week 1 evolution (textbook ch 22)

1. Bombardier Beetle: An Example of Adaptation and Natural Selection

The bombardier beetle demonstrates an extreme and effective defensive adaptation against predators such as wolf spiders. When attacked, instead of being eaten, the beetle releases a hot, noxious chemical spray.

Inside its abdomen, the beetle stores hydroquinones and hydrogen peroxide in two specialized structures called pygidial glands. When the beetle senses danger, these chemicals are released into a reaction chamber that contains the enzymes catalase and peroxidase, which act as catalysts.

The chemical reaction is exothermic, meaning it releases heat. This reaction heats the chemicals to nearly 100°C, producing a pulsating spray accompanied by a popping sound. The beetle can also rotate the tip of its abdomen, allowing it to aim the spray accurately at predators. This makes the defence both powerful and precise.

Importantly, this defence did not appear suddenly. Many arthropods already produce hydroquinone irritants, and the enzymes involved existed in ancestral species. The beetle’s defence system evolved through modification of pre-existing traits, likely progressing from a simple ooze or mist into the highly specialized, explosive spray seen today. Because this defence strongly improves survival, natural selection would strongly favour individuals with increasingly effective deterrents.

2. Darwin’s Three Major Observations About Life

Charles Darwin sought to explain three broad, observable features of life:

  1. Adaptation – Organisms are well suited to their environments.

  2. Unity of life – Many organisms share similar characteristics.

  3. Diversity of life – There is an enormous variety of species.

These are observations, not explanations. Evolution provides the explanation for these patterns.

3. Definition of Evolution

Darwin described evolution as descent with modification, meaning that present-day species descend from ancestral species that were different from them.

In modern biology, evolution is more precisely defined as a change in the genetic composition (allele frequencies) of a population over successive generations.

Evolution can be viewed in two ways:

  • Pattern: the observable changes in organisms over time (fossils, similarities, distributions).

  • Process: the mechanisms that cause those changes, such as natural selection.

4. Early Views on Life and Classification

Scala Naturae (Aristotle)

Aristotle proposed that species were fixed and unchanging, arranged on a ladder called the scala naturae, from simple to complex. Each species was believed to have a permanent and perfect place.

These ideas aligned with traditional religious beliefs that species were individually created.

Linnaeus

Carolus Linnaeus developed binomial nomenclature and a nested classification system (genus, family, order, etc.). Although Linnaeus believed species were created, his classification system later supported evolutionary thinking because it naturally grouped organisms by similarity.

Darwin later argued that these similarities reflected common ancestry, not independent creation.

5. Fossils, Geology, and Change Over Time

Cuvier and Catastrophism

Georges Cuvier studied fossils and discovered that different rock layers contained different species. Older layers contained fossils increasingly different from modern organisms, showing that extinction had occurred.

Cuvier rejected evolution and instead proposed catastrophism, arguing that sudden disasters caused mass extinctions and that new species later migrated into affected areas.

Hutton and Lyell: Gradualism and Uniformitarianism

James Hutton proposed that Earth’s features formed through slow, continuous processes such as erosion. Charles Lyell expanded this idea, arguing that the same geological processes observed today operated in the past.

These ideas implied that Earth is very old, giving enough time for gradual biological changes. Darwin applied this reasoning to evolution.

6. Lamarck’s Hypothesis of Evolution

Jean-Baptiste Lamarck proposed one of the first mechanisms for evolution. He suggested two main principles:

  • Use and disuse: frequently used traits become stronger; unused traits weaken.

  • Inheritance of acquired characteristics: traits acquired during an organism’s lifetime are passed to offspring.

Lamarck used the example of giraffes stretching their necks to reach higher leaves. While Lamarck correctly recognized that species change over time, his mechanism was incorrect. Modern genetics shows that acquired traits are not inherited in this way.

7. Darwin’s Research and the Voyage of the Beagle

During the voyage of the HMS Beagle, Darwin collected and observed organisms across South America and the Galápagos Islands. He noticed several important patterns:

  • Species in South America resembled other South American species more than European species.

  • Fossils resembled living species from the same region.

  • Geological features and marine fossils high in the Andes suggested that Earth had changed gradually over long periods.

In the Galápagos Islands, Darwin observed closely related species that differed slightly from island to island. He proposed that organisms colonized the islands from the mainland and then diversified into new species adapted to local conditions.

8. Adaptation and Natural Selection

An adaptation is an inherited trait that increases an organism’s survival and reproductive success in a particular environment.

Darwin proposed natural selection as the mechanism of evolution, based on the following logic:

  • Individuals in a population show variation in heritable traits.

  • Organisms produce more offspring than can survive due to limited resources.

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

  • Over time, favourable traits become more common in the population.

Darwin was influenced by Thomas Malthus, who wrote that populations grow faster than resources, leading to competition.

Important clarifications:

  • Individuals do not evolve; populations do.

  • Natural selection only acts on heritable variation.

  • What traits are favoured depends on environmental conditions, which can change over time.

9. Descent with Modification and Evolutionary Trees

Darwin viewed life as a branching tree. Each branch point represents a common ancestor, and different branches represent evolutionary lineages.

  • Unity of life results from shared ancestry.

  • Diversity of life results from accumulated differences over time.

  • Extinction removes intermediate forms, creating gaps between groups.

Scientists estimate that over 99% of species that have ever lived are now extinct.

10. Artificial Selection vs Natural Selection

Artificial selection occurs when humans selectively breed organisms with desired traits, such as crops and domesticated animals. Darwin used artificial selection to show that selection can cause major changes over relatively short times.

Natural selection operates similarly but without human involvement, with the environment acting as the selecting agent.

11. Evidence for Evolution

Direct Observation

  • Soapberry bugs evolved different beak lengths in response to introduced plant species within decades.

  • Antibiotic resistance (MRSA) shows rapid evolution in bacteria due to strong selection pressures.

Natural selection does not create new traits; it selects existing variation.

Homology

  • Anatomical homologies (mammalian forelimbs) show shared structural patterns.

  • Embryological homologies (pharyngeal pouches, post-anal tail) appear during development.

  • Vestigial structures are remnants of ancestral traits.

  • Molecular homologies include shared genes and the universal genetic code.

Convergent Evolution

Unrelated species may evolve similar traits independently due to similar environments. These traits are analogous, not homologous.

Fossil Record

Fossils document transitions, such as the evolution of whales from land mammals, showing gradual structural change over time.

Biogeography

The geographic distribution of species reflects continental drift and colonization patterns. Island species are often endemic but closely related to nearby mainland species.

12. Evolution as a Scientific Theory

In science, a theory is a well-supported explanation that unifies a large body of evidence. Evolution is supported by fossils, genetics, direct observation, and biogeography.

While details continue to be refined, the theory of evolution by natural selection remains one of the most robust frameworks in biology.