Transcript Sect. 7.1

7.1 Darwin’s Theory
Key ideas
 What important observations did
Darwin make on his voyage?
 How did Darwin account for the
diversity of species and the
differences between similar species?
 How does natural selection lead to
evolution?
Darwin’s Observations
 December 1831, Charles Darwin set
sail from England on a five year trip
around the world.
 He observed plants and animals he
had never seen before.
 This observation led him to develop
the theory of evolution by natural
selection.
 Darwin’s observations included:
 Diversity of organisms
 Species: a group of similar organisms that
can mate with each other and produce
fertile offspring
 Remains of ancient organisms (fossils)
 Fossil: preserved remains or traces of an
organism that lived in the past
 Characteristics of organisms on the
Galapagos Islands
Examples of different species.
Fossils
Comparisons to South American
Organisms
 How are these
iguanas different?
Galapagos Organisms
 Darwin’s drawings
 Identify some
specific differences
in these finches’
beaks.
 These finches live
in different islands.
Adaptations
 Def: a trait that helps an organism
survive and reproduce in its
environment.
 Ex: milkweed – poisonous or have a
bad taste – avoid being eaten
 Ex: some plants have bright colored
flowers – attract insects
Evolution
 Darwin reasoned that plants or
animals that arrived on the GI faced
environmental factors that were
different from those on the mainland.
 Darwin hypothesized that the species
gradually changed (evolution) over
many generations and became better
adapted to the new environments.
Scientific theory
 Scienfic theory: A well-tested concept
that explains a wide range of
observations
 Darwin concluded that organisms on
the GI had changed over time, but
did not know HOW the changes had
happened.
Selective Breeding
 Humans have used selective breeding
to produce sheep and pigeons that fit
certain criteria.
 Darwin thought that a process similar
to selective breeding might happen in
nature.. But what process would push
this selective breeding in nature?
Meet the Supercow □ at 3:30
Natural Selection
 Darwin proposed that evolution
occurs in nature through natural
selection ( individuals that are better
adapted to their environment are
more likely to survive and reproduce
than other members of the same
species) in the book called, On the
Origin of Species by Natural
Selection.
 Overproduction – Malthus pointed out
that sometimes the food supply is not
large enough to feed all the offspring
of certain organisms- insects,
humans, etc.
 Darwin knew that overproduction occurs
in many species.
 Why don’t all the offspring survive?
 Genetic Variations: a difference
between individuals of the same
species
 Certain insects may be able to eat
foods that others might avoid
 This characteristic gives the insects
an advantage over others.
 Competetion – since food and other
resources are limited, the members of
a species must compete with each
other to survive.
 Selection – some variations make
individuals better adapted to their
environment making them more likely
to survive and reproduce
 Selection cont’d – Darwin proposed
that, over a long time, natural
selection can lead to change. Helpful
variations may gradually accumulate
in a species, while unfavorable ones
may disappear.
 Environmental factors - factors in the
environment can affect an
organisms’s ability to survive.
 Genes and natural selection – like
environmental factors, genetic
variation contributes to evolution.
Without variations, all members of a
species would have the same traits.
Why the big eyes??
NOT EVOLUTION…
human manipulation.
 Darwin could not explain what caused
variations or how they were passed on.
 Scientists later learned that variations can
result from mutation and the shuffling of
alleles during meiosis. Genes are passed
from parents to offspring.
 Because of this, only traits that are
inherited, or controlled by genes, can be
acted upon by natural selection