Next four lectures will be devoted to evolutionary biology.
The concept of evolution is a central unifying concept in biology -- links molecular biology, physiology, genetics, ecology and other branches of biology.
Dobzhansky: "Nothing in biology makes sense except in the light of evolution."
If one looks at life on earth, one can't help but notice the tremendous diversity of life forms that exist -- scientists estimate that there are currently somewhere between 8 million and 30 million different species living on earth -- millions more lived but are now extinct.
This observation leads to a fundamental question: What process has produced this diversity of life forms? One hypothesis is evolution.
Evolution -- a term that means different things to different people and a concept that is misunderstood by many people -- evolution is often equated with life forms giving rise to other life forms -- e.g. fish becoming amphibians, reptiles becoming birds, lower primates becoming higher primates and humans.
This notion is really the process of speciation -- new species arising from pre-existing species -- this an important part of evolution, but there is much more to evolution than just speciation -- we will discuss speciation in a subsequent lecture.
In the broad sense, the word "evolution" means "change" -- we can talk about the evolution of language, dance, cars, computers -- astronomers talk about "stellar" evolution -- changes in stars.
Let’s look at two definitions for the term "evolution" from a biological perspective.
1. Darwin’s definition: evolution is descent with modification.
Charles Darwin is the man most often associated with the concept of evolution -- "descent" refers to the inheritance of biological traits, while "modification" means "change" -- thus, according to this definition, biological evolution is change in the biological characteristics between generations.
2. Dobzhansky’s definition: genetic changes in populations through time.
Important points:
1. the population is the unit of evolution -- a population is a group of individuals of the same species at a particular place -- individuals do not evolve during their lifetimes.
2. evolution means change -- a dynamic process.
3. can infer that these "genetic changes through time" lead to new life forms arising from pre-existing life forms -- if this is so, then the various life forms on earth are related to one another -- "tree of life."
Are there alternatives to evolution as an explanation for how biological diversity has arisen?
Special Creation: life forms created by a supernatural being -- a belief with a long history and one that still enjoys considerable popularity.
Creation stories are inherent in most cultures and have close ties with religion.
Several important points about creation models:
1. There is not a single creation model, but many -- each culture/religion has it’s own creation model (myth) -- Judeo-Christian, Hindu, Moslem, each Native American tribe, etc.
2. Creation models are outside the realm of science -- cannot be approached using the scientific method --impossible to devise experiments to determine whether creation is even a possibility, let alone an actual event -- thus, because it is not testable, creationism is not a scientific hypothesis and cannot be studied using conventional scientific methods.
Despite being completely accepted by the scientific community, evolution remains controversial among the general public -- surveys indicate that a majority of Americans do not believe in evolution.
Often heard that evolution "is only a theory" -- recall that a theory is a hypothesis that has stood repeated testing without being falsified -- evolution is a hypothesis about the production of biological diversity -- it is supported by an enormous amount of data -- it has yet to be falsified -- many biologists feel that we should replace the phrase "theory of evolution" with "law or fact of evolution."
Let’s briefly consider the history of evolutionary thought -- begin by going back to the 18th century and by considering how people viewed the natural world.
General beliefs of the time were based on religious beliefs -- e.g. Book of Genesis.:
1. young earth -- 6,000 years old according to biblical scriptures -- Bishop Ushher calculated that man appeared on earth on October 24, 4004 B.C.
2. catastrophism -- geologic changes and extinctions due to violent catastrophes such as the Noahic Flood.
3. special creation -- all species ("kinds") produced by a single act.
4. immutability of species -- species "fixed" -- not capable of changing or becoming something different.
Towards the end of the 1700’s, scientists began being confronted with data that were difficult to reconcile with the scriptural notion of creation.
1. geology -- mountain building, cutting of canyons, strata thousands of feet thick, etc. all argued for an old earth. -- theory of uniformitarianism -- earth shaped by observable processes (e.g. erosion by wind and water) working over long periods of time -- questions "young earth" and "catastrophism."
2. biology -- late 1700’s was a time of extensive collecting by biologists -- specimens coming in from all over the world to Europe -- If creation occurred, how did all these organisms get from the Garden of Eden to the New World? Limitations to Noah's Ark -- Fossil record suggested widespread and numerous extinctions -- not evidence for "perfect world".
Jean Baptiste Lamarck (1744-1829) -- French naturalist -- early evolutionist -- late 1700's -- one of first to propose a mechanism for evolutionary change.
inheritance of acquired characteristics -- Lamarck maintained that species could change through the use or disuse of structures during the individual organism's lifetime -- basic idea:
1. the environment presents conditions which produce "needs" in organisms.
2. such needs lead to modifications of organs, part, and behaviors.
3. Used parts increase in size, disused parts decrease in size.
4. Modifications occur during the lifespan of the organism and are passed on to their offspring.
e.g. giraffes with short necks stretch to reach vegetation-- neck lengthened by a fraction of an inch and this change was passed on to next generation -- after many generations, giraffes have long necks.
Remember that this was well before Mendel and scientists of the time had no idea about how genetic traits were inherited-- now know that this doesn't work -- Lamarck's ideas not widely believed by his peers.
This brings us up to the beginning of the 1800’s -- next important person on the scene was, of course, Charles Robert Darwin.
Charles Robert Darwin was born in Shrewsbury, England on February 12, 1809 --- the same day that Abraham Lincoln was born -- His father, Robert Darwin, was a successful physician and his grandfather, Erasmus Darwin, was a physician, poet and noted naturalist who wrote about evolution at the end of the 1700's -- Darwin's family was very well off -- his mother was the daughter of Josiah Wedgewood, famous and wealthy for his innovations in the British pottery industry.
Darwin's academic career -- began as a medical student at the Univ. of Edinburgh -- lasted 2 years before becoming bored with lectures and revolted by operations -- in 1828, Darwin's father sent him to Cambridge to become a clergyman --- graduated in 1831 -- not a very good student, though he did well in natural history.
While at Cambridge, Darwin established a friendship with botany professor John Stevens Henslow -- shortly after graduation, Henslow recommended Darwin for a job as an unpaid naturalist on the H.M.S. Beagle, a surveying ship that was about to embark on a round-the-world voyage.
The Beagle sailed in December of 1831 and did not return to England until October of 1836. Voyage included the Cape Verde and Canary Islands, South America, the Galapagos Islands, Australia and Africa.
Darwin was apparently sea-sick from the moment the Beagle hit the English Channel until the voyage was over. While ashore, however, he was a relentless collector of plants, animals and fossils --- extended collecting trips into Amazonian rainforests, the Andes mountains, grasslands of Argentina, rich fossil beds of Patagonia .
Several things greatly influenced Darwin while on this voyage:
1. Read "Principles of Geology" , published in 1830 by Charles Lyell -- book strongly supported the doctrine of uniformitarianism -- argued for an old earth and against catastrophism and said that the features of the earth are the product of forces now observable operating over long periods of time -- Darwin wondered if the same was true for organisms?
2. Visited the Galapagos Islands off coast of Ecuador -- probably removed Darwin's last doubts about evolution -- small group of volcanic islands -- no land mammals or amphibians, but interesting reptiles and birds -- giant tortoises interesting because, while similar, tortoises on different islands had different shell morphologies -- could tell which island a tortoise came from just by looking at its shell-- also inhabited by 12-13 species of finch -- Darwin observed these birds doing very "unfinch-like things" -- e.g. one behaved like a hummingbird, another like a flycatcher, another like a woodpecker.
Given the fact that the islands are close together and similar in terms of their climate, Darwin wondered why these birds were different from one another, yet all were still finches -- evolution seemed to be the only answer -- Darwin theorized that an ancestral group of finches from the mainland had colonized the islands and then, in the absence of competitors (e.g. true hummingbirds, flycatchers, etc.) these colonizers gradually changed into the specialized finches that he saw -- here then is our first hint of "descent with modification."
Darwin returned to England in 1836 and began writing about his ideas on evolution in a series of notebooks ("Darwin's notebooks") in 1837 -- first wrote that "species are mutable" -- began using phrase "descent with modification."
Darwin needed a mechanism by which this "descent with modification" could be accomplished -- noted that man, plant and animal breeders, through selective breeding (artificial selection) was able to dramatically change domestic species (e.g. pigeons, cattle, vegetables, etc.)
How could this selection work in nature? In 1838, Darwin read an essay by the Reverend Thomas Malthus, written in 1826, entitled "Essay on a Principle of Population" -- Malthus was an economist -- said that human populations may grow geometrically, but the population is limited by resources such as food which only grow arithmetically -- this leads to a "struggle for existence" -- Darwin saw this could provide a mechanism for selection in nature if all populations were engaged in this "struggle for existence."
By the early 1840's, Darwin had written up a detailed treatment of his theory of evolution. However, instead of publishing, Darwin got sidetracked and spent 10 years and 4 volumes on a detailed study of barnacles--
In 1858, Darwin received a manuscript from another naturalist, Alfred Russel Wallace. In the manuscript, Wallace proposed a theory of evolution virtually identical to the one Darwin had been sitting on for nearly 20 years.
Darwin and Wallace delivered a joint paper on their theory in 1858 at the meeting of the Linnean Society. In the next 15 months, Darwin completed a 450 page essay that explained and elaborated on his idea. This essay was published in 1859 with the title "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life." -- we have come to call it simply "Origin of Species."
We can briefly summarize Darwin's theory in 6 major points:
1. Organisms display variation among themselves.
2. Many of these variations are heritable -- that is, they are inherited.
3. In nature, more offspring are produced than survive to reproduce --- this comes from Malthus.
4. Offspring that do survive, do so because they possess variations favored by the environment -- we call these variations "adaptations."
5. Surviving offspring will leave more offspring that will also be favored by the environment -- this is "natural selection."
6. Over time, these favored variations will accumulate -- this is "descent with modification" or evolution.
NOTICE: Evolution is the process of these favored variations accumulating -- here is our descent with modification and genetic changes in populations over time. Natural selection is the mechanism by which the process of evolution occurs.
As we shall see, there are other mechanisms in addition to natural selection by which evolution occurs.
Now, what happened when the Origin of Species was released in 1859?
1. Scientific community -- generally embraced Darwin's theory -- young scientists almost immediately adopted it as new and revolutionary -- older scientists took a little longer to accept -- e.g. Darwin's friend Charles Lyell accepted it after several years -- In the U.S., East Coast scientists quickly accepted Darwin -- unfavorable reaction by French scientific community -- there were those who argued against the theory on what appeared to be pretty good scientific grounds.
2. General Public -- controversial -- rejected by those having something to lose from Darwin's ideas (e.g. the clergy) -- many people who disagreed, as today, never took the time to read the Origin of Species -- others who read the book pretty much agreed with the arguments -- much of the controversy centered on human evolution, a topic hardly discussed in the Origin (later outlined in The Descent of Man, published in 1871) --- public debate most heated in Britain -- Darwin caricaturized unmercifully in the British press.
Heated debates -- Darwin's most famous defender was Thomas Henry Huxley ("Darwin's bulldog") -- debated Samuel Wilberforce, Anglican Bishop in 1860 -- Wilberforce began personal attacks on Darwin and asked Huxley whether he claimed descent from the monkeys on his grandfather's or grandmother's side -- Huxley reportedly replied that he would much prefer to be descended from a monkey than from a bishop of the Church of England.
Darwin's ideas were quickly picked up and largely misused by economists both from the political right and left --
1. Herbert Spencer -- coined term "survival of the fittest" -- espoused "social darwinism": unregulated economic competition leads to social progress -- clearly a way to justify imperialism, exploitation, racism and sexism.
2. Karl Marx -- also used the "struggle for existence" idea to support the idea of social progress occurring through revolutions by the proletariat -- offered to dedicate Das Kapital to Darwin, who politely declined.
Darwin’s theory was faced with a major criticism that Darwin was unable to refute -- how are "favorable variations" (adaptations) passed from one generation to the next?
Recall the widely held belief of the time that heredity involved the blending of "fluids" -- critics argued that these favorable variations would never become widespread because they would be "diluted" every generation because of blending inheritance.
Darwin had nothing to refute this with -- Mendel’s work would have allowed him to answer this criticism, but, Mendel’s work was not widely known -- we know that Mendel was aware of Darwin’s work, but Darwin was unaware of Mendel’s -- Darwin died in 1882 without addressing the criticism of how adaptations are inherited.
Around 1900, Mendel’s work was rediscovered -- its importance to Darwin’s theory was immediately seen and removed the last hurdle for complete acceptance of Darwin’s theory by scientists.
The 1900’s saw the development of the modern synthetic theory of evolution:
1. integration of Darwin’s theory with Mendelian genetics.
2. natural selection still the primary evolutionary mechanism.
3. other mechanisms (mutation, gene flow, and genetic drift) also responsible for evolutionary change.
Next time: The Evolutionary Process