Transcript The Evolution of Natural Selection

The Evolution of Natural Selection
• Born the same day as Lincoln, to prominent (but not
aristocratic) family in Kent, England. Father was a country MD.
• Darwin was not a splendid student. "You care for nothing but
shooting, dogs and rat-catching, and you will be a disgrace to
your family."
• Went to Edinburgh to study medicine, washed out, partly
because he couldn't stomach the brutal conditions.
• Then to Christ College, Cambridge, with intention of studying
for the clergy. Cambridge and theology were a good fit.
• Darwin did his rat-catching with the best;
 Adam Sedgwick (geology) ,
 Whewell (philosophy)
 J.S. Henslow, (botany).
It was Henslow who set him up for the voyage on the Beagle
The Voyage
• In 1831, signs on as captain’s companion
to Capt. Robert FitzRoy aboard the survey
ship, H.M.S. Beagle.
• Reported to have taken 2 books with him:
 William Paley's Natural Theology, which
argued for a literal interpretation of
natural history in light of the bible.
 Charles Lyell's Principles of Geology,
which argued for interpretation from
visible evidence.
• He left believing implicitly in the former,
and returned a convert to the latter.
The Voyage
Human 1. Fitzroy had educated Feugian natives in England, having picked them up in an
earlier voyage.
2. They were returned to Tierra del Fuego on this voyage, and observed over a
year. Darwin got to see the range of "adaptiveness' in humans, and to argue
this, and the place and nature of man in the system, with Fitzroy.
The Voyage
Geology 1. January 1835: Experienced an earthquake while in Chile - caused a 10 foot uplifting of the
coastline. Helped convince him of Lyell's views, especially after seeing fossil shells in
Andes.
2. Noted that fossils he collected resembled (but were not identical to) living organisms in
the same region. Not consistent with catastrophism.
3. Noted latitudinal replacement of species on the continent, with related forms being
geographically close to one another.
The Voyage
Biology -
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South American flora and fauna from different regions were distinct from the flora
and fauna of Europe.
Temperate species of South America were taxonomically closer to species living in
tropical regions of South America than to temperate species of Europe.
In Patagonia he collected giant fossil ground sloths, elephants, llamas and
armadillos- all clearly extinct, but very much akin to he living animals of the area.
Observations like these would eventually force him to reject the view that species
are immutable.
The Voyage
Galapagos Islands
Tremendous variation among the tortoises.
The Voyage
Galapagos Islands
Variation among the mockingbirds.
"...the different islands probably have their representative species or races of the Amblyrhynchus, as well as
the tortoise. My attention was first thoroughly aroused by comparing together the numerous specimens, shot by
myself and several other parties on board, of the mocking-thrushes, when, to my astonishment, I discovered that all
those from Charles Island belonged to one species (Mimus trifasciatus); all from Albemarle Island to M. parvulus;
and all from James and Chatham Islands...belonged to M. melanotis."
Genus and
species
Common Name
Distribution
Nesomimus
trifasciatus
Charles
Mockingbird
Champion & Gardner-near
Floreana
Nesomimus
macdonaldi
Hood
Mockingbird
Espanola
Nesomimus
melanotis
Chatham
Mockingbird
San Cristobal
Nesomimus
parvulus
Galapagos
Mockingbird
all other islands except
Pinzon
http://people.rit.edu/rhrsbi/GalapagosPages/mockingbird.html, Dr. Robert Rothman
The Voyage
Galapagos Islands
Variation among the finches.
The Voyage
Notebook “B”
The Voyage
Notebook “B”
“Each species changes”
“Heaven knows whether this
agrees with Nature: Cuidado”
The Evolution of Natural Selection
• Returned to England in 1836. Contemplates marriage to his
cousin, Emma Wedgwood).
The Evolution of Natural Selection
• Returned to England in 1836, married (Emma Darwin, nee
Wedgwood).
• Settled down as moderately wealthy country squire.
• Moderately famous, makes the rounds among the scientific
societies.
• Ultimately a recluse, worked on a variety of things, much of it
just cataloging the specimens from the Beagle. Conducting
experiments.
• Corresponds with the leading scientists and thinkers of his time.
The Evolution of Natural Selection
• Settled down as moderately wealthy country squire.
• Moderately famous, makes the rounds among the
scientific societies.
• Ultimately a recluse, worked on a variety of things, much
of it just cataloging the specimens from the Beagle.
Conducting experiments.
• Corresponds with the leading scientists and thinkers of
his time.
Darwin with eldest son, 1842
E.g. Thomas Robert Malthus.
Malthus was among first to recognize that population growth
is a multiplicative (or exponential) process. A couple have two
offspring, which go on to produce 2 offspring each, and so on:
2->4->8->16->32->64->128
Thomas Robert Malthus (1766 – 1834)
- English cleric
- Interested in all things regarding populations.
- Was Professor of History and Political Economy at East India Company
College in Hertfordshire.
Most famously known for his manifest:
An Essay on the Principle of Population (1798)
“I think I may make fairly two postulata. First, that food is necessary to the existence of
man. Secondly, that the passion between the sexes is necessary and will remain nearly
in its present state ... Assuming then my postulata as granted, I say, that the power of
population is infinitely greater than the power in the earth to produce subsistence for
man. Population, when unchecked, increases in a geometrical ratio. Subsistence
increases only in an arithmetical ratio. A slight acquaintance with numbers will show
the immensity of the first power in comparison of the second. By the law of our nature
which makes food necessary to the life of man, the effects of these two unequal powers
must be kept equal. This implies a strong and constantly operating check on population
from the difficulty of subsistence. This difficulty must fall somewhere and must
necessarily be severely felt by a large portion of mankind....”
Thomas Robert Malthus (1766 – 1834)
“This natural inequality of the two powers, of population, and of production in the earth,
and that great law of our nature which must constantly keep their efforts equal, form the
great difficulty that appears to me insurmountable in the way to the perfectibility of
society...The checks which repress the superior power of population, and keep its effects
on a level with the means of subsistence, are all resolvable into moral restraint, vice and
misery. “... this constantly subsisting cause of periodical misery has existed ever since we
have had any histories of mankind, does exist at present, and will for ever continue to
exist, unless some decided change takes place in the physical constitution of our nature.”
Joseph Dalton Hooker(1817 – 1911)
Alfred Russell Wallace
In 1858, he received a letter from Alfred Russell Wallace.
Wallace was a naturalist and explorer who had spent several
years in the Malay Archipelago, asking his advice about a paper
he (ARW) was planning to write (it was already written) which
outlined a theory of evolution essentially identical to CD's.
"Wallace map archipelago" by Alfred Russel Wallace, translator P.J. Veth (1814-1895). - Dutch translation of Alfred Russel Wallace, The Malay
Archipelago: 'Insulinde: het land van den orang-oetan en den paradijsvogel', Van Kampen, Amsterdam, 1870-1871. Translation by P.J. Veth
(1814-1895).. Licensed under Public Domain via Wikimedia Commons
The Abstract
Writes a 513 pp manuscript – “The Abstract”
The first printing of 1,250 copies sells out in the first
day.
The arguments he presents in favor of evolution
are still among the strongest ever given, and
many subjects he presented are still being
worked on today.
The Building Blocks to the Theory
Variation under domestication
• Upon his return to England, he joined
pigeon-fanciers groups and takes up pigeon
breeding.
• Produces 15 different varieties, all very
different from each other, but all from the
same species. Tells his buddy Lyell that if
they had been found in the wild, they might
be classified not only as separate species,
but separate genera! - which are groups of
species.
The Building Blocks to the Theory
Variation in barnacles
"If there are a continuous grade of varieties of
a plant that range from red flowers through
pink to white flowers, they are classified as
races of a single species. But if the
intermediate races go extinct, we will be left
with only the red and white extremes, and
classify them as distinct species. Varieties can
be interpreted as incipient species. (species
beginning to come into being) Likewise, the
difference between two species within a genus
may foreshadow the origin of two new
genera."
He wrote that in many of these cases, "natural
selection probably has come into play &
according to my views is in the act of making
two species."
The Building Blocks to the Theory
Heritability
It is the proportion of the variation observed
in a population that is due to variation in the
effects of genes.
The Building Blocks to the Theory
The struggle for existence
• Darwin recalls Malthus uncontrolled growth of
populations.
• He observed in nature that
there is not uncontrolled
growth. How come?
• Most individuals born don't
survive.
• On average, the most fit
individuals pass their genes onto
more offspring than less fit
individuals.
Incorrect
The Building Blocks to the Theory
Is Survival and Reproduction Nonrandom?
Great question by Darwin! He considered the null
hypothesis. Were the observations he was making
solely due to chance alone?
• Those individuals that possess the traits that
make them best suited for the environment (or
most successful at surviving) will go on to
reproduce.
• More importantly, they will produce more
offspring than those individuals that do not have
the trait that is as well suited.
• Natural selection requires more than surviving, it
means surviving to reproduce. And it means
reproducing more than other phenotypes.
The Building Blocks to the Theory
This leads to - Natural selection.
1. Natural selection is the differential success in
reproduction, and its product is adaptation of
organisms to their environment.
"This preservation of favorable variations and the
rejection of injurious variations, I call Natural
Selection."
2. Even slight advantages of some variations will
accumulate in a population over many
generations of being perpetuated by natural
selection.
3. Natural selection thus occurs from the interaction
between the environment and the inherent
variability in a population.
4. Variations in a population arise by chance, but
natural selection is not a chance phenomenon
since environmental factors set definite criteria
for reproductive success.
The Building Blocks to the Theory
To summarize natural selection
1. Thus, variation + inheritance + struggle for existence = evolution by natural selection.
If these three preconditions are met, evolution necessarily results.
2. Results in adaptation - a structure, behavior or other feature of an organism that
enables it to survive or reproduce more successfully than it otherwise would under the
existing environmental conditions.
The Building Blocks to the Theory
Unique features of Darwin's theory:
a. Entirely materialistic and mechanistic - depends entirely on forces or factors that can
be observed.
b. No limit to the amount of change possible.
c. Non-progressive - proper icon for history of life isn't ladder (Scala naturae), but tree.
Every branch has a history as long as any other. Each represents a lineage evolving
adaptively to survive and reproduce in its habitat (not trying to become human, or
anything else).
i. Species are not more or less advanced - no linear order to life.
ii. Evolution is not goal directed or teleological.
Important implication: Natural selection favors only successful individuals, not coexistence
of species. Contrast with popular views on the harmony of nature. Improvement of
one species can even  extinction of another.
Darwin’s Postulates
Postulate 1: Individuals Differ from one another.
Postulate 2: The variation is inherited
Postulate 3: Individuals vary in their success at surviving and reproducing
Postulate 4: Survival and Reproduction are non-random.
Darwin’s Postulates
Postulate 1: Individuals Differ from one another.
Peter and Rosemary Grant – Princeton University
Isla Daphne Major
Medium ground finch?
Darwin’s Postulates
Postulate 1: Individuals Differ from one another.
Variation in beak depth of
medium ground finches
(Geospiza fortis) in 1976.
Darwin’s Postulates
Postulate 2: The variation is inherited
Heritability of beak depth in medium
ground finch (Geospiza fortis).
Darwin’s Postulates
Postulate 3: Individuals vary in their success at surviving
and reproducing
Drought in 1977.
Medium ground finches did not even attempt to breed.
“The decline in the number of finches was simultaneous
with a decline in the availability of seed the birds eat.”
Darwin’s Postulates
Tribulus plant (Tribulus cistoides)
Darwin’s Postulates
Postulate 4: Survival and Reproduction are non-random.
Beak depth in surviving medium
ground finch (Geospiza fortis) before
and after the drought.
Darwin’s Postulates
Did evolution occur?
Beak depth of finches hatched the year
before the drought versus the year after
the drought.
The Nature of Natural Selection
 Natural Selection acts on the individual but its consequences are seen at the population
level.
• During the drought of 1977, the beaks of individual finches did not become larger.
• However, more finches with larger beaks survived the drought event.
• After the drought, the mean size of finches and the mean beak depth changed in the
population.
The Nature of Natural Selection
 Natural Selection acts on the phenotype, but the result is a change in genotype (change in
allele frequencies).
This is validation for the concept that for natural selection to occur, traits must be hereditary.
• Even if all of the variation in beak depth had been
environmental (the drought leaves behind finches with large
beakes), finches with deep beaks would have still been
favored in 1977.
• But the next generation of finches would not necessarily have
had deep beaks. The range of variation (or population mean)
might go back to what it was before the drought.
• The change in one generation must be passed on to the next.
The Nature of Natural Selection
 Natural Selection cannot predict the future (and neither can organisms)
o The phenotype of a generation is a product of the environment that preceded it. If the
environment stays the same, so will the phenotype of the population.
• But if the environment changes, there is no way the organisms would know that in
advance.
• The finches with deep beaks could not predict the rain and the abundance of small
seeds.
The Nature of Natural Selection
 Natural selection can produce new traits, even though it operates on existing traits.
Natural selection can only select from the range of variation that already exists in the population.
o Paradoxically, evolution can produce new traits.
• Not from the range of variation that exists in a starting
population, but,
• As a result of mutations that occur in each generation over
time.
• Coupled with meiosis and fertilization (recombination)
more variants are created for Natural Selection to act on.
Could you produce a Brassica with more trichomes
than in the initial population?
Long-term selection for oil content
of corn kernels.
The Nature of Natural Selection
 Occasionally, novel characteristics can be generated through preadaptation (exaptation).
 This occurs when a pre-existing trait is used in a novel way and is elaborated upon by selection.
Leaves of butterwort (P. moranensis) are
covered with glandular trichomes.
Butterwort plants
http://beyondthehumaneye.blogspot.com/2010
_03_01_archive.html
Non-glandular trichomes
http://beyondthehumaneye.blogspot.com/2010_0
3_01_archive.html
WEDNESDAY, MARCH 31, 2010
Botanical Flypaper
The Nature of Natural Selection -activity
 Natural Selection does not predict perfection
Fig. 1.Collection sites for G. affinis (A) and G.
hubbsi (B). Open symbols represent predatorfree populations, and filled symbols represent
predator populations. Outer labels of maps
represent latitude and longitude values.
Labeling beside collection localities identifies
populations and corresponds with those
presented in Fig. 2.
Langerhans R B et al. PNAS 2005;102:7618-7623
The Nature of Natural Selection
Divergence in gonopodium size, controlling for body size, between
predator regimes in G. affinis (A) and G. hubbsi (B). Data were
pooled across years for both species. Least-squares means ± 1
standard error from ANCOVAs are depicted. Open bars represent
predator-free populations, and filled bars represent predator
populations. Population names are as in Fig. 1.
Female mating responses to videos of a male with a small vs. large
gonopodium. (A) Females spent more time directly interacting with the
large-gonopodium male than the small-gonopodium male (P < 0.0001). (B)
Females approached the large-gonopodium male within one body length
more times than the small-gonopodium male (P = 0.0002). Means ± 1
standard error are depicted.
The Nature of Natural Selection
 Fitness is not Circular
“Survival of the fittest.” (Herbert Spencer, 1864)
Of course the fittest survive!
Of course cacti are more likely to survive and reproduce in the desert
– that’s because they have the traits that permit them to do so.
• The weakness in our thinking is we forget the fundamentals of how natural selection
works.
• For natural selection to operate, there must be a range of variation in the population for a
trait. The environment eliminates those individuals who do not possess the trait that
permits them to survive.
• Even if they do survive, they may not reproduce as much as a conspecific that has a
different trait.
Fitness can be measured = the number of offspring
The Nature of Natural Selection
 Natural Selection Acts on Individuals, not groups.
• Groups of individuals are not selected for.
• Neither are species.
• The environment acts on each individual separately. An individual that puts its life on the
line for the sake of another will be quickly selected against and eliminated from the
population.
• Self-preservation will be selected for.
• Group selection may seem to occur (kin selection) because individuals in the population
are closely related (e.g. social insects, Hymenoptera).
• Group selection is considered by many to be theoretically possible and has been
demonstrated in a laboratory setting.