for Evidence 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. The existence of evolution was proposed in the late 1700's and early 1800s by several scientists including Compte de Buffon and Erasmus Darwin This idea remained controversial for a long time, partially because it ran contrary to contemporary religious ideas and partially because no mechanism for evolution was known.
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
Linneus 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 embraced the principal uniformitarianismthe idea that geological processes in operation now operated similarly in the past. 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
Charles Darwin was a nineteenth-century English naturalist who developed the theory of evolution by natural selection. Alfred Russell Wallace was a nineteenth century naturalist and explorer who developed essentially the same theory of evolution by natural selection. Darwin is usually credited with the discovery, which is probably the most important intellectual development of his century, which was an age of important intellectual developments..
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. Darwin was the first to publish, and his ideas were more comprehensive, but either model of evolution was far superior to contemporary models, and to force European intellectuals to re-examine their place in nature.
1859: The Origin of Species
Darwin's manuscript contained several new ideas 1. All species evolved from earlier species. 2. The mechanism is natural selection members of a species possessing more desirable traits will have more offspring and survive to reproductive maturity. 3. Evolution is a GRADUAL process-it occurs over a very long span of time.
1) All organisms produce more offspring than can possibly survive 2) All organisms for vary a wide variety of different attributes and features-they also vary in reproductive success: some have more offspring than others. 3) Some variation is heritable. 4) Some of this variation must influence reproductive success 5) Desirable characteristics will thus be preferentially passed to offspring 6) Over vast spans of time, present day species have descended from a common ancestor.
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 Structures
Many 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
Comparative Development and Embryology
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
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 have an excellent fossil record-showing transitional 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 reemergence 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