Descent with Modification: A Darwinian View of Life

What is Evolution?

Evolution is a central concept in biology, explaining both the diversity and unity of life. It refers to the process by which populations of organisms change over generations through variations in their genetic material.

• Definition: Evolution is the change over time in the genetic composition of a population.

• Descent with Modification: Modern organisms are descended from ancestral species, but with modifications that have accumulated over time.

What is Adaptation?

Adaptation is a key outcome of evolution, allowing organisms to survive and reproduce in specific environments.

• Evolutionary Adaptation: The accumulation of inherited characteristics that enhance an organism’s ability to survive and reproduce in particular environments.

• Example: The beak shapes of Galápagos finches, which are suited to different food sources.

Pre-Darwinian Views on the Origin of Species

Aristotle and the Scala Naturae

Before Darwin, most Western philosophers believed species were fixed and unchanging.

• Aristotle (384–322 BC): Proposed that organisms could be arranged on a ladder of increasing complexity (Scala Naturae), with each species having a fixed place.

• Creationism: The belief that species were individually designed by a creator for specific purposes.

Carolus Linnaeus and Taxonomy

• Linnaeus (1707–1778): Developed the binomial system of nomenclature (genus and species) and organized species into hierarchical categories.

• This system did not imply evolutionary relationships, but rather patterns of creation.

Prevailing Beliefs Before Darwin

• The Earth was thought to be only about 6,000 years old.

• There was little time for significant change or evolution.

Influences on Darwin

Fossils and Paleontology

Fossils provided evidence that life on Earth had changed over time.

• Fossils: Remains or traces of organisms from the past, often found in sedimentary rocks formed from sand and mud.

• Strata: Layers of sedimentary rock that represent different periods in Earth’s history.

• Paleontology: The study of fossils, founded by Georges Cuvier (1769–1832).

• Cuvier observed that older strata contained fossils very different from current life forms, and that some species appeared or disappeared between layers.

Catastrophism vs. Gradualism and Uniformitarianism

• Catastrophism (Cuvier): The idea that sudden, short-lived, violent events (catastrophes) caused extinctions and the formation of new strata.

• Gradualism (Hutton, 1795): Proposed that geological features are the result of slow, continuous processes over long periods.

• Uniformitarianism (Lyell, 1797–1895): The same geological processes observed today have operated throughout Earth’s history at the same rate.

Implication: If geological change is slow and continuous, Earth must be much older than previously thought, allowing time for evolution.

Pre-Darwinian Theory of Evolution

Lamarck’s Hypothesis of Evolution

Jean-Baptiste Lamarck (1809) proposed one of the first mechanisms for how life changes over time.

• Compared living species with fossils and suggested lines of descent from older to younger forms.

• Envisioned a ladder of complexity, with organisms striving to become more complex.

• Two Principles:

  1. Use and Disuse: Body parts used extensively become larger and stronger; those not used deteriorate.

  2. Inheritance of Acquired Characteristics: Modifications acquired during an organism’s life are passed to offspring.

• Note: Lamarck’s mechanism was incorrect, but he recognized adaptation to the environment as a result of gradual change.

Darwin and the Voyage of the HMS Beagle

Darwin’s Journey

• Charles Darwin (1809–1882): Sailed on the HMS Beagle from 1831 to 1836, observing diverse plants and animals.

• Noted variations among species, such as beak shapes in Galápagos finches, which were adapted to different environments.

Evolution by Natural Selection

Darwin and Wallace

• Both independently developed the theory of evolution by natural selection.

• Darwin’s observations during the Beagle voyage and Wallace’s work in Indonesia led to similar conclusions.

Natural Selection

• Definition: A process in which individuals with certain inherited traits survive and reproduce at higher rates than others because of those traits.

• Natural selection acts on existing variation in the gene pool; it does not create new traits on demand.

• Example: The evolution of beak shapes in finches to exploit different food sources.

Artificial Selection

• Definition: The selective breeding of organisms by humans to encourage the occurrence of desirable traits.

• Artificial selection is analogous to natural selection but is guided by human choice rather than environmental pressures.

• Example: Breeding dogs for specific traits such as size or temperament.

Evidence for Evolution

Examples of Natural Selection

• Natural selection can be observed in real time, such as the development of HIV resistance or drug-resistant bacteria.

Homologous and Analogous Structures

• Homologous Structures: Structures in different species that are similar due to common ancestry (e.g., mammalian forelimbs).

• Analogous Structures: Structures that are similar in function but not in evolutionary origin (e.g., wings of birds and insects).

• Vertebrate Embryos: All vertebrates share similar developmental genes; differences arise from genes being switched on or off at different times.

Convergent Evolution

• Similarity between two organisms, structures, or molecules due to independent evolution along similar lines, not common ancestry.

• Example: The wings of bats and birds are analogous, not homologous.

Fossil Record

• Shows changes in organisms over time and the appearance and extinction of species.

• Provides evidence for both small-scale changes and the origin of major groups (e.g., cetaceans).

Biogeography

• The geographic distribution of species is influenced by factors such as migration and continental drift.

• Earth’s landmasses were once joined as Pangea (~250 million years ago) and have since drifted apart, affecting the distribution of organisms.

Evolution in a Historical Context

Catastrophism, Gradualism, and Uniformitarianism

• Catastrophism (Cuvier): Catastrophic events caused extinctions and the formation of new strata.

• Gradualism (Hutton): Slow, continuous processes (wind, water, earthquakes, volcanoes) shaped geological formations.

• Uniformitarianism (Lyell): The same natural processes operate today as in the past, at similar rates.

• Observations of geological formations and the rates at which processes occur suggest that Earth is at least millions of years old.

Additional info: The modern synthesis of evolutionary biology integrates Darwin’s theory of natural selection with Mendelian genetics, providing a comprehensive explanation for the mechanisms of evolution.