Historically have been two primary competing views about life:

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

Traditionally, most people accepted the fixity of species just as they accepted that the world today is pretty much the same now as in the past.

Theological argument for fixity under the Biblical concept of the Plenum ("fullness"):

• Ecclesiastes 1:9 and 3:14-15, if you want to look it up
• "Nothing new under the sun": nothing has been taken from Creation, nor removed from it

The discoveries of the early (18th and 19th Century) geologists put paid to the idea that the surface of the Earth was unchanging:

"These facts, unknown to the vulgar, but well known to all who observe nature, force the physical scientist to recognize that all the surface of our globe has changed; that it has had other seas, other continents, another geography." --Nicolas Boulanger (1722-1759)

"Life, therefore, has been often disturbed on this earth by terrible events - calamities which, at their commencement, have perhaps moved and overturned to a great depth the entire outer crust of the globe, but which, since these first commotions, have uniformly acted at a less depth and less generally. Numberless living beings have been the victims of these catastrophes; some have been destroyed by sudden inundations, others have been laid dry in consequence of the bottom of the seas being instantaneously elevated. Their races even have become extinct, and have left no memorial of them except some small fragments which the naturalist can scarcely recognise." --'Preliminary discourse', to Recherches sur les Ossemens Fossiles (1812), trans. R. Kerr Essay on the Theory of the Earth (1813), Baron Georges Leopold Chretien Frederic Dagobert Cuvier

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. Naturalist John Herschel (in an 1836 letter to Charles Lyell) wrote:

"I allude to that mystery of mysteries, the replacement of extinct species by others. Many will doubtless think your speculations too bold, but it is as well to face the difficulty at once. For my own part, I cannot but think it an inadequate conception of the Creator, to assume it as granted that his combinations are exhausted upon any one of the theatres of their former exercise, though in this, as in all his other works, we are led, by all analogy, to suppose that he operates through a series of intermediate causes, and that in consequence the origination of fresh species, could it ever come under our cognizance, would be found to be a natural in contradistinction to a miraculous process -- although we perceive no indications of any process actually in progress which is likely to issue in such a result."

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, Herschel, 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.

Transmutationism, a set of early evolutionary models, accepted by several prominent scientists by the late 1700s. Among them were Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamarck (normally known as Jean Baptiste de Lamarck) and Erasmus Darwin (doctor, scientist, surgeon, abolitionist, and INCREDIBLY rich).

Fossils demonstrated that the living component of the Earth changed through time, sometimes linking what are now distinct groups; shared anatomical features showed connections between groups; adaptations showed organisms "fit" to their environment; the nested hierarchy of classification showed structure to the distribution of anatomical features among organisms; natural hybrids demonstrated that species were not distinct inseparable kinds; embryology revealed organized similarities between distinct groups; the distribution of species over the surface of the Earth and through time showed similar groups in close regions. Together, all this showed that life had a history. The Transmutationists thus 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

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

This was the independent discovery of Charles Robert Darwin and Alfred Russel Wallace. Their model was called Natural Selection, and was analogous to "artificial selection" (e.g., domestication). Here is Natural Selection as described in Darwin's own words:

"As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form." -- Introduction to The Origin, first edition, 1859.

• 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
• The Importance of Time: "No one but a practiced geologist can really comprehend how old the world is, as the measurements refer not to the revolutions of the sun & our lives." -- Notebook E, late 1838 to 1839
• The Importance of Isolation: "Change of external conditions, and isolation either by chance landing of a form on an island, or subsidence dividing a continent, or great chain of mountains, and the number of individuals not being numerous will best favour variation and selection... Barrier would further act in preventing species formed in one part migrating to another part." -- Sketch, 1842
  • We now refer to this as allopatric speciation if the barrier entirely separates interbreeding, or peripatric speciation if one population is on the fringes of the original with only limited gene flow.

From Darwin and Wallace, we get the beginnings of modern evolutionary theory. It has five major components:

• Evolution is descent with modification: that is, the anatomical traits and other features of populations change over time from generation to generation
• These modifications occur relatively slowly on average: small incremental changes added up over many generations
• Populations may diverge into two or more distinct lineages (which may or may not produce their own descendant branches)
• All species share a common ancestry: thus, the shape of the history of lineages can be seen as a Tree of Life
• Much (although not all) evolutionary change is due to natural selection, which is the sole process for producing adaptations

III. Patterns of Evolution

With the discovery of evolution by natural selection, biologists from Darwin and Wallace's time onward have documented many different patterns and processes in evolution. Sometimes they refer to "microevolution" (changes within an species) and "macroevolution" (patterns on the larger scale; changes from one species to another, or between different lineages of ancestors and descendants). It is important to remember that "micro-" vs "macro-" is just a matter of scale and perception: at the level of individuals and populations, there is just variability, heritability, and superfecundity.

The most important pattern: the Tree of Life. Darwin and Wallace demonstrated the reality of Divergence through Time and Common Ancestry:

• Common Ancestry
  • Closely related species are close because their common ancestor diverged relatively recently in Earth history
  • Other species are more distantly related because of divergences of THEIR common ancestors even farther back in time
  • No separate origins for different groups; instead, patterns of common ancestry and diverging descendants
• Divergence from common ancestors
  • Two (or more) distinct variations in an ancestral population convey their own advantage against the rest of the population
  • Over time, these two (or more) variations will become more distinct from each other
  • If they diverge enough, they will no longer be able to mate with each other: will be different species
  • "We may look at Megatherium, armadillos & sloths as all offsprings of some still older type some of the branches dying out." -- Notebook B, mid 1837-February 1838
  • Divergence can also occur (perhaps more commonly!) if an ancestral population is divided into two or more by changes in geography: because natural selection works by chance survivals, it is unlikely that exactly the same variations of the ancestral population will survival in the two or more separated populations. Over time, if the populations meet again, the accumulation of variations may be significant enough that they are distinct species.

Other important patterns and processes:

• Sexual Selection, a variation of Natural Selection recognized by Darwin, where the variation is "being more sexy" (and thus have better than average chance of breeding, and thus passing on "sexiness", compared to other members of the population [increased reproductive success]). Explains many extravagent display structures and behaviors (such as peacock tails, bird song, lion manes, etc.)
• Correlated Progression: Ancestor and descendants form a lineage (historical line). Sometimes a particular life habit favors the slight increase in multiple different traits (e.g., longer and longer legs, more compact body, more effcient heart and respiration for fast running; longer and longer necks, longer legs, better cropping teeth and/or grasping tongue for browsing in trees; more and more streamlined body profile, more paddle like legs, more dorsal nostris, etc. in swimmers; etc.) Traits that go against the general trend will be selected against; traits that go with the general trend will be selected for. (For many people, this series of trends in adaptations represents the totality of evolution)
• Adaptive Radiation: If a population evolves some significant new adaptation, or colonizes a region without competitors, or is present when competitors die off, many different variations from that common ancestral population might survive (fill new or unoccupied "niches" (ways of life) in environment). Over a geologically short period time, a common ancestor can radiate into many very different descendant lineages.
  • Niche Partitioning: during an adaptive radiation, the early members of the divergence will (naturally) still be relatively similar to each other (and to their common ancestor) in terms of size, shape, behavior, etc. Over time, those variations in each lineage that are least like their relatives will more likely survive, because they will have less competition. Consequently, the different species will "partition" (divide up) the niches and the resources.
• Convergence: Some adaptations are mechanically advantageous and easy to produce developmentally. Different lineages of organisms can independently develop some of the same features, even though ancestors were quite different (i.e., streamlining in sharks, tunas, ichthyosaurs & dolphins).
• Living Fossils: Species do not have a fixed duration, but will persist until the evolve into something else and/or go extinct. In some cases, species (or genera) may persist for extremely long periods of time with no major changes.
• Exaptation: Formerly called "preadaptation", the co-option of a structure that previously had some entirely different function for a new use. Seems to be the more common pattern of evolution than the appearance of entirely novel structures. For example, the wings of birds and bats were initially arms and hands; the mouthparts of various arthropods were legs; etc.
• Heterochrony: Evolution by changes in rate of development from embryo to adulthood. Two major forms of heterochrony:
  • Paedomorphosis: descendant populations will retain some juvenile features into adulthood
  • Peramorphosis: descendant populations will develop structures beyond the adult form of ancestor
• Extinction:
  The termination of a lineage. (If a species "dies out" by evolving into another species, this is more properly called a pseudoextinction). Extinctions occur throughout Earth History. What is more remarkable is Mass Extinction: the geologically-sudden disappearance of many diverse groups of organisms, which are not immediately replaced by ecological equivalents. Some mass extinction events seem to correlate with asteroid impacts; many with major volcanic episodes; others with glaciation.
• Co-evolution: Selection of one species due to activity of an interactor leads to counter-selection in response of the first species
  • For example, plant species develop traits (shapes, colors, tastes of nectar) that favor a select few number of pollinators, thereby promoting greater chance of getting their own pollen rather than some other plant's
  • Or, in the Galápagos: drier islands have fewer small plants, so tortoises preferentially feed on Opuntia cacti. Cacti on these islands have evolved taller woody trunks, and in response the tortoises have evolved a "saddle-backed" shell that allows them to reach higher than dome-backed ancestors.
• Living Fossils: Species do not have a fixed duration, but will persist until the evolve into something else and/or go extinct. In some cases, species (or genera) may persist for extremely long periods of time with no major changes.
• Exaptation: Formerly called "preadaptation", the co-option of a structure that previously had some entirely different function for a new use. Seems to be the more common pattern of evolution than the appearance of entirely novel structures. For example, the wings of birds and bats were initially arms and hands; the mouthparts of various arthropods were legs; etc.

And here is another summary of evolution and how it works (and how it ISN'T like the parody-version of evolution which Creationists claim scientists believe):

And yet another:

And its sequels:

To Lecture Notes.

Last modified: 17 January 2024