Download Principles of Heredity

Evolution
A Scientific Explanation for Similarities
and Differences between Organisms
Definitions
• Evolution is a change in the characteristics of a
population over time
• Two characteristics identified by Darwin’s work
– Descent with modification: species diverge from
a common ancestor
– Natural selection: individuals with favorable traits
accumulate within a population
• Evolution is a change in the genetic
composition of a population from generation to
generation
– Can include mechanisms other than natural
selection such as genetic drift and gene flow
Populations evolve as natural selection acts on its individuals.
Prior to Darwin
• Prevailing view that species were unchanging
– Aristotle proposed a “scale of nature”, that
organized fixed species from least to most
complex
– Special Creation as the origin of species was
reinforced by religious thought and interpretation
of scientific evidence, such as the classification
scheme of Linnaeus
• Differing views of the history of the earth as
seen in the fossil record
– Catastrophism: events in past occurred suddenly
and are unlike mechanisms of today
– Uniformitarianism: mechanisms of change are
constant over time
Evolutionary Thought Before Darwin
•
Lamarck developed ideas for changes
among species
– Use and disuse – Individuals retain
characteristics that are required for their
survival, leads to adaptation
– Inheritance of acquired traits – Individuals
inherit traits that were developed over the
lifetimes of their ancestors
– Innate drive for species to become more
complex
A Flowchart of Evolutionary Reasoning
Variations exist
between members
of a population
Relatively constant
resources and
population over time
At least some
variation is hereditary
More offspring are
born than survive and
reproduce
observations
Individuals whose
inherited traits give a
higher probability of
survival and reproduction
leave a greater number of
offspring
inferences
Favorable traits will
accumulate in the population
over generations
2
Identify the inference
that shows
1. Evolution
2. Natural Selection
1
Darwin’s
Observations
Islands with saddle
shell giant
tortoise had tall
prickly pear cacti.
Adaptation:
characteristic that
favors survival
and reproduction
Darwin’s Observations
Galapagos finches had beak shapes suited to
food sources on specific islands.
Cactuseater
Seed-eater
Insect-eater
Darwin’s Observations
Galapagos finches resembled the
grassquit found on the coast of Ecuador.
Recent studies suggest that the finch
ancestor may have originated from the
Caribbean islands.
Explaining the long neck of a giraffe
Giraffes stretched
their necks to reach
food on tall trees and
passed the longer
necks onto their
offspring
Lamarck versus Darwin
Evolution as a Change in Genetic Composition
Natural selection led to an increase in
dark colored peppered moths in
industrial regions of Britain.
How is this trait inherited?
• Wing color is the result of a single gene with two
alternative forms called alleles
• One allele codes for dark color, the other codes for
light color
• Each moth inherits two alleles for wing color
• Only one dark allele is needed for the moth to have
dark wings since the dark color allele is dominant
over the recessive light color allele
• Two light color alleles are required for a moth to have
light wings
• If birds remove light winged moths from the
population, dark winged moths survive and pass the
dominant allele for dark wings to their offspring
• The increase in frequency of the dark wing allele is
evidence of evolutionary change
How is this trait inherited?
Phenotype
Genotype
(trait)
(inherited alleles)
Dark moth
DD
Dark moth
Dd
Light moth
dd
Homozygous
(identical alleles)
Heterozygous
(different alleles)
homozygous
The homologous chromosomes carrying alleles of
the same gene separate during Meiosis I so that
each gamete receives only one of the two alleles.
How is this trait inherited?
Genotype for Dark moth
Genotype for Light moth
Dd
D
d
dd
d
Dd
dd
d
Dd
dd
Genotype for Dark moth
Genotype for Dark moth
Dd
D
d
50% dark moths
50% light moths
Dd
D
DD
Dd
d
Dd
dd
75% dark moths
25% light moths
Birds remove half
of light wing moths
dd
Dd
Dd
dd
Reproduction occurs
dd
dd
Dd
Dd
dd
dd
dd
dd
dd
Dd x dd 50% Dd + 50% dd
Dd x dd 50% Dd + 50% dd
dd
dd
dd x dd  0% D_ + 100% dd
Total Dd = 100/300 = 33.3%
dark wing moths
dd
20% dark wing
moths
Simulating
Natural Selection
33.3% dark wing
moths
Natural selection acts on the phenotype.
Evolution is the resulting change in genotypes within the population.
Evidence for Evolution: Artificial Selection
Evidence for Evolution: Adaptation
Object Mimicry provides camouflage
for evading predators
Treehoppers
Walking stick
Evidence for Evolution: Direct Observation
Increase in guppy coloration with predator change
Evidence for Evolution: Direct Observation
Resistance of HIV virus to drug therapy
Evidence for Evolution: Fossil Record
Living organisms resemble extinct fossil forms
Evidence for Evolution: Fossil Record
Progressive changes can be seen from
simpler to more complex organisms
Evidence for Evolution:
Comparative Anatomy
• Structures that do not show a common
origin
– Analogous structures: superficially similar
• Structures that show a common origin
– Homologous Structures: same
evolutionary origin despite differences in
function
– Vestigial Structures: serve no purpose but
are homologous to structures in related
organisms
Analogous and Homologous Structures
Not homologous;
analogous
Not homologous;
not analogous
Homologous;
analogous
Homologous;
not analogous
Homologous Structures
Flying
Swimming Running Grasping
Vestigial
Structures
Remnants of
hindlimb seen in
boa and whale
Functional hindlimb
in salamander
Vestigial Structures
Evidence for Evolution: Developmental Biology
Evidence for Evolution: Biochemistry
Molecular similarities between different organisms
Evidence for Evolution: Molecular Biology
Similarities in sequence measured by ease of
separating hybrid DNA strands by heat
Evidence for Evolution: Genetics
• Mutation generates diversity
• Meiosis and Fertilization generate new
combinations due to
– Crossing Over
– Alternate patterns of chromosome
segregation
– Unique chromosomes of fertilizing
sperm combined with unique
chromosomes of egg
Evidence for Evolution: Biogeography
Plants and animals of each continent are distinctive
Evidence for Evolution: Convergent Evolution
Unrelated organisms in similar environments
show some of the same adaptations
Evidence for Evolution:
Convergent Evolution
Yellow-Throated
Longclaw of Africa
Cactus of
North America
Euphorb of
Africa
Meadowlark of
North America
Other Patterns: Divergent Evolution
Different phenotypes arise as related species
encounter environmental differences
Other Patterns: Coevolution
Interacting species adjust together
to maintain a relationship
Predators
and their Prey
Flowering plants
and their Pollinators
Applying Your Knowledge
1. Adaptation
2. Homologous Structures
3. Analogous Structures
4. Divergent Evolution
5. Convergent Evolution
A. Leads to similar traits in unrelated
species
B. Traits that have a common origin
C. A trait that makes a species
survival more likely is called a(n)