CHAPTER 19: DESCENT WITH MODIFICATION

Observations About Life

  • Many organisms share common traits (unity).

  • Differences between organisms lead to a rich diversity of life (biodiversity).

  • Organisms possess traits that make them well adapted to their environments.

Historical Context of Evolution

Charles Darwin

  • Published On the Origin of Species by Means of Natural Selection on November 24, 1859.

    • Key Points:

    • Today's organisms descended from ancestral species.

    • Natural selection is a mechanism for evolutionary change in populations.

Evolution: A Pattern and Process

  • Evolution: Defined as descent with modification, a process where species change over time due to accumulated differences from ancestors as they adapt to their environments.

    • Evolution as a Pattern:

    • Determined by collected scientific data outlining facts (observations of change in organisms over time).

    • Evolution as a Process:

    • Mechanisms causing observable changes include natural selection, mutation, gene flow, and genetic drift.

Pre-Darwinian Thought

Philosophers and Early Scientists

  • Aristotle (384-322 BCE): Viewed species as fixed entities, unchanging over time.

    • Concept of “scala naturae”: Organisms arranged on a ladder of increasing complexity, influenced by literal translations of the Old Testament.

  • Carolus Linnaeus (1707-1778):

    • Swedish botanist who developed a binominal classification system.

    • His system categorized organisms hierarchically, with closely related organisms grouped together.

  • Georges Cuvier (1769-1832):

    • Fundamental to paleontology, studying ancient life through fossils.

    • Observed that older rock strata contained fossils that were dissimilar to modern organisms.

    • Proposed that large extinction events were common, leading to repopulation by different organisms.

    • Did NOT support evolution; believed new organisms originated from nearby areas.

  • James Hutton (1726-1797):

    • Scottish geologist advocating for gradualism, the idea that significant geological changes occur through slow processes over long periods.

  • Charles Lyell (1797-1875):

    • Expanded Hutton's work, proposing uniformitarianism, where geological features arise from natural processes acting over time, influencing Darwin's thinking about Earth’s age and gradual change.

Jean-Baptiste de Lamarck (1744-1829)

  • French biologist who proposed a mechanism of evolution based on fossil records.

  • Lamarck's Principles:

    1. Use and Disuse: Traits that are used become larger/stronger, while those that are disused diminish.

    2. Inheritance of Acquired Traits: Organisms can pass useful modifications to their offspring.

The Voyage of the Beagle

  • Darwin, encouraged by his mentor John Henslow, joined Captain Robert FitzRoy on the HMS Beagle for a scientific expedition.

    • Initially studied medicine before transitioning to natural history and theology through his Cambridge studies.

Adaptations

  • Adaptation: An inherited characteristic enhancing survival and reproduction in a specific environment.

    • Raised the possibility that accumulated adaptations could lead to the emergence of new species.

Natural Selection

  • Definition: The process where individuals with advantageous inherited traits (adaptations) tend to survive and reproduce at higher rates.

    • Natural selection is not a choice made by individuals; rather, it is a function of environmental pressures promoting the survival of the fittest.

On the Origin of Species

  • By 1844, Darwin compiled extensive data and finished his manuscript detailing descent with modification through natural selection.

  • Hesitated to publish due to expected backlash, but was encouraged by Lyell amid fears that others might publish similar ideas first.

Alfred Russel Wallace (1823-1913)

  • British naturalist working in the Malay Archipelago, arrived at similar conclusions of evolution through natural selection.

  • Sent his manuscript to Darwin for feedback.

Publication

  • On July 1, 1858, Lyell presented Wallace’s manuscript alongside excerpts from Darwin’s unpublished works at the Linnean Society of London.

  • Darwin's On the Origin of Species was published on November 24, 1859. While primarily credited to Darwin, Wallace and others contributed to the theory.

Decent with Modification

  • Encapsulates two primary concepts:

    1. Descent: Shared ancestry resulting in common traits.

    2. Modification: The process where differences accumulate over time leading to new characteristics.

Trees of Life

  • Darwin envisioned the history of life as a branching tree, where different organisms emerged from a common trunk, with living organisms represented distinctly from extinct ones.

Artificial Selection

  • Darwin utilized the concept of artificial selection to illustrate his theory of evolution, demonstrating how agricultural practices had altered plants from their wild ancestors.

Observations and Inferences in Natural Selection

First Observation

  • Members of a population exhibit variations in inherited traits.

First Inference

  • Individuals with advantageous traits are more likely to survive and reproduce, leading to higher offspring counts.

Second Observation

  • Organisms often produce more offspring than their environment can sustain.

Second Inference

  • This leads to survival advantages for those with beneficial adaptations, which will be favored and accumulate over generations.

Capacity for Over-reproducing

  • Darwin's notion of over-reproduction was influenced by Malthus’s essay, which argued that human suffering arises from resource scarcity due to population density.

Lamarck’s Giraffes vs. Darwin’s Adaptations

  • Lamarck's theory of the giraffes' long necks contrasted with Darwin’s, which explained that environmental changes select for longer necks positively.

Heritable Traits - Adaptations

  • Adaptations can be inherited by offspring, influencing how future generations adapt. Favorable traits will accumulate over time due to environmental stresses such as predation and resource availability.

Key Features of Natural Selection

  1. Individuals with advantageous heritable traits survive and reproduce at higher rates than those lacking them.

  2. Over time, beneficial traits become more common within a population.

  3. Environmental changes or migrations may shift which traits are advantageous, possibly resulting in new species.

Limitations of Natural Selection

  1. Natural selection acts at the population level, not the individual.

  2. Only traits that vary within the population are affected.

  3. Favorable traits are dependent on the present environment.

Evidence of Evolution

Direct Observations

  • Real-time evolution can be observed, such as the emergence of drug-resistant bacteria.

Homology

  • Homology: Genetic similarities due to common ancestry, identifiable through anatomical or molecular structures, often seen in vestigial features.

Example of Homology: Penguin Knees

  • Penguins possess knee structures, demonstrating retention of ancestral features even in specialized organisms.

Convergent Evolution

  • Occurs when distantly related organisms independently develop similar traits, often due to analogous environmental pressures.

Evidence – Fossil Record

  • Fossil records illustrate:

    1. Substantial differences between extinct and present-day organisms.

    2. Patterns of extinction events over time.

    3. Evolution of new groups from single ancestors.

Evolution of Cetaceans

  • Examination of cetaceans provides insights into the common ancestry of species through fossil records.

Evidence - Biogeography

  • Biogeography: The study of species distribution across geographic areas, influenced by continental drift and animal migration, and focuses on endemic species like Darwin's finches.