Unit 5 Evolution
Ch. 15 Darwin’s Theory of Evolution
The Puzzle of Life’s Diversity
• Evolution - modern organisms have
descended from ancient organisms
(change over time)
• Theory - a well-supported, testable
explanation of phenomena that have
occurred in the natural world
Voyage of the HMS Beagle
• 1831
• Charles Darwin contributed most to our
understanding of evolution
• He made observations & collected
evidence that led him to propose a
hypothesis about the way life changes
over time.
Voyage of the Beagle
• That hypothesis, now supported by a large
amount of evidence, has become the
Theory of evolution
Darwin’s Observations
• Darwin collected the preserved remains of
ancient organisms - Fossils
• Some of these fossils resembled
organisms that were still alive
• Others looked unlike any creature ever
seen
Darwin’s Observations
• The Galapagos Islands influenced Darwin
the most
• He observed that the characteristics of
animals & plants varied among the
different Islands.
Darwin’s Observations
• Each island had a different climate
although they were very close.
• He studied tortoises.
• Each tortoise’s shell had a different shape
depending on which island it came from.
• He wondered if the animals living on
different islands were once related….
Views before Darwin
• Earth was created only a few thousand
years ago.
• Since creation, neither earth nor its
species had changed
• Darwin was influenced by several
individuals.
An Ancient, Changing Earth
• Hutton & Lyell helped scientists
recognize that Earth is millions of years
old.
• They also noted that the processes that
changed Earth in the past are the same
processes that are changing Earth now.
Lamarck’s Evolution Hypothesis
• The year that Darwin was born, Lamarck
published his hypothesis
• He proposed that by selective use or
disuse of organs, organisms acquired or
lost certain traits during their lifetime
• Over time, this process led to change in a
species
Lamarck’s Explanation
•Tendency toward
perfecton
•Use and Disuse
•Inheritance of
Acquired traits
Population Growth
• English economist, Malthus, published a
book, noting that babies were being born
faster than people were dying
• He stated that if the human population
continued to grow unchecked, sooner or
later there would be insufficient living
space & food for everyone
Darwin Presents His Case
• In 1859, Darwin published the results of
his work in a book, On the Origin of
Species.
• In his book, he proposed a mechanism for
evolution called natural selection.
• He stated that evolution has been taking
place for millions of years, & continues in
all living things
Darwin Presents His Case
• Species on Earth today descended from
ancestral animals in various habitats.
• Heritable variation- differences that are
passed from parents to offspring.
• Variations were thought to be unimportant.
Inherited Variation & Artificial
Selection
• Artificial selection - nature provided the
variation, & humans selected those
variations that they found useful.
• It has produced diverse plants & animals
by selectively breeding for different traits.
Evolution by Natural Selection
• Struggle for existence - the members of
each species compete regularly to obtain
food, living space, & other necessities of
life
– Predators that are faster & better at catching
prey are more likely to survive
Evolution by Natural Selection
• Fitness - the ability of the organism to
survive & reproduce in its specific environ.
– Fitness is the result of adaptations
Evolution by Natural Selection
• Adaptation - any inherited characteristic
that increases an organisms’ chance of
survival
– Successful adaptations allow
organisms to become better
suited to their environ. & thus better able to
survive
Evolution by Natural Selection
• Individuals that are better suited to their
environ., with adaptations that enable
fitness, survive & reproduce most
successfully - Survival of the Fittest
Evolution by Natural Selection
• Since it is similar to artificial selection,
Darwin referred to survival of the fittest as
- Natural Selection
• In both AS & NS, only certain individuals of
a population produce new individuals
Evolution by Natural Selection
• However, in NS, the traits being selected,
& therefore, increasing over time,
contribute to an organism’s fitness
• NS takes place without human control or
direction
Evolution by Natural Selection
• NS results in changes in the inherited
characteristics of a population, that
increase a species’ fitness in its environ.
• Over time, NS produces organisms that
have different structures, & occupy
different habitats
Evolution by Natural Selection
• As a result, species today look different
from their ancestors
• Each living species has descended, with
changes, from other species over time Descent with Modification
Evolution by Natural Selection
• Descent with modification also implies that
all living organisms are related to each
other
• Common descent - all species (living &
extinct) were derived from common
ancestors
Evidence of Evolution
• Darwin argued that living things have been
evolving on Earth for millions of years
• Evidence of this could be found: in the
fossil record, the geographical distribution
of living species, homologous structures of
living organisms, & similarities in early
development
Evidence of Evolution
• The Fossil Record:
– Darwin noticed that the sizes, shapes, &
varieties of related organisms preserved in the
fossil record, changed over time
Evidence of Evolution
• Geographic Distribution of Living Species:
– Darwin realized that similar animals in different
locations were the product of different lines of
evolutionary descent
Evidence of Evolution
• Homologous Body Structures:
– Homologous structures - structures that have
different mature forms but develop from the
same embryonic tissues
– Not all homologous structures serve important
functions
– Organs of many animals are so reduced in size
that they are just vestiges, or traces, of
homologous organs
Homologous Structures
Evidence of Evolution
• Homologous Body Structures:
– Vestigial organs - may resemble miniature legs,
tails, or other structures, a trace of a
homologous structure
Evidence of Evolution
• Similarities in Early Development:
– The early stages or embryos, of many animals
with backbones are very similar