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Evolution
Reading:
Evolution 101 from the University of California Museum of Paleontology's Understanding Evolution
website. It begins here: http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_01
Some of the illustrations used in these notes are also from this website.
Curious about Alfred Wallace? See:
http://www.newyorker.com/arts/critics/atlarge/2007/02/12/070212crat_atlarge_rosen?currentPage=all
Charles Darwin
credited with “discovering” evolution (hopefully you'll
have a more nuanced description of what Darwin did
by the end of this module)
Wrote On theOrigin of Species
In this book, Darwin did did two things:
(1) established that species have changed over time
(descent with modification)
(2) proposed a mechanism for how that process
took place (natural selection)
Darwin did not arrive at the theory of natural selection
out of the blue. He drew on an existing knowledge
base as well as his own observations.
What was known about heredity
by the early 1800s:
Generally accepted that both plants and animals are sexual organisms
males use sperm/pollen to pass traits; females use eggs
need for pollen in sexual reproduction by plants is well known
Also by the early 1800s, farmers and stockbreeders had some rules of thumb on practical genetics:
1. Stable varieties nearly always breed true.
2. You can mate two different parents to generate hybrids, which can be identical to one parent or
combine features of both parents. (hybrid x hybrid crosses result in extreme variation in the offspring
hybrids don t breed true)
Sometimes you get sports (mutants), even in stable varieties. (The sports could be backcrossed with
normals to create new stable varieties.)
Problem for Darwin, Wallace, etc.: no understanding of the genetic basis of variation and inheritance
until Mendel's work is found in 1900
But, a prevalent idea in the 19th century was fixity of species:
each species is a separate creation by a creator
species have not changed over time
That species are not fixed was, a numerous present-day commentators have put it, “a common enough
heresy at the time.”
Erasmus Darwin (Charles Darwin's grandfather): wrote a two volume book (Zoonomia) and poems
about how organisms have changed over time
Buffon (a.k.a. Laclerc): in his 44 volume treatise on natural history, provided evidence for change over
time among organisms
Lamarck: hereditary change over time occurred due to characteristics acquired during the lifetime of
an organism. Lamarck's proposed mechanism for species change is sometimes shortened to
“inheritance of acquired characteristics.” Lamarck's mechanism focused on organisms striving to
acquire new traits or better versions of existing traits. This is not the modern day view of modified
traits arising by the random occurrence of mutations.
Remember: science is a process to acquire information to understand the natural world
==> make observations, formulate hypotheses, make predictions, do experiments, explain results in a
manner consistent with observations
If Lamarck is correct about how species change over time, then we should be able to alter an existing
trait and then see that new version passed to offspring.
==> Removed mouse tails for several generations. Tails never disappeared from subsequent
generations; tails didn't even get shorter over time. Conclusion: Lamarck disproved.
mid-1800s: Alfred Russel Wallace and Charles Robert Darwin
independently proposed that inherited change over time occurs by natural selection
Examples of variation in a domesticated plant species as a result of artificial
selection. All of these crops are the same species, Brassica oleracea.
Wallace and Darwin were two of the many Europeans who went to New World tropics in the 19th
century as naturalists
Darwin was on an official survey mission with the British Navy. He originally joined the
mission as the captain's social companion but became the de facto naturalist.
Wallace went with a friend (Henry Walter Bates). They backed by a vendor in Britain who
would sell their collections.
Both Wallace and Darwin collected multiple individuals of the same species
saw extensive variation within a single species
saw similarities between species in adjacent locales
observed geographic distributions of species (biogeography)
Wallace and Darwin shared two sources of information for their eureka moment
observing biogeography of similar species on nearby islands:
Darwin, the Galapagos Islands
Wallace, the Malay archipelago
reading Thomas Malthus's “Essay on the Principle of Population” :
resources limit the numbers in a population; the human population will outstrip its food
supply unless the population is held in check by war, disease, etc.
Wallace and Darwin realized that natural populations are held in check by some limiting factor in their
environments; only those individuals that survived the “check” will reproduce
Side note: Many textbooks imply that Darwin and Wallace agreed to a joint presentation of their
independent conclusions regarding natural selection. Here's the short version of the story. The
financing for Wallace's travels was the same as when he went to South America. He collected
specimens to sell to collectors in Europe. In 1858, while recovering from a recurrence of malaria, he
couldn't go out collecting. So, Wallace spent the time mulling over his various ideas about species. As
he thought about species variation and geographic distributions, he also was thinking about what
Malthus had written about human populations. Wallace basically tied things together as what we
would now call natural selection. He wrote up his theory (the Ternate essay, named for the island he
was on at the time) and sent it to Darwin, asking for feedback. (Darwin had purchased specimens from
Wallace, and they had developed an intermittent correspondence on natural history topics.) Darwin, on
reading Wallace's essay, realized that Wallace had come to the same conclusions that Darwin had first
written about (but kept between himself and a few close friends) in 1842. Darwin sent Wallace's work
to Charles Lyell, asking for advice. Lyell and Joseph Hooker (the director of Kew Gardens) arranged
for two items from Darwin (an unpublished essay from 1844 and an 1857 letter to American botanist
Asa Gray) and Wallace's essay to be read at the July 1858 meeting of the Linnean Society. Lyell and
Hooker attended the meeting. Darwin was indisposed and stayed home. Wallace was still island
hopping in the Malay Archipelago and knew nothing about the arrangement as the letter informing him
had yet to arrive.
Natural Selection
more offspring are produced than can survive
individuals within a population vary; at least some of the variation can be inherited
differential reproduction and survival of members of the population
Therefore, over time, the most advantageous traits for reproduction and survival (adaptations) will
increase in a population
Adaptation
Adaptations can be structural, physiological, &/or behavioral.
An adaptation aids in fitting an organism to its environment.
examples
the butterfly egg mimics on the passionflower tendrils
The creosote bush produces allelopathic chemicals which reduce competition for water and
minerals.
Darwin's part of the natural selection story
Darwin in the Galapagos
was showing a govt official the tortoises that he had recently collected
this person said that he (Darwin) must have been on island X in order to get those particular
tortoises
Darwin realized that the different islands have distinct tortoise species and that he should label all of his
collections as coming from a specific island, rather than the Galapagos as a whole. Darwin looks at his
earlier collections and has to re-label the one he made of Galapagos birds (which became known as
Darwin's finches).
Darwin's Finches
illustration of the finches from Origin of Species
Darwin thought that his birds from the Galapagos
were different kinds of birds: finches, wrens,
woodpecker, etc. After returning to Britain, an
ornithologist told him that all were finches.
Darwin starts to think about a process that could
result in beaks adapted to the food source
available on each finch's native island.
Natural Selection
result of variation, differential reproduction, and heredity
it is not planned; there are no goals; there is no "progress" up an evolutionary ladder
it does not produce perfection; an organism is “good enough” to survive long enough to produce
offspring
Can natural selection be used to make predictions about what might be observed in nature?
==> pollination of Darwin’s orchid
Darwin’s Orchid
In 1862, Darwin was sent some flowers of
Angraecum sesquipedale (Madagascar star orchid
or, as it later became known, "Darwin's orchid").
This orchid has impressively long spurs with
necataries at the end of the spur. Darwin
predicted that a moth with a 10-11 inch long
proboscis must exist as only such a creature could
both reach the nectar and be positioned while
doing so to collect or deliver pollen. A few years
later, Fritz Müller found moths close to that size
in Brazil, so it was possible that such a moth could
exist on Madagascar. The moth, Xanthopan
morgani praedictum (named in honor of Darwin’s
prediction), was discovered in 1903 on
Madagascar by W. Rothschild and K. Jordan.