Download Theory of Evolution and Natural Selection

Natural Selection and Evolution
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Std/Obj
 Know why natural selection acts on the
phenotype rather than the genotype of an
organism.
 Know how natural selection determines the
differential survival of groups of organisms.
 Know a great diversity of species increases the
chance that at least some organisms survive
changes in the environment.
 Know new mutations are constantly being
generated in a gene pool.
 Know why alleles that are lethal in a homozygous
person may be carried in a heterozygote and thus
maintained in a gene pool.
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Evidence for Evolution
(change over time)
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Fossil record
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2.
Earth is older than
previously believed
Mechanisms of heredity
–
modern genetics

molecular evidence
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–
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All cells have DNA,
RNA, ribosomes, the
same 20 amino acids,
and use ATP as an
energy carrier
DNA similarities
ex. Human and Chimp
phylogenetic trees
Diff in # aa
Human beta chain
0
Gorilla
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Gibbon
2
Rhesus monkey
8
Dog
15
Horse, cow
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Mouse
27
Chicken
45
Frog
67
Sea slug
127
Soybean
124
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Phylogenetic Tree
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3. Comparative
Anatomy
Homologous structures
•
Have same
evolutionary origin,
with similar
structure but
different function.
Analogous structures
•
Have different
structure and
similar function,
but different
evolutionary origin
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4.
5.
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Contributers
 Carolus Linnaeus - developed a classification of plants
and animals
 Jean-Baptiste Lamarck - first scientist who put his beliefs
into a system that attempted to explain evolution.
 Thomas Malthus - concerned with relationship of human
population growth to food supplies.
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Charles Darwin (1809-1882)
 Evolution is descent with
modification
 Ship’s naturalist on the HMS
Beagle (1831-1836)
 Fossils of Patagonia: similarity
of fossil and living forms
(e.g., armadillos).
 Fauna of Galapagos Islands:
each island had similar
physical conditions but distinct
species of mockingbirds,
tortoises.
divergence in isolation
from a common ancestor
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Theory of Evolution by Natural Selection
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5.
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All species produce offspring at a faster rate than food
supplies can increase.
Except for identical twins, no two individuals are exactly
alike.
In each generation, more individuals are produced than
can survive.
Individuals that possess certain traits have an advantage
over those that do not.
The environment determines whether a trait is beneficial.
Traits are inherited and passed on to the next generation.
Over time, successful variations accumulate in a
population, and a new species may appear.
Geographical isolation may lead to the formation of a
new species.
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Fitness
 Selection is variation in the average reproductive
success among different phenotypes.
 Fitness is a quantitative measure of reproductive
success.
 Selection is variation in average fitness among
different phenotypes.
 Phenotypes will be differentially affected by
selection if they have consistent (average)
differences in fitness.
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Population genetic models of
selection
 Usually assume that the phenotypes are
completely determined by the genotype.
 They therefore typically assign fitness
values directly to genotypes.
 Sophisticated pop. gen. models don’t make
this assumption, and neither do quantitative
genetic models.
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 Natural selection acts on individuals, but the
consequence is a change in the population phenotypic
distribution
 Natural selection acts on phenotypes, but changes
the genotypic distribution (without underlying
genetic variation, evolution will not occur)
 Each generation is the product of selection on their
parents
 Natural selection acts on existing traits. New traits
arise as a modification of old features and in
combination with new mutations
-adaptations
 Natural selection is non-random, but not progressive
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Darwin vs Lamarck
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Agents of Evolutionary Change
 Mutation
– Mutation rates are generally so low they have little effect on
proportions of common alleles.
 ultimate source of genetic variation
 Gene flow
– movement of alleles from one population to another
 tend to homogenize allele frequencies -% of any specific allele in the gene
pool (all alleles of a population’s genes)
 Nonrandom mating
– assortative mating - phenotypically similar individuals mate
 Genetic drift – statistical accidents
– Frequencies of particular alleles may change by chance alone.
 important in small populations
– founder effect - few individuals found new population (small allelic pool)
– bottleneck effect - drastic reduction in population, and gene pool size
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Genetic Drift - Bottleneck Effect
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 Selection – Only agent that produces adaptive
evolutionary change
– artificial - breeders exert selection
– natural - nature exerts selection
 variation must exist among individuals
 variation must result in differences in numbers of
viable offspring produced
 variation must be genetically inherited
– natural selection is a process, and evolution is an outcome
– Selection pressures:
 avoiding predators
 matching climatic condition
 pesticide resistance
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Interactions Among Evolutionary Forces
 Levels of variation retained in a population
may be determined by the relative strength
of different evolutionary processes.
 Gene flow versus natural selection
– Gene flow can be either a constructive or a
constraining force.
 Allelic frequencies reflect a balance between gene
flow and natural selection.
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Natural Selection Can Maintain Variation
 Frequency-dependent selection
– Phenotype fitness depends on its frequency
within the population.
 Negative frequency-dependent selection favors rare
phenotypes.
 Positive frequency-dependent selection eliminates
variation.
 Oscillating selection
– Selection favors different phenotypes at different
times.
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Heterozygote Advantage
 Heterozygote advantage will favor
heterozygotes, and maintain both alleles
instead of removing less successful alleles
from a population.
– Sickle cell anemia
 Homozygotes exhibit severe anemia, have abnormal
blood cells, and usually die before reproductive age.
 Heterozygotes are less susceptible to malaria.
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Sickle Cell and Malaria
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Forms of Selection
 Disruptive selection
– Selection eliminates intermediate types
– Leads to the evolution of 2 new species
 Directional selection
– Selection eliminates one extreme from a phenotypic
array
 Stabilizing selection
– Selection acts to eliminate both extremes from an array
of phenotypes
– Can reduce variation in pop
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Patterns of Evolution
Divergent evolution-the process of two or more
related species becoming more and more dissimilar
ex. Adaptive Radiation-an ancestral species evolves
into an array of species to fit a number of diverse
habitats
-a type of birds in Hawaiian Islands have the
same body size and shape, but different color
and beak shapes
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Rates of Evolution
 Large populations evolve very slowly or almost not at all.
– Advantageous mutations pass very slowly through a pop.
 Small populations can evolve very rapidly.
– Advantageous mutations can be passed very quickly through the
population.
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