AP Biology
Notes: Darwin

Many Greek philosophers believed in the gradual evolution of life.  However, the two that 
influenced Western culture most, Plato (427-347 B.C.) and his student Aristotle (384-322 B.C.),
held opinions which were inconsistent with a concept of evolution.

                Plato, whose philosophy is known as idealism, believed that there were two
                coexisting worlds: an ideal, eternal, real world and an illusionary imperfect world 
                that humans perceive with their senses.

                Aristotle questioned the Platonic philosophy of dual worlds, but his beliefs also 
                excluded evolution.   
                       *Recognizing that organisms vary from simple to complex, he believed that 
                         they could be placed on a scale of increasing complexity.  On this ladder of 
                         life, each form had its allotted rung and each rung was occupied. 
                       *In this view of life, species were fixed and did not evolve. 
                       *The scala naturae view of life prevailed over 2000 years.

Creationist 
        *Natural Theology, a philosophy that the Creator's plan could be revealed by studying 
         nature, dominated European and American biology even as Darwinism emerged.
        *For natural theologians, adaptations of organisms were evidence that the Creator had 
         designed every species for a particular purpose.
        *Natural theology's major objective was to classify species revealing God's created 
         steps on the ladder of life.

Carolus Linnaeus (1707-1778)
    *Known as the father of taxonomy-the naming and classifying of organisms-he developed 
     a system of  binomial nomenclature still used today.
    *He adopted a system for grouping species into categories and ranking the categories into
      a hierarchy.  For example, similar species are grouped into a genus; similar genera are 
      grouped into the same order.
    *The clustering of species in taxonomic groups did not imply evolutionary relationships to 
       Linnaeus, since he believed that species were permanent creations.
      "God creates, Linnaeus arranges".

Curvier
    Fossils- Relics or impressions of organisms from the past preserved in rock
                 Most fossils are found in sedimentary rocks, which:
                   *Form when new layers of sand and mud settle to the bottom of seas, lakes and 
                    marshes, covering and compressing older layers into rock.
                   *May be deposited in many layers (strata) in places where shorelines repeatedly 
                    advance and retreat.  Later erosion can wear away the upper (younger) strata, 
                    revealing older strata which had been buried.

           The fossil record thus provides evidence that Earth has had a succession of flora 
           and fauna.

         Even with paleontological evidence, Cuvier was an effective opponent to the 
         evolutionists of his day.
                    He reconciled the fossil evidence with his belief in the fixity of species by 
                    speculating that boundaries between fossil strata corresponded in time to 
                    catastrophic events, such as floods or droughts.
                    This view of Earth's history is known as catastrophism.

Catastrophism- Theory that major changes in the Earth's crust are the result of catastrophic
    events rather than from gradual processes of change.

Hutton

Gradualism- Principle that profound change is the cumulative product of slow, continuous process.
                -Competed with Cuvier's theory of catasttrophism
                -Proposed by James Hutton (1975), a Scotttish geologist.  He proposed that it 
                  was possible to explain the various land forms by looking at mechanisms currently 
                  operating in the world.

               Ex: Canyons form by erosion from rivers, and fossil-bearing sedimentary rocks 
               form from particles eroded from the land and carried by rivers to the sea.

Lyell

                -Charles Lyell, a leading geologist of DDarwin's time, expanded Hutton's gradualism
                  into the theory known as uniformitarianism.

Uniformitarianism- Theory that geological processes are uniform and have operated from 
    the origin of the Earth to the present.
                    It was Lyell's extreme idea that geological processes are so uniform that their 
                    rates and effects must balance out through time.

Lamarck 

Only Jean Baptiste Lamarck (1744-1829) developed and published (1809) a comprehensive 
    model which attempted to explain how life evolved.

            Lamarck was in charge of the invertebrate collection at the Natural History Museum 
            in Paris, which allowed him to:
                        Compare modern species to fossil forms, and in the process, identify several 
                        lines of descent composed of a chronological series of older fossils to younger 
                        fossils in modern species.

Lamarck proposed a mechanism by which specific adaptations evolve, which included two related principles:

1. Use and disuse. Those body organs used extensively to cope with the environment become larger and stronger while those not used deteriorate.

2. Inheritance of acquired characteristics. The modifications an organism acquired during its lifetime could be passed along to its offspring.

Lamarck deserves credit for proposing that:

1. Evolution is the best explanation for both the fossil record and the diversity of life.
2. The Earth is ancient.

Darwinian Revolution

At the beginning of the 19th century, natural theology still dominated the European and American intellectual climate. In 1809, the same year Lamarck published his theory of evolution, Charles Darwin was born in England. When he was 16, he was sent by his father to the University of Edinburgh to study medicine, which he found boring. He left without a degree and enrolled at Christ College, Cambridge University to prepare for the clergy.

The Voyage of the Beagle

The HMS Beagle, with Darwin aboard, sailed from England in December 1831. The voyage's mission was to chart the poorly known South American coastline. While the ship's crew surveyed the coast, Darwin spent most of his time ashore collecting specimens of the exotic and diverse flora and fauna.

Darwin observed the various adaptations of plants and animals that inhabited the diverse environments of South America: Brazilian jungles, grasslands of the Argentine pampas, desolate islands of Tierra del Fuego, and the Andes Mountains. Darin noted that:

1. The South American flora and fauna from different regions were distinct from the flora and fauna of Europe.

2. Temperate species were taxonomically closer to species living in tropical regions of South America than to temperate species of Europe.

3. The South American fossils he found were distinctly South American in their resemblance to the living plants and animals of that continent.

Geographical distribution was particularly confusing in the case of the fauna of the Galapagos. Darwin found that most animal species on the Galapagos are unique to those islands, but resemble species living on the South American mainland. Darwin collected 13 types of finches from the Galapagos, and although they were similar, they seemed to be different species.

Darwin and Adaptation

Darwin believed that the origin of new species and adaptation were closely linked processes. In particular, he believed that:

1. Two populations of a species could be isolated in different environments and diverge as each adapted to local conditions.

2. Over many generations, the two populations could become dissimilar enough to be designated separate species. This is what happened with the Galapagos finches; their different beaks are adaptations to specific foods available on their home islands.

In 1844, Darwin wrote a long essay on the origin of species and natural selection. In June 1858, Darwin received a letter from Alfred Wallace, who was working as a specimen collector in the East Indies. Accompanying the letter was a manuscript detailing Wallace's own theory of natural selection which was almost identical to Darwin's. The letter asked Darwin to evaluate the theory and forward the manuscript if it was thought worthy of publication. Wallace's paper, along with excerpts from Darwin's unpublished 1844 essay, were presented to the Linnaean Society of London on July 1, 1858. The next year, Darwin finished and published The Origin of Species. He succeeded where previous evolutionists had failed not only because science was moving away from natural theology, but because he convinced his readers with logic and evidence.

The Two Main Points of The Origin of Species

I. Descent with modification

Darwin used the phrase "descent with modification" instead of evolution in the first edition of The Origin of Species. He perceived a unity in life with all organisms related through descent from some unknown ancestral population that lived in the remote past. Diverse modifications accumulated over millions of years, as descendants of this common ancestor moved into various habitats. 

II. Natural selection and adaptation

Darwin's book focused on the role of natural selection in adaptation. Ernst Mayr of Harvard University dissected the logic of Darwin's theory into three inferences:

1. Natural selection is this differential success in reproduction, and its product is adaptation of organisms to their environment.

2. Natural selection occurs from the interaction between the environment and the inherent variability in a population.

3. Variations in a population arise by chance, but natural selection is not a chance phenomenon, since environmental factors set definite criteria for reproductive success.

Darwin was already aware of the struggle for existence caused by overproduction, presented in an essay written by the Reverend Thomas Malthus. Malthus held that much of human suffering was a consequence of human populations growing faster than the food supply. This capacity for overproduction is common to all species, and only a fraction of new individuals complete development and leave offspring of their own; the rest die or are unable to reproduce.

Summarizing Darwin's view of evolution:

1. The diverse forms of life have arisen by descent with modification from ancestral species.

2. The mechanism of modification has been natural selection working gradually over long periods of time.

Evidence of Evolution

Biogeography

It was biogeographical evidence that first suggested common descent to Darwin, because the biogeographical patterns he observed only made sense in the light of evolution.

Biogeography - the geographical distribution of species

Why are two islands with similar environments in different parts of the world not populated by closely related species, but rather by species more closely related to those from the nearest mainland even when that environment is quite different?

The Fossil Record

Darwin was troubled by the absence of transitional fossils linking modern life to ancestral forms. Even though the fossil record is still incomplete, paleontologists continue to find important new fossils, and many key links are no longer missing. For example, fossilized whales link these aquatic mammals to their terrestrial ancestors.

Comparative Anatomy

Anatomical similarities among species grouped in the same taxonomic category are a reflection of their common descent.

Homologous structures: structures that are similar because of common ancestry

Other evidence from comparative anatomy supports that evolution is a remodeling process in which ancestral structures that functioned in one capacity have been modified as they take on new functions.

Vestigial organs: rudimentary structures of marginal or no use to an organism

Vestigial organs are remnants of structures that had important functions in ancestral forms but are no longer essential. For example, the remnants of pelvic and leg bones in snakes show descent from a walking ancestor, but have no function in the snake.

Comparative Embryology

Closely related organisms go through similar stages in their embryonic development. Vertebrate embryos go through an embryonic stage in which they possess gill slits on the sides of their throats. In fish, the gill slits form gills; in humans, they form the eustachian tubes that connect the middle ear with the throat.

Molecular Biology

An organism's hereditary background is reflected in its genes and their protein products. Siblings have greater similarity in their DNA and proteins than do two unrelated organisms of the same species. Likewise, two species considered to be closely related by other criteria should have a greater proportion of their DNA and proteins in common than more distantly related species. Molecular biology has also substantiated Darwin's idea that all forms of life are related to some extent through branching descent from the earliest organisms. Even taxonomically distant organisms have some proteins in common. For example, cytochrome c (a respiratory protein) is found in all aerobic species. Cytochrome c molecules of all species are very similar in structure in function, even though mutations have substituted amino acids in some areas of the protein during the course of evolution.