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

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Endless Forms Most Beautiful

  • Dead-leaf moth (Oxytenis modestia):

    • Blends in with its forest floor habitat in the Peruvian rain forest.

  • Lepidopterans (moths and butterflies):

    • Diverse group with over 120,000 species.
    • Juvenile stage (caterpillars): Well-developed head and chewing mouthparts.
    • Adult stage: Three pairs of legs and two pairs of wings covered with small scales.

  • Key observations about life:

    • Organisms are suited for life in their environments.
    • Shared characteristics (unity) of life.
    • Rich diversity of life.

Descent with Modification: A Darwinian View of Life

  • Chapter 22 introduces the concept of descent with modification, a Darwinian view of life.

  • Evolution:

    • Descent with modification: Earth’s many species are descendants of ancestral species that were different from present-day species.
    • Change in the genetic composition of a population from generation to generation (Concept 23.3).

  • Two ways to view evolution:

    • Pattern: Revealed by data from biology, geology, physics, and chemistry, showing that life has evolved over time.
    • Process: Mechanisms that cause the observed pattern of change.

  • Evolution is a unifying theory that explains and connects observations about the living world.

  • Ongoing discoveries continue to shape our understanding of evolution.

The Darwinian Revolution

  • The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species.
  • Darwin’s revolutionary proposal developed over time, influenced by others' work and his travels.
    • Ideas had deep historical roots.

Historical Context of Evolutionary Thought

  • Aristotle (384–322 BCE):

    • Viewed species as fixed (unchanging).
    • Recognized “affinities” among organisms.
    • Arranged life-forms on a ladder of increasing complexity called scala naturae (“scale of nature”).

  • Old Testament account of creation: Species were individually designed by God and therefore perfect.

  • Carolus Linnaeus (1707–1778):

    • Swedish physician and botanist who sought to classify life’s diversity for the glory of God.
    • Developed binomial format for naming species (e.g., Homo sapiens).
    • Adopted a nested classification system, grouping similar species into general categories.
    • Did not ascribe resemblances among species to evolutionary kinship but to the pattern of their creation.
    • Darwin later argued classification should be based on evolutionary relationships.

Ideas About Change over Time

  • Fossils: Remains or traces of organisms from the past, found in sedimentary rocks in strata (layers).

  • Paleontology: Study of fossils, developed by Georges Cuvier (1769–1832).

    • Cuvier noted that the older the stratum, the more dissimilar its fossils were to current life-forms.
    • Observed extinctions were a common occurrence, but opposed the idea of evolution.
    • Speculated that boundaries between strata represented sudden catastrophic events.

  • Gradual change:

    • James Hutton (1726–1797) proposed that Earth’s geologic features could be explained by gradual mechanisms.
    • Charles Lyell (1797–1875) incorporated Hutton’s thinking, proposing that the same geologic processes are operating today as in the past and at the same rate.
    • These ideas strongly influenced Darwin.

  • Lamarck’s Hypothesis of Evolution:

    • Jean-Baptiste de Lamarck (1744–1829) proposed a mechanism for how life changes over time.
    • Compared living species with fossil forms, finding lines of descent.
    • Two principles:
      • Use and disuse: Parts of the body used extensively become larger and stronger, while those not used deteriorate.
      • Inheritance of acquired characteristics: An organism can pass these modifications to its offspring.

  • Lamarck thought organisms have an innate drive to become more complex.

  • Today's genetics refute Lamarck's mechanism: Experiments show traits acquired by use during an individual’s life are not inherited.

Darwin's Research

  • Charles Darwin (1809–1882) was born in Shrewsbury, England, and had an interest in nature.
  • Enrolled at Cambridge University, intending to become a clergyman.
  • Became protégé of botany professor John Henslow.
  • Henslow recommended Darwin to Captain Robert FitzRoy for a voyage around the world on the HMS Beagle.
    • Darwin served as a conversation partner to the captain.

The Voyage of the Beagle

  • Darwin embarked from England in December 1831.
  • Observed features of organisms suited to diverse environments.
  • Noted plants and animals in temperate regions of South America resembled species in the South American tropics more than those in Europe.
  • Fossils resembled living organisms of South America.
  • Experienced an earthquake in Chile, observing rocks being thrust upward, reinforcing Lyell's ideas.

Darwin's Focus on Adaptation

  • Darwin observed mockingbirds on the Galápagos Islands, noting differences between species on individual islands.
  • Hypothesized that the Galápagos were colonized by organisms from South America that diversified.
  • Observed adaptations, inherited characteristics enhancing survival and reproduction.
  • Darwin began to perceive adaptation to the environment and the origin of new species as closely related processes.
    • Example: finches found on the Galápagos Islands (see Figure 1.20). The finches’ various beaks and behaviors are adapted to the specific foods available on their home islands
      *Explanation of adaptations centered on natural selection, where individuals with certain inherited traits survive and reproduce more due to those traits.

Natural Selection in Action

  • By the early 1840s, Darwin had worked out the major features of his hypothesis and wrote an essay in 1844.
  • In June 1858, Alfred Russel Wallace sent Darwin a manuscript with a hypothesis of natural selection nearly identical to Darwin’s.
  • On July 1, 1858, Lyell and a colleague presented Wallace’s paper, along with extracts from Darwin’s unpublished 1844 essay, to the Linnean Society of London.
  • he quickly finished his book, titled On the Origin of Species by Means of Natural Selection (commonly referred to as The Origin of Species), and published it the next year.

Ideas from The Origin of Species

  • Darwin amassed evidence that descent with modification by natural selection explains the unity of life, the diversity of life, and the striking ways in which organisms are suited for life in their environments.

Descent with Modification

  • Darwin discussed descent with modification, a phrase summarizing his view of life.
  • Organisms share many characteristics (unity of life) due to descent from a common ancestor.
  • Descendants living in various habitats gradually accumulated diverse modifications (adaptations).
  • Over time, descent with modification leads to the rich diversity of life.

Visualizing Evolutionary History

  • Darwin viewed the history of life as a tree with multiple branchings from a common trunk out to the tips.
  • Each fork represents the most recent common ancestor.
  • Extinction events and branching processes explain morphological gaps between related groups.
  • Scientists estimate that over 99% of all species that have ever lived are now extinct.
  • Fossils of extinct species document the divergence of present-day groups.

Artificial Selection, Natural Selection, and Adaptation

  • Darwin proposed natural selection to explain observable patterns of evolution.

  • He discussed artificial selection: humans modifying species by selecting and breeding individuals with desired traits.

  • Darwin's argument based on two observations and two inferences:

    • Observation #1: Members of a population vary in their inherited traits.
    • Observation #2: All species can produce more offspring than their environment can support, and many fail to survive and reproduce.
    • Inference #1: Individuals with traits giving them a higher probability of surviving and reproducing tend to leave more offspring.
    • Inference #2: Unequal ability to survive and reproduce leads to the accumulation of favorable traits in the population over generations.

  • Key connection between natural selection and overreproduction, inspired by Thomas Malthus's essay.

  • The rest are eaten, starved, diseased, unmated, or unable to tolerate physical conditions of the environment such as salinity or temperature.

  • Darwin reasoned that if artificial selection can bring about dramatic change in a relatively short period of time, then natural selection should be capable of substantial modification of species over many hundreds of generations.

Key Features of Natural Selection

  • Natural selection operates through interactions between individual organisms and their environment.
  • Natural selection can amplify or diminish only those heritable traits that differ among individuals in a population.
  • Environmental factors vary from place to place and over time.

Evidence for Evolution

  • Evolution is supported by an overwhelming amount of scientific evidence.
  • Four types of data document the pattern of evolution:
    • Direct observations
    • Homology
    • The fossil record
    • Biogeography

Direct Observations of Evolutionary Change

  • Biologists have documented evolutionary change in thousands of scientific studies.
  • Natural Selection in Response to Introduced Species:
    • Herbivores often have adaptations to feed on primary food sources.
    • Soapberry bugs use a “beak” to feed on seeds within fruits.
    • In southern Florida, they feed on balloon vines, while in central Florida, they feed on goldenrain trees.
    • soapberry bugs that feed on goldenrain tree, natural selection would result in beaks that are shorter than those in populations that feed on balloon vine (Figure 22.13).
      Observed changes in beak lengths had important consequences: In Australia, for example, the increase in beak length nearly doubled the success with which soapberry bugs could eat the seeds of the introduced species. Furthermore, since historical data show that the goldenrain tree reached central Florida just 35 years before the scientific studies were initiated, the results demonstrate that natural selection can cause rapid evolution in a wild population.

The Evolution of Drug-Resistant Bacteria

  • Drug-resistant pathogens are an example of ongoing natural selection dramatically affecting humans.
  • Bacteria and viruses produce new generations in a short time, causing rapid proliferation of resistant strains.
  • Example: Staphylococcus aureus and methicillin-resistant S. aureus (MRSA).
    *In 1943, when penicillin became the first widely used antibiotic.However, by 1945, more than 20% of the S. aureus strains seen in hospitals were already resistant to penicillin. Then, in 1959, doctors began using a promising new anti- biotic, methicillin. But within two years, methicillin-resistant strains of S. aureus were observed.
  • Some S. aureus populations included individuals able to synthesize cell walls using an enzyme unaffected by methicillin.
    • These individuals survived, reproduced, and spread MRSA.
  • MRSA could initially be controlled by other antibiotics, but strains became resistant to multiple antibiotics (multidrug-resistant strains).
    These examples highlight three key points about natural selection. First, natural selection is a process of editing, not a creative mechanism.

*Second, in species that produce new generations in short periods of time, evolution by natural selection can occur rapidly— in just a few years (S. aureus) or decades (soapberry bugs).
*Third, natural selection depends on time and place..

Homology

  • A second type of evidence for evolution comes from analyzing similarities among different organisms. evolution is a process of descent with modification: Characteristics present in an ancestral organism are altered (by natural selection) . Similarity resulting from common ancestry is known as homology.

Anatomical and Molecular Homologies

The view of evolution as a remodeling process leads to the prediction that closely related species should share similar features—and they do. For example, the forelimbs of all mammals, including humans, cats, whales, and bats, show the same arrangement of bones from the shoulder to the tips of the digits, even though the appendages have very different functions: lifting, walking, swimming, and flying (Figure 22.15).

  • vestigial structures are remnants of features that served a function in the organism’s ancestors. For instance, the skeletons of some snakes retain vestiges of the pelvis and leg bones of walking ancestors.
  • Biologists also observe similarities among organisms at the molecular level. All forms of life use essentially the same genetic code, suggesting that all species descended from common ancestors that used this code.
  • Consider the tetrapods (from the Greek tetra, four, and pod,
    foot), the vertebrate group that consists of amphibians, mam- mals, and reptiles. Like all vertebrates, tetrapods have a back- bone. But unlike other vertebrates, tetrapods also have limbs with digits (see Figure 22.15).

A Different Cause of Resemblance: Convergent Evolution

envolution, the independent evolution of similar features in different lineages.

  • Although they evolved independently from different ancestors, these two mammals have adapted to similar environ- ments in similar ways. In such examples in which species share features because of convergent evolution, the resemblance is said to be analogous, not homologous. Analogous features share similar function, but not common ances- try, while homologous features share com- mon ancestry, but not necessarily similar function.

The Fossil Record

  • A third type of evidence for evolution comes from fossils. The fossil record docu- ments the pattern of evolution, showing that past organisms differed from present- day organisms and that many species have become extinct.
    Fossils can also shed light on the origins of new groups of organisms. An example is the fossil record of cetaceans, the mammalian order that includes whales, dolphins, and por- poises. Early fossils provided strong support for a hypothesis based on DNA sequence data: that cetaceans are closely related to even-toed ungulates, a group that includes hippopotamuses, pigs, deer, and cows.

Biogeography

  • A fourth type of evidence for evolution comes from the field of biogeography, the scientific study of the geographic distribu- tions of species. The geographic distributions of organisms are influenced by many factors, including continental drift, the slow movement of Earth’s continents over time.

Scientists have constructed evolutionary trees for horses based on anatomi- cal data. These trees and the ages of fossils of horse ancestors suggest that the genus that includes present-day horses (Equus) originated 5 million years ago in North America. Geologic evidence indicates that at that time, North and South America were not yet connected, making it difficult for horses to travel between them. Thus, we would predict that the oldest Equus fossils should be found only on the conti- nent on which the group originated—North America.

What Is Theoretical About Darwin’s View of Life?

Keep in mind that the scientific meaning of the term theory is very differ- ent from its meaning in everyday use. The colloquial use of the word theory comes close to what scientists mean by a hypothesis. In science, a theory is more comprehensive than a hypothesis.