ORGANIC EVOLUTION
Chapter 6
Pre-Darwinian Ideas
• Lamark (1744 – 1829) – Inheritance of Acquired
Characteristics
- organisms, by striving to meet the demands of their
environments, acquire adaptations and pass them
by heredity to their offspring.
- ex. Giraffe evolved its long neck by stretching
• Lyell (1797 – 1875) – Uniformitarianism
- the laws of physics and chemistry have not
changed throughout the history of the Earth
- past geological events occurred by natural
processes similar to those observed today
Darwin’s Voyage
• Charles Darwin sailed around the world for 5 years on the
HMS Beagle.
• collected specimens and made observations in his journal
• Did research on the Galapagos Islands:
- unique character of the plants and animals
- animals and plants were related to those of the
South American mainland
- each island often contained a unique species
related to forms on other Galapagos Islands
• After Beagle returned, Darwin worked on his Theory of
Evolution for 20 years
Darwinian Theory of Evolution: The
Evidence
• Perpetual Change:
- The living world is always changing
- Perpetual change can be seen most directly in the
fossil record
- Fossils are deposited in stratified layers
- The fossil record allows us to view evolutionary
change across the broadest scale of time
- Fossil records have shown us trends (directional
changes in the features or patterns of diversity in a
group of organisms)
• Evolutionary trends:
- increased size
- elaboration of molars
- loss of toes
• Common Descent
- all plants and animals have descended from an
ancestral form
- Homology (major source of evidence) – the same
organ in different organisms under every variety of
form and function.
- Embryology
• Multiplication of Species
- genetic variation present within a species,
especially variation that occurs between
geographically separated populations, provides the
material from which new species are formed
(speciation).
- biological features that prevent different species
from interbreeding are called reproductive barriers
- Reproductive barriers between populations usually
evolve gradually (must be kept separated for long
periods of time)
- Allopatric speciation – the separated populations
evolve independently and adapt to their respective
environments, generating reproductive barriers
between them as a result of their separate
evolutionary paths.
- allopatric speciation can happen two ways:
vicariant speciation
founder event
- Sympatric speciation – different individuals within a
species become specialized for occupying different
components of the environment.
- Parapatric speciation – separate species form
where diverging lineages are mostly nonoverlapping in geographic distribution but make contact
along a narrow borderline.
- Adaptive radiation – production of several diverse
species from a common ancestral species.
- especially when many species arise within
a short period of time (few million years)
• Gradualism:
- small, continuous changes in phenotypes (can lead
to major differences if accumulated over many
thousands to millions of years).
- Punctuated equilibrium – phenotypic evolution is
concentrated in brief events, followed by long
periods of evolutionary stasis.
• Natural Selection:
- the major process by which evolution occurs
- Darwin developed his theory of natural selection as
a series of five observations and three inferences:
Observation 1 – organisms have great potential
fertility.
- organsisms produce more offspring than can
survive
Observation 2 – natural populations normally remain
constant in size, except for minor fluctuations.
Observation 3 – natural resources are limited
Inference 1 – a continuing struggle for
existence exists among members of a
population (food, shelter, space)
Observation 4 – Populations show variations among
organisms (no two individuals are exactly alike)
Observation 5 – some variation is heritable (offspring
tend to resemble their parents)
Inference 2 – varying organisms show
differential survival and reproduction favoring
advantageous traits (= natural selection)
Inference 3 – over many generations, natural
selection generates new adaptations and new
species.
- long-term “improvement” of
populations
Neo-Darwinism
• The biggest weakness in Darwin’s theory was his failure
to identify the mechanism of inheritance.
- Darwin saw heredity as a blending phenomenon in
which the hereditary factors of parents blended
together in their offspring.
- Darwin also used Lamark’s hypothesis that an
organism could alter its heredity through use and
disuse of body parts and through the direct
influence of the environment
Neo-Darwinism
• August Weismann rejected Lamarkian inheritance by
showing experimentally that modifications of an organism
during its lifetime do not change its heredity.
• He revised Darwin’s theory
• We now use the term neo-Darwinism to identify Darwin’s
theory that has been revised by Weismann
Microevolution
• Microevolution is the study of genetic change occurring
within natural populations.
• Occurrence of different allelic forms of a gene in a
population is called polymorphism.
• All alleles in a population form the gene pool.
• The frequency of a particular allelic form of a gene in a
population is called its allelic frequency.
• Hardy-Weinberg Equilibrium:
- a hypothetical population will remain in genetic
equilibrium unless disturbed by recurring mutations,
natural selection, migration, nonrandom mating,
or genetic drift.
- if Hardy-Weinberg is occurring, evolution is not!!
- How genetic equilibrium is upset:
Genetic drift – random change in allele
frequencies (really affects small populations)
Nonrandom mating – if mating is nonrandom,
then genotypic frequencies will change.
- sexual selection
Migration – individuals leaving or coming into
a population will disrupt the allelic and
genotypic frequencies
Natural Selection – favorable traits are passed
on to the next generation, thus changing the
frequency of alleles and genotypes
- three types of natural selection
(stabilizing, directional, and disruptive)
Mutations – changes in DNA or genes can
change allelic and genotypic frequencies.
Macroevolution
• Evolutionary change on a grand scale, encompassing the
origin of novel designs, evolutionary trends, adaptive
radiation, and mass extinction.