships between different species or groups. They aid in understanding how organisms are related and how they have evolved over time.

Cladograms:

  • Cladograms are diagrams that show relationships among species based on shared characteristics. They do not necessarily indicate the amount of time that has passed or the extent of evolutionary change.

  • Clades: A clade is a group of organisms that includes a common ancestor and all of its descendants. Each branching point (node) in a cladogram represents a common ancestor that gave rise to two or more species.

  • Derived Traits: These are characteristics that are present in an organism but were absent in the last common ancestor. Cladograms are built based on these shared derived traits (synapomorphies).

Reading a Cladogram:

  • Root: The base of the cladogram represents the common ancestor of all organisms depicted in the diagram.

  • Nodes: Points where two branches split, representing a common ancestor shared by the species at the ends of the branches.

  • Outgroup: A species or group that is closely related to, but not part of, the group of species being studied. It helps determine the evolutionary direction of traits.

Example:

Imagine a cladogram of mammals that includes humans, dogs, and cats. If dogs and cats share a node, it means they share a more recent common ancestor with each other than with humans.

Phylogenetic Trees:

Phylogenetic trees are more detailed than cladograms and provide information about the evolutionary timeline. They can show the length of time that has passed since species diverged from a common ancestor.

  • Branches: Represent evolutionary lineages.

  • Length of branches: In some phylogenetic trees, the length of the branches can represent time or the amount of evolutionary change.

Differences between Cladograms and Phylogenetic Trees:

  • Cladograms show relationships between species based on shared traits but do not provide a sense of time or amount of evolutionary change.

  • Phylogenetic trees can incorporate both the degree of evolutionary change and the time scale over which divergence occurred.

Building Cladograms and Phylogenetic Trees:

Scientists use multiple sources of data, including:

  • Morphological Data: Physical traits, like bone structures.

  • Molecular Data: DNA sequences, protein similarities, and other genetic information.

  • Fossil Record: Helps determine the timeline of divergence between species.

Monophyletic, Paraphyletic, and Polyphyletic Groups:

  • Monophyletic Group (Clade): Includes a common ancestor and all its descendants (e.g., all mammals).

  • Paraphyletic Group: Includes a common ancestor and some, but not all, of its descendants.

  • Polyphyletic Group: Includes species with different ancestors, grouping them based on similar traits that evolved independently (e.g., winged organisms like bats, birds, and insects).

Example:

Consider a phylogenetic tree for vertebrates. At a certain node, amphibians diverge from reptiles, mammals, and birds. Further up, another split occurs, dividing reptiles from mammals and birds. This illustrates the common ancestry of these groups and the order of their divergence.

Key Concepts in Phylogenetics:

  • Ancestral Trait: A trait that was present in a common ancestor of a group of organisms.

  • Derived Trait: A trait that evolved in one or more members of a group but was not present in the common ancestor.

  • Homology: Traits inherited from a common ancestor.

  • Homoplasy (Convergent Evolution): Traits that appear similar between species due to similar environmental pressures, not common ancestry (e.g., wings in bats and birds)

Flashcards to add

MRSA evidence 1.2

Staph evidence 1.2