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Workshop21_Principles_Fall2023_Notes (1).pdf
1. 7th Principle of Evolution: in a competitive population,
‘superior’ individuals contribute more to next generation.
The seventh principle, this one also first proposed by Darwin & Wallace, states
that in a population that is at carrying capacity…
…meaning there is intrapopulation competition…
…individuals that are superior competitors will survive & reproduce better.
In other words, in a competitive population, superior individuals produce more
offspring than inferior individuals, on average.
Note, this does not mean that all less competitive individuals will die without
reproducing.
Rather, the hypothesis is that over time, the benefits of superiority will influence
long-term patterns of reproductive success.
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2. Surplus reproduction & intragenerational competition
Surplus
Further, remember that surplus reproduction causes competition among
offspring in many populations.
This surplus usually occurs annually, following the reproductive season (see
workshop 20 preparation slides).
Thus, for many populations, competitive circumstances occur each year,
following the reproductive season.
In this example, Darwin & Wallace would have predicted that if different giraffe
parents are unequal & produce a surplus of young during the breeding season,
more offspring that exhibit beneficial traits inherited from more competitive
parents will survive.
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3. pictures.ezpics.net/
Hypothesis 7a: survival is not ‘random’
This seventh principle can be subdivided into two sub-principles or hypotheses.
The first hypothesis (7a) is that survival of offspring is not random.
Random events are unpredictable & do not seem to follow a definite plan or
pattern.
In other words, they happen for no reason.
The idea that survival is not random (i.e., happens for a reason) relates to the
concept of superiority, which we have already discussed.
Superior offspring (in this context) are those more likely to survive longer.
With regard to reproduction (Principle 7), it is important to note that an individual
must survive to adulthood to reproduce.
Offspring that die before adulthood never get the chance to reproduce.
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4. H7a re-stated: survival is ‘deterministic’
i917.photobucket.com
Another way to state hypothesis 7a is that survival in general (freak accidents
aside) happens for a reason.
This is to say that survival is deterministic, meaning that it is predictable.
In other words, deterministic events are ultimately determined by a detectable
cause.
Detectable - able to be discovered or identified (i.e., empirical).
The assertion is that if we can determine the traits that make an individual
superior…
…then we can use that knowledge to predict which individuals will survive
longer.
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5. Deterministic survival: given the ‘right’ knowledge,
survival is ‘predictable’
www.hdfinewallpapers.com/
In other words, if we know enough about giraffes to know what causes them to
die…
…& if we know the qualities of giraffes that reduce the risk of death…
…then we should be able to predict with some accuracy which offspring are
more likely to survive longer.
Any offspring that doesn’t survive to adulthood cannot contribute offspring to the
next generation.
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6. Thought question: use deductive reasoning to devise
study methods that could help determine factors
enhancing survival in giraffes.
img.burrard-lucas.com/
Thought question – consider how you might study factors that affect survival
probabilities in giraffes.
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7. http://www.gutenberg.org/
Inductive reasoning:
Feeding advantage (H1)?
Deductive reasoning:
Possible test?
www.pinterest.com
Generation X →→→→→→→→ Generation Y
Height Advantage Hypothesis
Many hypotheses regarding giraffes center around their impressive height.
First hypothesis:
Biologists have long believed that understanding the reason for the height of
giraffes would help explain how evolution works.
One popular hypothesis is that taller giraffes have better access to food high up.
This could give taller individuals a competitive advantage.
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8. U. Ill. Chicago –
BIOS 100 lecture
material online,
2004
Feeding advantage?
One study on this subject found that contrary to this hypothesis, giraffes did not
commonly feed at great height.
As shown here, in this study, giraffes tended to feed more at chest height.
This suggests even young giraffes should have ample feeding opportunity once
they reach the chest height of an adult.
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9. https://elements.envato.com/
Excessive feeding advantage?
In addition, it has been argued that a simple feeding advantage cannot explain
the massive height of giraffes.
Simmons & Altwegg (2010) report giraffes are 2.5 m taller than any potentially
competing browser.
Thus, a giraffe much shorter would still have a browsing advantage.
Note here, even a seated giraffe towers over sympatric herbivores.
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10. Falsification: goal of deductive reasoning, eliminate
unsupportable alternative hypotheses
www.thesafaricollection.com
Although this evidence doesn’t prove that taller giraffes do not have a foraging
advantage…
…it does weaken the feeding-height hypothesis as a complete explanation for
giraffe height.
More study is needed to determine the effect of feeding height on survival.
Further, other hypotheses could also be considered.
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11. 1056
Thomas C. Chamberlin
(1843 – 1928)
www.library.wisc.edu
Method of multiple working hypotheses –
• reduce bias
• Increase objectivity
Importantly, some researchers have argued that it is a best practice to always
employ multiple hypotheses.
We have already discussed the goal of scientists to avoid bias & remain
objective in their research.
Thomas Chamberlin (among others) argued that having multiple working
hypotheses in a research program is a good way to avoid bias.
The reason is, that researchers are less likely to become attached to a single
hypothesis & may keep a more open mind.
At the same time, because they are considering more options, researchers are
also more likely to eventually reach more valid findings.
12. Scientific method
• Multiple hypotheses
• Falsification
• Supportable hypotheses
remain for ongoing
study
Fig. 1.3
Your textbook (Chapter 1) outlines this approach.
In an ideal research setting, researchers begin an investigation with as many
hypothesis as they can devise.
Then, the research program works to eliminate (i.e., falsify) as many
hypotheses as possible to eventually reach the most supportable hypothesis.
This figure is an oversimplification, because in a real research program, there
are usually at least two additional complications.
First, as research progresses, new hypotheses are likely to arise, so the figure
should also show new hypotheses being added over time.
Second, in biology, which is very complex, multiple hypotheses may gain
support at the same time.
In such cases, full explanations of a phenomenon will require understanding all
supportable hypotheses & interactions among them (not just one hypothesis).
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13. 1058
Smartsheet.com
Science – iterative process:
sequential studies sustained to
approach goal, target, or result
As we previously discussed, an important feature of science is that it is an
iterative process.
This means science never stops moving forward.
Each good study adds new information & understanding.
But this also leads to additional questions in need of study.
Thus, the figure on the last slide (Fig. 1.3) is missing a place for hypotheses to
be revised as well as for new hypotheses to arise.
All but the simplest or most fortuitous studies will typically lead to revisions of
hypotheses &/or development of new hypotheses.
14. images.fineartamerica.com/
Hypothesis 7b (inductive reasoning): reproductive
success differs among mature individuals (adults).
▪ Health?
▪ Attractiveness?
▪ Fecundity?
▪ Offspring
health?
Thinking back to the 7th principle of evolution, which states that in a population
that is at carrying capacity, individuals that are superior competitors will
reproduce better…
…the second sub-principle (hypothesis 7b) is that for individuals that do survive
to adulthood, they do not all have equal reproductive success.
Individuals that reach adulthood & become reproductively mature are not all
equally good at reproduction.
They might differ in health, which can affect survival & reproductive ability.
They might differ in attractiveness to the opposite sex.
They might differ in fecundity (more fecund individuals are better able to
reproduce more offspring).
The health of their offspring might differ, affecting offspring survival.
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15. H7b re-stated:
breeding success
isn’t ‘random’
Nickgarbutt.com
Another way to state this hypothesis is that breeding success or failure in
general (freak accidents aside) happens for a reason (is not random).
Individuals that produce more offspring typically have some advantage.
Individuals that produce fewer offspring typically have some disadvantage.
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16. Given the ‘right’ knowledge,
breeding success can be
predicted (deterministic).
www.pinterest.com
Generation X →→→→→→→→ Generation Y
This is to say that breeding success is deterministic, meaning that it is
predictable, if we can identify the traits that favor it.
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17. What could we study to determine factors influencing
reproductive success (discuss)?
pixels.com
This sort of study would require studying the reproductive biology of giraffes.
All aspects of giraffe health would also be good to know, because general health
of any animal can strongly affect their breeding condition.
In modern biology, genetics would be central to this study, to determine heritable
traits that affect reproductive success.
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18. Generation X →→→→→→→→ Generation Y
Inductive reasoning:
Male to male combat (H3)
Deductive reasoning:
Possible test?
i.ytimg.com/
Height Advantage Hypothesis
With these factors in mind, we might consider other possible explanations for
superiority in giraffes, besides taller individuals feeding better.
Second hypothesis:
In giraffes (as in many ungulates), males fight each other for the right to breed.
Interestingly, male giraffes use their necks to fight (see photo)…
…so some researchers suggest that greater neck length (& overall height)
provides a breeding advantage.
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19. Simmons & Altwegg (2010)
Sexual
selection?
lintworm, 17 Sep 2019
Some evidence supports this hypothesis.
Here, the relation of neck mass to body mass for males (filled points) shows that
as males grow…
…the neck grows in mass more rapidly than the body.
In females (hollow points), neck mass increases at the same rate as body mass.
This suggests males grow larger necks for a purpose beyond that needed by
females.
This purpose could be to improve ability to fight other males.
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20. Simmons & Altwegg (2010)
https://www.express.co.uk/
More specifically, the heads of males are used as clubs when males fight each
other for reproductive dominance.
The photo shows one male swinging his head at another that is deftly avoiding
the head-head contact.
The graph shows that, as with necks, the heads of male giraffes gain mass at a
rate faster than overall body mass as individuals grow.
Again, this is also faster than observed for females.
This suggests males grow larger heads for a purpose beyond that needed by
females.
This purpose could be to improve ability to fight other males.
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21. c2.staticflickr.com
Sexual dimorphism
Female
Male
Thus, it appears there is support for sexual selection as a partial cause of male
neck & head mass.
However, with regard to this hypothesis, females also have long necks (although
the necks are smaller & grow at the same rate as the body).
Thus, some other hypothesis is needed to explain long necks in females.
It is likely that whatever can explain long necks in females would also apply to
males, even if long necks are also an advantage for male-male fighting.
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22. sciencing.com
Inductive reasoning:
Protective vantage point (H2)
Deductive reasoning:
Possible test?
Third hypothesis:
If height is a general benefit (potentially explaining the great height of giraffes)…
…another advantage of height for taller individuals could be better ability to see
over vegetation & other obstacles.
For example, it could be beneficial to spy predators or suitable forage from a
distance.
That is, greater height may provide a more protective vantage point or help
make giraffes more efficient foragers.
It may also help giraffes better avoid intrapopulation competition by allowing
them to see each other & space out efficiently.
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25. Science limitations: hypotheses cannot be “proven”
➢ requires replication, confirmation, & iteration
➢ best available information
➢ Multiple hypotheses could be valid
Marianne Morin (flickr.com)
This giraffe example helps illustrate another aspect of science as a discipline.
Science is always advancing & always building on existing knowledge.
But, at any point in time, science relies on the best available knowledge.
New findings & new techniques (often aided with new technology) can update
knowledge.
Further, as we have already discussed, it is common for multiple hypotheses to
have some validity.
For giraffes (for example), more research is still needed to fully understand the
reason for their long necks & how this relates to their reproductive success.
It is possible each of the presented hypotheses has some validity and that
additional factors also play a role.
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26. 8th Principle of Evolution: prevalence of ‘favored’ traits
increases from one generation to the next.
www.orbitz.com
The eighth principle of evolution follows logically from the seventh.
If individuals that have an advantage tend to be more successful breeders (on
average)…
…then the qualities that make them successful will increase in the next
generation.
This is because the offspring of favored individuals will compose a
relatively high proportion of the next generation.
And, some (or many) of these offspring will have inherited some of the superior
traits that made their parents more successful.
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27. Favored: endowed with
special advantages or gifts
en.wikipedia.org
Can favored traits guarantee
reproductive success?
Darwin used the term ‘favored’ to express the idea of superior versus inferior
individuals within a population.
Recall the analogy Darwin drew with artificial selection.
In selective breeding, human breeders favor certain qualities.
By controlling which individuals reproduce, breeders manipulate qualities that
will be most prevalent in offspring.
By comparison, in nature, niches that populations occupy favor certain qualities.
Thus, in natural selection, the niche plays the role of a selective breeder,
influencing which individuals have greater success than others.
Nevertheless, it is important to remember that nature is complex & messy.
It’s still possible for a superior individual to die without reproducing due to an
accident or bad luck.
However, on average, it is expected that superior individuals will succeed better.
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28. Advantage: condition or
circumstance creating a favorable
or superior position
www.pinterest.com
Increased probability of
reproductive success
In other words, the advantage gained by individuals with favored qualities does
not guarantee their success, but does increase their probability of success.
Probability – the likelihood of something happening or being the case.
In a population of hundreds or thousands of individuals, this means that many
advantaged individuals will succeed (even if some do not).
Similarly, individuals at a disadvantage may also succeed if conditions are not
too harsh, but they will succeed less, on average.
That is, compared to favored individuals, disadvantaged individuals will be less
likely to reach adulthood.
Further, disadvantaged individuals surviving to adulthood will breed less often as
adults &/or have fewer offspring when they breed.
Further, the offspring of disadvantaged individuals will likely to survive at a lower
rate than offspring of favored individuals.
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29. Principles of Evolution
1. Animal types exist
2. Types form populations & fill niches
3. Populations are composed of unique individuals*
4. Individuals are not equals
5. Superiority depends on niche
6. Overpopulation = intraspecific competition
7. ‘Favored’ individuals survive & reproduce better
8. ‘Favored’ traits increase from generation to generation
IMPLICATIONS
These eight principles add up to the crux of the theory of natural selection
proposed by Darwin & Wallace, independently but at the same time, in 1858.
The key emphasis of the theory is a focus on individuals within populations.
Populations evolve based on variation among individuals (Principle 3).
That is, if it is true that populations do contain unique individuals, then principles
4-8 are reasonable inferences that follow.
Because of individual variation, the traits present in a population can change
over time (across generations)…
…based on the relative reproductive success of favored over disadvantaged
individuals.
Note, principles in green (1-4) were proposed before Darwin & Wallace, but
formed the basis for their theory.
Principles in blue (5-8) were new insights proposed by Darwin & Wallace.
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30. Beak shape
a) Heritability
Ian & Kate Bruce
Fig. 21.2
We have already looked at examples that illustrate how populations can adjust
to their niches over time (recall the stickleback & guppy studies from last class).
Here is another example from your textbook.
As we have studied in lab, beak size in songbirds is related to the size of seed a
bird can eat.
In class, we have also already seen how this occurs within Galápagos finches.
Within a population of Galápagos finches (shown here), individuals have slight
differences in beak depth.
Further, beak depth is a heritable trait, meaning parents with bigger beaks have
offspring that also have bigger beaks (graph).
Points on this graph indicate pairs of parents (X axis) with their offspring (Y
axis).
Given this information, we can predict that if one beak depth is favored over
another in a given place & time…
…individuals with the favored depth will survive best & reproduce more
successfully.
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31. Beak shape
b) Natural
selection
(precipitation)
Ian & Kate Bruce
Fig. 21.2
Wet period
Continuing with this example, on this graph, points indicate generations.
More specifically, each point represents mean beak depth in a generation (error
bars indicate variation within the generation).
Interestingly, mean beak depth is not constant, but changes from one generation
to another.
Note, the red boxes indicate periods in which precipitation was either
exceptionally high or low.
In dry years (left box), larger seeds became the predominant food supply, so
deep-beaked individuals were favored.
As a result, beak depth increased from one generation to the next in this period.
Thereafter, the population persisted for several generations with relatively deep
beaks.
However, in wet years (right box), small seeds became abundant, which
changed the situation so that shallow-beaked individuals were favored.
Thus, during the wet period, beak depth decreased & eventually returned back
to where it began at the start of the study.
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32. 1077
Camouflage &
natural selection
-
Timema cristinae
Farkas et al. 2013
In another example, the stick insect Timema cristinae has two color-pattern
morphs.
One color pattern (striped, A) is most abundant on the shrub Adenostoma
fasciculatum (C).
The other color pattern (solid, B) is most abundant on the shrub Ceanothus
spinosus (D).
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Timema cristinae
Solid color pattern
AARON COMEAULT
The solid color pattern provides great camouflage on the shrub Ceanothus
spinosus!
35. 1080
Montejo-Kovacevich et al. 2020
This figure shows the landscape in which Timema cristinae was studied (right).
The host plants are interspersed along the hillside.
Note the person standing on the lower-right center.
The figure on the left provides an indication of how plants were distributed.
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Farkas et al. 2013
Timema cristinae
Predation risk
Jerry McFarland/ Flickr (CC-BY-NC-2.0)
Studies have shown that camouflage is crucial for Timema cristinae to avoid
predation.
In the study illustrated here (right), solid green individuals were transplanted to
Adenostoma fasciculatum (upper right).
Transplants were susceptible to predators like the California scrub-jay
Aphelocoma californica (pictured).
Attraction of predators to the plants with transplanted individuals increased
overall predation there.
In comparison (lower right), Adenostoma fasciculatum with striped individuals
that are well camouflaged…
…had little predation on T. cristinae or on other arthropods.
Thus, the alternative color patterns are strongly specific to alternative host
shrubs (Adenostoma fasciculatum or Ceanothus spinosus).
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Nosil et al. 2018
Timema cristinae
Negative
frequency-
dependent
selection
Further, constant high predation threat from birds creates the scenario that the
more abundant color pattern of Timema cristinae becomes a target for
predators.
This negative frequency-dependent selection means that whichever color
pattern becomes more abundant…
…receives greater predation pressure.
Here, Nosil et al. (2018) introduced individuals of both patterns to the plant
Adenostoma fasciculatum to track their survival.
Based just on camouflage, this test should have favored the striped pattern.
Indeed, striped individuals did survive well when they composed 20% of the
population (red bars).
However, when striped individuals were 80% of the population (black bars),
survival was low despite their better camouflage.
This is because their overall abundance attracted predators that then focused on
them as prey, even though individuals were relatively difficult to detect.
Thus, an advantage of maintaining two color patterns is that a population can
spread out among two species of plant, reducing the likelihood of either color
becoming a focus of predators.
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Nosil et al. 2018
Timema cristinae
Frequency-
dependent
population
fluctuations
Over 18 years, Nosil et al. (2018) saw fluctuations of striped individuals on both
host bushes (see graph)…
…although striped individuals were most abundant on Adenostoma
fasciculatum.
Whenever the striped portion of the population increased, this increase evidently
increased predation.
Heightened predation in turn led to a decrease in the striped morph.
Because the measure shown is a frequency within the population versus the
solid color pattern…
…the solid color pattern showed the opposite fluctuation trend.
That is, at times when the striped morph was decreasing, the solid morph was
increasing & thus facing increasing predation pressure.
40. Natural selection
sorts from
individual variation!
They key insight that Darwin & Wallace each had, independently, was that
ecological conditions naturally sort out which individuals are superior or
inferior based on how well an individual’s qualities (i.e., traits) suit them for
success.
This is why individual variation is key, because without individual variation,
there can be no natural selection.
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41. 1086
Natural selection summary / review
clarkscience8.weebly.com
This figure provides a nice summary of natural selection.
This shows the step-by-step process of natural selection.
Study carefully.
Review previous slides (& textbook) as needed.
42. Thought question: describe the step-by-step process of
natural selection (see example).
Figs. 21.3 & 21.4
Pennsylvania
Michigan
For review, practice describing the step-by-step process of natural selection
(last slide):
The peppered moth is a textbook example of natural selection (pp. 446-447,
Chapter 21).
Study this example as presented in your textbook.
Also review the previous slide.
Then describe the step-by-step process of natural selection.
Because there is much confusion about natural selection, it is important to
develop a clear understanding.
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43. 1088
Unit VII: inheritance & diversification of life
cdn.isciencetimes.com/
We have already discussed how science always moves forward.
Although the breakthrough made by Darwin & Wallace was revolutionary, it was
only a gateway for evolutionary research.
Modern understanding of natural selection & evolution has dramatically
advanced since 1858.
Many, many advances have been made & many things that were unknown in
the 19th Century now are known.
New understanding still occurs & will continue for the foreseeable future.
44. ‘Principles’ following Darwin &
Wallace:
Modern understanding of
inheritance & selection
For the remainder of the semester, we will learn about advances made since
Darwin & Wallace.
As we go, we will also continue to study the basic fundamentals of animal
biology.
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45. 15 decades of research + advancing technology:
connect natural selection & inheritance
www.yalescientific.org/
Importantly, more than 150 years have now passed since 1858.
Consider how technology & understanding have changed!
One of the key mysteries that existed in 1858 was that of inheritance.
Anyone could see that inheritance existed, certainly selective breeders saw it.
However, no one had a clue how inheritance occurred!
This was an important stumbling block to full understanding of natural selection
& evolution.
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46. 1091
cdn.c.photoshelter.com/
www.goodfood.com.au/
USFWS Mountain-Prairie
19th Century biologists already knew about fertilization of gametes – eggs &
sperm.
Fertilization – process by which male & female gametes are fused together,
initiating the development of a new organism (offspring).
They knew that mixing gametes (shown here for walleye) produced zygotes
(fertilized eggs) that would develop into offspring.
Without magnification or chemical analysis, this was far as researchers could
get in their understanding.
47. 9th Principle of Evolution: traits pass from parents to
offspring through gametes
www.fishpondinfo.com
The knowledge that new offspring are produced from gametes was very
important.
This focused researchers on understanding how parents can pass their traits to
offspring through gametes.
In the wild, fertilization is observable for many species that fertilize eggs
externally.
Here is an example with toads.
Thus, this 9th principle of evolution became a research focus in the late 19th
Century & into the 20th Century.
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48. 1093
http://2.bp.blogspot.com
Friedrich Miescher
(1869)
Discovers nucleic acids
en.wikipedia.org/
In the time of Darwin & Wallace, chemists were already at work.
Miescher is the biochemist credited with isolating & naming the macromolecules
responsible for inheritance – nucleic acids.
We have already learned what nucleic acids are & how they are built.
However, at the time of their discovery, the role of nucleic acids in inheritance
was not yet known.
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www.goodreads.com
Edouard Van Beneden (1875) Oskar Hertwig (1876)
alchetron.com
Observe
details of
fertilization
At the same time, researchers were beginning to study fertilization.
Van Beneden was first (1875) to report that the nucleus of a sperm is released
into an egg…
…& that the sperm & egg nuclei fused to form the new nucleus for the zygote.
Zygote - a diploid cell resulting from the fusion of two haploid gametes; a
fertilized ovum.
The following year, Hertwig made similar observations, but in more detail.
He pioneered the use of sea urchin eggs, which were easy to observe.
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www.invitra.com
A critical observation was that the sperm nucleus fuses with the egg nucleus.
This observation helped focus researchers on the cell nucleus as the source of
inheritance.
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upload.wikimedia.org/
Albrecht Kossel
(1885-1901)
isolates & names nitrogenous
bases: adenine, cytosine,
guanine, thymine, & uracil
Meanwhile, after their discovery, people began to study nucleic acids.
Over a period of years, Kossel led the research that discovered the
nitrogenous bases of nucleic acids.
We are already very familiar with these from studying DNA & RNA.
Remember, nitrogenous bases determine the behaviors of different
nucleotides.
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upload.wikimedia.org
Carl Correns
(1900)
Hypothesizes chromosomes
linked to heredity
Is genetic material proteins or
nucleic acids?
At this time, Correns was one of the first to suggest chromosomes were
somehow linked to heredity.
Recall, chromosomes of many organisms can be seen under a light microscope
(as you saw in lab).
Thus, early biologists were able to view chromosomes before they could view
finer details within cells.
However, as you have learned, chromosomes are composed of DNA &
protein.
Nobody knew whether, if chromosomes did confer inheritance, it was through
DNA, protein, or both.
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Virus structure: protein & DNA
library.cshl.edu/
A.D. Hershey & Martha Chase
Fig. 26.3
This uncertainty remained for 50 more years until Hershey & Chase (right)
devised a way to figure it out.
These researchers studied viruses.
Virus anatomy is relatively simple (see figure).
Viruses are composed of protein & DNA.
Hershey & Chase realized this simple model could provide a means to
determine which of the two macromolecule types (protein or nucleic acid)
conferred inheritance.
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Viral cycle separates protein from
DNA
Fig. 26.6
National Geographic
In the life cycle of a virus, there is separation of DNA from protein.
A virus does not have a chromosome, so the two macromolecules are not
intertwined.
We now know that when a virus infects a cell, it injects DNA into the cell.
The protein sheath stays outside.
The study of Hershey & Chase revealed this information.
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SpringerLink
Hershey & Chase (1952):
• DNA includes P, absent in
protein
• Protein includes S, absent in
DNA
Waters Corporation
At the time of their study, Hershey & Chase did not know which was which, but
they determined a way to find out.
They knew (as you have learned) that DNA includes phosphorous, which is
present in the phosphate group of every nucleotide (upper figure).
In contrast, they also knew that protein includes sulphur because it is present
in certain amino acids (lower figure)…
…including methionine, which is typically the beginning link in a polypeptide.
However, DNA does not include S & protein does not contain P!
This created a potential distinction that could be used to determine which
macromolecule viruses injected into cells!
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simple.wikipedia.org
Hershey & Chase (1952): radioactive P (DNA) or S (protein)
Hershey & Chase used radioactive isotopes of these two elements, which they
could later detect, to label the DNA & Protein.
Recall our definition of an isotope:
Isotope: A form of a chemical element in which the atoms have the same
number of protons but a different number of neutrons.
Radioactive isotope: The nucleus is unstable & decays, emitting radiation, these
are often the heavier isotopes, such as 35S & 32P
Hershey & Chase grew some viruses in a medium with radioactive
phosphorous to label the DNA (top row).
They grew others in a medium with radioactive sulphur to label the protein
(bottom row).
For each virus strain, they then allowed the viruses to infect bacteria so that they
could determine whether protein or DNA was injected into the bacterial cells.
58. 1103
Karen Steward
Hershey & Chase (1952):
viral inheritance
Hershey & Chase knew that whichever substance was injected into the bacteria
caused the development of new viruses within the cell (see life cycle).
Hence, whichever macromolecule made bacterial cells radioactive would clearly
be the macromolecule of inheritance.
Presumably, this would also be the macromolecule of inheritance for other
organisms.
59. 1104
Infected bacteria contain P: DNA = genetic material!
Fig. 14.2
Once they had allowed infection to occur, Hershey & Chase put each bacterial
culture into a blender.
Blending shook the sheaths off the cells, separating the sheaths from the cell
material.
Centrifuging spun down the cell contents, separating them from the fluids that
held the sheaths.
Hershey & Chase could detect radioactivity.
In the experiments using radioactive sulphur & normal phosphorus, they
detected radioactivity in the fluid (supernatant).
This indicated the virus sheaths were made of protein & because the sheaths
were not injected into bacterial cells…
…protein was not the macromolecule of inheritance!
In the experiments using radioactive phosphorous & normal sulphur, they
detected radioactivity in the bacterial cells (centrifugal pellet)…
…indicating DNA was injected into the bacterial cells.
In other words, DNA was shown to be the macromolecule of inheritance for
a virus!!
60. 1105
www.vedantu.com
Although prior studies had suggested DNA was the macromolecule of
inheritance…
…the Hershey & Chase study provided the first more widely accepted support
for the role of DNA in inheritance.
This key breakthrough did not occur until almost 100 years after Darwin &
Wallace proposed the theory of natural selection!
Importantly, this discovery occurred only 71 years ago (1952)!!
In other words, only the last two generations of biologists & doctors (or so)
have conducted research with the understanding of what molecules
confer inheritance!!!
Think about all of the advances in biology & medicine that have happened since
then.
Scientific understanding is powerful!
