Historically have been two primary competing views about life:

• Species do not change, but are fixed.
• Life changes over time.

Much new consideration of topic during Age of Enlightenment and afterwards:

• New knowledge of diversity of life, particularly of marine forms and foreign animals & plants
• Beginning to realize the existence of a geologic past and fossil (extinct) organisms

Prior to knowledge of geological ancient past and its extinct creatures, could only relate modern animals to each other; and thus assumed any transitions had to be between known modern forms.

With rise of knowledge of both fossil creatures and geologic time, new models of evolution were proposed. In these models the earlier forms were the ancestors of modern taxa.

While some thinkers once thought that life as we see it now is the way it has always been, the discovery of the fossil record showed that strange creatures once roamed the Earth that are no longer there. How to explain these observations? Two main possibilities:

• The successive appearance and disappearance of different forms through time, without genetic connection (as supported by Owen, Cuvier, and others)
• Transmutationism: direct lineal relationships between ancestor and descendant species. So living species are descendants of earlier distinct species, which themselves were the descendants of even earlier ones. "Transmutationism" became known as "evolution" after the work of Darwin and Wallace.

The Initial Evidence for Transmutationism/Evolution

• Homologies: the same anatomical structures ("body parts") are repeated in different organisms. This allows us to recognize how they differ from each other, and how they resemble each other.
• Living things can be grouped using a nested hiearchy based on shared presence of homologous stuctures of similar form
  • System of classification codified by Carolus Linnaeus (18th Century Swedish botanist)
  • Many of his principles, such as Latin names for organisms, and the use of genus and species still used today
  • However, species are not fixed entities. They vary across their range, and they can often hybridize with closely related forms
• Adaptations: any structure or behavior which allows an organism to interact with its environment in certain specific ways
• Analagous structures: non-homologous structures found two or more organisms that are adapted for the same function
• Vestigial structures: anatomical features which have some significant adaptive function in some forms, but are reduced and non-functional (or nearly non-functional) in a related form
• Transitional fossils: extinct species intermediate in morphology between now-distinct groups
• Presence of similar phases and morphologies in embyros of organisms which are very different and distinct as adults
• Biogeography: the non-random distribution of living things over the surface of the Earth, both now and in the geologic past
• William "Strata" Smith's Stratigraphic Principle of Fossil Succession, indicating a History to Life

Fossils demonstrated that the living component of the Earth changed through time; shared homologies showed connections between groups; adaptations showed organisms "fit" to their environment. Transmutationists already accepted the central tenets of Evolutionary Theory:

• The Diversity of Living Things is the Product of Descent with Modification
• New species are the modified descendants of previously existing species

Transmutationist models:

• Spontaneous generation of new lineages of organisms throughout time; thus, many living things represent seperate origins at different points in Earth History
• Within each lineage, "driving forces" impel organisms towards improvement (i.e., simple forms become complex) down predetermined pathways
• Inheritance is from use and disuses: characters acquired during the lifetime of an individual are passed onto descendants

Problems with these ideas, however:

• Spontaneous generation doesn't work
• "Driving forces" never identified, and are more metaphysical than naturalistic
• Continuity of lineages through long periods of Earth history, rather than appearance, transformation, and reappearance:
  • Also, fossils documented linkages between groups rather than separation
• Inheritance doesn't happen by use & disuse; transformations to adult are not passed onto offspring

II. On the Origin of Species by Means of Natural Selection

The discovery of the primary mechanism of evolution was the work of two English naturalists:

• Charles Robert Darwin
• Alfred Russel Wallace

• Both studied natural history, including geology, in the UK
  • Thus, both were familiar with fossil organisms and with the (then-new) ideas of geologic time
• Both traveled to distant lands (Darwin to South America, the Galápagos Islands, and various other localities in the Pacific Ocean; Wallace to Amazonia and Indonesia)
• Both made collections of organisms, and so had direct experience with the varieties of nature

Their model was called Natural Selection, and was analogous to "artificial selection" (e.g., domestication). Darwin and Wallace's observations:

• Variability: There is variation in all populations.
  • No two members of a population are totally identical.
  • Some sources of variation include age and sexual differences; the results of factors that happened during the lifetime (differences of nutrition, disease, accident, etc.); individual difference in inherited traits; etc.
  • The idea that individual variation was significant was a blow to previous models of Nature. Most earlier natural historians believed in perfect types, and thought variation was degeneration from those types. Darwin and Wallace documented that the variation is the reality, and the "perfect types" were just myths.
• Heritability: Some (but not all) variation is inherited.
  • Causal mechanism of inheritance unknown in Darwin's time.
  • Discovery by Gregor Mendel of genetics came later, and discovery of DNA came later still
  • Heritable traits are coded in DNA and passed on to descendents
    • Note that DNA is NOT a "blueprint" as commonly thought: it is a set of instructions for putting bodies together and maintaining them after they've been built
    • Each little instruction is called a gene: a piece of code that helps the cell to build a protein
    • Most genes have slightly different versions called alleles that produce different end products
    • It is these alleles (one copy for each gene per parent) that is passed on to offspring
    • Different combinations of alleles result in different traits being expressed (that is, different phenotypes). Depending on the particular combination of alleles an offspring gets, they might have the same trait as their mother, their father, or something different than either.
    • This was the major source of individual variation that Darwin & Wallace never knew about!
  • Mutations are new variations in heritable traits, caused by miscopied DNA (duplication of parts of genes; miswritten code; etc.)
    • Some mutations may be deleterious (they result in harm to the organism)
    • Many mutations may be neutral (they don't benefit the organism in an obvious way, nor hurt it)
    • A small number of mutations may wind up being beneficial (the variation they produce allow it to do better somehow in the world)
• Superfecundity: Organisms produced far more offspring than can possibly survive
  • Application of demographer Thomas Malthus' reproductive excess concept to Nature
  • Violated another previously-held belief: that Nature was perfect and everything had its place

Thus, IF some variation gives the individual a slight advantage (bigger, stronger, smaller, smarter, less tasty, whatever) at surviving; and IF that variation is heritable; THEN there is a somewhat better than average chance that organisms with that variation will survive to bear the next generation. Over the long expanse of geologic time, the accumulation of these variations will change the population from one form to another: the origin of species.

Hence,

* Natural Selection is the differential survival and reproduction of variants in a population resulting in a net change in phenotype of the descendants. *

(Short form: "Natural selection is the differential survival and reproduction of variants in a population.")

(Even shorter form with a 20th Century slant: MUTATION PROPOSES, SELECTION DISPOSES)

If Evolution can be summarized as "no one is identical to their parents", then Natural Selection can be summarized as "no one is identical to their siblings, either; plus, life's hard!"

Key points of Natural Selection:

• Does NOT happen to individuals, only to populations (lineages)
• Analogous to "artificial selection" (domestication), but operates:
  • On all traits rather than a few (humans can keep alive crops, farm animals, or pets that might otherwise die in the wild; obviously, wild plants and animals don't have that help!)
  • Over vast amounts of geologic time, rather than just a few generations
• Does NOT require simple things evolving into complex: sometimes a simplified mutation of a structure might be advantageous than the ancestral complex one (hence, vestigial organs)
• Cannot evolve towards something with a goal in mind; only favors variations that are advantageous at the time of selection

"Survival of the Fittest"?: Not as such. Phrase not in the earlier editions of the Origin, nor was it coined by Darwin. Comes from economist/philosopher Herbert Spencer:

• Unlike popular idea, evolutionary fitness is NOT being the biggest, strongest, fastest, etc.
• Instead, Fitness = Reproductive Success
• So a great grandmother with dozens of children, grandchildren, and great grandchildren is far more "fit" (in evolutionary terms) than all the childless Nobel prize winners and Olympic athletes put together!

• Homologies are shared structures inherited from common ancestors
• Adaptations are produced by the net effect of differential survival of particular variations in the populations, favoring a particular interaction with the environment
• Life is organized by nested hierarchy because it IS a tree of descendants branching out from common ancestors
• The Principle of Fossil Succession works because Life had a single, unique, non-repeating history
• Vestigial organs exist because variants with reduced versions of these organs have a better than average survival rate (perhaps because physiological resources aren't being used up for less-used structures) than those with fully developed organs
  • However, vestigial organs represent existence of ancestor with more fully developed version of that organ: vestigial legs on whales & snakes indicate legged whale- and snake-ancestors
• Common pattern of embryology between closely related forms because they share very recent common ancestors; more distantly related organisms with different forms of development because longer times between divergence
• Fossil record of groups intermediate between distinct modern groups represent common ancestors (or relatives of the actual common ancestors) of these groups
• Because life has a history of divergence from common ancestors, regions with shared history of particular creatures will have closely related species
• New species are the modified descendants of older species

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Last modified: 10 January 2007