Natural selection occurs through a struggle for existence where individuals compete for limited resources. Those best suited to the environment will survive and pass on their genes, while others will die without reproducing. Over generations, this process of natural selection can cause populations to evolve and change through descent with modification from ancestral species. Key mechanisms include adaptation, fitness, and common descent of all living things from shared ancestors. Evidence for evolution comes from the fossil record, geographic distribution, homologous structures, vestigial organs, and comparative genetics.

1. Natural Selection
Struggle for Existence
Survival of the Fittest
Descent with Modification
Results of Natural Selection

2. Natural Selection
 Struggle for
Existence – members of
each species compete
regularly to obtain food,
living space, and other
necessities of life
 From Malthus’ work,
Darwin concluded that high
birth rates and a shortage of
life’s basic needs would
force competition for
resources.

3. Natural Selection
 Survival of the Fittest –  Adaptation – any inherited
some individuals are better suited characteristic that increases an
for their environment than organism’s chance of survival
others; those best suited will  Could be anatomical, behavioral,
survive physiological
 Evolutionary adaptation – any
genetically controlled
 Fitness – ability of an individual physiological, anatomical, or
to survive and reproduce in its behavioral trait that increases an
specific environment individual’s ability to pass along its
 Result of adaptations genes
 Evolutionary Fitness –
organism’s success in passing
genes to the next generation
 Where fitness varies, natural
selection can act.

4. Natural Selection
 Descent with Modification – every  Implies that all
living species has descended, with changes, living organisms are
from other species over time related to one
another
 Principle of
Common
Descent – all
species (living
and extinct) were
derived from
common
ancestors
 Allows for a
single “tree of
life” that links all
living things

5. Result of Natural Selection…
 …changes in the
inherited
characteristics of
a population
 These changes
increase a
species’ fitness
in its
environment.
 Acts on
populations, not
individuals. It is
populations that
evolve over time.

6. Summary of Darwin’s Theory of
Evolution
 Individual organisms differ (variations exist in a population).
 Some of this variation is heritable.
 In nature, there is a tendency for overpopulation.
 Organisms produce more offspring than can survive. There is a high death
rate.
 Only the organisms with the variations that are suitable to the environment
survive.
 Organisms must compete for limited resources.
 Each unique organism has unique advantages and disadvantages in the struggle
for existence.
 The organisms that survive get to reproduce and pass on their genes.
 The process of Natural Selection causes species to change over time.
 It is the population that changes to match the environment – not individuals.
 Species alive today are descended with modification from ancestral species.

7. Evidence of Evolution
 Fossil Record
 Any preserved remains of ancient organisms
 Darwin Proposed that the earth was millions of years
old and from looking at the fossil records in older rocks
and fossils from younger rocks scienists could
document that life on earth had changes over time.

8. Evidence (continued)
 Geographic Distribution
 Different species living in the same
region/environment have similar body structures and
behaviors
 Homologous Body Structures
 Structures that have different mature forms
but develop from the same embryonic tissue;
look the same but used differently
 Strong evidence that all four-limbed
vertebrates have descended, with modification
from common ancestors.

10. Evidence (continued)
 Vestigial organs
 Organs that are so reduced in size that
they are just traces of homologous organs
in other species.
Example:
 Whale hips
 Human appendix

11. Evidence (continued)
 Comparative Genetics
 DNA from different
organisms works the
same
 Early development is
similar