Transcript Lecture 15

Evolution
The Process of Evolution (Variation, Heredity
and Differential Reproduction)
Important things to recall
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DNA
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deoxyribonucleic acid, the
macromolecule, composed of two
polynucleotide chains in a double
helix, that is the carrier of genetic
information in all cells
Genes
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Heredity unit containing specific
genetic information. A gene can
mutate to various forms called alleles.
Alleles
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Any of the alternative forms of a given
gene
Allele Frequency
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The relative proportion of all alleles of
a gene that are of a designated type.
DNA (gene) → RNA → protein → trait
Definition of Evolution
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Any process of change over time.
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Classically
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A change in the relative frequencies of heritable
traits within a population across generations.
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Modern
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A change in the distribution of relative
frequencies of genes (which code for heritable
traits) within a population across generations.
Classical Evolution
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A change in the relative frequencies of heritable traits within a
population across generations
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Relative frequency:
50% Tall plants
50% Short plants
55 generations later
25% Tall plants
75% Short plants
Modern Evolution
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A change in the
distribution of relative
frequencies of genes
(which code for heritable
traits) within a
population across
generations
Differential Survival of Traits
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Differential survival of traits in a population means that some
characteristics will become more frequent while others occur less
or are lost.
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There are four known processes that affect the survival of a
characteristic; or, more specifically, the frequency of an allele:
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Mutation – error in duplication
Genetic drift - stochasticity
Gene Flow – movement of genes from pop. to pop.
Natural selection
The production and redistribution of variation is produced by
three of the four agents of evolution: mutation, genetic drift, and
gene flow. Natural selection, in turn, acts on the variation
produced by these agents.
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Mutation directly changes gene frequencies
Mutation occurs in the trait
An example of a spontaneous mutation
during the development of plant leaves
For evolution to occur via this
mechanism, what has to be true of the
mutation?
The mutation has to be
heritable
The mutation of fruit flies with four
wings is an inherited mutation
The population size is small
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Genetic drift – random fluctuations in the allele
frequencies
Only 5 of
10 plants
leave
offspring
Generation 1
p (frequency of R) = 0.7
q (frequency of r) = 0.3
Only 2 of
10 plants
leave
offspring
Generation 2
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0
Gene flow
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Immigration or emigration occurs based on the trait
Microevolution
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Microevolution is the occurrence of small-scale
changes in gene frequencies in a population over a
few generations, also known as change at or below
the species level.
Mechanisms of Evolution (cont.)
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Natural Selection
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Natural selection, the last of the four forces, is based on
three principles:
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(a) there is VARIATION within a species and this
variation is HERITABLE
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(b) parents have more offspring than can survive
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(c) surviving offspring have favorable traits
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The mechanism by which it operates is termed survival of
the fitter meaning differential mortality and fertility.
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Differential mortality is the SURVIVAL rate of
individuals before their REPRODUCTIVE AGE. If they
survive, they are then selected further by differential
fertility – that is, their total genetic contribution to the
next generation.
VARIATION
HERITABLE
SURVIVAL TO REPRODUCE
Darwin’s Observations
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Biogeography
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Agriculture
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Evidence of organisms no longer
present
Evidence that many living
organisms were not present in the
past
Geology
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Selective breeding
Fossils
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It is the study of distributions of
organisms, both past and present,
and of related patterns of variation
over the earth in the numbers and
kinds of living things
New ideas about the age of the
Earth via plate tectonics
Economics
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Competition for resources and the
effects of overpopulation
Evolution by Natural Selection
(a mechanism of evolution)
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Population level:
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If variation exists
and
If variation is heritable
and
If differential reproduction (differential selection) exists
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Then over time, those variations that enhance the ability of
the organism to reproduce will increase in any population
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Natural Selection leads to ADAPTATION
Adaptation
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A biological adaptation is an anatomical structure,
physiological process or behavioral trait that has
evolved over a period of time by the process of
natural selection that increases the likely hood of
producing larger numbers of offspring or its
reproductive success.
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A heritable characteristic of an organism that helps it
to survive and reproduce in a particular
environment.
Adaptation vs. Acclimatization
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Evolutionary biologists refer to an adaptation as a trait
that evolved as the result of natural selection.
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Physiologists use “adaptation” when they mean
“acclimatization” –
 A nerve “adapts” to a continual stimulus or
 our eyes “adapt” to darkness
does not involve genetic change!
Adaptation refers to traits that are heritable
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Acclimation: Changes in the structure or
physiology of an individual over its lifetime
Examples:
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Increasing muscle
mass via weightlifting
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High altitude acclimation:
But, neither of these involves genetic change!
Human example of an adaptation:
Sickle Cell Anemia
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In North America, sickle cell
anemia is uncommon and a
disadvantage.
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However, in other parts of the
world, specifically areas where
malaria is common, the
occurrence of sickle cell anemia
is greater.
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Why?
Sickle Cell Anemia
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Because the heterozygous (SS, Ss, ss) Sickle Cell Anemia
genotype gives a higher resistance to malaria, however a
homozygous genotype is still a disadvantage.
Sickle Cell Anemia
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Because the heterozygous (SS, Ss, ss) Sickle Cell Anemia
genotype gives a higher resistance to malaria, however a
homozygous genotype is still a disadvantage.
DNA (gene) → RNA → protein → trait
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Flattened tail – aids in
swimming
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Long, sharp claws – Aid in
clinging to rocks
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Salt gland – Allows
drinking of salt water
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Diving adaptations. While
diving they:
 Reduce blood flow to
body surface- helps
retain heat
 Lower metabolic rate –
conserves O2
Imperfect Adaptations
trait
performance
fitness
feeding
trait
defense
fitness
thermoregulation
Multi-tasking may force an evolutionary compromise
Can’t simultaneously optimize distinct functions.
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Adaptation – organisms are not perfectly
adapted ( have to make due with your
genetics)
 Examples: panda thumb
All traits evolve from something else, so carry
historical baggage thus can be “contrivances” (make
do!)
Homologous
structures which have a common origin.
Divergent Evolution
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Divergent evolution is the
process of two or more related
species becoming more and
more dissimilar.
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The red fox lives in mixed
farmlands and forests, where
its red color helps it blend in
with surrounding trees.
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The kit fox lives on the plains
and in the deserts, where its
sandy color helps conceal it
from prey and predators.
Convergent Evolution
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Convergent evolution is an evolutionary process
in which organisms not closely related
independently acquire some characteristic or
characteristics in common, or the evolution of
species from different taxonomic groups toward
a similar form.
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This usually reflects similar responses to similar
environmental conditions.
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Structures that are the result of convergent
evolution are called analogous structures or
homoplasies; they should be contrasted with
homologous structures which have a common
origin.
Wings