How Populations Evolve - 2.files.edl.io
Transcript of How Populations Evolve - 2.files.edl.io
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Lectures by Chris C. Romero
PowerPoint® Lectures for
Essential Biology, Third Edition
– Neil Campbell, Jane Reece, and Eric Simon
Essential Biology with Physiology, Second Edition
– Neil Campbell, Jane Reece, and Eric Simon
CHAPTER 13
How Populations Evolve
Figure 13.1
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Charles Darwin and The Origin of Species
• Charles Darwin’s On the Origin of Species by
Means of Natural
• Two concepts: “descent with modification” and
natural selection.
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• The basic idea of natural selection is that:
- Success in reproduction leads to an accumulation
of favored traits in the population over generations
- Darwin based this off of:
- Species tend to produce excessive numbers of
offspring (only a small percentage will survive)
- The variation among individuals of a population
Example: The evolution of pesticide resistance in
hundreds of insect species
Figure 13.2
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• The Darwinian revolution in its historical context
1831-36: Darwin travels around the world on the
HMS Beagle
1837: Darwin begins his notebooks on the origin of
species
1844: Darwin writes his essay on the origin of species
1858: Wallace sends his theory to Darwin
1858: The Origin of Species is published
Grand Canyon
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• Jean Baptiste Lamarck was a French naturalist who
suggested that the best explanation for the
relationship of fossils to current organisms was that
life evolves.
• He explained evolution as a process of adaption.
(evolutionary adaption)
• He also proposed that acquired characteristics are
inherited.
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The Voyage of the Beagle
• Darwin sailed from Great Britain on the Beagle in
December 1831.
• Main mission: chart poorly known stretches of the
South American coastline.
Figure 13.4
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• On his journey on the Beagle, Darwin
– Collected thousands of specimens.
– Observed various adaptations in organisms.
– The plants and animals living in temperate
regions of South America seemed more closely
related to species living in tropical regions of that
continent than to species living in temperate
regions of Europe.
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• Darwin was intrigued by the geographic
distribution of organisms on the Galapagos Islands.
Figure 13.5
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Descent with Modification
• Darwin made two main points in The Origin of
Species:
• Descent of diverse species from common
ancestors.
• Natural Selection as the mechanism of Evolution
Figure 13.6
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Evidence of Evolution
• Fossils
• Biogeography
• Comparative Anatomy
• Comparative Embryology
• Molecular Biology
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The Fossil Record
• Over millions of years, deposit piles up and
compress older sediments.
• If organisms that have been swept into the water
die, could be compressed too and may leave
imprints into the rock.
Figure 13.7
Figure 13.8
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Comparative Anatomy
• Comparison of body structures between different
species.
• Certain anatomical similarities show signs of
evolutionary history.
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• Homology: similarity due to common ancestry.
• Arms, forelegs, flippers, and wings of different
mammals are variations on a anatomical themes
that has become adapted to different functions.
Figure 13.11
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Comparative Embryology
• Comparative Embryology- The comparison of
structures that appear during the development of
different organisms
Figure 13.12
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Molecular Biology
Molecular Biology says:
• If two species have genes with nucleotide
sequences that match closely, they might have been
inherited from a common ancestor.
• The greater the number of sequence differences,
the less likely the share a common ancestor.
Figure 13.13
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Natural Selection
• Darwin studied finches that lived on different
Galapagos islands and concluded that:
• The finches began to differ from one another
because they adapted to their environment.
o Different beak shapes (adapted to specific foods).
o Might become dissimilar enough to be classified
as different species.
Figure 13.14
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Natural Selection in Action
• Examples of natural selection include
– The evolution of pesticide resistance in hundreds
of insect species: a pesticide does not create
resistant individuals, but selects for resistant
insects that were already present in the
population.
– The development of antibiotic-resistant bacteria:
doctors have recently documented an increase in
drug-resistant strains of HIV.
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Three General Outcomes of Natural Selection
• Directional selection-occurs when the environment
changes in a particular way, there is therefore a
selective pressure for species to change in response
to the environmental change.
– Populations do not decide to adapt to the change
in environment, the adaptations, mutations and
combination of alleles must already be present.
– species shifts the phenotypic curve of a
population by selecting in favor of extreme
phenotype.
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• Disruptive (diverging) selection- occurs when the
environmental changes produces selection
pressures which favor the two extremes of a
characteristic.
• Example: sickle cell anemia – in areas where
malaria does not exist, it is a disadvantage but in
parts of the world where malaria is common,
sickle cell anemia is advantageous. Therefore
the population in malaria affected areas have a
higher proportion of the sickle cell anemia allele
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• Stabilizing selection – this occurs when the environment doesn’t
change, natural selection doesn’t have to cause change. There
are many species which are thought to have remained the same
over the course of millions of years
• Maintains variation for a particular trait within a narrow
range
• Occurs in a relatively stable environment to which
populations are already well adapted
• This evolutionary conservatism works by selecting against
the more extreme phenotypes.
Example: Stabilizing selection keeps the majority of human birth
weights between 3 and 4 kg
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin CummingsFigure 13.28
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Sources of Genetic Variation
• Mutations: a mutation in a gene may substitute one
nucleotide for another, but the change will be
harmless if it does not affect the function of the
protein the DNA encodes.
• Sexual recombination: plants and animals depend
on sexual recombination for the genetic variation
that makes adaption possible. The two sexual
processes of meiosis and random fertilization
shuffle alleles and deal them out to offspring in
fresh combinations.
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Mechanisms of Microevolution
• The changes in allele frequencies that occur over time within a
population
• The main causes of microevolution are:
– Genetic drift- a change in the gene pool of a small
population due to chance
– Gene flow- A population may gain or lose alleles when
fertile individuals move into or out of a population or
when gametes are transferred between populations
– Mutations- Changes in an organism’s DNA.
– Natural selection- The offspring with the best traits will
survive and reproduce
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Genetic Drift
• Can cause a population to shrink down. A change
in the gene pool of a population due to change.
• where gene drift occurs happens in 2 ways.
– Bottleneck Effect
– Founder Effect
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin CummingsFigure 13.22
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The Bottleneck Effect
• Genetic drift resulting from a drastic reduction in
population size.
• Ex: disasters killing people, producing a small
surviving population with different genetic makeup
as the original population.
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin CummingsFigure 13.23
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin CummingsFigure 13.24
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The Founder Effect
• Random change in the gene pool that occurs in a
small colony of a population.
• Effect contributed to evolutionary divergence of
the finches on the Galapagos Islands.
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Genetic Drift and Hereditary Disorders in Human Populations
• 1854 – 15 people found a British colony on Tristan
da Cunha.
• One of the colonists carried a recessive allele called
retinitis pigmentosa
• Of the 240 descendants who still lived on the
islands in 1960’s, 4 had retinitis pigmentosa and 9
were known to be carriers
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Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin CummingsFigure 13.25
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Gene Flow
• The gain or loss of alleles from a population by the
movement of individuals or gametes into or out of
the population.
• Another source of evolutionary change.
• Gene flow tends to reduce differences between
populations.
• Has become an important agent of evolutionary
change in previously isolated human populations.
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Mutation
• Changes in an organism’s DNA.
• A mutation transmitted in gametes can immediately
change the gene pool of a population by
substituting one allele for another.
• For one gene locus, however, mutation does not
have much effect on the population.
• Over the long term, mutation is, in itself, essential
to evolution because it’s the original cause of
genetic variation.
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Natural Selection: A Closer Look
• Of all causes of microevolution, only natural
selection promotes adaptation.
• All individuals in a population is supposed to
equal in their ability to survive and reproduce
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Darwinian Fitness
• It’s not about “survival of the fittest” but survival
to sexual maturity. Production of fertile offspring is
the only score that counts in natural selection
Example: The biggest, fastest, toughest frog in the
pond has a fitness of zero if it’s sterile.