Lecture 1: Evolution

Lecture 1: Evolution

Course Overview and Introduction

This lecture introduces the foundational concepts of evolution, a central theme in biology. It covers the evidence for evolutionary change, mechanisms driving evolution, and addresses common misconceptions.

• Major Course Topics:

  • Evolution and the diversity of life

  • Plant and animal form and function

  • Organisms in their environment

Organizing Biodiversity

Historical Perspectives on Biodiversity

Biologists have developed various frameworks to classify and understand the diversity of life. These frameworks reflect changing views on the nature of species and their relationships.

• Plato: Typological thinking – species are fixed and unchanging.

• Aristotle: Typological thinking plus a scale of nature (hierarchical ranking).

• Lamarck: Change through time, with a scale of nature (species can evolve).

• Darwin and Wallace: Change through time and common ancestry (species are related through evolutionary history).

Evolution

The Pattern of Evolution

Evolutionary biology seeks to answer whether and how species have changed over time. Multiple lines of evidence support the concept of evolution.

• Evidence of Change Through Time:

  • Extinction: Fossil evidence shows that many species that once existed are no longer present.

  • Transitional Forms: Fossils reveal intermediate forms between major groups, such as the evolution of whales from land mammals.

  • Vestigial Traits: Structures that have lost their original function, such as the human coccyx or goose bumps.

• Evidence That Species Are Related:

  • Groupings of Similar Species: Closely related species often share similar traits and are found in similar geographic regions.

  • Homologies: Similarities among species due to shared ancestry.

The Fossil Record

Fossils are physical traces of organisms that lived in the past and provide direct evidence of evolutionary change.

• Examples:

  • 110-million-year-old ammonite shell

  • 50-million-year-old bird tracks

  • 20,000-year-old sloth dung

• Earth's Age: About 4.6 billion years

• Earliest Life: About 3.85 billion years ago

Extinction

Extinction events are documented in the fossil record and demonstrate that species are not fixed.

• Example: Fossil sloths compared to present-day sloths show evolutionary change.

Transitional Forms

Transitional fossils illustrate evolutionary steps between major groups.

• Example: Evolution of whales from land mammals:

  • Pakicetus (~50 mya)

  • Ambulocetus (~49 mya)

  • Rhodocetus (~47 mya)

  • Basilosaurus (~38 mya)

Vestigial Traits

Vestigial traits are remnants of structures that served important functions in ancestors.

• Examples:

  • Human coccyx (tailbone)

  • Capuchin monkey tail (used for balance and locomotion)

  • Human goose bumps vs. erect hair in chimps (insulation, emotional display)

Groupings of Similar Species

Closely related species often share similar traits and geographic distributions, supporting the idea of common ancestry.

• Example: Galápagos mockingbirds show distinct species on different islands, all sharing a common ancestor.

Homology

Homology refers to similarity due to shared ancestry. It can be structural, developmental, or genetic.

• Structural Homology: Similar bone structures in the limbs of turtles, humans, horses, birds, bats, and seals.

• Developmental Homology: Embryos of chicks, humans, and cats all have gill pouches and tails at certain stages.

• Genetic Homology: Genes such as Aniridia in humans and eyeless in fruit flies have similar amino acid sequences (90% identical).

Table: Types of Homology

Type	Description	Example
Structural	Similar anatomical features	Forelimb bones in vertebrates
Developmental	Similar embryonic structures	Gill pouches in vertebrate embryos
Genetic	Similar DNA or protein sequences	Hox genes in animals

Homology and Analogy

Not all similarities are due to common ancestry. Analogous traits arise from convergent evolution, where unrelated species independently evolve similar solutions.

• Convergent Evolution: Independent evolution of similar traits in distantly related lineages.

• Example: Dolphins (mammals) and ichthyosaurs (reptiles) both evolved streamlined bodies and fins for swimming, but are not closely related.

What is Evolution?

Definition and Mechanisms

Evolution is the change in the genetic makeup of a population over time. Several mechanisms drive evolutionary change.

• Mechanisms:

  • Natural selection

  • Genetic drift

  • Gene flow

  • Mutation

The Process of Evolution: Natural Selection

How Does Natural Selection Work?

Natural selection is a key mechanism of evolution, acting on heritable variation within populations.

• Conditions for Natural Selection:

  1. Variation exists between individuals.

  2. Some variations are heritable.

  3. There is differential survival and reproduction due to those heritable differences.

• Fitness: Genetic contribution to the next generation, measured by the number of offspring produced.

Adaptation

Adaptation is the process by which traits that increase fitness become more common in a population over time.

• Genes that lower fitness become less common.

• Genes that increase fitness become more common.

• The average fitness of the population increases.

Evolution in Action: Antibiotic Resistance

Evolution can be observed in real time, such as the development of antibiotic resistance in bacteria.

• Example: Mycobacterium tuberculosis infections worldwide show increasing resistance to antibiotics.

• Process:

  1. Existing genetic variation

  2. Exposure to antibiotic (selection)

  3. Differential survival/reproduction

  4. Adaptation and evolution

• Graph: Percentage of ICU patients with S. aureus infections resistant to vancomycin increased after vancomycin use began.

Evolution in Action: Galápagos Finches

Natural selection can be studied in wild populations, such as the medium ground finch (Geospiza fortis) on Daphne Major island.

• Experiment:

  • Question: Did natural selection occur when the environment changed?

  • Hypothesis: Beak characteristics changed in response to drought.

  • Null Hypothesis: No change in beak characteristics.

  • Setup: Weigh and measure all birds before and after drought.

  • Result: Average beak depth increased after drought, indicating natural selection.

• Genetic Basis: Higher Bmp4 expression in embryos leads to deeper adult beaks.

Important Points and Common Misconceptions

Individuals Do Not Evolve

Evolution occurs at the population level, not within individuals.

• Acclimatization: Short-term physiological adjustment (e.g., to high elevation).

• Adaptation: Genetic change in a population over generations.

Evolution Is Not Progressive

Evolution does not produce a "ladder" of life; it results in a branching tree of related species.

• Lamarckian View: Ladder of life, with humans at the top.

• Darwinian View: Tree of life, with all species sharing a common ancestor.

Evolution Is Not Goal-Directed

Organisms do not evolve because they "need" a particular adaptation; evolution is not purposeful.

Individuals Do Not Act "For the Good of the Species"

Selection acts on individuals, not for the benefit of the species as a whole. Altruistic behaviors are selected against unless they increase individual fitness.

Not All Traits Are Adaptive

Some traits persist due to genetic, tradeoff, or historical constraints.

• Genetic Constraints:

  • Genetic correlation

  • Amount of genetic variation

• Tradeoffs: Some adaptations may have negative side effects.

• Historic Constraints: Evolution is limited by ancestral traits.

Summary Table: Mechanisms of Evolution

Mechanism	Description	Example
Natural Selection	Traits that increase fitness become more common	Antibiotic resistance in bacteria
Genetic Drift	Random changes in allele frequencies	Bottleneck effect in small populations
Gene Flow	Movement of genes between populations	Migration of individuals between populations
Mutation	Random changes in DNA sequence	New alleles arise in a population

Key Equations

• Fitness: $\text{Fitness} = \text{Number of offspring produced}$

• Change in allele frequency (simplified): $\Delta p = p' - p$ where $p$ is the initial allele frequency and $p'$ is the allele frequency after selection.

Conclusion

Evolution is a central concept in biology, supported by extensive evidence from fossils, homologies, and observed changes in populations. Understanding the mechanisms and correcting misconceptions is essential for further study in biology.