Charles Darwin – Life & Times, Influences & Friends 09/09/2013
Born 1809 in Shrewbury, England – died 1882
History of science/medicine/money in family
Father – physician and investor
Mother – daughter of Josiah Wedgewood (member of Lunar Society), a wealthy investor
Grandfather – Erasmus Darwin (17311802): a physician, inventor, member of Lunar Society, and published
work on enlightenment and improvement
Published “Zoonomia”: defended early evolutionary theory which according to all life (plant and animal
origin) originated from primitive “filaments”
Had innate tendencies for selfimprovement over time
Similar to JeanBaptiste de Lamarck (17441829)
Charles didn’t think highly of the book
Didn’t until University years realize how much money he was going to inherit – hard for him to want to
continue to learn – worried that he was going to be a disgrace to the family after his father berated him
about not being able to focus.
Enjoyed hunting, sporting, and hobbies of the upper class
Became a Medical student at Edinburgh
Member of the Plinian Society: small studentrun group which met “underground” to read papers on natural
science and discuss them.
Introduced to Lamarck’s work: transformisme /transmutationism
Species capable of infinite change over time
Organisms have internal drive towards adapting to their environment
Changes made during one organisms lifetime can be inherited by their children
Species changes change’s over time can be track along in par with changing
Decided medicine wasn’t for him (weak stomach/no interest) – urged by his father to the Clergy
Went to Christ’s College, Cambridge University
Didn’t learn squat there besides from William Paley
Argued intelligent design Life was so complex that some greater being must have been creator.
Other Friends at Cambridge
Didn’t go to Sedgwick’s geology lecture
Befriended Henslow – Angelical priest & professor of Botany
William Whewell – Philosopher of science
Met lots of other men who hunted for sport and shit
Accompanied geologist Sedgwick on a geological trip to North Wales – left early to go hunting
Invited by Capt. Fitzroy to accompany him as the ship’s “naturalist” on a voyage on the Beagle – last 5
Left England in 1831
Back to London
Gave talks at geological society
Published “Journal of Researches”: an account of the voyage
Read some philosophy
Studied and organized his journals.
Journals contained a lot about geological, anthropological, botanical, zoological, philosophical, and
Taken as a whole, it suggested a transmutationist view of natural history
Read “Essay on the Principle of Population” by Malthus
Pops increase geometrically while food supplies increase only arithmetically – pops outstrip the resources –
restraint on reproduction
Conditions preserve favorable dispositions over unfavorable ones – overtime led to the formation of a new
Sitting on the Theory of Natural Selection
Had idea but afraid of prejudicing his thought by committing to it
Wrote sketch of the idea in 1842 – expanded by 230 pages by 1844 but didn’t publish it Made fellow of the Royal Society (1839)
Married his cousin, Emma Wedgewood
Moved to Down House
To ill for the city – moved to the country
Spent 8 years studying barnacles – finished in 1854
Cement his role as a naturalist
Catalogue the vast variations of their structure
Distress caused by his theory because his wife was a devout Anglican
Hostile reactions from previous papers published on transmutationist theories
Wallace developed his own theory of evolution that was very similar to Darwin’s – drove Darwin to finally
Semi shared credit – Wallace mostly gave Darwin the credit
Waited out the debates
Publication of the Descent of Man
Was an opponent of evolutionary ideas
Changed by the 1860 – became a defender of Darwin’s theory
Published “Evidence of Man’s Place in Nature”
Advance acceptance of evolution
Criticized the book published before Darwin on evolution
Universe doesn’t change
Verbally attacked Huxley in a debate on Darwinian theory
Died April 19 1882, at the age of 73
Buried at Westminster Abbey Chapter 1 – The Origin of Species 09/09/2013
“Variations under domestication” Chapter 1 – The Origin of Species 09/09/2013
Darwin argues that there is a genealogical pattern of relationships among the different varieties of particular
groups of domesticated plants/animals
Argues that the different varieties each had a distinct (wild) ancestor type.
Hoping to set up an analogy b/w artificial selection and natural selection
If his genealogical view is correct about natural selection – demonstrates what accumulative selection can
Produce variety species from common ancestors
Needs to show that there is sufficient variation in nature for natural selection to operate upon
Argues against the traditional view that little variation occurs in a species/type – rather continual variation
(structural, behavioral, etc.)
He begins with the claim that older domesticated plants and animals exhibit more variability than
plants and animals in nature
Setting up his view about what causes variations to occur
No idea about mechanism of inheritance – only observations
Thought that variations were caused due to pressures on the reproductive organs/systems of organisms
When under domestication – subjected to different “conditions of life” – affect reproduction
Requires several generations under new conditions of life before variation can be seen
Once begun, generally will continue for many generations
Does not cease
Oldest cultivated plants (wheat) yield new varieties
Oldest domesticated animals are still capable of being improved/modified
“variability caused by reproductive elements being affected prior to the act of conception
domestication introduces new “conditions of life” Chapter 1 – The Origin of Species 09/09/2013
plants that grow vigorously but don’t seed
His conclusion – variants result in some way from a perturbed reproductive system
More evidence due to perturbed reproduction system
Presents of “sports” in plants – sudden variants from the parents
Common view of the time
More common in cultivated plants than in nature
“Conditions of life” affect pollen or parental ovule.
Doesn’t want to overstate the effects of “Conditions of life”
Laws of reproduction of growth and of inheritance that are largely determinate of variation
If conditions of life were largely responsible for variation, we would see the same variation in all offspring,
but siblings differ from each other.
Darwin appeals to the Lamarckian mechanism of use and disuse (“habit”) as a source of variation
Points to ducks’ wings weigh less and the bones more than wild ducks
Udders of domesticated cows/goats grow larger in areas whe=re they are habitually milked.
Droopy rabbit ears – don’t need to be alert
Are such differences acquired through “habit” (like Lamarck thought?)
If not, wouldn’t they be relevant to Darwin’s argument for natural selection
Argument for the heritability of variations
Darwin notes that his theory can only work on variations that can be inherited, and then tries to convince
the reader of the abundance of such variability
Notes that rare variations occurs in a parent and then reoccurs in the offspring, so improbable that we
suppose its inherited
Strange structures can be inherited then common ones can be inherited. Noninheritance = anomaly
Inheritancemechanism (mystery for Darwin’s time)
Child reverts to trait of the grandfather or gmom
Some traits passed to some sex, some to both Chapter 1 – The Origin of Species 09/09/2013
Some only appear in adolescents
Laws regulation variation
Correlation of growth: change in one part of organism is correlated w/ changes in another, seemingly
Blue eyed cats are often deaf
White sheep are affected by poisons that don’t bother black sheep
Mystery but in Darwin’s time acknowledged as causally relevant to variation
Reversion as an argument for immutability of species
“Reversion to parental type”: when domesticated varieties revert to ancestral form when “turned loose”
Casts doubt on the very occurrence of reversion by noting the lack of controlled experiments on it:
The organisms would have to be kept in unchanging conditions, w/ enough of them together to allow free
Even if “reversion” to an ancestral type happens, it is natural selection that is responsible
Variants of the same domesticated species = different species in the same genus in nature
Similarity 1: Different variants in the same domesticated species often differ from one another in some
dramatic way w/ respect to one particular feature.
Same things observed in nature among closely related species of the same genus
Similarity 2: Different variants of the same domesticated species are often classed by expert as mere
varieties and by another expert as a descendant of ancestors from another species. Same thing occurs in
the classification of species w/in the same genus.
Conclusion: “If any marked distinction existed b/w domestic races and species, this source of doubt could
not so perpetually recur”
Argues that “doubtful forms” (hard to classify as genuine species) are more the rule than the exception
Set up his argument that there is no clear line b/w varieties and species
Argues that domesticated varieties of a given type are related by common descent.
Sets this up as a model for the relationship he believes holds b/w varieties and species in nature
An opponent might concede that varieties come into being through “crossing” of two distinct species
But, we need something more than mere crossing to get the great variety of new species that we observe.
To get a new breed intermediate b/w species would take long and careful process of selection Chapter 1 – The Origin of Species 09/09/2013
Himself a breeder of pigeons – received specimen of bones and skin from correspondents around the
Gives his reasons for thinking that varieties of pigeons have descended from the rock pigeon a common
view among naturalists in his day, but uncommon among pigeon fanciers
Concludes by appealing to the incredulity of pigeon fanciers and cattle breeders with respect to the claim of
common descent as a caution to those who would too quickly dismiss the idea of common descent of
species in nature.
Darwin now turns to the process of selection to investigate how the various races of domesticated species
have been produced
Different conditions of life and traits/behaviors acquired by habit might be responsible for some variation
We can’t account however, in this way for the differences b/w ex) the carrier pigeon and the tumbler pigeon
or the work horse and the race horse.
We know in many of these cases that these useful traits were deliberately bred – Distinct breeding
The key to the existence of these different varieties is “man’s power of accumulative selection: nature gives
successive variations; man adds them up in certain directions useful to him.”
In addition to deliberate, methodical selection, there is also unconscious selection
The result of every dog owner trying quite naturally to get the best dog she can and afterwards breeding her
own best dogs w/o wishing to change the breed in any way
This process, over/continued through centuries, would improve and modify any breed.
Circumstances favorable to human’s power of selection
High degree of variability
Large population size (increases the chance for a useful variant to arise)
Favorable “conditions of life” so that the breeder isn’t forced to allow all the animals to breed (preventing
That the plant or animal be useful to humans, so that we pay close attention to its qualities
Summary of Chapter 1:
In selection under domestication we see
Dramatic change over time in traits and behaviors
Much variation in structures and behaviors upon which a breeder might focus their efforts Chapter 1 – The Origin of Species 09/09/2013
The accumulation of effects over time, when selection is deliberate, and when selection is not deliberate
This power of selection is the main driver of transmutation – of an ancestor form change into various distinct
Thus, Darwin has set up his analogy for natural selection Chapter 2 – Variation Under Nature 09/09/2013
3 elements of Darwin’s Theory
Prodigious amount of heritable variation among individuals in nature
Struggle for existence among organisms
Some of the variations confer an advantage in survival and reproduction
In support of his theory:
Darwin wishes to est. the character of variability and the sheer amount of variety that is found in nature.
He is developing the “natural” side of his analogy with artificial selection
Naturalists studying plants and animals are more interested in their similarities when they engage in
classification. Darwin has to argue for the differences!
Those who believe in the immutability of species would have no reason to attend to the differences from the
archetype. Any such differences are monstrous, generally associated with illhealth, and should be ignored
For his theory to work:
Lot of variation
The variation has to found in all structures, physiological features, all behaviors
Variation must be inherited
Hitherto, any notice of variation had mostly led to discussions about proper classification
Darwin refers to examples of controversial classifications to establish a different point – his theory
“Two varieties of the same species…”
“is x a species or a variety?”
His point is that there is no genuine difference b/w varieties and species
Classificatory decisions are arbitrary
Takes on not just various creation stories by also Aristotle.
There are just populations of varying individuals that (depending on selection pressures)
May be further diverging from ancestors
May be diverging along various paths in response to different pressures
No clear cut of static classes of organisms sharing a common set of essential properties. Chapter 2 – Variation Under Nature 09/09/2013
The reason Darwin focuses on the controversial cases of classification (and the objective difficulty of
classification) is b/c it supports his view of evolution through natural selection.
It’s a continuum : (Individuals – slight varieties – marked varieties – more permanent varieties – subspecies
Phenomenon 1: Species in large, widespread genera seem to exhibit more variability that species in
smaller more localized genera.
Support for Theory
This is predicted on Darwin’s theory of how variation occurs – a) widespread species are exposed to a
wider range of conditions; b) the more individuals, the more potential variation.
Darwin’s observations about genus size if true, would support his theory because it would be predicted by
his theory as follows: If a genus contains many species, then we know that the conditions have been
productive of variability. We would therefore expect that the species within this genus would continue to
exhibit variability. This prediction is confirmed: Species in large genera have proportionally more varieties
that species in small general.
There doesn’t appear to be a strong correlation b/w genus size and the range and the size of species of the
sort Darwin was envisioning
However, it does stand to reason that a large species would be more productive of varieties simply because
of the number of individuals that could exhibit mutations. The more widespread it was, the more likely those
varieties would develop on their own trajectory into marked varieties (incipient species).
The difference b/w species in large genera are less pronounced. They are less sharply delineated than the
species in smaller genera.
Support for Theory:
These species resemble varieties – as is predicted by Darwin’s theory : The larger genera (for reasons
given above) produce more variability. They are exposed to more conditions of life, they produce more
varieties (i.e., incipient species) and therefore more newly minted species
Species in large genera cluster like satellites around a type in the same way that varieties w/in a species
cluster like satellites around a type – this is why classification is so difficult
Darwin’s theory can be understood as entailing that varieties are incipient species and that species are
simply varieties that are further along an evolutionary trajectory.
Species that cluster around a type also resemble varieties in having a restricted range Chapter 2 – Variation Under Nature 09/09/2013
Support: This similarity b/w species and varieties again supports the expectation on Darwin’s theory that we
should find species that are newly derived from varieties of their ancestor species. Chapter 3 – Struggle for Existence 09/09/2013
Second of his threepart theory
“How have all those exquisite adaptations of one part of the organism to another part, and to the conditions
of life, and one distinct organic being to another being, been perfected?” (60)
Ex exquisite adaptation) Mudskipper
Fish can jump out of water; their gills hold water so they can breathe when out of water; walk using their
“How is it that varieties, which I have called incipient species, become ultimately converted into good and
distinct species, which in most cases obviously differ from each other far more than do the varieties of the
“How do these groups of species, which constitutes what are called distinct genera, and which differ from
each other more than do the species of the same genus, arise?”
“I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural
Selection, in order to mark its relation to man’s power of selection.”
Theory defines nature in a dark light
God’s not benevolent – rather struggle to survive
The Malthusian formulation generalized
Thomas Malthus in “Essay on Population” argued that the rate of increase of humans outstrips the rate of
increase of human food sources, leading to a struggle for existence.
Darwin sees this principle at work throughout nature – causing a struggle for existence
Among individual of same species
Among individuals of different species
With the physical conditions of life (ex. plants) Chapter 4 – Natural Selection 09/09/2013
“any variation in the least degree injurious would be rigidly destroyed.”
“This preservation of favorable variations is natural selection”
Darwin Begins w/ a hypothetical case – a country undergoing physical change, such as a climate change.
Immediate change in proportional #s of the inhabitants
Some species may go extinct
Change in the number of Xs cause a change in the number of Y’s
Emigration and immigration
Resulting disturbance of the relations among the former inhabitants
If an island, no immigration, but more scope for evolution to work on the variations that arise w/in any
“vacuums” that are created
Change in “conditions of life” affect reproductive systems of organisms and cause increases in variability
Not that natural selection needs dramatic climate change and the resulting extinctions,
emigrations and immigrations, or an increase in the usual amount of variability.
There are always new and unoccupied places for natural selection to fill up by modifying and improving
some of the varying inhabitants SLIDES
Darwin’s theory vs. Paley’s theory
“How strange it is that a bird, under the form of a woodpecker, should have been created to prey on insects
on the ground; that upland geese, which never or rarely swim, should have been created with webbed feet;
that a thrush should have been created to dive and feed on subaquatic insects. But on the view of each
species constantly trying to increase in number, with natural selection always ready to adapt the slowly
varying descendants of each to any unoccupied or illoccupied place in nature, these facts cease to be
strange, or perhaps haven anticipated.”
Paley’s static/harmonious view vs. Darwin’s precarious theory.
The inhabitants of any region are not all perfect for their environs, since to be perfected would be to be
capable of resisting invasion.
Darwin returns to his analogy w/ artificial selection, not to contrast it w/ natural selection:
Focused on what is apparent
Select for our own good
Treats various Characters the same, regardless of natural fitness to externals
Protects even the weak Chapter 4 – Natural Selection 09/09/2013
Works over time
VS (Natural Selection)
Can work on what is unseen to us
Select what is good for that kind of organism
All characters selected for or against are tested against externals
Discards the weak
Works over ages
“It may be said that natural selection is daily and hourly scrutinizing, throughout the world,
every variation, even the slightest; rejecting that which is bad, preserving and adding up all that
is good; silently and insensibly working, whenever and wherever opportunity offers, at the
improvement of each organic being in relation to its organic and inorganic conditions of life
Correlation of Growth
In cases where two traits s and y are correlated, if x is undergoing selection, y must at least be neutral.
Just by Y’s association with x, it will become more prevalent.
EX) the poncho chicken – frizzled
Sometimes traits of the immature organism are selected for but are also correlated w/ changes in the traits
of the mature organism that might not be independently selected for
Levels of selection debate
In social animals, natural selection will adapt the structure of each individual for the benefit of the
community “if each in consequence profits by the selected change” (p.87)
Can this explain the sterile worker ant
Appears to be the colony which in undergoing selection
This marked division of labor among the individual ants don’t promote the survival and fitness of individual
worker ants, yet it does promote the survival of the colony as a whole and contributes to the queen’s
Can altruism be explained by natural selection?
Selfsacrificing behaviors like sharing resources or defending another individual/group territory
Group selection Chapter 4 – Natural Selection 09/09/2013
Not passed down – cultural trait?
No such thing as “pure” altruism
The “selfish gene”
Perhaps selection is really at the level of the genotype or even of individual genes that code for particular
phenotypic traits – Richard Dawkins
Organisms are just vehicles for the propagation of genes. Our physical and behavioral traits are the result of
selection on these replicators. These traits exist insofar as they contribute to the successful replication of
More complex genes help along this process.
Selection resulting from competition b/w males for females
Type of natural selection
Darwin: “less rigorous” than natural selection since the result of losing the competition is not death but
rather fewer off spring
Not necessarily “vigor” or “strength” that is selected
Malemale competition vs. female choice
Malemale leads to selection for traits of strength offensive “weapons” and defensive “weapons”
Ex) rams butting heads
Female choice leads to selection for coloration, pretty songs, attractive phenotypes
Ex) Those hilarious birds from AP Bio videos.
Possible natural selection at the beginning – able to overcome ridiculous tales (ex peacock)
Chapter 4 continued
Darwin’s intro and explanation of the notion of natural selection in the first half of chapter 4
Levels of selection debate
Group selection Chapter 4 – Natural Selection 09/09/2013
Sexual selection as a type of natural selection
Ex) flowering plants and their pollinators
This leads to the evolution of the nectaries – that part of the plant that produces the nectar that insects
How could this lead to their being separate sex plants – one with only female flowers, and the other with
only male flowers?
Insects attracted to the nectar are dusted w/ pollen, carrying pollen from flower to flower. Once the plant
developed to have sufficient nectar to be attractive to bees, another process could take place – division of
Assuming that it is advantageous to the plant to produce only stamens in one flower and only pistils in
another (division of labor) – more efficient
Also you have to assume that this will help reduce the likelihood of selffertilization (which is assumed to be
less desirable than cross pollination)
You would expect to see separate male and female flowers/plants
Gradualism of Natural Selection
Throughout this section Darwin emphasizes the gradualness of this process
For support, he reminds his reader that Lyell’s gradualist view in geology were once thought implausible but
were now considered correct.
How do distinct species form?
Divergence of character
Darwin has explained how natural selection operates, identifying the conditions favorable to its operation
Discussion of islands vs. large expanses
Discussion of intercrossing vs. self fertilization
Discussion of extinction as the result of failure or inability to adapt to competition or to conditions.
There is competition b/w similar life forms in the same location Chapter 4 – Natural Selection 09/09/2013
NS will favor competitive ability
For that have greater competitive ability that similar forms (competing for same resources) will tend to
outnumber their competitors
They will grow and spread, using their superior competitive ability to displace occupants of nearby niches.
As they occupy these nearby niches, and displace the former occupants, they will undergo new selection
pressures that will adapt them to their new niche
The result is that the descendents of these successful occupiers will diverge in form from their ancestors
Will also diverge in form from other “splinter groups” of those ancestors which successfully occupied other
nearby niches. Descendents of those splinter groups will in turn, begin to diverge from their ancestors in
ways that make them more adapted to their niche
Result: Eventual production of new types from a common ancestor.
“How then does the lesser difference b/w varieties become augmented into the greater
difference b/w species?” (pg 111)
His analogy: he describes divergence of character in artificial breeding first, then explains how it could
happen in nature
“The more diversified the descendents of any one species become (in structure, constitution, and habits),
will be better enabled to seize on many and widely diversified places in the polity of nature, and so be
enabled to increase in numbers”
Division of labor:
Adam Smith’s idea that there is increased productivity when there is diversification in manufacturing in labor
Applied to organisms – organ systems specialization w/in a organism
Applied to species – increased productivity on a parcel of land (Darwin)
Darwin vs. Creationists
If special creation were true, species would be perfectly adapted to their environments
Yet we see that non0indigenou plants thrive and often outcompete indigenous
So, they occupy different niches, allowing for coexistence or eventual takeover (invasive)
Darwin trying to establish that:
Modified descendents of species succeed so much better that they become diversified.
We get an explanation of the diversity and the distribution of the forms of life, how from variation we get
wellmarked varieties, and form wellmarked varieties, good distinctive species
Anagenesis: a modern term for evolution w/in FUCKING SLIDES Philosophical Issues in the Theory of Evolution Through Natural Selection
Some Philosophy of science issues:
Does science aim at a true description of the world?
Does it simply aim for predictive and explanatory power?
What makes a theory scientific? What makes for a good scientific theory?
What is the nature of causation?
What kinds of explanation are legitimate?
Is biology like other sciences? Does it have laws like physics? If not, is that a problem for biology?
Individual organisms in a species vary
There is a severe struggle for existence in nature
Thus, it is likely that some of the variation b/w individuals will promote the welfare of the bearer
And, such beneficial variations will give an advantage in the struggle for existence
Those individuals w/ beneficial variations that will give an advantage in the struggle for existence will, as a
result, tend to live longer and tend to leave more offspring
Offspring tend to resemble their parents
Thus, those variations that are beneficial in the struggle for existence will be preserved in the offspring
What natural selection hath wrought
Over time, this process leads to the development of adaptive traits including the most exquisite adaptations
Eventually, we see the development of new species (by process of divergence of character – Darwin Ch.4)
We’ve seen how Darwin knows that in order to demonstrate that his theory is true, he needs to do more
than simply provide this argument
Following Herschel’s guide to good scientific methodology, Darwin uses analogy in his argument for natural
He also knows that his theory won’t work unless the variation required by natural selection has the right
In particular, we’ve seen Darwin argue for the abundance of variability in nature
Lewen’s brings up a point that we haven’t seen Darwin discuss explicitly, though he touches on it when he
talks of “the correlations of growth” Philosophical Issues in the Theory of Evolution Through Natural Selection
The traits that are supposed to undergo selection have to have some degree of independence so that they
can undergo modification w/o harmful effects on the rest of the organism
Modularity of Traits:
To some extent the province of evolutionary developmental biology (evodevo)
“Modern Synthesis” (19361947) of genetics and evolutionary biology, classically focused on the frequency
of alleles in populations
We can now look at how developmental pathways may have constrained/enabled evolution
Now learning more at the molecular level, about which traits are affected together and which can be
Other potential problems for Darwin’s theory:
Objections raised by Henry Fleming Jenkin
Even assuming we have unlimited variability of species:
To what extent do offspring resemble their parents?
.5 and .5? Right in the middle? Only the better adapted parent?
Sure there are abundant variation, but how often is it favorable?
If rare, not likely to be preserved
Have to arise regularly to have a chance of preservation
If the individuals with the favorable variations live and breed, what happens to the favorable traits?
Will likely have an average mate Jenkins believed that traits were “blended” in the offspring – so the
offspring are intermediate b/w parents. These offspring then mate w/ average individuals, and their
offspring move even closer to average, etc.
Some of Jenkin’s assumptions are false, but his objections worried Darwin and forced him to think about
issues of which he was rather ignorant
The distributions of traits in populations
Explanation of a thing’s existence and/or why does it have the properties that it does (purpose, role, or goal
Appeals to purpose or function to explain
Makes perfect sense for human behavior
Ex) corset – in order to appear fashionable and impress people with her trim waist – doing this to achieve a
goal (but is achieved only after wearing the corset) Philosophical Issues in the Theory of Evolution Through Natural Selection
The cause is actually her intention to have this effect and the intention precedes the behavior
Ex) waiting in line to vote – wait in line and then vote.
Ex) Why does this flower have the appearance of a female insect?
To attract the male of the species to act as a pollinator.
Biology is full of teleological explanation
Anytime you talk about it’s function – you are giving a teleological explanation
Basic categories are defined in terms of their function, not their structure, not merely their effects
Even structural terms have their meaning in terms of the role that structure plays in some adaptive process
the purpose they serve in some larger system that contains them:
Molecular level: condons, introns, transcription factors, genes
Anatomical level: flagellum, vacuole, valve, vessel, heart, lung
Ecological level: predator, parasite slides
Problem w/ functional talk and teleological talk: cant appeal to a future even to explain something that is the
case. Backwards causation?
Modern science deals with causes that precede their effects, not purposes, ends, what Aristotle, called
This is an apparent problem. Slides
TE as based on conditional facts:
Following philosopher Daniel Dannett:
Natural selection is a process that take facts about organisms/environment relations in the form: “If species
were to have a trait, then they would survive and reproduce better”
Thus we can answer questions about what this trait is for, by saying if species had trait, they would be more
successful at “ging”
Selected effects account for function (Larry Wright)
Trait T is for “ging” if T or its precursors were selected for as a result of their usefulness in attaining G
No backward causation
Distinguishes what a trait is for, from other nonfunctional effects (heart makes “lubdub” sound)
Casual Role account of function
Terms like “heart” and “gene” have no teleological content, either implicitly or explicitly. They refer instead to
components of larger systems to whose behavior they make a causal contribution. The component’s
function is thus relative to a larger system and I only defined in terms of the causal role it plays in that
Useful addition to selected effect theory because it allows, for example, paleontologist and
anatomists, to describe traits in a way that is neutral as to their etiology Population Genetics and Natural Selection 09/09/2013
Darwin’s seven step argument is elegant and convincing, but did adaptations and diverse species
actually evolve through the mechanisms of NS
We saw that certain things have to be true if Darwin’s theory provides a true description of evolution
Some of those things Darwin was in a position to argue positively for
Others he wasn’t
He needed to make testable claims about the rate at which favorable variations occur
He needed to make testable claims about the rate at which favorable variations would be inherited
In short, he needed statistical tools he just didn’t have
R.A. Fisher, JBS Haldane, and Sewell Wright (1920s, 30s, and 40s) developed statistical tools for
Comparison of trait distribution in parent and offspring generations
Which patterns of inheritance could give rise to adaptation by selection
How frequency of a trait arising in a population affects the likelihood of its being preserved.
With these tolls you can make predictions about how the frequency of a trait may change over generations
once you’ve make certain assumptions
Frequency of variations, favorableness, presence of alternative variations, linked traits, etc.
Given these assumptions, what would the theory of NS predict about the relative frequency of the traits in
Lewens points out that it isn’t entirely obvious that modern evolutionary biology’s concept of NS requires a
“struggle for existence”
We don’t need to assume scarcity of resources, that one individual’s success means another’s failure. A
particular trait can still become more prevalent in a population through NS.
Selection as a positive force
This def of selection helps us to see why selection is a positive and creative force in evolution – not merely
a “winnower” or a “sieve”
Selection M can lead to the increase of the proportions of Ms over Fs in the population
Assume that Ms are more likely than Fs to have even better memory than the average M whereas Fs are
more likely to have offspring with even worse memory than the average F.
Ms increase in the population so the likelihood of M+ goes up as well = more adaptation
Talking more about traits like M and F rather than individuals who have traits
Darwin talked more about the fit individuals being selected for – win struggle for existence
Common contemporary approach tends to focus on traits and their changing and relative frequency
in populations. Traits get selected for Evolutionary Drift 09/09/2013
“And what makes for good science”
Modern Idea of selection
Factor that gives rise to an increase of the frequency of a given type of trait over another type of trait w/in a
population. The trait is being selected for
We are talking about trains rather than individuals who have the traits
Modern conception: the “level of selection” is not really on the individuals being selected for.
Darwin talked of more or less fit individuals being selected for – winning the struggle for existence
Helps us see why selection is a positive and creative force in evolution – not merely weeding out bad traits
No need to assume winners and losers in struggle for existence.
Selection vs. Evolutionary Drift
The element of chance in the process of evolution (evolutionary drift)
Contrasted w/ evolution through natural selection
Evolution – change in the proportion of traits over time
Such changes can happen through other processes besides selecting for them.
Drift resulting from chance elements in sexual reproduction:
The proportion of alleles in the offspring generation can differ from the parent generation just by the
chance element in which of a parent’s 2 copies of a gene get passed along to a giver offspring
Such changes in the distribution of traits that do not take the population in the direction of greater
adaptations are generally described as a “drift”
Drift: Founder Effects
Small isolated population that doesn’t carry the full range of genetic variation in the full population.
Proportion of traits are different as well
Drift: Small Population Effects
Compared to larger populations, we’d expect that the actual percentage to diverge more from that
prediction because of the greater effects of chance events
The larger the population, the closer to 60% the actual proportion of a trait in a population should be.
In a large pop, the offspring of other couples will “even out” this deviation from the expected distribution of
In a small pop, this is likely not to happen, effecting the overall distribution of traits
Large ramifications on the following generation and trait prevalence.
Original population is exposed to catastrophic reduction in population resulting in chance survivors. This
leads to a new population of just the chance survivor genes.
Philosophical questions here
What are the grounds of the alleged objective probability of a trait becoming dominant?
How do we distinguish b/w drift and selection? Evolutionary Drift 09/09/2013
Need some prior notion of an ecological “problem” that a trait solves successfully.
Biologists are divided over the extent to which drift has played a role in the evolution of adaptations and of
Is it equally important force? Especially given how speciation tends to result from random events
that isolate populations, giving rise to founder effects. What is Scientific Evidence 09/09/2013
Darwin’s own views about what makes for a good scientific theory:
We’ve noted how he emphasizes the several large classes of facts that the theory of natural selection is
able to explain
Known as “inference to the best explanation” IBE
A very intuitive notion of how theories are supported by data in everyday life this seems to be how we
Anytime a hypothesis explains the facts better than rival hypotheses we tend to think it is true
Inference to the best explanation
God produced each species individually to suit its surroundings
Species are evolved from common ancestors through the process of natural selection
Example of best explanation
South American coast and Galapagos:
Considerable dissimilarity in conditions of life; geology; height; climate
Considerable similarity in the inhabitants, in the species observed
Between Galapagos and cape verde Archipelagos:
Considerable similarity in volcanic soil; climate; height; size of islands
Considerable dissimilarity in the inhabitants.
Which hypothesis suits these facts best?
Special creation offers no real explanation. If anything, it would assume that similar creatures would be
placed in similar environments
Darwin’s theory predicts that the Galapagos would likely receive immigrants from South America and the
Cape Verde archipelago would likely receive immigrants from Africa. These different islands would be
predicted to have different inhabitants, despite their similarity.
Is inference always a good one?
We need to say what makes an explanation a good explanation and why a hypothesis that explains so
many things is thereby more likely to be true.
A hypothesis has a high “likelihood” when that hypothesis makes a certain set of observation probable
Thus a good explanation is one that, if it were true, would make the events or facts we are trying to
What else makes a good explanation
Herschel’s verae causae
Whewell’s “consilience of inductions”
Especially when those other phenomena are of a different kind than those the theory was intended to
Tripartite structure of the origin: What is Scientific Evidence 09/09/2013
The existence of the causes posited
The adequacy of these causes for producing the phenomena observed
That these causes actually were responsible for producing the observed phenomena
Here is where consilience comes in and can support a hypothesis over rivals that can be shown to exist and
to be adequate.
NS and Common Ancestry
Lewens points out that many of the facts that Darwin points to as better explained by “his theory” than by
special creation are better explained by “common ancestry” than by special creation
But this leaves it opens as to whether the process that led from those common ancestors to their diverse
species was driven by natural selection
In fact, many people came to accept Darwin’s arguments for evolution and common ancestry, but