Week 3 - Evolution, Speciation, and Phylogenetics: Foundations of Biological Diversity

Vertebrate Radiation and Geologic Time

Major Events in Vertebrate Evolution

Vertebrate radiation refers to the diversification of vertebrate lineages over geologic time, resulting in the wide variety of vertebrate species observed today. This process is mapped against the backdrop of Earth's history, with major evolutionary events corresponding to different geologic eras and periods.

• Key Groups: Jawless fish, cartilaginous fish, bony fish, amphibians, reptiles, birds, and mammals.

• Geologic Time Scale: Major vertebrate groups appeared and diversified during the Paleozoic, Mesozoic, and Cenozoic eras.

• Example: The transition from aquatic to terrestrial life is marked by the evolution of amphibians from lobe-finned fish.

Additional info: The geologic time spiral visually represents the progression of life from the Precambrian to the present, highlighting mass extinctions and adaptive radiations.

Key Observations of Life

Patterns in Biological Diversity

Biologists have identified three fundamental observations about life:

• Adaptation: Organisms are suited to the environments in which they live.

• Unity: There are shared characteristics among life forms, indicating common ancestry.

• Diversity: There is an amazing diversity of life, resulting from evolutionary processes.

• Example: Bats, butterflies, and hummingbirds all have wings, but these structures evolved differently (convergent evolution).

Phenotypic Variance

Sources of Variation in Populations

Phenotypic variance explains why individuals and populations differ. It is the result of both genetic and environmental factors.

• Equation:

$V_P = V_G + V_E + V_{GE} + V_{error}$

• $V_P$: Phenotypic variance

• $V_G$: Genetic effects

• $V_E$: Environmental effects

• $V_{GE}$: Gene-environment interactions

• $V_{error}$: Error variance

Foundations of Evolutionary Theory

Historical Context and Key Figures

• Charles Darwin: Naturalist on HMS Beagle, formulated the theory of natural selection, published "On the Origin of Species" (1859).

• Alfred Russel Wallace: Independently conceived the theory of evolution by natural selection.

• Influence of Malthus: Both Darwin and Wallace were influenced by Malthus' essay on population growth and competition for resources.

Descent with Modification

Core Principles of Evolution

• All species, living or extinct, have descended from preexisting species.

• Species can change over time, leading to the diversity of life.

Theory of Natural Selection

Mechanisms Driving Evolution

• Overproduction: More offspring are produced than can survive.

• Genetic Variation: Individuals in a population have different traits.

• Struggle for Existence: Individuals compete for limited resources.

• Differential Reproduction: Individuals with advantageous traits are more likely to survive and reproduce.

Mechanisms of Evolution

Processes That Change Allele Frequencies

• Natural Selection: Non-random increase of beneficial alleles.

• Sexual Selection: Traits that increase mating success become more common.

• Mutation: Random changes in DNA introduce new alleles.

• Genetic Drift: Random changes in allele frequencies, especially in small populations (e.g., bottleneck effect, founder effect).

• Gene Flow: Movement of alleles between populations increases genetic diversity.

• Horizontal Gene Transfer: Transfer of genes between different species, common in bacteria.

Microevolution and Populations

Changes Within Populations

• Microevolution: Changes in allele frequencies within a population over time.

• Allele: Alternative versions of a gene.

• Population: Group of interbreeding individuals in the same area.

• Example: Antibiotic resistance in bacteria evolves through mutation and horizontal gene transfer.

Speciation

Formation of New Species

• Speciation: The process by which one species splits into two or more species.

• Biological Species Concept (BSC): Species are groups of populations whose members can interbreed and produce viable offspring.

• Barriers to Reproduction: Must evolve for speciation to occur.

Mechanisms of Speciation

• Allopatric Speciation: Geographic isolation leads to divergence.

• Sympatric Speciation: New species arise within the same geographic area, often through polyploidy in plants.

Homology, Analogy, and Phylogenetics

Understanding Evolutionary Relationships

• Homology: Similarities due to common ancestry (e.g., vertebrate forelimbs).

• Analogy: Similarities due to convergent evolution, not common ancestry (e.g., wings of bats and birds).

• Homoplasy: Similar appearance, but not due to shared ancestry or function.

Building Phylogenetic Trees

• Taxonomy: Naming and classifying organisms (Linnaean system, binomial nomenclature).

• Systematics: Study of evolutionary relationships (phylogeny).

• Cladistics: Grouping organisms by shared derived traits into clades.

Table: Linnaean Classification Example

Rank	Example (Human)	Example (Cat)	Example (Whale)
Kingdom	Animalia	Animalia	Animalia
Phylum	Chordata	Chordata	Chordata
Class	Mammalia	Mammalia	Mammalia
Order	Primates	Carnivora	Cetacea
Family	Hominidae	Felidae	Balaenopteridae
Genus	Homo	Felis	Balaenoptera
Species	Homo sapiens	Felis catus	Balaenoptera musculus

Molecular Evidence for Evolution

Genetic Data and Phylogenies

• All living things share a common genetic code, supporting common ancestry.

• DNA and protein sequence comparisons clarify evolutionary relationships.

• More similar sequences indicate more recent common ancestry.

• Example: The hemoglobin gene is found across many vertebrates, reflecting shared ancestry.

Summary Table: Mechanisms of Evolution

Mechanism	Description	Effect on Diversity
Natural Selection	Non-random survival and reproduction	Can increase or decrease
Mutation	Random changes in DNA	Increases
Genetic Drift	Random changes in allele frequencies	Decreases (especially in small populations)
Gene Flow	Movement of alleles between populations	Increases
Horizontal Gene Transfer	Genes transferred between species	Increases

Additional info: Modern phylogenetics uses computational tools to analyze DNA sequence data and construct evolutionary trees, providing robust evidence for evolutionary relationships.