Download Natural selection

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Sylvia S. Mader
Immagini e
concetti
della biologia
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
B4 Microevolution
and
macroevolution
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Population genetics
A gene pool is the sum of all the alleles in a population.
Microevolution is evidenced by allele frequency changes
within the gene pool.
Hardy-Weinberg equilibrium describes microevolution in
non-evolving populations.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Population genetics
Ex.: a population of 100 turtles (trait: neck length)
Homozygote LL = 36%; Heterozygote Ll = 48%; Homozygote ll = 16%
Allele frequencies
L frequency = (36 + 36 + 48)/200 = 120/200 = 0.6 L
l frequency = (48 + 16 + 16)/200 = 80/200 = 0.4 l
Punnett square for the II generation
p2 + 2pq + q2 =1
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Hardy-Weinberg principle
G.H. Hardy and W. Weinberg (1908):
“it is possible to calculate genotype and allele frequency of
a population using the formula p2 + 2pq + q2 = 1”.
Microevolution does not occur (Hardy-Weinberg
equilibrium) if the following conditions are satisfied:
•no mutations
•no gene flow
•random matings
•no natural selection
•no genetic drift
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Hardy-Weinberg principle
Generally all frequencies do change between generations
and microevolution (Hardy-Weinberg disequilibrium) does
occur.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Hardy-Weinberg principle
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Genetic variation
Both mutations and sexual recombination produce genetic
variation.
Mutation rate is very low: 1 mutation every 105 cell
divisions. Mutations are the primary source of genetic
differences in prokaryotes.
Random matings and gene flow (movement of alleles
between populations due to migration) help microevolution.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Natural selection
In stabilizing selection extreme phenotypes are negatively
selected, intermediate phenotypes are favored.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Natural selection
Stabilizing selection helps maintain harmful alleles.
The case of the sickle-cell disease
Heterozygote advantage causes sickle-cell allele to be
maintained, even though the homozygous recessive is lethal.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Natural selection
In directional selection an extreme phenotype is favored.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Natural selection
In disruptive selection two or more extreme phenotypes
are favored over the intermediate one.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Genetic drift
Effects of the random changes in allele frequencies in a
gene pool (genetic drift) are unpredictable.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Genetic drift
Two mechanisms are important in genetic drift:
•Bottleneck effect (for species close to extinction) prevents
the majority of genotypes from participating in formation of
the next generation.
•Founder effect occurs when rare alleles by the founders
occur at high frequency in an isolated population.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
New species and biodiversity
Speciation occurs when:
•one species splits into two or more species;
•one species becomes a new species over time, as resulted
from the changes in the allele frequencies in the genetic
pool.
Macroevolution depends on speciation.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Evolutionary
species concept
Every species has its own
evolutionary history and a
species can be recognized
by diagnostic traits.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Biological species concept
Members of a same species are reproductively isolated
from the members of other species. They can only
reproduce with members of their own species.
Similar phenotype but different species
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Reproductive barrier
Reproductive barriers contribute to maintain genetic
differences between species.
Prezygotic isolating mechanisms prevent reproductive
attempts.
•Geographic isolation: species live in different habitats.
•Temporal isolation: species reproduce at different
periods.
•Behavioral isolation: different courtship mechanisms.
•Mechanical isolation: incompatibility due to size or
morphology.
•Gametic incompatibility: gametes transfer but do not
form zygotes.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Reproductive barrier
Postzygotic isolating mechanisms prevent hybrid
offspring from breeding.
•Zygotic mortality: eggs are fertilized, but the zygote
does not develop.
•Hybrid sterility: hybrid zygote develop, but the resulting
adult is sterile.
•F2 sterility: although hybrids are fertile, further hybrid
generations (F2) are inviable or sterile.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Reproductive barrier
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Geographic barrier
Ernst Mayr (1942) described
the allopatric speciation.
“Populations separated by
geographic barriers will
differentiate genetically and
phenotypically”.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Geographic barrier
Adaptive radiation
A single ancestral species may
evolve into several new species
showing different phenotypic
traits with which they adapt to
different environments.
Hawaiian honeycreepers
Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
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Non-geographic barrier
Sympatric speciation (without geographic isolation)
mostly occurs in plants, as they develop a condition called
polyploidy (more than two sets of chromosomes).
Polyploid individuals are reproductively isolated as they
cannot reproduce with parental 2n plants.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Non-geographic barrier
Autoploidy
Occurs when an haploid gamete fuses with a diploid
gamete, resulting in a triploid plant, which is sterile.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Non-geographic barrier
Alloploidy
Occurs when two different but related species of plants
hybridize and the chromosome number doubles.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Gradual or rapid speciation
Gradualistic model: speciation occurs due to gradually
changing environmental conditions.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Gradual or rapid speciation
Punctuated equilibrium model: periods of equilibrium are
interrupted by rapid speciation.
If the environment changes rapidly, new species suddenly
arise.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Speciation in the history
Burgess Shale fossils represent Precambrian marine life 540 MYA.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Adaptation
Evolution proceeds for adaptations to the environment.
Natural selection is opportunistic, not goal-oriented.
Species able to adapt to the changing environment have
more offsprings.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Artificial selection
Teosinte was the ancestor of modern corn. Thousand of
years of artificial selection has changed the species.
Today corn is allotetraploid which accounts for the size.
Teosinte (Zea mexicana)
Modern corn (Zea mays)
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Development genetics and speciation
Eyes development, limb development and shape
determination are controlled by the same genes in different
headed animals.
Pax6 gene is involved in the eyes development of many different animals
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Evolution
Differential gene expression can cause
dramatic changes in body shape and organs.
Differential gene expression and/or new
functions for old genes can explain
evolution, including human evolution.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012