Download GAD EvoTalk

Evolutionary Biology Overview
Presentations for Georgia Academic Decathlon Coaches
29 August 2008, Scott Reese & Michael Dias
What is evolution?
In its most basic sense, evolution is about changes over time. It suggests that organisms
have some kind of common ancestry, some universal suite of common traits. Our
understanding of evolution today often relates to changes in allelic frequencies in a
population reproducing over time.
Why should we care about evolution?
Referenced quote from former Russian geneticist, Theodosius Dobzhansky:
“Nothing in biology makes sense except in the light of evolution.”
Biological science consists of dramatically different disciplines, with specializations at
the molecular, cellular, organismal, and ecological levels. Why are all of these different
disciplines in the same department? Evolution brings all the different biology disciplines
together. There is a common language.
How much medical research is done on humans? Only about 10%. The rest is
done on rats, cats, dogs, pigs, monkeys. The fact is, we can advance our knowledge of
human physiology by studying the same processes in other animals. This is based on
common ancestry that yields physiological similarities. By and large, we make many of
our knowledge gains in medical science because evolution tells us where to look. A field
known as evolutionary medicine helps us understand and respond to pathogenic diseases,
cancer, and genetic disorders.
Evolutionary thought before Darwin
Charles Darwin was not the first to posit the idea of evolution. The idea of transmutation,
that species can change, had been around prior to Darwin’s time of the mid 19th century.
Charles’ Darwin’s grandfather Erasmus had previously written on the possibility of
transmutation. More widely known, Jean-Baptiste de Lamarck is typically cited in
biology textbooks, but most remembered for being wrong in his idea of “inheritance of
acquired characteristics.” His notion of transmutation was that organisms develop traits
in one generation, then pass it on to the next generation. The classic example is
Lamarck’s explanation of how the giraffe got its long neck. Early giraffes were more like
what we’d call a little horse, and Lamarck posulated that in reaching for higher leaves,
they stretched their necks, and somehow this physical change was then passed on to the
offspring giraffes. Today we laugh at this and know that it can’t work that way, but what
we should remember Lamarck for is his championing of the idea that species could
change. In his day, the big names in the field (Cuvier, Linneus, Owens, Paley) rejected
the transmutation hypothesis. Over his life, Lamarck gave a huge push for evolutionary
thinking. Although he got the mechanism wrong, he nevertheless promoted the idea that
species can change, so when Darwin and Wallace proposed a mechanism for speciation,
the scientific community was open to it.
2
The other historical change in thinking that would impact Darwin was the view of
the Earth’s age. The longstanding notion of catastrophism, that the Earth was shaped by
dramatic events, and what happened in the past does not happen anymore, came under
question. Charles Lyell summarized several decades of research into his three volume,
Principles of Geology, published in 1833. In this work, Lyell lays out the case for
uniformitarianism, the idea that catastrophic processes were not responsible for the
landforms of Earth's surface. This idea was diametrically opposed to the ideas of the late
1700s, which were based on a biblical interpretation of the history of the Earth. Instead,
the theory of uniformitarianism suggested that the landscape developed over long periods
of time through a variety of slow geologic processes. Today we think life has existed on
Earth for about 4.5 billion years. From this perspective, we can investigate the past or
natural history of life. That was a big transition in our thinking. History credits Lyell’s
view of the planet as paving the way for Darwin. In fact, Charles Darwin read Lyell’s
book on his voyage.
Darwin’s Voyage of the Beagle1
Charles Darwin comes into his own on the Beagle voyage. Her Majesty’s Ship, The
Beagle was a 16 gun brig, a rather small vessel with the charge of mapping the coast of
South America. This survey expedition set sail from Plymouth Sound on December 27,
1831, under the command of Captain Robert Fitzroy. You have to keep in mind that there
was a rather rigid social structure in England at this time, and as such, the ship captain
would be rather isolated on the ship, unable to bunk, dine, or even converse with the ship
crew. Thus there was reason to worry about ship captains becoming lonely, and
despondent when long at sea, despairing at times to suicide. As a matter of fact, Fitzroy’s
father had committed suicide. Moreover, the previous captain of The Beagle had
committed suicide. Thus it was imperative that a member of the British aristocracy join
the voyage as the “cabin boy.” This honorary position was open for an intellectual and
aristocratic equal to the captain, whose primary job would be to chat and dine with the
captain. Though Charles Darwin had just completed theological studies at Cambridge, he
had also cultivated his knowledge and skills as a naturalist through his adolescents and
early adulthood. Thus, when Darwin’s parents purchased for him the position on the
Beagle voyage, he was quite competent to serve as ship’s naturalist. In his autobiography,
Darwin summed up the impact of the voyage on his life.
The voyage of the Beagle has been by far the most important event in my
life and has determined my whole career. …I have always felt that I owe
the voyage the first real training or education of my mind. I was led to
attend closely to several branches of natural history, and thus my powers
of observation were improved. …I had brought with me the first volume
of Lyell’s Principles of Geology, which I studied attentively; and this book
was of the highest service to me in many ways. …During some part of the
day I wrote my journal, and took much pains in describing carefully and
vividly all that I had seen. …As far as I can judge of myself, I worked to
the utmost during the voyage from the mere pleasure of investigation, and
An engaging summary of Darwin’s voyage, with graphics is available at
http://www.aboutdarwin.com/voyage/voyage03.html
1
3
from my strong desire to add a few facts to the great mass of facts in
natural science. But I was also ambitious to take a fair place amongst
scientific men.2
Although Darwin was seasick throughout the voyage, he still managed to conduct
his sorting, labeling, and writing. At sea he trailed trawls astern and gathered marine life.
Every chance he got he went ashore to make geological observations and collect
specimens. His energy and spirit soared on land. He hired horses and guides, arranged for
camping trips into the interior, climbed mountains, and rented bungalows for weeks at a
time when Fitzroy and crew were busy mapping rivers. Always Darwin collected,
dazzled by the richness, the strangeness and variety of what he found. At intervals he
packed up crates of specimens and shipped them off to Henslow in London, who not only
took good care of all this material, but also showed some of it to colleagues, and read
extracts from Darwin’s letters at meetings of the Philosophical Society. Unbeknownst to
Darwin, he was emerging in England as a respected young scientist.
Influence of Lyell and Geologic Observations
Lyell’s Principles of Geology were encountered by Darwin as he traveled about in
South America. Everywhere he looked he found evidence of the slow continuing
processes that formed the basis of Lyell’s uniformitarian argument. In the Andes, he saw
stratified layers of rock laid down by lava flows. At Valdivia, Chile, Darwin experienced
a severe earthquake, impressing him with the fragility of the earth’s crust. He found
marine fossils on upland plateaus, indicating those places has once been ocean bottoms.
At Cape Verde Islands on Santiago, Darwin noted a horizontal white band of shells in a
cliff face along the shore of Porto Praya and pondered why this layer was over forty feet
above sea level. Perhaps a gradual upward movement of land raised the strata of marine
fossils. More catastrophic movements of the earth would have surely broken this
horizontal line of shells.
Amid continual evidence of slow geologic processes, Darwin observed most
varied and peculiar life forms.
The sheer magnitude of the variety made him wonder whether differences
between one animal and the next should be ascribed to an over-generous
God or to some more natural cause. The animals of Australia were so
different to animals in the rest of the world that he pondered whether two
distinct creators had been at work. In the Galapagos Islands he was told
that the finches and tortoises differed marginally from one island to
another. But why should God create different finches to populate each
island in such a small archipelago? Why should the biology of the
Galapagos resemble that of mainland South America but not that of the
2
Charles Darwin and Thomas Henry Huxley, Autobiographies, (London: Oxford University Press, 1974),
pp. 44-46.
4
distant Cape Verde Islands, even though the climate and environment are
similar?3
Darwin’s Observations
Three key observations were foundational to Darwin’s thought process: comparison of
organisms on mainland Africa with that of the Cape Verde Islands, comparison of organisms on
the South American mainland to those of the Galapagos Islands, and how organisms varied
within the Galapagos Islands. Observations made in the Galapagos likely gave Darwin the first
firm suspicion to species immutability. At this point in the journey, Darwin had seen enough to
appreciate that Cape Verde and Galapagos islands had similar geological features and similar
environmental conditions. At the time the prevailing thought was that in similar environments
you should see similar animals. That is not what Darwin found. He found Cape Verde animals
similar to but more diverse than those on mainland Africa. He also found Galapagos animals
similar to those on mainland South America yet different from those found on the Cape Verde
islands.
Galapagos Finches and Adaptive Radiation
When Darwin explored the different Galapagos Islands, he found greater diversity
within; lots of environmental niches or different roles that organisms could fulfill as they
went about seeking food and shelter and mates. The classic example of Darwin’s finches
helps us understand the idea of adaptive radiation – the evolution from one species of
plants or animals or microbes to a number of different forms. As the original population
increases in size, it spreads out from its center of origin to exploit new habitats and food
sources. Eventually this results in a number of populations each adapted to its particular
habitat. In time these populations will differ from each other sufficiently to become new
species. A good example of this process is the evolution of the Australian marsupials into
species adapted as carnivores, herbivores, burrowers, fliers, etc. On a smaller scale, the
adaptive radiation of the Galapagos finches provided Darwin with crucial evidence for
his theory of evolution.
Influence of Malthus
Upon his return to England in October, 1836, Darwin consulted experts in analyzing his
collection, and did in fact find that 6 to 8 distinct finch species were collected in the Galapagos
(today we recognize 13). Darwin worked for two years on his “transmutation notebooks” as he
sorted and classified the collection from the Beagle voyage. It was then that he read Thomas
Malthus’ Essay on the Principles of Population. Darwin asserts that the reading of Malthus was
his moment of discovery.
I happened to read for amusement Malthus on Population, and being well
prepared to appreciate the struggle for existence which everywhere goes
on from long-continued observation of the habits of animals and plants, it
3
Vernon Blackmore and Andrew Page, Evolution: The Great Debate (Oxford: Lion Publishing, 1989), p.
66.
5
at once struck me that under these circumstances favorable variations
would tend to be preserved, and under unfavorable ones to be destroyed.
The result of this would be the formation of new species.4
Where others had considered the struggle only as a means of keeping a species
unchanged, Darwin saw the struggle to survive in favor of the “more fit” variants. Darwin
saw that the concept of struggle could lead to individuals with greater reproductive
success spreading the traits that contributed to this fitness through a population. Thus,
from Malthus, Darwin derived his evolutionary mechanism of natural selection.
Environmental pressures in nature (such as competition for food) select for particular
variants within the population. Species can change through differential reproductive
success in the population. The more fit members of the population (the more successful
reproducers) are better adapted, and in time this supports the evolution of different
species.
In The Origin of Species by Means of Natural Selection published in 1859,
Darwin provides four chapters explaining natural selection and ten more chapters
presenting over fifty examples of natural selection.5 Darwin’s theory of common descent
was rapidly accepted by the scientific community at this time, largely because it
explained the Linnaean hierarchy of life and many findings of comparative anatomy. In
contrast, it took eighty years before the concept of natural selection was universally
adopted. The genetic basis for population thinking had to develop and displace
typological thinking, the concept that organisms of a species conform to a specific norm,
a view that considers variation abnormal.
http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html
Darwin's theory of evolution has four main parts:
1. Organisms have changed over time, and the ones living today are different from those that
2.
3.
4.
4
5
lived in the past. Furthermore, many organisms that once lived are now extinct. The world is
not constant, but changing. The fossil record provided ample evidence for this view.
All organisms are derived from common ancestors by a process of branching. Over time,
populations split into different species, which are related because they are descended from a
common ancestor. Thus, if one goes far enough back in time, any pair of organisms has a
common ancestor. This explained the similarities of organisms that were classified together -they were similar because of shared traits inherited from their common ancestor. It also
explained why similar species tended to occur in the same geographic region.
Change is gradual and slow, taking place over a long time. This was supported by the fossil
record, and was consistent with the fact that no naturalist had observed the sudden
appearance of a new species. [This is now contested by a view of episodes of rapid change and
long periods of stasis, known as punctuated equilibrium].
The mechanism of evolutionary change was natural selection. This was the most important and
revolutionary part of Darwin's theory, and it deserves to be considered in greater detail.
Charles Darwin, Autobiography, (New York: Henry Schuman, Inc., 1950), p. 54.
Full text available at http://www.talkorigins.org/faqs/origin.html
6
The Process of Natural Selection
Natural selection is a process that occurs over successive generations. The following is a summary of
Darwin's line of reasoning for how it works (see Figure).
o
If all the offspring that organisms can produce were to survive and reproduce, they would soon
overrun the earth. Darwin illustrated this point by a calculation using elephants. He wrote:
"The elephant is reckoned the slowest breeder of all known animals, and I have taken
some pains to estimate its probable minimum rate of natural increase; it will be safest
to assume that it begins breeding when 30 years old and goes on breeding until 90
years old; if this be so, after a period from 740 to 750 years there would be nearly 19
million elephants descended from this first pair."
o
The Process of Natural Selection



This unbounded population growth resembles a simple geometric series (2-4-8-16-32-64..) and
quickly reaches infinity.
As a consequence, there is a "struggle" (metaphorically) to survive and reproduce, in which only a
few individuals succeed in leaving progeny.
Organisms show variation in characters that influence their success in this struggle for existence.
7


Individuals within a population vary from one another in many traits. (Animal behavioralists
making long-term studies of chimps or elephants soon recognize every individual by its size,
coloration, and distinctive markings.)
Offspring tend to resemble parents, including in characters that influence success in the struggle to
survive and reproduce.
Parents possessing certain traits that enable them to survive and reproduce will contribute
disproportionately to the offspring that make up the next generation.
To the extent that offspring resemble their parents, the population in the next generation will
consist of a higher proportion of individuals that possess whatever adaptation enabled their parents
to survive and reproduce.
The well-known example of camouflage coloration in an insect makes for a very powerful, logical
argument for adaptation by natural selection. Development of such coloration, which differs
according to the insect's environment, requires variation. The variation must influence survival and
reproduction (fitness), and it must be inherited.
During the early and middle 20th Century, genetics became incorporated into evolution, allowing
us to define natural selection this way:
Natural Selection is the differential
reproduction of genotypes.
Natural Selection Requires...
For natural selection to occur, two requirements are essential:
There must be heritable variation for some trait. Examples: beak size, color pattern,
thickness of skin, fleetness.
There must be differential survival and reproduction associated with the possession of that
trait.
Unless both these requirements are met, adaptation by natural selection cannot occur.
Content in this box taken from a University of Michigan course website, link below:
http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html
Darwin provided the mechanism, natural selection, by which populations of
species change over time. His key insight was that species can change because members
of a population have more reproductive success than others because of some adaptation
to the environment.
In the early half of the 20th century, evolutionary biology as a science grew
beyond what Darwin gave us. In Darwin’s day of the latter half of the 19th century two
different schools of thought existed: the geneticists thought in terms of mutations causing
variation within a population, while the transmutationists were more Lamarckian in their
8
thinking of some gemmules floating in the blood and providing a blending-type
inheritance of traits from the parents. In the 1930s, we began to see that these fields are
not as disparate as we thought. In a couple decades, from 1932 to 1953, “The Modern
Synthesis” developed. This was the merging of Darwinian evolution with the new field of
population genetics. This knowledge synthesis added the following knowledge of
population genetics to natural selection:
Population is a group of species with the potential to breed, Any given gene may
have variations of alleles. Meiosis assorts these alleles into gametes. Segregation
(separation) of parental alleles sets up variability about a mean, which distributes
as a bell-shaped curve (normal distribution). There is a continuum of variation.
Alleles are passed on to offspring intact. It is not the case that children are a
blending of their parents phenotypes. Alleles are transferred in whole units to
sperm and egg and then to zygote.
This sets up a situation where we can talk about Darwinian evolution in genetic
terms. More offspring more of a trait. Evolution is about sex. Evolution is not
about survival of the fittest merely for meeting habitat and food (resource) needs,
but fitness is all about reproductive success, passing on one’s genes. If an
organism is born today, reproduces tomorrow, and dies the next day, it is as fit as
one who reproduces every 80 years.
Geographical Isolation (Allopatric Speciation)
New species usually develop due to geographical isolation (allopatric speciation). This
occurs when a population of the same species is divided by a physical barrier (water,
mountains, desert, or distance). This may occur due to migration, dispersal or geologic
events like erosion, floods, volcanic eruption. If the two environments differ significantly,
the two populations will experience different selection pressures, and over time will
evolve differently. If the descendents of the originally separated population diverges to
the point that the two populations can no longer interbreed to produce fertile offspring,
you’ve got 2 new species where at first there was one.