Chapter 22: Descent with Modification - A Darwinian View of Life

Descent with Modification: A Darwinian View of Life

• Descent with modification: Species accumulate differences from their ancestors as they adapt to different environments over many generations. This results in shared characteristics and the diversity of life.

Timeline of Evolutionary Thought

• 1795: Hutton proposes gradualism.
• 1798: Malthus publishes "Essay on the Principle of Population."
• 1809:
  • Lamarck publishes his hypothesis of evolution.
  • Charles Darwin is born.
• 1812: Cuvier publishes his studies of vertebrate fossils.
• 1830: Lyell publishes Principles of Geology.
• 1831-1836: Darwin travels around the world on HMS Beagle.
• 1844: Darwin writes his essay on descent with modification.
• 1858: Wallace sends Darwin his hypothesis of natural selection while studying species in the Malay Archipelago.
• 1859: Darwin publishes The Origin of Species.

Darwin's Focus on Adaptation

• Darwin observed many examples of adaptations during the voyage of the Beagle. Adaptations enhanced the survival and reproduction rate of organisms in specific environments.
• Adaptations are inherited characteristics that enhance an organism’s survival and reproduction in specific environments
• Darwin proposed natural selection as an explanation for adaptation.
• Natural selection: A process in which individuals with certain inherited traits tend to survive and reproduce at higher rates because of those traits.

Ideas from The Origin of Species

• Descent with modification by natural selection explains:
  • The unity of life
  • The diversity of life
  • The ways organisms are suited to life in their environments

Descent with Modification

• Darwin used descent with modification to describe his view of life. All organisms are related by descent from a common ancestor that lived in the past.
• Related organisms living in different habitats gradually accumulated diverse modifications to fit them to specific ways of life.

Darwin's View of the History of Life

• Darwin viewed the history of life as a tree with multiple branchings from a common trunk.
• Labeled branches represent groups of organisms living in the present day.
• Unlabeled branches represent extinct groups.
• A fork represents the most recent common ancestor of all lines of evolution branching from that point.

Artificial Selection, Natural Selection, and Adaptation

• Humans modify species through artificial selection, breeding only individuals with desired traits.
• Crops, livestock animals, and pets often bear little resemblance to their wild ancestors.

Key Features of Natural Selection

• Individuals with certain heritable traits survive and reproduce at a higher rate than other individuals.
• Natural selection increases the frequency of adaptations that are favorable in an environment.
• If the environment changes, natural selection may drive adaptation to new conditions, giving rise to new species.
• Individuals do not evolve; it is the population that evolves over time.
• Natural selection can only increase or decrease heritable traits that are variable in a population.
• The environment varies from place to place and over time; favorable traits vary with the environment.

Evidence for Evolution

• Evolution is supported by an overwhelming amount of scientific evidence.
• New discoveries continue to fill the gaps identified by Darwin in The Origin of Species.
• Four types of data document the pattern of evolution:
  • Direct observations
  • Homology
  • The fossil record
  • Biogeography

Homology

• Homology: Similarity resulting from common ancestry, is another type of evidence for evolution.
• Related species can have characteristics that have an underlying similarity yet function differently.

Anatomical and Molecular Homologies

• Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor. For example, the forelimbs of all mammals have the same arrangement of bones, but different functions.
• Vestigial structures are remnants of features that served a function in the organism’s ancestors. For example, snakes arose from ancestors with legs; the skeletons of some snakes retain vestiges of pelvis and leg bones.
• Molecular homologies include the genetic code shared by all life and specific genes that are shared between vastly different organisms.
• In some species, homologous genes may acquire new functions, or lose function entirely.
• Similarities in such genes are evidence of inheritance from a common ancestor.

Homologies and “Tree Thinking”

• Characteristics shared by many species date to a deep ancestral past; homologies that evolved more recently are shared only within smaller groups.
• For example, tetrapods, like all vertebrates, have a backbone. Unlike other vertebrates, all tetrapods also have limbs with digits.
• Evolutionary trees are diagrams that reflect hypotheses about the relationships among groups.
• Homologies form nested patterns on the tree.
• Relatedness is determined by the recent common ancestor, not the proximity of groups on the tree.
• Evolutionary trees show relative timing of events, not actual dates.
• Evolutionary trees are made using many different data sets, including both anatomical and DNA sequence data.
• Well-supported trees can be used to make predictions about organisms.

Convergent Evolution

• Convergent evolution is the evolution of similar, or analogous, features in distantly related groups.
• Analogous traits arise not through common ancestry, but through independent adaptation to similar environments. For example, the sugar glider is an Australian marsupial that superficially resembles the flying squirrel, a North American eutherian.

Biogeography

• Evidence from biogeography, the scientific study of the geographic distribution of species, provides support for evolution.
• Species distributions are influenced by continental drift, the gradual movement of Earth’s landmasses.
• For example, 250 million years ago, all landmasses formed a single large continent called Pangea. By 20 million years ago, they had drifted apart to form the continents near their present locations.

What Is Theoretical About Darwin’s View of Life?

• In science, a theory accounts for many observations and data and attempts to explain and integrate a great variety of phenomena.
• Darwin’s theory of evolution by natural selection integrates diverse areas of biological study and stimulates many new research questions.
• Ongoing research adds to our understanding of evolution.

Observations and Inferences

• Observations
  • Individuals in a population vary in their heritable characteristics.
  • Organisms produce more offspring than the environment can support.
• Inferences
  • Individuals that are well suited to their environment tend to leave more offspring than other individuals.
  • Over time, favorable traits accumulate in the population.