Descent with Modification: A Darwinian View of Life

Historical Context for Evolutionary Theory1.Western culture resisted

evolutionary views of life

2.Theories of geologic gradualism

helped clear the path for

evolutionary biologists

3.Lamarck placed fossils in an

evolutionary context

CH 22

• On November 24, 1859, Charles Darwin published On the Origin of Species by Means of Natural Selection.

• Connecting what had once seemed a bewildering array of unrelated facts.

Introduction

•Two main points in The Origin of Species:

•Today’s organisms descended from ancestral species.

•Natural selection provided a mechanism for evolutionary change in populations.S

• The Origin of Species challenged a worldview that had been accepted for centuries.

• The key classical Greek philosophers who influenced Western culture, Plato and Aristotle, opposed any concept of evolution.

• Plato believed in two worlds: one real world that is ideal and perfect and an illusory world of imperfection that we perceive through our senses.

• Aristotle believed that all living forms could be arranged on a ladder (scala naturae) of increasing complexity with every rung taken with perfect, permanent species. Species are permanent and do not change.

Western culture resisted evolutionary views of life

• In the 1700s, the dominant philosophy, natural theology, was dedicated to studying the adaptations of organisms as evidence that the Creator had designed each species for a purpose.

• Carolus Linnaeus, a Swedish botanist, developed taxonomy, a system for naming species and grouping species into a hierarchy of increasingly complex categories.

• He believed that classification would reveal a divine plan.

Carolus Linnaeus(1707-1778)

• Darwin’s views were influenced by fossils, the relics or impressions of organisms

from the past, mineralized in sedimentary rocks.• Sedimentary rocks form when mud and sand

settle to the bottom of seas, lakes, and marshes.

• New layers of sediment cover older ones, creating layers of rock called strata.

• Fossils within layers show that a succession of organisms have populated Earth throughout time.

• Cuvier recognized that extinction had been a common occurrence in the history of life.

• Instead of evolution, Cuvier advocated catastrophism, that boundaries between strata were due to local flood or drought that destroyed the species then present.

• Later, this area would be repopulated by species immigrating from other unaffected areas.

• Strongly opposed evolution.

Paleontology, the study of fossils, was largely developed by Georges Cuvier, a French anatomist.

-realized that each stratum of earth is characterized by different fossils.

Theories of geologic gradualism helped clear the path for evolutionary

biologists• In contrast to Cuvier’s catastrophism, James Hutton, a Scottish geologist, proposed a theory of gradualism in 1795.

• Earth had been molded, not by sudden, violent events, but slow, gradual change

• Wind, water and weather formed the Earth.

• Proposed that the Earth was VERY old.

James Hutton

•Later, Charles Lyell, a geologist, proposed a theory of uniformitarianism.

Charles Lyell

Geological processes had not changed throughout Earth’s history.Earth was very old!

Uniformitarianism is the assumption that the natural processes operating in the past are the same as those that can be observed operating in the present

-contrast with catastrophism

• Hutton’s and Lyell’s observations and theories had a strong influence on Darwin.

• First, if geologic changes result from slow, continuous processes, rather than sudden events, then the Earth must be far older than the 6,000 years assigned by theologians from biblical inference.

• Second, slow and subtle processes persisting for long periods of time can add up to substantial change.

• In 1809, Jean Baptiste Lamarck published a theory of evolution based on his observations of fossil invertebrates in the Natural History Museum of Paris.

Lamarck placed fossils Lamarck placed fossils in an evolutionary in an evolutionary

contextcontext

•Lamarck thought that he saw what appeared to be several lines of descent in the collected fossils and current species.•Each was a chronological series of older to younger fossils leading to a modern species.

1809-Darwin was born

• Central to Lamarck’s mechanism of evolution were the concepts of use and disuse of parts and of inheritance of acquired characteristics.

• Proposed that body parts used extensively to cope with the environment became larger and stronger, while those not used deteriorated.

• These modifications acquired during the life of an organism could be passed to offspring.

• A classic example of these is the long neck of the giraffe.

• Lamarck’s theory was a visionary attempt to explain both the fossil record and the current diversity of life through its recognition of the great age of Earth and adaptation of organisms to the environment.

• However, there is no evidence that acquired no evidence that acquired characteristics can be inheritedcharacteristics can be inherited.

• A lizard that didn't use it legs would eventually not have legs and its offspring wouldn't have legs

• A giraffe stretched its neck to reach higher leaves, and this stretched neck would be a trait inherited by its offspring

• Acquired traits (e.g., bigger biceps) do not change the genes transmitted by gametes to offspring.

Inheritance of Acquired Characteristics

Alfred Russel Wallace -1858-emphasis was based on the idea of competition for resources as the main force in natural selection

January 8, 1823 – November 7, 1913

•He independently proposed a theory of natural selection which prompted Charles Darwin to publish his own more developed and researched theory sooner than he had intended. •“Father of biogeography".

Publications

•Wallace, Alfred Russel (2000; originally published 1869). The Malay Archipelago. Singapore: Periplus Press. ISBN 962-593-645-9.

•Wallace, Alfred Russel (1870). Contributions to the Theory of Natural Selection.

•Wallace, Alfred Russel (1876). The Geographical Distribution of Animals.

•Wallace, Alfred Russel (1898). Vaccination A Delusion. Swan Sonnenschein & Co.

• Charles Darwin (1809-1882) was born in western England.

• While Darwin had a consuming interest in nature as a boy, his father sent him to the University of Edinburgh to study medicine.

• Darwin left Edinburgh without a degree and enrolled at Christ College at Cambridge University with the intent of becoming a clergyman.

• At that time, most naturalists and scientists belonged to the clergy and viewed the world in the context of natural theology.

Edinburgh

Cambridge

CLIP

• Darwin received his degree in 1831.

• After graduation Darwin was recommended to be the conversation companion to Captain Robert FitzRoy, who was preparing the survey ship HMS Beagle for a voyage around the world.

• FitzRoy chose Darwin because of his education, and because he was of the same social class, and was close in age to the captain.

• The main mission of the five-year voyage of the Beagle was to chart poorly known stretches of the South American coastline.

Field research helped Darwin frame his view of life

• He collected He collected thousands of thousands of specimens of the specimens of the exotic and diverse exotic and diverse flora and fauna of flora and fauna of South America.South America.

• Darwin explored the Darwin explored the Brazilian jungles, the Brazilian jungles, the grasslands of the grasslands of the Argentine pampas, Argentine pampas, the desolation of Tiera the desolation of Tiera del Fuego, and the del Fuego, and the heights of the Andes.heights of the Andes.

Darwin had the freedom Darwin had the freedom to explore extensively to explore extensively on shore while the crew on shore while the crew surveyed the coast.surveyed the coast.

• Darwin noted that the plants and animals of South America were very distinct from those of Europe.

• Organisms from temperate regions of South America were more similar to those from the tropics of South America than to those from temperate regions of Europe.

• Further, South American fossils more closely resembled modern species from that continent than those from Europe.

• The origin of the fauna of the Galapagos, 900 km west of the South American coast, especially puzzled Darwin.• Darwin noted that

while most of the animal species on the Galapagos lived nowhere else, they resembled species living on the South American mainland.

• While on the Beagle, Darwin read Lyell’s Principles of Geology.• Lyell’s ideas and his observations on the voyage led

Darwin to doubt the church’s position that the Earth was static and only a few thousand years old.

• Instead, he was coming to the conclusion that the Earth was very old and constantly changing.

• After his return to Great Britain in 1836, Darwin began to perceive that the origin of new species and adaptation of species to the environment were closely related processes.• For example, clear differences in the beak among the 13

types of finches that Darwin collected in the Galapagos are adaptations to the foods available on their home islands.

• By the early 1840s Darwin had developed the major features of his theory of natural selection as the mechanism for evolution.

• In 1844, he wrote a long essay on the origin of species and natural selection, but he was reluctant to publish his theory and continued to compile evidence to support his theory.

• In June 1858, Alfred Wallace, a young naturalist working in the East Indies, sent Darwin a manuscript containing a theory of natural selection essentially identical to Darwin’s.

Alfred Wallace

1. Evolution is the explanation for life’s unity and diversity.

2. Natural selection is the cause of adaptive evolution.

The Origin of Species developed two main pointstwo main points:

*the occurrence of evolution and natural selection as its mechanism

Descendents of these ancestors accumulated diverse accumulated diverse modifications ormodifications or adaptations adaptations that fit them to specific ways of life and habitats.

Central to Darwin’s view of the evolution of life is descent with descent with

modificationmodification•All present day organisms are related through descent from unknown ancestors in the past.

This evolutionary tree of the elephant family is based on evidence from fossils.

Descent with modificationDescent with modification

Darwin’s main ideas can be summarized as follows.

1. Populations tend to grow exponentially, overpopulate, and exceed their resources.

2. Overpopulation results in competition and struggle for existence.

3. In any population, there is variation and an unequal ability of individuals to survive and reproduce.

4. Only the best-fit individuals survive and get to pass on their traits to offspring.

5. Evolution occurs as advantageous traits accumulate in a population.

The other major point that Darwin pioneered is a unique mechanism of evolution - the theory of natural selectionnatural selection.

For example, related species of insects called mantids have diverse shapes and colors that evolved in different environments.

• In each generation, environmental factors filter heritable variations, favoring some over others.

• Differential reproduction -- whereby organisms with traits favored by the environment produce more offspring than do organisms without those traits -- results in the favored traits being disproportionately represented in the next generation.

• This increasing frequency of the favored traits in a population is evolution.

Peppered Peppered MothMoth

Two versions

of the moth-

Black and

peppered

During the

industrial

revolution on

England

Population of

moths changed

• Darwin’s views on the role of environmental factors in the screening of heritable variation were heavily influenced by artificial selection.

• Humans have modified a variety of domesticated plants and animals over many generations by selecting individuals with the desired traits as breeding stock.

Natural selection can only amplify or diminish

heritable variations, not variations that an

individual acquires during its life, even if these

variations are adaptive.

• In general, natural selection operates not to create variation, but to edit existing variation.

• For example, resistant insects are favored and non-resistant individuals are not when insecticides are applied.

• Natural selection favors those characteristics in a variable population that fit the current, local environment.

• Fossil record

Other evidence of evolution

Rock layer

Isotope dating-radioactive dating• Half-life

• 14C

• Earth

• 4.6 billion years old

• Anatomical similarities between different species.

• Ex: forelimbs of human, cats, whales, and bats share the same skeletal elements, but different functions because they diverged from their common ancestor.

Homologous structures Reflect common ancestry

• Bat’s wing and a fly’s wing have the same function; However they have different ancestors.

• They have simply adapted to similar environments.

• Convergent evolution

Analogous structures

While the sugar glider and flying squirrel have adapted to the same mode of life, they are not closely related.

• Instead, the sugar glider from Australia is more closely related to other marsupial mammals from Australia than to the flying squirrel, a placental mammal from North America.

• The resemblancebetween them is another example ofconvergentevolution.

Analogous structures

• Structures that have marginal, if any, importance to a current organism, but which had important functions in ancestors.

Vestigial structures

They are usually reduced in size

Whale pelvis

Vestigial structures

Comparative Embryology

•Similar stages in embryonic development.

•Ex: All vertebrates embryos go through a stage in which they have gill pouches on the sides of the throats.

•Fish-gills

•Humans-eustachian . tubes

Sometimes, homologies that are not obvious in adult organisms become evident when we look at embryonic development.• For example, all vertebrate embryos have

structures called pharyngeal pouches in their throat at some stage in their development.

• These embryonic structures develop into very different, but still homologous, adult structures, such as the

• gills of fish or

• the Eustacean tubes that connect the middle ear with the throat in mammals.

Comparative Embryology

Molecular biology allows links between organisms that have no macroscopic anatomy in common:

1. All species of life have the same basic genetic machinery of RNA and DNA and the genetic code is essentially universal.

-The language of the genetic code has been passed along through all the branches of the tree of life ever since the code’s inception in an early life-form.

2. All aerobic organism contain the polypeptide cytochrome C

-Amino acids sequence of CytoC can show how closely related two species are.

The geographical The geographical distribution of distribution of

species -- species -- biogeographybiogeography -- -- first suggested first suggested

evolution to evolution to Darwin.Darwin.

Theory of Continental drift:

200mya-Pangaea

Today-7 continents

Wagner

• Species tend to be more closely related to other species from the same area than to other species with the same way of life, but living in different areas.

• For example, even though some marsupial mammals (those that complete their development in an external pouch) of Australia have look-alikes among the eutherian mammals (those that complete their development in the uterus) that live on other continents, all the marsupial mammals are still more closely related to each other than they are to any eutherian mammal.

eutherian mammals

Beaver

NORTH AMERICA

Muskrat

Capybara SOUTH AMERICA

Coypu

Distribution of Living Species

Beaver

Muskrat

Beaver andMuskrat

Coypu

Capybara

Coypu andCapybara

• Island and island archipelagos have provided strong evidence of evolution.• Often islands have many species of plants and

animals that are endemic, or found nowhere else in the world.

• As Darwin observed when he reassessed his collections from the Beagle’s voyage, these endemic species are typically related more closely to species living on the nearest mainland (despite different environments) than those from other island groups.

All of the 500 or so endemic species of Drosophila in the Hawaiian archipelago descended from a

common ancestor that reached Kauai over 5 million years ago.

• Natural selection is widely accepted in science because its predictions have withstood thorough, continual testing by experiments and observations.• However, science is not static and arguments

exist among evolutionary biologists concerning whether natural selection alone accounts for the history of life as observed in the fossil record.

• The study of evolution is livelier than ever, but these questions of how life evolves in no way imply that most biologists consider evolution itself to be “just a theory.”

All of that being said, be aware that…..

Timeline

BC AristotleSpecies are permanent, arranged on a ladder.

1700s

Carolus Linnaeus

Taxonomy, classification shows relationships, “classification would reveal a divine plan”

1700s

Georges Cuvier

Studied fossils, opposed evolution, “catastrophes responsible for changes in organisms”

1795 James Hutton

Gradualism, Earth molded by weather, Earth has a long history

1798 Thomas Malthus

Populations outgrow their food supplies, causing competition

1809Jean

Baptiste Lamarck

Inheritance of acquired characteristics.

1833 Charles Lyell

Geologist, Earth more than 6,000 years old, Principles of Geology, uniformitarianism

1858Alfred Russel Wallace

Proposed natural selection as the mechanism for evolution

1859 Charles Darwin

Theory of Natural Selection, “On the Origin of the Species by Natural Selection”