Evolution: Descent with Modification

Theory

• a theory accounts for many observations and data and attempts to explain and integrate a great variety of phenomena

Darwin Introduces a Revolutionary Theory

• A new era of biology began on November 24, 1859, the day Charles Darwin published On the Origin of Species by Means of Natural Selection

• The Origin of Species focused biologists’ attention on the great diversity of organisms

Darwin

• Darwin made two major points in his book:– Many current species are descendants of ancestral

species– Natural selection is a mechanism for this

evolutionary process

Resistance to the Idea of Evolution

• The Greek philosopher Aristotle viewed species as fixed and unchanging: Scala Naturae

• The Old Testament holds that species were individually designed by God and therefore perfect

Resistance to the Idea of Evolution

• Carolus Linnaeus interpreted organismal adaptations as evidence that the Creator had designed each species for a specific purpose

• Linnaeus was a founder of taxonomy, the branch of biology concerned with classifying organisms

Fossils, Cuvier, and Catastrophism

• The study of fossils helped to lay the groundwork for Darwin’s ideas

• Fossils are remains or traces of organisms from the past, usually found in sedimentary rock, which appears in layers or strata

• Paleontology, the study of fossils, was largely developed by French scientist Georges Cuvier

• Cuvier advocated catastrophism, speculating that each boundary between strata represents a catastrophe

Gradualism

• Gradualism is the idea that profound change can take place through the cumulative effect of slow but continuous processes

• Geologists Hutton and Lyell perceived that changes in Earth’s surface can result from slow continuous actions still operating today

• This view strongly influenced Darwin’s thinking

Lamarck’s Theory of Evolution

• Lamarck hypothesized that species evolve through use and disuse and the inheritance of acquired traits

• The mechanisms he proposed are unsupported by evidence

Charles Darwin

• During his travels on the Beagle, Darwin collected specimens of South American plants and animals

• He observed adaptations of plants and animals that inhabited many diverse environments

• stop at the Galápagos Islands near the equator west of South America

Charles Darwin

• Darwin perceived adaptation to the environment and the origin of new species as closely related processes

• From studies made years after Darwin’s voyage, biologists have concluded that this is indeed what happened to the Galápagos finches

Cactus eater. The long, sharp beak of the cactus ground finch (Geospiza scandens) helps it tear and eat cactus flowers and pulp.

Insect eater. The green warbler finch (Certhidea olivacea) used its narrow, pointed beak to grasp insects.

Seed eater. The large ground finch (Geospiza magnirostris) has a large beak adapted for cracking seeds that fall from plants to the ground.

Charles Darwin

• The Origin of Species, an essay on the origin of species and natural selection was published in 1859

• Darwin developed two main ideas:– Evolution explains life’s unity and diversity– Natural selection is a cause of adaptive evolution

Darwin

• descent with modification – perception of the unity of life– all organisms are related through descent from an

ancestor that lived in the remote past• In the Darwinian view, the history of life is like

a tree with branches representing life’s diversity

Natural Selection and Adaptation• Evolutionary biologist Ernst Mayr has dissected the logic of

Darwin’s theory into three inferences based on five observations– Observation #1: For any species, population sizes would increase

exponentially if all individuals that are born reproduced successfully

– Observation #2: Populations tend to be stable in size, except for seasonal fluctuations

– Observation #3: Resources are limited– Inference #1: Production of more individuals than the environment

can support leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving

• Observation #4: Members of a population vary extensively in their characteristics; no two individuals are exactly alike

• Observation #5: Much of this variation is heritable• Inference #2: Survival depends in part on inherited traits;

individuals whose inherited traits give them a high probability of surviving and reproducing are likely to leave more offspring than other individuals

• Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over generations

Natural Selection

• Natural selection is differential success in reproduction from interaction between individuals that vary in heritable traits and their environment

• an increase over time in adaptation of organisms to their environment

• If an environment changes over time, natural selection may result in adaptation to these new conditions

Homology

• Homology is similarity resulting from common ancestry

• Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor

Homology, Biogeography, and the Fossil Record

• Comparative embryology reveals anatomical homologies not visible in adult organisms

Biogeography

• Darwin’s observations of biogeography, the geographic distribution of species, formed an important part of his theory of evolution

• Some similar mammals that have adapted to similar environments have evolved independently from different ancestors

• Analogous structures

Convergent Evolution: similar problem, similar solution

Fossil Record

• The succession of forms observed in the fossil record is consistent with other inferences about the major branches of descent in the tree of life

• The Darwinian view of life predicts that evolutionary transitions should leave signs in the fossil record

• Paleontologists have discovered fossils of many such transitional forms