Evidence for Evolution

Reading: Freeman, Chapter 23, 26

The Fact of Evolution– Evolution-the progressive change of organisms as they

descend from ancestral species-is a fact. By now, the evidence for it is overwhelming and ubiquitous.

• It is of such obvious clinical significance in medicine that to deny it is irresponsible.

• That said, any explanation for its existence and mode of action is a scientific theory, which must be testable and, in theory, falsifiable.

• Darwin’s theory of natural selection, combined with other mechanisms of evolution discovered since Darwin, form what is known as the “modern synthesis”, the current scientific paradigm in the biological sciences.

– It provides a central explanation for phenomena in such diverse fields as paleontology and developmental biology, medicine and psychology.

– The existence of evolution has been proposed several times in history. For instance, the ancient Greek scientist, Animaxander, proposed a theory of evolution.

– In terms of modern science, it was first advanced proposed in the late 1700’s and early 1800s by several scientists including Compte de Buffon and Erasmus Darwin.

– The idea of evolution remained controversial for a long time, partially because it ran contrary to contemporary religious ideas and partially because no mechanism for evolution was known.

– Darwin and Wallace’s theory of evolution by natural selection was the first plausible, widely-accepted mechanism for evolutionary change.

– By now it is well-tested, supported by hundreds of independent scientific investigations.

– It is also falsifiable-aspects of Darwin’s theory of evolution have been successfully challenged, others supported. This is the case for the other mechanisms of evolution as well.

Examples of the clinical significance of evolutionary biology to medicine HIV. HIV is a retrovirus of enormous medical concern.

Because of evolutionary studies, we know that two separate lineages of this retrovirus passed into the human population from African Apes in the mid 20th century.

This knowledge has alerted us to the danger of emergent diseases from other animal hosts, a reason for our concern about SARS and bird flu.

In addition, it is an understanding of evolutionary biology that has enabled us to develop a therapy for HIV.

The so-called “triple therapy” HIV treatment is an example of evolutionary medicine. – A single drug will not work against the disease because the virus

evolves so quickly, it attains resistance to every drug we have within a few months.

– By using three drugs simultaneously, we subvert the evolution of the virus…evolving resistance to one drug means loosing resistance to another.

Antibiotic resistance is an evolutionary phenomenon of tremendous clinical significance.

Early in the 20th century, a variety of antibiotics, used to treat bacterial diseases, were developed.– An understanding of evolution is helpful to understand

where these antibiotics come from to begin with…many, such as penicillin, were evolved by fungi, over millions of years, to kill off their bacterial competitors.

– Humans have co-opted them for our own purposes. Since the 20th century, the bacterial pathogens have

evolved resistance to our antibiotics, because extensive use of these drugs has caused very strong natural selection in favor of mutations which favor antibiotic resistance.– For instance, various strains of Neisseria gonorrheae have

evolved resistance to penicillins, tetracyclines, spectinomycin and floroquinolones.

Natural Selection as the Mechanism of Evolution:– Scientific understanding of evolution came out of its infancy in

1859, when theories of evolution by natural selection by Charles Darwin and Alfred Russel Wallace became widely known.

– We now know of other mechanisms of evolution, including genetic drift and mutation, but natural selection is the only mechanism capable of producing adaptation.

– Natural Selection was not immediately accepted-it took until the1930s for Darwin’s ideas to be synthesized with a modern understanding of genetics for widespread acceptance.

Intellectual stepping-stones to developing a theory of evolution Linnaeus and Taxonomy Malthus and the Principle of Population Lyell and Uniformitarianism Lamarck and the fist comprehensive

theory of evolution The Voyage of the Beagle Wallace and Darwin

Linneus and Taxonomy Carolus Linnaeus was a sixteenth-century Swedish

physician and Botanist. He founded the science of taxonomy, the branch of

biology concerned with naming and classifying living things.

He developed the two part system of binomial nomenclature we use today.

His genera were clustered into increasingly broader categories; families, classes, phyla, and kingdoms…although he did not believe in evolution by descent, this pattern does provide a framework for thinking about evolution from a common ancestor.

Malthus Thomas Malthus, an eighteenth

century economist, published “An Essay on the Principle of Population” in 1798.

This document had profound implications. Simply stated: people tend to have more children than can possibly survive, and human populations have historically been kept in check by famine, starvation, and disease.Darwin read this essay and was strongly influenced: he noted that every species has more offspring than can be expected to survive.

How Old is the Earth? From a scientific standpoint, the age of the Earth was

essentially unknown until the 19th century. Early ideas varied greatly, some cultures, such as

classical Hindu society, thought of the Earth as incredibly old.

Christian theology limited the age of the Earth to a few thousand years, because of the biblical account of creation as lasting seven days, and the geneologies included in the book of numbers..

Based upon the old testament, the Archbishop James Usher calculated that God created the Earth in 4004BC. This left little time for incredibly slow, gradual processes like evolution...

Hutton, Lyell and Uniformitarianism The English geologist, James Hutton

proposed that it was possible to explain geological land formations by processes that are currently in operation, such as erosion and sedimentation.

Canyons were cut by the erosion of streams, layers of sediment were deposited at the edge of river deltas, these processes occurred slowly over a very long time-this idea was called gradualism.

The English geologist, Charles Lyell was a contemporary of Darwin’s.– He was a proponent of Hutton, and went a bit farther,

embracing the principal uniformitarianism-the idea that geological processes in operation now operated similarly in the past, at about the same rate.

The Uniformitarian view of nature, requires vast amounts of time to explain the present state of the Earth.

Jean Baptiste Lamarck Jean Baptiste de Lamark developed the first

comprehensive model of evolution. Lamarck was a French Zoologist, curator of the

invertebrate collection at the Paris museum. Lamarck saw many different lines of descent

among the fossil invertebrates he encountered: instead of Aristotle’s single scala natura, there were many.

He proposed that organisms increased in complexity through time because of an innate tendency.

Lamarck proposed that interactions of organisms and environment drove the process of evolution.

He followed the widely accepted notion that characteristics acquired during an individual’s lifetime could be passed to one’s offspring.

He proposed that patterns of use and disuse drove the evolution of adaptations. In stretching their necks to reach leaves high in the treetop, giraffes acquired slightly longer necks, and passed these longer necks to their offspring.

According to Lamarck, every organism was continually striving for greater complexity, a clam strove to be a better clam, etc.

Lamarckian evolution can be disproved by experiment, specifically, we now know that acquired characters cannot be passed to offspring, also, evolution carries no innate tendency toward increasing complexity, but Lamarck’s theory was an important prelude to Darwin’s, it opened the door to thinking that organisms can and do change over the course of time.

The Voyage of the Beagle Much of Charles Darwin’s inspiration for his theory of

evolution by natural selection came from his voyage on the HMS Beagle, in 1831.

He saw an incredible diversity of species, with adaptations to a wide variety of environments; Brazilian rainforests, Chilean deserts, oceanic islands, etc.

The Galapagos islands particularly impressed him; most of the species there live nowhere else in the world, yet their closest living relative is on the mainland a few hundred miles away.

He was to spend the next 27 years developing a theory to explain what he saw.

Darwin and Wallace Alfred Russell Wallace, a nineteenth century naturalist

and explorer, an expert on collecting specimens for resale in Europe, developed essentially the same theory of evolution by natural selection as Darwin.– An active man, he sat down to write it recovering from

a bout of malaria, when he was unable to go out and explore.

The two shared credit for the discovery, a rare example of diplomacy in 19th century science.

• Darwin is better known today, because he amassed a considerable amount of evidence to support his ideas. Wallace’s arguments were more intuitive and contained a less-extensive battery of examples.

Darwin had spent much of his life amassing the evidence he needed to support his model of evolution.

• He was finally goaded into publishing when he came across a manuscript by Wallace which contained many of the same ideas.

– Both theories had very broad implications, forcing European intellectuals to re-examine their place in nature.

• By proposing a mechanism for the evolution of the human species, its mind, and its achievements, that is not supernatural, it removed the need for a divine “prime mover” from science.

– Such a creator, or “prime mover” had been an element of Western science, since Roman times or earlier, and had been removed from physics and astronomy centuries earlier.

1859: The Origin of Species

Darwin’s manuscript contained several new ideas, ideas not found in earlier notions of evolution;– All species evolved from earlier species.– The mechanism is natural selection; members of

a species possessing more desirable traits will have more offspring and survive to reproductive maturity.

– Evolution occurs over a very long span of time.

The Origin of Species makes this argument, structured logically…– All organisms produce more offspring than can possibly

survive– All organisms vary for a wide variety of different attributes

and features-they also vary in reproductive success: some have more offspring than others.

– Some variation is heritable.– Some of this variation must influence reproductive success– Given that the above are true…desirable characteristics will

thus be preferentially passed to offspring• This is a logical conclusion of the first four points

Darwin concluded, based upon intuitive grounds, that, over vast spans of time, present day species have descended from a common ancestor. The book contained no mechanism for speciation, however.

Evidence for Evolution The gradual evolution of life on the planet, and

their descent from a common ancestor, is a fact. – Darwin’s theory of evolution is a comprehensive body

of evolution that attempts to explain how this occurred. One of the hallmarks of a truly revolutionary

scientific theory is that it brings together many previously unexplained patterns under a single body of theory.– Like Newton’s theory of universal gravitation, Darwin’s

theory of evolution created a new scientific paradigm.

Some of the original evidence for evolution:

Embryology Vestigial and Homologous structures Biogeography The Fossil Record

Embryology Closely related species go through similar stages of

development, although the adults may not resemble each other very closely.

For instance, all vertebrate embryos develop gill pouches at some stage, even though in many species, they are lost later. This is suggestive of a common origin for vertebrates.

Embryological development is often suggestive of evolution: birds have many developmental features in common with reptilian ancestors, land vertebrate embryos have many features suggestive of an aquatic existence (gill pouches, a notochord, blocks of segmented muscle).

Snake Chicken Possum Cat Bat Human

Vestigial StructuresMany species retain structures that only make sense in light of their ancestry.These structures are typically reduced and nonfunctional, but they are inherited from ancestors, in whom they were important to survival or reproduction

If members of a taxonomic unit share a common ancestry, it is reflected in their development:

Two of the many examples:– limb bud development in whales– extraembryonic membranes of the amniote egg

Comparative Development and Embryology

Homologous Structures Closely related species frequently have

homologous structures: structures that are similar in their fundamental layout and construction, although they may serve very different purposes.– For example, the forelimbs of mammals are

constructed from the same skeletal elements: The wings of a bat, a whale, a human, a dog, etc. all contain the same bones, despite their different uses.

This suggests that common ancestry, rather than design, plays a role in the construction of species.

The Fossil Record The succession of forms in the fossil record

clearly suggests that organisms change through time, and have descended from a common ancestor.

Different groups appear in the fossil record at different times, with a general trend toward the simplest organisms appearing the earliest..this is at odds with the view that they were all created at the same time.

Many forms have gone extinct, another observation that is at odds with the view that each species was specially created for a purpose.

In some cases, a direct line of descent, and change through time, can be observed in fossils. Foraminifera, small oceanic protozoans, leave a continuous fossil record in oceanic sediments. It is possible to trace their gradual evolution over millions of years.

Since Darwin’s day, our knowledge fossil record has improved tremendously, we can trace the evolution of many different groups through fossils: horses, for instance, have a superb fossil record, showing many instances of speciation and many intervals of evolutionary change.

Example-Whales havean excellent fossil record-showingtransitional forms

Biogeography The distribution of living plants and animals suggests

that organisms adapted to one environment can invade a new environment, and develop specific adaptations to the new conditions. On the HMS Beagle, Darwin noted that in South America, temperate species tended to resemble their South American tropical relatives, rather than temperate species in Europe. On the Galapagos, most species had a recognizable ancestor from the coast of Ecuador, but species there had numerous adaptations specific to the climate of the Islands.

Wallace observed the same pattern in many different parts of the world.

Modern Evidence

Since Darwin’s time, there have been hundreds of studies of evolution.

Natural selection has been measured in many organisms in the field, and in laboratory populations.

An understanding of evolution has also become important to combating disease.

Example-DDT resistance in mosquitoes The misuse of DDT, and the re-

emergence of malaria as an important human pathogen, is perhaps one of the greatest public health failures of the century– it could possibly have been prevented if the

evolution of mosquitoes had been taken into account

In nonresistant insects, DDT is a very effective insecticide-causing massive mortality and very strong selective pressure in favor of any mutation that might lead to resistance

Indiscriminate spraying (when there was no particular need to control the organism) led to the rapid evolution of pesticide resistance. Five Anopheles species were resistant by 1956 and 38 by 1968.

Resistance takes many forms-some of this genetic variation was probably present in the mosquito population before the use of DDT, but in the absence of DDT, these variants are selected against.

1) Chemical adaptation: enzymes evolve that break down the pesticide.

2) Behavioral adaptation: They evolved to move from inner, sprayed walls to outer, unsprayed walls. They evolved sensitivity and avoid the pesticide.

These data are from Bangkok-the R allele is resistant, the + allele is not.Note that the + allele becomes more common in the absence of DDT spraying

Adaptation Natural Selection as the mechanism for

adaptation was Darwin’s most important contribution.

There are other forces of evolution (most of which were discovered after Darwin), but natural selection is the only evolutionary mechanism that can produce adaptation.

Some examples of adaptation are very impressive.

Find the mantis in this picture

The Variation Problem For Natural Selection to be effective, there must

be genetic variation upon which selection acts. Darwin discussed the origin of variations

extensively in “On the Origin of Species..”, but he did not know how variation persists.

Although he was a contemporary of Mendel’s, Darwin did not know Mendelian genetics (his work was not well understood at the time).

The current theory of genetics, blending inheritance, suggested that useful adaptations would blend into the population and become diluted.

The synthesis of Darwin’s theory with Mendelian genetics led to our modern understanding of Evolution.

Several early twentieth century evolutionary biologists are widely credited with developing our modern understanding:

R.A. Fisher J.B.S. Haldane Sewall Wright Theodosius Dobzhanski Thomas Hunt Morgan