Biology Chemistry Computer Engineering Electronics Mathematics Physics Science Home

UNIT 4:  EVOLUTION – Origins of Evolutionary Science

 

More than one million species have already been identified, while millions more wait to be discovered – most of which are insects and smaller organisms.  Despite the wide diversity in physiology, morphology, behaviour, and survival strategies on this planet, all life forms are fundamentally similar at the cellular and molecular level.  Throughout the history of this planet these fundamentally similar molecules and cells have grouped together in a variety of different ways to make up tissues of various types and function.  Evolution is the theory that attempts to explain the transformation of these cells and tries to establish similarities between different organisms in this process.   It also strives to explain why these transformations have take place over time.  Evolutionary biology not only provides answers to questions about the past, but it also has direct applications in the health sciences, agriculture, industry, and conservation.

 

A. Evolutionary Theory

 

·         the theory of evolution is not just an idea or an assumption

·         it is the process in which significant changes in the inheritable traits of a species occur over time

·         it exists today because of the following extensive work that has been done by scientists for many centuries:

o        careful observations

o        formulation of hypotheses

o        analyzing of data from diverse sources

o        drawing inferences from the data

·         despite the existing strong evidence, this theory is not universally accepted

·         cultural and religious beliefs quite often conflict with this theory

·         the common belief from the 1600s to the mid 1800s, was that the Earth was created by God on Sunday, October 23, 4004 B.C.

·         most people believed that the natural environment was static, and that life forms were immutable

·         today, most people understand that the Earth is a changing – a dynamic environment where change is both natural and unavoidable

·         the most compelling evidence of changing life forms over time is the fossil record

·         by 1859, scientists were able to accumulate enough information from the fossil record to formulate a viable explanation for the mechanism of evolution

·         the general consensus today is that the history of life on Earth has been one of continual change over billions of years

 

B. Evidence from the Past

 

·         Tuscany mountain seashell remains were discovered and analyzed as early as the late 1500s by da Vinci

·         since these were found hundreds of kilometers away from any sea he concluded that the Earth’s surface must have changed dramatically over time

·         mineralized remains of living organisms were studied in detail by Nicholas Steno in 1669

·         fossils are defined as any preserved remains or traces of an organism or its activity

·         most of the time these consist of bones (since they preserve the best) or impressions on solidified rock

·         fossils are commonly formed when the bodies of organisms become trapped in sediments, which become compressed into strata, or layers, and eventually harden into sedimentary rock

·         slow decomposition of an organism causes living cells to be replaced by mineral precipitates, resulting in a permineralized fossil

·         if conditions are favorable, organisms may preserve intact – these are usually found in tar pits, volcanic ash, peat bogs, permanently frozen ground, and amber (hardened tree sap)

·         ideal fossilization conditions are rare – soft tissue decomposes quickly

·         organisms with hard shells or bones and that live in or near aquatic environments are more likely to become fossilized than soft-bodied and land-dwelling species

·         large land animal fossils are rare – hard-bodied marine organisms such as clams and snails are more common

·         the most abundant are microfossils – microscopic remains of tiny organisms or structures that have hard and resistant exoskeletons

 

 

 

·         fossils demonstrate:

 

o        great diversification of species

o        extinction of many life forms

o        patterns of evolution

 

·         the scientific study of fossil remains is called paleontology

·         an 18th century anatomists by the name of Baron Georges Cuvier extensively studied fossils of species that he deemed extinct

·         this view contradicted the common belief that fossils were of living species that had not yet been discovered

·         the fossil record today consists of more than 250 000 identified species – only 1% of these consist of organisms still exist today

·         Cuvier was the first to observe that each deposition layer of fossil finds contained distinct species that were not found in layers above or below it

·         Cuvier’s findings contradicted his own belief in creation – he believed that species did not change over time and

·         to explain his observations, he suggested the idea of  catastrophism – multiple global catastrophes occurring at different times, resulting in widespread extinctions and the replacement of extinct species with newly created species

·         Cuvier introduced the concept relative age – a chronology of rock strata corresponding to a sequence of fossils found in the layers (basically, the older fossils are found in deeper deposits, and more recently formed deposits contained younger fossils)

·         until the 19th century scientists had no way of calculating the absolute age of fossils

·         in 1866, through rigorous mathematical and scientific calculations, Thomson Kelvin theorized that the once-molten Earth was cooling down, and that it had an absolute age of 400 million years – he later revised it to 20 million

·         with the discovery of radioactive decay – the release of subatomic particles from the nucleus of an atom, resulting in a new daughter isotope and free subatomic particles – and radiometric techniques, scientist have been able to determine absolute ages of rock

·         the oldest rock on Earth actually comes from the Canadian Shield north of Yellowknife – 3.9 billion years

·         all meteorites that have hit the Earth are estimated at about 4.6 billion years old

·         moon rocks collected on Apollo missions have been dated at about 4.53 billion years old

·         all evidence about other bodies in the solar system and universe suggest that the Earth is about 4.6 billion years old

 

RADIOMETRIC DATING

 

 

Example:         A sample of igneous rock contains small amounts of radioactive potassium and argon.  Using the ratio of 40Ar to 40K, it is determined that only 25% of the original parent potassium isotope remains.  How old is the rock sample?

 

                        Figure 5, p. 514, shows that it requires two half-lives for the ratio of parent-to-daughter isotopes to change from 100%:0% to 25%:75%.  Given the half-life for 40K of 1.3 billion years, found in Table 1, p. 514, this igneous rock formed 2.6 billion years ago.

 

Homework:       1-2, p. 515, and 1-9, p. 516

 

C. Early Ideas About Evolution

 

·         by the late 1700s, James Hutton proposed the theory of actualism – the same geological processes occurring in the present also occurred in the past (Figure 1, p. 517)

·         in 1830, Charles Lyell published his principles of uniformitarianism – the Earth’s surface has always changed and continues to change through similar, uniform, and very gradual processes

·         using fossil formations, erosion and sedimentation principles and processes, Lyell was able to conclude that changes were slow and gradual, not sudden and catastrophic, and that natural laws and processes are constant and eternal

·         Hutton’s and Lyell’s work formed the foundation on which other scientists could build theories about the history of life forms on Earth

·         During the second half of the 18th century, Georges Buffon proposed that species could change over time and that these changes lead to new organisms

·         by the late 1800s Linnaeus and the grandfather of Charles Darwin, Erasmus Darwin, were among the first to introduce theories of extinction and evolution – the problem was that no mechanisms were in place to explain how the process of evolution might occur

 

LAMARCK’S VIEW OF ADAPTATION AND INHERITANCE

 

·         one of the first to recognize that the environment played a key role in evolution was Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamarck

·         his reasoning was that in order for species to survive, they must be able to adapt to changing environmental conditions

·         Lamarck did not have the same beliefs as Buffon

·         he reasoned that single species could not give rise to additional species; instead, each species gradually became more complex, and that new simple species were continually being created by spontaneous generation from non-living matter

·         this principle helped him develop the theory of acquired traits – changes in individuals resulting from interactions with the environment that could be passed on to future generations

·         the classic example to help explain “Lamarckism” views is that of the giraffe

o        giraffes had to strive to obtain food higher in trees, so that over their lifetime, continual stretching might have led to a slight elongation of the neck

o        this longer neck, acquired by the parent giraffes, would be passed on to the offspring

·         although it has been established that acquired traits cannot be inherited Lamarck does deserve credit for recognizing that the environment is the driving force of evolutionary change

 

·         Table 1, p. 518 summarizes the most significant early ideas about evolution

 

Homework:       1-5, p. 518

 

D. Darwin’s Voyage of Discovery

 

·         even though Darwin was only 22 years old, and he had very little field work experience, he was recommended to partake in a surveying voyage by his professor because of his amazing observation skills and ability to note details in natural history

·         Darwin’s voyage was initially scheduled to take two years, but lasted almost five!!

·         Figure 3, p. 520 illustrates the route of the HMS Beagle that set sail in 1831

·         Darwin collected thousands of specimens and many fossils

·         the fossils collected in South America (seen in Figure 4b, p. 520 and Figure 5a, p. 521) showed similarities in both structure and patterns of distribution as the modern animal versions (Figure 4a and 5b)

·         along with detailed observations about animal distribution, physiology, morphology, etc., Darwin also collected geological data that supported Lyell’s assertions about land formations

·         when he reached the Galapagos Islands, Darwin noted that, although the plants and animals closely resembled species in South America, those of the islands had unique traits

·         he also found that animals collected from each island possessed traits that were unique to that particular island

·         after his journey, he went from believing in the immutable nature of species to proposing one of the most controversial theories in the history of science

 

 

 

E. Darwin’s Input

 

·         Darwin’s observations collected from his historic voyage around the world were irrefutable and too convincing and powerful to ignore

·         Darwin’s extensive collection of anatomical oddities, as well as the molecular biological data that researchers have today, strongly suggest that all organisms possess an evolutionary past

 

o        animals living in the grasslands of South America or Australia showed more similarities to animals living in a nearby tropical rainforest than to grassland species on other continents

o        fossils found represented ancestral forms of the living organisms

o        13 distinct species of finches were collected and identified from the Galapagos Islands – it was concluded that a single ancestral species transported from a nearby land might have given rise to a number of similar but distinct new species, especially when isolated on separate islands

o        remote islands were inhabited by birds and other flying organisms – mammals, reptiles, and amphibians that were not able to survive the travel to distant islands were entirely absent

o        body parts of organisms with entirely different functions were similar in structure (Figure 2, p. 523)

o        organisms with homologous features likely shared a recent common ancestor, while those with analogous features did not

o        some animals possessed rudimentary and nonfunctioning structures that were homologous to fully functioning structures in closely related species – he called these structures vestigial features

o        all species possess inherited variations that can be selected to change the species in desirable ways -- if people could alter the characteristics of a species through artificial selection practices, then the environment could have a similar selective effect on wild species – artificial selection offered a model for how evolution might operate in nature

o        there must be intense competition among individuals of the same species to survive – the favorable forms survived to reproduce, and the less favorable died off

 

HOMOLOGOUS VERSUS ANALAGOUS FEATURES

 

ü       particular structures that share a common origin but may serve different functions in modern species are called homologous features (i.e. bat wings and human arms)

ü       structures that are similar in function but not in origin or anatomical structure are called analogous structures (i.e. bird wings and butterfly wings)

ü       homology can also be noticed during embryonic development (Figure 3, p. 524) – some features that are common at the embryonic stage among various animals, serve not function as the organisms grow

§         for example, some frog species are born with physical features that make them fully adapted to life on land, however, their embryonic development shows the transformations through a “tadpole” like form, with a tail, gills, and other features clearly adapted for life in the water

 

VESTIGIAL FEATURES AND ANATOMICAL ODDITIES

 

ü       some examples include:

o        beetle wings underneath fused covers, human ear muscles, vestigial hips on some snakes and whales (Figure 5, p. 525), vestigial toes on dogs, pigs and horses (Figure 4, p. 524), human “Goosebumps”

ü       in addition to morphological data, modern researchers have found very large numbers of vestigial genes in the DNA of living organisms – although these genes do not function, they bear a very close resemblance to functioning genes

 

ARTIFICIAL SELECTION

 

ü       humans have been artificially selecting desirable traits in various species (plant or animal) for centuries

ü       humans have been improving domesticated plant and animal species for thousands of years by selecting offspring with desirable traits as breeding stock for succeeding generations, creating extraordinary diversity of breeds of farm animals, cats, and dogs

ü       Figure 6, p. 526 shows the results of artificial selection in corn seeds to increase the percentage of oil content per seed

ü       given the appropriate length of time, small changes in species could accumulate over many thousands of generations

 

THE STRUGGLE TO SURVIVE

 

ü       principle put forth by Malthus in 1798 stating that nature produces far more offspring than are able to survive

 

Homework:       1-10, p. 528

F.  The Theory of Evolution by Natural Selection

 

·         it took 7 years after his voyage to formulate his theory of natural selection

·         Darwin was so worried about his work not being published that he gave a copy of his manuscript to his wife, just in case he died

·         Darwin was reluctant to publish his work, even though he had over 20 years of collected evidence and data to support his theory, while another scientist, Alfred Russell Wallace, who independently arrived at the same conclusions as Darwin, was extremely anxious to publish his work that took him only two days to write

·         finally, a year and a half later, on July 1, 1858, Darwin published his work and titled it “On the Origin of Species by Means of Natural Selection or The Preservation of Favoured Races in the Struggle for Life”

 

NATURAL SELECTION

 

·         Darwin’s theory of natural selection is based on the following observations:

 

  1. Individuals within a species vary in many ways
  2. Some of this variability can be inherited
  3. Every generation produces far more offspring than can survive and pass on their variations
  4. Populations of species tend to remain stable in size
  5. Members of the same species compete with each other for survival – intraspecific competition
  6. Individuals with more favourable variations are more likely to survive and pass them on – survival is not random
  7. As these individuals contribute proportionately more offspring to succeeding generations, the favourable variations will become more common – this is natural selection

 

OBSTACLES TO OVERCOME

 

·         some scientists did not share the view that the Earth was many millions of years old

·         fossil evidence was scarce and there were too many gaps in the theory of evolution through natural selection – i.e. descent with modification over time

·         there were no fossils that linked the ancestral forms or morphologies to the modern day living descendents

·         Darwin’s theory required the Earth to be billions of years old, not millions – radiometric dating technology to support the actual age of the Earth was not available then

·         most variations among individuals of the same species were considered by most scientists as subtle – not significant enough to lead to the production of an entirely new structure, such as an eye or a wing, where none had existed before

·         the common belief was that inherited traits from each parent were blended in the offspring, therefore unusual or rare variations would become diluted over time rather than becoming prominent – this was a belief that even Darwin shared with his opponents and he called it the weak link in his theory

 

THE SOLUTION TO DARWIN’S OBSTACLE

 

·         Gregor Mendel’s pea plant experiments provided the answer to Darwin’s problem

·         Mendel’s work provided a foundation for an understanding of inheritance

·         this foundation was the basis for modern molecular revelations about how mutations and genetic recombination are the source of inheritable variation

·         mutation and genetic recombination also offer evidence for biological evolution of life on Earth

·         Figure 4, p. 532 presents a model of a hypothetical evolutionary scenario for the evolution of larger claws in crabs over time – this is the basis of Darwinism

 

Homework:       1-9, p. 533

 

Back to Top