Samenvatting
Quiz 1 Summary Complexity - a guided tour
A detailed summary of the book 'Complexity a guided tour' by Melanie Mitchell. Chapters 2, 3, 5, 7.
[Meer zien]Voorbeeld 4 van de 39 pagina's
Voorbeeld 4 van de 39 pagina's
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CHAPTER 5 - EVOLUTIONSecond law of thermodynamics: the total entropy of an isolated system will always increase
until reaches its maximum value (logically)
Counterexample = life: living systems have become vastly more complex and intricate
rather than more disordered and entropic
Evolution: process of gradual (and sometimes rapid) change
PRE-DARWINIAN NOTIONS OF EVOLUTION
Until 18th century
- Biological forms do not change over time
- All organisms were created by a deity (God)
- All organisms have largely remained in their original form since their creation
George Louis Leclerc de Buffon
- French zoologist
- Historie Naturelle - mid 1700’s
- Similarities between different species
- Earth much older than the Biblical 6000 years
- All modern organisms evolved from a single ancestor (no mechanism for evolution yet)
Erasmus Darwin
- Grandfather Charles Darwin
- 18th century
- Scientist and poet
- All species from a single ancient ancestor
- Proposed mechanisms for evolution (basis for natural selection by Charles Darwin)
Jean-Baptiste Lamarck (1744-1829)
- Most famous pre-Darwinian evolutionist
- French aristocrat and botanist
- Philosophie Zoologique - 1809
- New types of organisms are spontaneously generated from inanimate matter
- These species involve via the ‘inheritance of acquired characteristics’
- Organisms adapted to their environment during their lifetimes
- These acquired adaptations were then passed directly to the organisms’ offspring
- For example birds that had to stretch their legs to keep their bodies out of the
water, made their legs longer.
- Evolution entails a tendency to progression, in which organisms evolve to be
increasingly advanced with humans at the pinnacle of this process
- Charles Darwin was impressed by Lamarck → in addition to natural selection,
the inheritance of acquired characteristics was one of the mechanisms of
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, evolution
- Darwin and Lamarck didn’t have a theory of how such inheritance could take place
Sigmund Freud
- If the instinctual life of animals permits of any explanation at all, it can only be this: that
they carry over into new existence the experience of their kind; that is to say, that they
have preserved in their minds memories of what their ancestors experienced
ORIGINS OF DARWIN’S THEORY
Charles Darwin (1809-1882):
- Collected plants, animals and fossils + did a lot of reading, thinking and writing while
being on the ship the Beagle for 5 years
- Inspired by
- Principles of Geology (Charles Lyell) → geological features (mountains,
canyons, rocks) arise from gradual processes of erosion, wind, myriad floods,
volcanic eruptions and earthquakes rather than from Biblical Noah’s flood
- Essay on the principle of population (Thomas Malthus) → human population
growth leads to competition for food and other resources, Darwin adapted
these ideas to explain evolution of all living organisms via a continual struggle
for existence
- the Wealth of Nations (Adam Smith, free-market) → invisible hand in
economics, whereby a collection of individuals acting in their own self-interest
produces maximum benefit for the entire community
- Observations in South America and elsewhere → struck by tremendous
variation among living beings and by the apparent adaption of different
species to their environments
- For example: birds on Galapagos Islands, where geographical isolation imposed
by different islands led to evolution of many different species from a small
number of ancestors
- Important ideas Darwin
- Gradual change over long periods can produce very large effects
- Population growth combined with limited resources creates a struggle for
existence
- Collections of individuals acting in self-interested ways produce global benefit
- Life seems to allow almost infinite variation
- Species’ particular traits seem designed for the very environment in which the
species lives
- Species branch out from common ancestors
- Coherent theory: evolution by natural selection
- Individual organisms have more offspring than can survive, given limited food
sources
- The offspring are not exact copies of the parents but have some small amount of
random variation in their traits
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, - The traits that allow some offspring to survive and reproduce will be passed on
to future offspring, thus spreading in the population
- Very gradually, through reproduction with random variation and individual
struggles for existence, new species will be formed with traits ideally adapted
to their environments
- Darwin waited for years to share his ideas because his theories would bring
unhappiness to religious people (including his wife)
- His main conclusion was hard for him too, because he once considered
becoming a country parson → conclusion: ‘I am almost convinced (quite
contrary to the opinion I started with) that species are not (it is like
confessing a murder) immutable
- Philosophical implications:
- Darwin wrote: ‘Plato says in Phaedo that our ‘necessary ideas’ arise from the
pre-existence of the soul, are not derivable from experience - read monkeys for
pre-existence.’
- Competition → centrepiece of evolution and motivator in science
- Darwin was about to be scooped
- In 1858, by English naturalist Alfred Russell Wallace (on the tendency of
varieties to depart indefinitely from the original type)
- They published their work together
- Patrick Matthew has published a book (on naval timber and arboriculture) very
much like Darwin’s evolution by natural selection → so who actually is
responsible for the idea of evolution by natural selection?
- Why does Darwin get all the credits?
- He was at that time a more famous and respected scientist
- Darwin’s book, unlike the works of Wallace and Matthew, contained a more
coherent set of ideas and tremendous amount of evidence supporting those
ideas
- Darwin turned natural selection into an extremely well-supported theory
Summary major ideas of Darwin
- All species descend from a common ancestor
- The history of life is a branching tree of species
- Natural selection occurs when the number of births is greater than existing resources
can support so that individuals undergo competition for resources
- Traits of organisms are inherited with variation. Variation is in some sense random, that
is, there is no force or bias leading to variations that increase fitness (Darwin himself
accepted Lammarck’s view that there are such forces). Variations that turn out to be
adaptive in the current environment are likely to be selected, meaning that organisms
with those variations are more likely to survive and thus pass on the new traits to their
offspring, causing the number of organisms with those traits to increase over subsequent
generations.
- Evolutionary change is constant and gradual via the accumulation of small, favorable
variations
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, The result of evolution by natural selection is the appearance of ‘design’ bu with no designer. It
comes from chance, natural selection, and long periods of time.
Entropy decreases (living systems become more organized, seemingly more designed) as a
result of the work done by natural selection. The energy for this work comes from the ability of
individual organisms to metabolize energy from their environments (like sunlight and food).
MENDEL AND THE MECHANISM OF HEREDITY
Not explained by Darwin:
- How are traits passed on from parent to offspring?
- How comes variation in those traits, upon which natural selection acts, about?
DNA is the carrier of hereditary information (1940s)
Gregor Mendel (1822-1884):
- Australian monk and physics teacher with interest in nature
- Experiments on generations of related pea plants to see whether he could verify
Lamarck’s claims
- Results disconfirmed Lamarck’s speculations and revealed surprising facts
- Experiment pea plants
- He looked at different traits, like smoothness and color of seeds, shape of pea
pod, color of pods and flowers, height of stems, etc..
- Each trait (of character) could have one of two distinct forms (color could be
green or yellow, stem height could be tall or dwarf)
- Results
- The plant’s offspring did not take on any traits that were acquired by the parents
during their lifetimes, so Lamarckian inheritance did not take place
- Heredity took place via discrete factors that are contributed by the parents:
one factor being contributed by each parent for each trait (so each
parent contributes either a factor for tall stems or dwarf stems) →
factores correspond to genes
- The medium of inheritance, whatever it was, seemed to be discrete, not
continuous as was proposed by Darwin and others
- For each trait, the plant has a pair of genes (he did not use this word)
- Each gene of the pair encodes a value for the trait, this value is calle an allele
- Three possibilities for the allele pairs encoded by two genes: both alleles
the same (tall/tall or dwarf/dwarf), or different (tall/dwarf, equivalent to
dwarf/tall).
- For each trait, one allele is dominant and the other is recessive
- Tall/tall will always be tall; tall/dwarf will be tall since tall is dominant, so
only dwarf/dwarf will be dwarf.
- Example: 2 tall/dwarf cross-pollinate. Both parents are tall, but there is a
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