Evolution—The Theory and Its Supporting Evidence Chapter 7

Some of the evidence for evolution –comes from fossils –such as this 2.3-meter-long Ceresiosaurus –which belonged to a groupof Triassic marine reptiles known as nothosaurs. On display at Glacier Garden, Lucerne, Switzerland Evidence for Evolution

During Charles Darwin’s five-year voyage –( ) on the HMS Beagle, –he visited the Galápagos Islands –where he made important observations –that changed his ideas about –the then popular concept called the fixity of species an idea holding that all present-day species had been created in their present form and had changed little or not at all Darwin fully accepted –the Biblical account of creation before the voyage Darwin and the Galápagos

Map showing the route (red line) followed –by Charles Darwin when he was aboard –HMS Beagle from 1831 to 1836 The Galápagos Islands –are in the Pacific Ocean west of Ecuador Route of HMS Beagle

The Galápagos Islands –are specks of land –composed of basalt –in the eastern Pacific The Galápagos Islands

During the voyage Darwin observed –that fossil mammals in South America –are similar yet different from present-day –llamas, sloths, and armadillos –that the finches and giant tortoises living –on the Galápagos Islands vary from island to island –and still resemble ones from South America, –even though they differ in subtle ways These observations convinced Darwin –that organisms descended with modification –from ancestors that lived during the past –the central claim of the theory of evolution Darwin Developed the Theory

Darwin’s finches from the Galápagos Islands –arranged to show evolutionary relationships Galápagos Finches –Notice that beak shape –varies depending on diet Berry eater Insect eaters Cactus eaters Seed eaters

Evolution –involving inheritable changes in organisms through time is fundamental to biology and paleontology –Paleontology is the study of life history as revealed by fossils Evolution is a unifying theory like plate tectonic theory –that explains an otherwise –encyclopedic collection of facts Evolution provides a framework –for discussion of life history Why Study Evolution?

Many people have a poor understanding –of the theory of evolution –and hold a number of misconceptions, –which include: evolution proceeds strictly by chance nothing less than fully developed structures –such as eyes are of any use there are no transitional fossils –so-called missing links –connecting ancestors and descendants humans evolved from monkeys –so monkeys should no longer exist Misconceptions about Evolution

In 1859, Charles Robert Darwin ( ) –published On the Origin of Species in which he detailed –his ideas on evolution –formulated 20 years earlier –and proposed a mechanism for evolution Darwin

Plant and animal breeders –practice artificial selection –by selecting those traits they deem desirable –and then breed plants and animals with those traits –thereby bringing about a great amount of change Observing artificial selection –gave Darwin the idea that –a process of selection among variant types –in nature could also bring about change Thomas Malthus’ essay on population –suggested that competition for resources –and high infant mortality limited population size Natural Selection

Darwin and Alfred Russel Wallace ( ) –read Malthus’ book –and came to the same conclusion, that a natural process –was selecting only a few individuals for survival Darwin’s and Wallace’s idea –called natural selection –was presented simultaneously in 1859 Darwin and Wallace

Organisms in all populations over produce Inheritable variations are passed down such as –size, speed, agility, visual acuity, digestive enzymes, color, and so forth. Some variations are more favorable than others some have a competitive edge in acquiring resources and/or avoiding predators Not all young survive to reproductive maturity –Those with favorable variations –are more likely to survive Successful Reproduction of their favorable variations Natural Selection—4 Steps

According to the Darwin-Wallace theory –of natural selection, giraffe’s long neck evolved Naturally Selected Giraffes –because ancestors with longer necks –had an advantage –and reproduced more often

One misconception about natural selection –is that among animals –only the biggest, strongest, and fastest –are likely to survive –These characteristics might provide an advantage but natural selection may favor –the smallest if resources are limited –the most easily concealed –those that adapt most readily to a new food source –those having the ability to detoxify some substance –and so on... Not only Biggest, Strongest, Fastest

Species is a biological term for a population –of similar individuals that in nature interbreed –and produce fertile offspring Species are reproductively isolated –from one another Goats and sheep do not interbreed in nature, –so they are separate species Yet in captivity –they can produce fertile offspring Species

Speciation is the phenomenon of a new species –arising from an ancestral species It involves change in the genetic makeup –of a population, –which also may bring about changes –in form and structure During allopatric speciation, –species arise when a small part of a population –becomes isolated from its parent population Speciation

A few individuals of a species on the mainland –reach isolated island 1 –Speciation follows genetic divergence in a new habitat. Allopatric Speciation

Later in time, a few individuals of the new species colonize island 2 –In this new habitat, speciation follows genetic divergence. Allopatric Speciation

Speciation may also follow colonization of islands 3 and 4 Invasion of island 1 by genetically different descendants of the ancestral species! Allopatric Speciation

More than 20 species of Hawaiian honeycreepers have evolved –from a common ancestor as they adapted to diverse food sources on the islands Honeycreeper Speciation

Divergent evolution occurs –when an ancestral species –gives rise to diverse descendants –adapted to various aspects of the environment Divergent evolution leads to descendants –that differ markedly from their ancestors Convergent evolution involves the development –of similar characteristics –in distantly related organisms Parallel evolution involves the development –of similar characteristics –in closely related organisms Styles of Evolution

In both convergent and parallel evolution, –similar characteristics developed independently –in comparable environments Styles of Evolution

Divergent evolution of a variety –of placental mammals from a common ancestor Divergence accounts for descendants –that differ from their ancestors and from one another Divergent Evolution

Convergent evolution takes place –when distantly related organisms give rise to species Convergent Evolution –that resemble one another –because they adapt –in comparable ways

Parallel evolution –involves the independent origin –of similar features in related organisms Parallel Evolution

Microevolution is any change in the –the genetic make-up of a species, and –involves changes within a species Macroevolution involves changes –such as the origin of a new species –or changes at even higher levels –For example, the origin of birds from reptiles The cumulative effects of microevolution –are responsible for macroevolution Microevolution and Macroevolution

Evolutionary trends are a series of adaptations –to changing environment –or in response to exploitation of new habitats Some organisms –show little evolutionary change –for long periods Lingula is a brachiopod –with a shell, at least, –that has not changed –significantly since the Ordovician Adaptations

Several organisms have shown –little or no change for long periods If these still exist as living organisms today –they are sometimes called living fossils For example: –horseshoe crabs –Latrimaria (fish) –Gingko trees Some of these are generalized and can live under a wide variety of enviroinments “Living Fossils”

Latimeria –belongs to a group of fish –once thought to have gone extinct –at the end of the Mesozoic Era A specimen was caught off the coast of East Africa in 1938 A Living Fossil

A Second Living Fossil Ginkgos –have changed very little –for millions of years

Perhaps as many as 99% of all species –that ever existed are now extinct Organisms do not always evolve –toward some kind of higher order of perfection –or greater complexity Vertebrates are more complex –but not necessarily superior –In some survival sense, bacteria –have persisted for at least 3.5 billion years! Natural selection yields organisms adapted –to a specific set of circumstances –at a particular time Extinctions

The continual extinction of species –is referred to as background extinction It is clearly different from mass extinction –during which accelerated extinction rates –sharply reduce Earth’s biotic diversity Extinction is a continual occurrence –but so is the evolution of new species –that usually quickly exploit the opportunities –another species’ extinction creates Mammals began a remarkable diversification –when they began occupying niches –the extinction of dinosaurs and their relatives left vacant Background and Mass Extinction

The mass extinction of dinosaurs –and other animals at the end of Mesozoic Era –is well known, but the greatest mass extinction –occurred at the end of the Paleozoic Era –when more than 90% of all species died out Mass Extinction

Darwin cited supporting evidence –for evolutionary theory such as classification embryology comparative anatomy geographic distribution fossil record, to a limited extent He had little knowledge –of the mechanism of inheritance, –and biochemistry and molecular biology –were unknown at his time Evidence in Support of Evolution

Since Darwin’s time, studies from additional fields –in biochemistry –molecular biology –more complete and better understood fossil record have convinced scientists that the theory –is as well supported by evidence –as any other major theory Scientists still disagree on many details, –but the central claim of the theory –is well established and widely accepted Evidence in Support of Evolution

An idea can only be a truly scientific theory –if testable predictive statements –can be made from it No theory in science is ever proven in the final sense, –although substantial evidence may support it All theories are always open –to question, revision and occasionally –to replacement by a more comprehensive theory Is the Theory of Evolution Scientific?

By predictive, we do not mean that –it can predict the future No one knows which existing species –will become extinct, or what descendants –of any particular organism, if any, –will look like in millions of years from now Nevertheless, we can make a number of predictions –about the present-day natural world –and about the fossil record –that should be consistent with evolutionary theory –if it is correct Theories Must Be Predictive

If evolution has taken place, –closely related species such as wolves and coyotes –should be similar in anatomy and biochemistry, –genetics, and embryonic development The oldest fossil-bearing rocks should have –very different fossils than organisms of today Some Predictions from Evolution

Suppose that contrary to evolutionary prediction –wolves and coyotes were not similar –in terms of their biochemistry, genetics –and embryonic development, then –our prediction would fail –and we would at least have to modify the theory If other predictions also failed, –for example, if mammals appeared in the fossil record before fishes –then we would have to abandon the theory –and find a better explanation for our observations Since the theory of evolution can “be falsified,” –it is truly scientific Testable

Classification uses a nested pattern of similarities Carolus Linneaus ( ) proposed –a classification scheme –in which organisms receive a two-part name –consisting of genus and species –for example, the coyote is Canis latrans Linnaeus’s classification is an ordered list –of categories that becomes more inclusive –as one proceeds up the hierarchy Classification

Kingdom –Phylum Subphylum –Class »Order Linnaean Classification the coyote, Canis latrans Animalia –Chordata Vertebrata –Mammalia »Carnivora Canidae –Canis latrans Family –Genus Species Most inclusive Least inclusive

Subphylum vertebrata –including fishes, amphibians, reptiles, birds and mammals, –have a segmented vertebral column Only warm- blooded animals with hair/fur and mammary glands are mammals Classification — shared Characteristics

18 orders of mammals exist including order Carnivora The Family Canidae are doglike carnivores and the genus Canis includes only closely related species Coyote, Canis latrans, stands alone as a species Coyote, Canis latrans

Coyote (Canis latrans) and wolf (Canis lupus) –share numerous characteristics –as members of the same genus They share some but fewer characteristics –with the red fox (Volpes fulva) –in the family Canidae All canids share some characteristics with cats, –Bears, and weasels in the order Carnivora –which is one of 18 living orders –of the class Mammalia Shared characteristics –are evidence for evolutionary relationships Coyote and Wolf

If all existing organisms descended with modification –from ancestors that lived during the past, all life forms should have fundamental similarities: –all living things consist mainly of carbon, nitrogen hydrogen and oxygen –their chromosomes consist of DNA –all cells synthesize proteins in essentially the same way Biological Evidence Supporting Evolution

Biochemistry provides evidence –for evolutionary relationships Blood proteins are similar among all mammals –Humans’ blood chemistry is related most closely to the great apes then to Old World monkeys then New World monkeys then lower primates such as lemurs Biochemical test support the idea –that birds descended from reptiles a conclusion supported by evidence in the fossil record Evolutionary Relationships

Homologous structures –are basically similar structures –that have been modified for different functions –They indicate derivation from a common ancestor. Analogous structures are structures –with similarities unrelated –to evolutionary relationships –that serve the same function –but are quite dissimilar –in both structure and development Structures with Similarities

Forelimbs of humans, whales, dogs, and birds –are superficially dissimilar, –yet all are made up of the same bones, Homologous Structures –have similar arrangement –of muscles, nerves and blood vessels, –are similarly arranged with respect to other structures, –have similar pattern of embryonic development

Wings of insects, birds and bats –serve the same function but differ considerably –in structure and embryological development Are any of these wings –both analogous and homologous? Analogous Structures Yes, bird and bat wings

Vestigial structures are nonfunctional remnants –of structures in organisms that were functional –in their ancestors Vestigial Structures Why do dogs have tiny, –functionless toes on their feet (dewclaws)? Ancestral dogs had five toes –on each foot, –all of which contacted the ground As they evolved –they became toe-walkers with only four toes on the ground –and the big toes and thumbs were lost or reduced –to their present state

The Eocene-aged whale, Basilosaurus, –had tiny vestigial back limbs –but it did not use limbs to support its body weight. Remnants of Rear Limbs in Whales

Small-scale evolution can be observed today. For example –adaptations of some plants to contaminated soils –insects and rodents developing resistance to new insecticides and pesticides –development of antibiotic-resistant strains of bacteria Variations in these populations –allowed some variant types –to live and reproduce, –bringing about a genetic change Evolution in Living Organisms

The fossil record consists –of first appearances of various organisms –through time One-celled organisms appeared –before multicelled ones –plants appeared before animals –invertebrates before vertebrates Fish appeared first followed –in succession by amphibians, –reptiles, mammals, and birds What do We Learn from Fossils?

Times when major groups of vertebrates appeared in the fossil record Thickness of spindles shows relative abundance Advent of Various Vertebrates

Fossils are much more common –than many people realize However the origin and initial diversification –of a group is generally the most poorly represented But fossils showing the diversification –of horses, rhinoceroses, and tapirs –from a common ancestor are known as are ones showing the origin –of birds from reptiles and the evolution –of whales from a land-dwelling ancestor Fossils Are Common

This cladogram shows the relationship among –tapirs, rhinoceroses, and horses Horses and Their Relatives

These might seem an odd assortment of animals –but fossils and studies of living animals –indicate that they shared a common ancestor As we trace these animals back –in the fossil record, –differentiating one from the other –becomes increasingly difficult The earliest members of each group –are remarkably similar, –differing mostly in size and details of their teeth As their diversification proceeded –the differences became more apparent Horses and Their Relatives