Chapter 15: Darwin’s Theory of Evolution – Notes
Evolution - change over time, is the process by which modern organisms have descended from ancient
organisms.
Voyage of the Beagle
 British Naturalist, Charles Darwin traveled around the world on board HMS Beagle.
 He observed and collected evidence & proposed hypothesis about the way life changes over time and
how living thing evolve from other living things.
Darwin's Observations
Patterns of Diversity
Darwin observed that plants and animals seemed well suited to the environment they inhabited. At the
same time he was puzzled by the fact that continents having similar kind of environments inhabit
entirely different species of organisms.
Living Organisms and Fossils
Fossil - preserved remains or evidence of an ancient organism
Some of those fossils resembled organisms that were still alive and others looked completely unlike any
creature he had ever seen.
The Galápagos Islands
Darwin noted that although they were close together, the islands had very different climates.
He observed that the characteristics of many animals and plants varied noticeably among the different
Galápagos Islands.
e.g The giant tortoises varied in predictable ways from one island to another, birds from different islands
had differently shaped beaks.
Darwin wondered if animals living on different islands had once been members of the same species. The
separate species would have evolved from an original ancestor species after becoming isolated from one
another.
Ideas That Shaped Darwin's Thinking
Hutton’s theory of Geological Change
James Hutton (1785) proposed that geologic forces, over extremely long periods of time shape Earth. He
estimated Earth to be millions-not thousands – of year old.
Lyell's Principles of Geology
Charles Lyell (1833) stressed that scientists must explain past events in terms of processes that they can actually
observe. Since processes (volcano, Erosion) that shaped the Earth millions of years earlier continue in the
present.
This understanding of geology made Darwin realize that it would have taken many years for life to change in
the way he suggested. This would have been possible only if the Earth were extremely old.
Lamarck's Theory of Evolution
French Naturalist Lamark (1809) recognized that
 living things have changed over time
 all species were descended from other species
 organisms were somehow adapted to their environments
Lamarck proposed that by selective use or disuse of organs, organisms acquired or lost certain traits during their
lifetime. These traits could then be passed on to their offspring.
Over time, this process led to change in a species.
1. Tendency Toward Perfection
All organisms have an innate tendency toward complexity and perfection. Hence they are
continually changed and acquire features that help them live more successfully in their
environments.
2. selective use or disuse of organs
Organisms respond to change in the environment by developing new organs or by changing the
structure of an old organ.
This adaptation to the change in the environment is called an acquired trait.
3. Inheritance of Acquired Traits
Lamark proposed that the acquired trait could be inherited and new generation receives beneficial
structure.
4. Evaluating Lamarck's theory
His explanation about how traits are inherited was found to be incorrect.
He did not know that an organism's behavior has no effect on its heritable characteristics.
Population Growth – Thomas Malthus
English economist Thomas Malthus (1798) noted that babies were being born faster than people were dying and
reasoned that if the human population continued to grow unchecked, sooner or later there would be insufficient
living space and food for everyone.
Darwin realized that this reasoning applied strongly to plants and animals than it did to humans.
Because humans produce far fewer offspring than most other species do.
In other species majority of offspring die and only a few of those offspring that survive succeed in reproducing.
Natural Variation and artificial selection
Natural variation is differences among individuals of a species in nature. This variation could be inherited.
Darwin noted that breeders used variation to improve their crops, livestock through a technique called selective
breeding.
Darwin termed this process as artificial selection, nature provided the variation, and humans selected those
variations that they found useful.
Evolution by Natural Selection
The Struggle for Existence
According Malthus high birth rates and a shortage of life's basic needs would force organisms to
compete for resources. The members of each species struggle to exist by competing for food, living
space, and other necessities of life.
Survival of the Fittest
According to Darwin ability of an individual to survive and reproduce in its specific environment is
Fittness.
The fitness is the result of adaptations.
Adaptations can be anatomical, or structural, characteristics, or an organism’s physiological processes,
or functions, or complex features, such as behavior.
An adaptation is any inherited characteristic that increases an organism's chance of survival.
Successful adaptations, enable organisms to become better suited to their environment and thus better
able to survive and reproduce.
Individuals with characteristics that are not well suited to their environment—that is, with low levels of
fitness—either die or leave few offspring.
Darwin referred to the survival of the fittest as Natural selection
Natural selection
 The traits being selected—increase over time —contribute to an organism's fitness in its environment.
 Takes place without human control or direction.
 Cannot be seen directly; it can only be observed as changes in a population over generations.
Similarity between artificial selection and natural selection
Only certain individuals of a population produce new individuals
Descent with Modification
Over long periods, natural selection produces organisms that have different structures, establish different niches,
or occupy different habitats.
As a result, species today look different from their ancestors.
Each living species has descended, with changes, from other species over time. Darwin called it as Descent
with Modification.
It also implies that all living organisms are related to one another. All species—living and extinct—were
derived from common ancestors. This is the principle known as common descent.
Considering both the concepts “Descent with Modification” and “common descent” a single “tree of life”
links all living things.
Evidence of Evolution
1. Fossil record
Examining fossils from sequential layers of rock, one could view how a species had changed and produced
different species over time.
Fossil age determined by
 Position in the sediment - oldest layer at the bottom
 Amount of radioactive isotopes in organism- e.g.C12, C14 etc
2. Geographical distribution of living species
Darwin discovered that finches he collected in the Galápagos were similar, yet distinctly different from one
another and also different from the similar species on the mainland.
He concluded that the finches could have descended with modification from a common mainland
ancestor.
Where as entirely different species of animals that had similar structures and behaviors inhabiting the
continents having similar kind of environments must have descended from different ancestor. Since they
were living under similar ecological conditions, and were exposed to similar pressures of natural
selection they ended up evolving common features.
Regardless of their appearance, two species are closely related when they share a common ancestry.
3. Anatomical Evidence
Homologous structures of living organisms

Body parts with same basic structure but different function

Structures have different mature forms but develop from same embryonic tissues are called as
homologous structures.

Indicates common ancestry

e.g. Foreleg – same set of bones for vertebrate.
Analogous structures of living organisms

Body parts with similar function but different structure

Indicates NO common ancestry

e.g. wings if insect and wings of bat
Vestigial

A structure reduced in size

No known function

Left over from other structure

e.g. tailbone for humans
4. Embryological Evidence
 Compares development of embryos
 Similar embryologycommon ancestry.
 Embryo looks similar for longer period – Closely related species
 Embryonic cells develop in same order and pattern to produce tissues and organs which produce
homologous structures
5. Biochemical Evidence
 The sequence of amino acids that make up proteins are similar similar DNA strandsorganism show
evolutionary relationship.
 e.g. Humans and Chimpanzees (97% similar)
Summary of Darwin's Theory
1.
2.
3.
4.
5.
Individual organisms differ, and some of this variation is heritable.
Organisms produce more offspring than can survive, and many that do survive do not reproduce.
Because more organisms are produced than can survive, they compete for limited resources.
Each unique organism has different advantages and disadvantages in the struggle for existence.
Individuals best suited to their environment survive and reproduce most successfully. These organisms
pass their heritable traits to their offspring. Other individuals die or leave fewer offspring.
6. Over long periods, natural selection causes changes in the characteristic of species, such as in size or
form. New species arise and some disappear.
7. Species alive today are descended with modification from ancestral species that lived in the distant past
8. This process, by which diverse species evolved from common ancestors, unites all organisms on Earth
into a single tree of life.