An overview of lines of evidence for evolution
(or evolution in a nutshell)
1. Populations and species change over
time = evolution happens.
Learning objectives:
To assess types of evidence for evolution,
including:
2. All species are related through common
ancestry = the tree of life connects
living and extinct species.
1. Evidence of change through time
2. Evidence of shared ancestry
3. Evidence for the action of natural selection
Background reading: Chapters 2-3 in Freeman
Up next: Population Genetics (review Hardy-Weinberg)
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Major contributions of Darwin’s work:
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Evolution Defined
3. Evolution happens through the action
of natural selection = differences in
survival and reproduction among
individuals lead to changes in the
characteristics of populations.
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1. Evidence of change through time
Evolution: Change in allele frequencies through
time, OR changes in the traits of a species
through time, OR descent with modification
The Fossil Record
Fossils are traces of organisms that lived in the past.
Where do fossils form?
A few things to keep in mind (or they will
haunt you):
•
•
•
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Evolution is not directed or directional - change
does not imply improvement.
Natural selection is not the only force that can
bring about change in allele frequencies.
Sometimes, change happens by chance.
Example:
The fossil record provides evidence of change in the form
of extinctions, and evidence of transitions in structures
through time.
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Patterns of Change Through Time:
Extinction
Patterns of Change Through Time:
Extinction
Extinction in the Fossil Record
Fossils provide evidence for the existence of forms of life
that have not existed in historic times.
The simple existence of fossils indicates that the
distribution, abundance and composition
of the fauna and flora has changed
through time.
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Extinction and the ‘law of
succession’
The ‘law of succession’, first noted
by 18th century paleontologists,
describes a pattern in which fossils
tend to be found in the same
geographical areas as their extant
relatives.
Example:
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Patterns of Change Through Time:
Transitional Forms
While the closest modern day
terrestrial relatives of whales
belong to the group of mammals
that include cows and hippos,
the fossils record provides a
clear picture of the transition
from terrestrial to aquatic
living.
Note the gradual loss of
terrestrial limbs, general change
in form, including skull shape.
Most compelling is the existence
of transitional forms, fossils that
have traits intermediate between
older and more recent species.
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Transitional
Forms in Whales
In some groups, the sequence of
fossil forms provides clear signs
of change through time.
Fabulous examples exist in the
fossil records of horses and
whales.
Reconstruction of an extinct
giant kangaroo from fossil
evidence from Australia
Horse feet
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Evidence of change through time:
Vestigial Traits
2. Evidence of shared ancestry
Some fossil and living organisms show evidence that their
ancestors once possessed functional traits that no longer
function (or partially function) in living representatives.
These vestigial traits include such things as the reduced
wings of ostriches and kiwis, the human tailbone,
‘goosebumps’.
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Similarity often reflects common
ancestry:
Given that organisms change
through time, the similarities
between some groups of
organisms is often found to
reflect shared recent common
ancestry. When we study
patterns of DNA variation, we
find evidence of common
ancestry between similar forms
that our intuition often suggests
are closely related.
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Evidence of Relatedness:
Homology
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Evidence of Relatedness:
Structural Homology
Homology
Homology is a powerful concept that describes shared
similarities in structures, DNA sequences, behaviours,
developmental sequences, due to shared common ancestry.
Evidence of shared homology is pervasive in nature: e.g.
the fact that life forms share the same genetic code
(with a few exceptions), that eukaryotes have DNA
organized into chromosomes, that eukaryotes share basic
cell structures, that vertebrates with limbs all have four
limbs, etc. are all examples of homologies.
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DNA-based
phylogeny of the
carnivores
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Structural Homologies
Structural homologies can be seen in a host of structures
across the tree of life. For example, in the limbs of
vertebrates.
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Molecular evidence of homology:
The genetic code
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Molecular evidence of (non-)
homology: developmental biology
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Pax6 controls eye development in
animals from flat worms to humans!
see figures 3.23, 3.25, 4.6
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Evidence of Relatedness:
Genetic Homology
Genetic Homologies
Genetic homologies exist for a number of genetic regions.
The human genetic condition aniridia (in which the iris is poorly
developed),
as well as the mutant phenotypes small eye in mice and
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eyeless in Drosophila are all caused by mutations in Pax6.
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(You can read more
about the effects of
variation in Hox and
HOM genes and their
role in the
diversification of
animals in section 19.2
and Fig. 19.3 of
Freeman)
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Adaptation
(noun)
3. Evidence for natural selection
Natural Selection: The process whereby some
individuals contribute more offspring to the next
generation as a consequence of possessing some trait
or traits that enhance their ability to survive or
reproduce.
Artificial Selection: The process of selection whereby
certain traits in an organisms are considered favorable
and are selectively bred by humans (humans decide
who survives and reproduces).
We’ll next go through an example of each from recent
literature.
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head
thorax
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Dispersal in island plant populations – Cody & Overton 1996
When we see the spines on a cactus, we understand that
these structures would help deter herbivores. The cryptic
coloration and shape of a katydid (below) is also clearly
adaptive.
A evolutionary biologists, we ask what evidence we can
gather to show the steps involved in producing these
adaptations?
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Evolution in Natural Populations
Evolution in action
Evolutionary change can
be seen in human
lifetimes in nature and
in artificial experiments.
Traits that enhance
an organisms
fitness are called
adaptations. Many
traits in nature are
clearly adaptive,
for example, there
is a dead leaf
praying mantis in
this image.
• Studied weedy, wind-dispersed
plants located on the islands off
the west coast of Vancouver
Island
• Censused the plant populations
of 200 islands and a region of
the mainland over a 10-year
period
• Extinction and recolonization
events occurred frequently on
the islands.
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Evolution in Natural Populations
Dispersal in island plant populations – Cody & Overton
1996
Studied two species:
A BC example: change in dispersal
ability of plants on islands
The two species under study have fruits that are
adapted for wind dispersal.
Hypochaeris radicata
Lactuca muralis.
Individuals with a relatively higher pappus
pappus
volume/ seed weight) are better dispersers
(their seeds will travel farther), and may have
an advantage in colonizing islands, but
may be at a disadvantage once a population
is established on a small island.
William S. Justice @
USDA-NRCS PLANTS Database
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© Markku Savela
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A BC example: change in dispersal
ability of plants on islands
They found that old island
populations had decreased
dispersal ability relative to
the mainland populations.
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They also saw that
newly founded
populations had higher
dispersal ability than
both mainland and older
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island populations.
Seed22
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Speciation and selection?
(An example of artificial selection in the lab)
While there have been numerous demonstrations of the
action of natural selection in nature and in the lab, the
theory of evolution by natural selection also implies that
natural selection plays a role in the birth of new species
(speciation). Contemporary evidence (natural or
experimental) for this is more challenging to acquire.
Speciation is harder to catch ‘in action’, because it often
happens over much longer time scales.
Provisional definition of species (until we revisit the topic
later in the term): A species is an interbreeding group of
organisms that is reproductively isolated from (does not
interbreed with) other groups of organisms.
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Selection and speciation?
Selection and speciation?
Habitat selection in Drosophila
(Rice & Salt 1988, 1990)
Habitat selection in Drosophila
(Rice & Salt 1988, 1990)
• Two papers by Rice and Salt (1988, 1990) impose artificial selection
on populations of fruit flies (Drosophila melanogaster) to test the
hypothesis that selection on habitat preferences can lead to the
development of reproductive isolation, a key step in speciation.
• The researchers set up an artificial habitat in which flies were
free to choose a habitat. Flies were found to take at least 8-12
hours to make three choices about the habitat where they would
reproduce. The choices involved light (light or dark), direction (up
versus down), and smell (ethanol or acetaldehyde). They could
choose any combination of these three features of the
environment. In addition, researchers selected flies based on
whether they developed early (10 days) or late (14 days).
• They asked whether over successive generations, flies would evolve
a preference for a given habitat type, and thus show evidence of
the beginning of reproductive isolation.
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• Flies were attached to the maze as
pupae, and upon emergence as adults
they entered the maze and chose
Up/ down
choice
Acetaldehyde/
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choice
• Experimenters choose time period:
early (E), middle (M), and late (L) –
selection for development time
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Habitat selection in Drosophila
Habitat selection in Drosophila
•
Experimental larvae were mixed
and placed together in the maze
to start the next generation.
• Flies mated within the maze
•
• Control lines: 120 females chosen
randomly
Controls were run through the
maze separately.
•
Offspring of mothers collected
from 5E and half of the controls
were raised on a chemical that
turned their eyes brown. This
allowed the researchers to track
any change in habitat preference
over successive generations).
• Selected lines – 60 females each from:
• dark, up, acetaldehde, early (5E)
• light, down, ethanol, late (4L)
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– lightness or darkness (left
vs. right) = selection for
phototaxis
– up or down = selection for
geotaxis
– acetaldehyde (white vial) or
ethanol (black vial) =
selection for chemotaxis
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% of flies with mothers from 5E
(as measured by the % of flies
at 5E and 4L with brown eyes)
Results of Habitat selection in
Drosophila
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Dotted line: 5E
Solid line: 4L
Control lines
5E
4L 5E
5E
Selected lines
4L
1
Generations
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The flies have
accumulated
differences in
habitat
preference
that lead to
reproductive
isolation = the
start of
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speciation!
Results of Habitat selection in
Drosophila
These results show that over time, the flies in the experimental lines
evolved stronger habitat preferences, that led to reproductive
isolation. Because mating occurred near the food vials (after the
flies had chosen a habitat), flies from 5E tended to mate with other
Control
flies from 5E, just because they shared
thelines
same habitat choice.
Over time, this lead to a stronger tendency for the offspring of flies
5Echoices.
collected at 5E to repeat their mother’s habitat
Note that when flies from 5E and 4L were mixed together for mating
trials outside the maze, they showed no mating preferences - the
mating patterns was entirely driven by habitat choice.
4L
5E 5E
4L
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