7.2 Darwin’s Theory
Darwin’s Theory
Published “The Origin of Species” in 1859
1) All species of organisms living on Earth today are
descended from ancestral species species
evolve over time
2) The mechanism that causes species to change is
natural selection
Descent with Modification Process in which
descendants of organisms spread into various
habitats over millions of years and accumulated
different adaptations
Natural Selection Individuals with inherited
characteristics best suited to their environment
leave more offspring than those that are less
adapted
Natural Selection
1) Struggle for existence
Excessive offspring, limited resources
Competition leading to only some offspring surviving
2) Variation
Differences among members of the same species
are heritable
Some variations are better suited to an environment
and so these individuals will leave behind more
offspring
If this occurs over many generations, each new
generation has a higher proportion of individuals
with the advantageous trait
3) Environment
Individuals with variations best suited to an
environment are selected for
Supported by genetics
From these three key points Darwin developed his
theory of …
Natural Selection the process by which
individuals with inherited characteristics well
suited to their environment leave more offspring
on average than do individuals with adaptations
less suited to the environment.
Survival of the Fittest
Fitness ability of an individual to survive and
reproduce in its specific environment
Survival of the fittest the differences in
rates of survival and reproduction
Note: Natural selection does not make
organisms “better” because what works in one
environment may not work in another
Fossil Record
Fossil: preserved remains/markings left by
organisms; mostly found in sedimentary rock
Fossil Record: chronological collections of life’s
remains in the rock layers; deeper the fossil, the
older it is
Palaeontologist: scientist who studies fossils
Fossil evidence supports the fact that present day
animals evolved from ancient ones and shows the
extinction of animals
Limits animals that do not possess hard tissue
rarely become fossilized
Fossilization-in action
http://www.bbc.co.uk/sn/prehistoric_life/dinosaurs/
making_fossils/makingfossils/index.shtml
Fossil Record showing
evolution of whales from land
mammals
Geographic Distribution
Difference and similarities between
organisms around the world
1) Closely related but different
i.e. Galapagos finches all descended from
one mainland species geographic
isolation allowed for adaptations that were
best suited to local environment to flourish
2) Distantly related but similar
similar habitats select for similar
adaptations
Darwin’s Finches
Comparative Anatomy
Similarities in structure that have adapted
different functions
e.g. forelimb of mammals all consist of the
same skeletal parts
Homologous Structures: similar structures
that share a common ancestor
Supports idea that evolution is a remodelling
process structures functioning one way in
ancestral species become modified
Forelimbs in human, cat, whale
and bat
Anatomy
Vestigial Structures: remnants of structures
that were important in ancestral species but
have no clear function now; often reduced in
size i.e. whales have hip bones, humans have
appendix, tailbone, (goosebumps!), wisdom
teeth
Analagous Structures: distantly related
species develop structures that are anatomically
different but perform same function i.e. wing of
bee and birds
Comparative Development
Comparing the embryos of organisms
shows that they have similar developmental
stages
Molecular Biology
The greater the number of similarities in DNA
and protein sequences, the more likely that the
species share a common ancestor
e.g. hemoglobin of gorilla and human has
only one different amino acid; cytochrome C
is identical in chimpanzees and humans
Evidence for Natural
Selection
Artificial Selection: selective breeding of
domesticated plants and animals to produce
offspring with traits that are valued by humans
Breeders create a great deal of change in few
generations, so natural selection could do the same
in long periods of time, with the environment doing
the selecting
Changes in Beak Shape of
Finches
One set of finches shows cycles of natural
selection in beak size that changes as El Nino
comes and goes the wet seasons provide
many small seeds that favour small beaks
and the dry seasons provide large seeds that
favour large beaks
Antibiotic Resistance of
Bacteria
When patients do not finish an antibiotic
treatment they leave behind some bacteria that
are resistant, which then reproduce and
become more common
e.g. streptomyocin resistant tuberculosis;
methicillin resistant staphylococcus aureus
(MRSA)