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Evolution by Natural Selection
• Struggle for Existence
– Members of each species compete for food,
space, and necessities of live.
– Predators that can run faster catch more prey
– Prey that are better camouflaged avoid being
caught.
Evolution by Natural Selection
• Survival of the Fittest
– Survival depends on how well-suited an
organism is to its environment
– Fitness: ability to survive and reproduce
– Adaptation: any inherited characteristic that
increases an organism’s chance for survival.
– Darwin called survival of the fittest natural
selection
– Underwater Astonishments
Evidence of Evolution
• Fossil Record
– Diff. layers of rock
form a different times
in the Earth’s history.
– Fossils show gradual
change over time.
Evidence of Evolution
• Biogeography
– Finches found all over the world evolved
different adaptation to survive in their habitat.
– Finches on the Galapagos evolved from a
mainland species.
– Darwin found that animals in similar habitats
evolved similar characteristics.
Evidence of Evolution
• Homologous Body Structures
– Structures that develop from the same
embryonic tissue but have different mature
forms.
– Suggest evidence that organism descended with
modification from a common ancestor.
Evidence Evolution
• Similarities in Early
Development
– Early embryo stages of
many vertebrates look
very similar.
– The same groups of
embryonic cells,
develop in the same
order, in similar
patterns, to produce
homologous structures.
Significance of biochemical
differences
• Vestigial structures – structures that have no
function in the living organism but may have
been used by its ancestors.
• Ex: human appendix,
python leg bones
•DNA evidence – the more similar the
genes are, the more closely related
organisms are thought to be.
•Direct observation – We’ve seen
evolution occur in cases like:
•Bacteria becoming resistant to
antibiotics
•Insects that become resistant to
pesticides
17.3 The Process of Speciation
• Species - group that can interbreed and produce fertile
offspring.
• Speciation – the formation of new species, occurs
whenever reproductive isolation develops
17.3 The Process of Speciation
Reproductive isolation develops by:
• Behavioral Isolation
• Geographic Isolation
• Temporal Isolation
Behavioral Isolation
• Two populations are capable of interbreeding
• Differences in mating rituals prevent interbreeding
• Ex: Similar birds will not interbreed b/c of
different mating songs
Geographic Isolation
• 2 populations are physically separated by
barriers
– Rivers
– Mountains
– Bodies of water
Temporal Isolation
• Two or more species reproduce at different
times
• Ex: One form of cicada emerges every 17
years, the other emerges every 13 years.
17.3 The Process of Speciation
How do new species evolve?
1. Founders arrive – A
few finches travel from
South America to one of
the islands. There, they
survive and reproduce.
2. Geographic Isolation – Some
birds from species A cross to a
second island. The two
populations no longer share a
gene pool.
17.3 The Process of Speciation
3. Changes in the Gene
Pool – Seed sizes on the
second island favor birds
with larger beaks. The
population on the second
island evolves into a
population, B, with larger
beaks.
4. Behavioral isolation – If a few
population-B birds cross back to
the first island, they will not mate
with the birds of population A.
The gene pools are now separate.
Populations A and B are separate
species.
17.3 The Process of Speciation
5. Ecological Competition –
As species A and B compete
for seeds on the first island,
they continue to evolve. A
new species, C, may evolve.
6. Continued Evolution –
The process continues,
leading to the formation
of all 13 finch species on
the Galapagos.
Chapter 17: Evolution of Populations
17.1 Genes and Variation
Populations and Gene Pools
• Population – a group of individuals of the same species
that mate and produce offspring
• Gene pool – all genes and the alleles for those genes
present in a population
• Allele frequency – the number of times an allele occurs in
a gene pool compared to the total number of alleles in that
pool for the same gene.
17.1 Genes and Variation
Populations and Gene Pools
• Take home message: Evolution, in genetic terms, involves
a change in the frequency of alleles in a population over
time.
• Note: Although natural selection acts on individuals it is
the population that evolves, not individuals
17.2 Evolution as Genetic Change in Populations
Genetic Drift
• Genetic Drift = random change in allele frequency
• Acts on small populations
• These chance occurrences can cause an allele to
become more or less frequent in a population
17.2 Evolution as Genetic Change in Populations
Genetic Bottlenecks
• Genetic Bottleneck = a change in allele frequency
following a dramatic reduction in the size of a
population
• Could result from disease, rapid climate or
environmental change
• Can reduce a populations genetic diversity
17.2 Evolution as Genetic Change in Populations
Founder Effect
• Founder Effect = when allele frequencies changes as
a result of the migration of a small subgroup of a
population
• Creates a new gene pool that could be different from
the parent population
17.1 Genes and Variation
Single-Gene and Polygenic Traits
• Single-Gene Trait
– Controlled by one gene
that has two alleles
– Two distinct
phenotypes
– Ex: bands or no bands
on snails
• Polygenic Trait
– Controlled by more
than one gene
– Many possible
genotypes and
phenotypes
– Ex: Human Height
Stabilizing selection
Directional selection
Disruptive selection