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Evolution (PCB 4674).
Chapter 3. Darwinian natural selection
Main topics of lecture:
I: Natural Selection: Darwin's four postulates
1.- Introduction
2.- The evolution of beak shape in Galápagos finches
3.- The nature of natural selection
II: The evolution of Darwinism
4.- Main initial problems of Darwinism
5.- The modern synthesis
I: Natural Selection: Darwin's four postulates
1.- Introduction
1.1.- Natural selection is the logical outcome of four postulates, which Darwin laid out in
his introduction to "On the origin of species by means of natural selection" (published in
1859). He considered the rest of the book "one long argument" in their support. The four
postulates are as follows:
(1) Individuals within species are variable
(2) Some of these variations are passed on to offspring
(3) In every generation, more offspring are produced than can survive
(4) The survival and reproduction of individuals are not random:
Individuals who survive and go on to reproduce are those with the most
favorable variation. They are naturally selected
1.2.- The main consequence of these four postulates is that the characteristics of
populations change from one generation to the next. As a result, the characteristics
of the population will change slightly with each succeeding generation. This is
Darwinian evolution: gradual change in populations over time
1.3.- Darwin referred to individuals who make up a greater percentage of the population
in the next generation as more fit. In doing so he introduced the concept of fitness.
Darwinian fitness is the ability of an individual to survive and reproduce in its
environment. Biologists use the word adaptation to refer to a trait or characteristic of
an organism that increases its fitness relative to individuals without the trait.
1.4.- One of Darwin's theory of evolution is that each of the four postulates - and
their logical consequence - can be verified independently. In the next section, we examine
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each of the four postulates by reviewing and ongoing study of finches in the
Galápagos Islands
2.- The evolution of beak shape in Galápagos finches
2.1.- There are 14 species of finches on the Galápagos islands. Two traits do show
remarkable variation among these 14 species: the size and the shape of their beaks
(Fig. 3.1). The beak is the primary tool used by birds in feeding, and the enormous
range of beak morphologies reflects the diversity of foods they eat.
Figure 3.1.: Phylogeny of the 14 species of Galápagos finches.
The photo show the extensive variation in beak size and shape
among species
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2.2.- We will test the four postulates of Darwin using data that the team of Grant and
Grant have gathered on the medium ground finch, Geospiza fortis, on Isla Daphne Major.
This is a tiny island close to the island of Santa Cruz. The island has about 1,200 individuals.
This species is primarily a seed eater. Beak size is correlated with the size of seeds harvested.
In general, birds with bigger beaks eat larger seeds, and birds with smaller beaks eat
smaller seeds.
Testing Postulate 1: Are populations variable?
2.3.- All the finch characteristics that the team of Grant and Grant measured clearly
conform to Darwin's first postulate. For instance measurements of beak depth of G. fortis
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indicated that this character varies in the individuals of the Isla Daphne Major (Fig 3.3)
Figure 3.3.: Beak depth in medium ground finches. The histogram
shows the distribution of beak depth in medium ground finches
on Daphne Major in 1976, at the start of the Grant study. A few
birds have shallow beaks, less than 8 mm deep. Most birds have
medium beaks, 8 to 11 mm deep. A few birds have deep beaks,
more than 11 mm deep
Testing Postulate 2: Is some of the variation among individuals heritable?
2.4.- Within a population, individual finches could vary in beak depth because the environments
they have experienced are different or because their genotypes are different, or both. For instance
variation in the amount of food that individual birds happened to have received as chicks can
lead to variation in beak depth among adults. Likewise injuries or abrasion against hard seeds
or rocks can also affect beak size and shape
2.5.- To estimate whether at least part of the variability among finch beaks is genetically based the
heritability of this trait was estimated. Heritability is the proportion of the variation observed
in a population that is due to variation in the effects of genes. It varies between 0 (all variation
in this population is due to the environment and therefore all the individuals of the population are
genetically identical for a particular trait) and 1 (the levels of genetic variation for a particular trait in this
popualtion are so high that each individual has a different genotype for this trait). Comparisons of
beak depth of G. fortis young after they had attained adult size to the average bill depth of their
mother and father show a very strong correlation (Fig. 3.4).
2.6.- Parents with deep beaks tend to have offspring with deep beaks, and parents with
shallow beaks tend to have chicks with shallow beaks. This is evidence that a large
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proportion of the observed variation in beak depth is genetic, and can be transmitted to
offspring
Figure 3.4.: Heritability of beak depth in medium ground finches.
The red line and circles are data from 1978, and the blue line
and circles are from 1976 data. The results from the two years
are consistent. Both show a strong relationship between the
beak depth of parents and their offspring
Testing Postulate 3: Is there an excess of offspring, so that only some individuals
live to reproduce
2.7.- In 1977, there was a severe drought at Isla Daphne Major. Instead of the normal 130 mm
of rainfall during the wet season, only 24 mm fell. Over the course of 20 months, 84% of
the medium ground finches of this island disappeared (Fig. 3.5 a). There was a strong correlation
between population size and seed availability. This sort of mortality is not unusual, Rosemary
Grant has shown that 89% of Geospiza conirostris individuals die before they breed.
Figure 3.5.: Decline of ground finch population and available seeds,
during the 1977 drought
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2.8.- In every natural population studied, more offspring are produced each generation than
survive to breed. The reproductive capacity or biotic potential of organisms is astonishing
(Table 3.1). It has been shown that in most populations, some individuals are more
successful at mating and producing offspring than others. Variation in reproductive
success represents an opportunity for selection, as does variation in survival.
Table 3.1. This Table gives the number of offspring that a single individual (or pair of individuals
for sexual species) can produce under optimal conditions, assuming that all progeny survive to
breed, over various time intervals
Aphis fabae (an aphid)
Elephant
524.000.000.000 in one year. Source: Gould, 1977
19 million in 750 years. Source: Darwin, 1859
Housefly
191 x 1018 in 5 months. Source: Keeton, 1972
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Mycophila speyeri (a fly that
feeds on mushrooms)
Staphylococcus aureus (a
bacterium)
Starfish
20,000/squared foot in 35 days. Source: Gould, 1977
Cells would cover the Earth 7 feet deep in 48 hours. Source: Audesirk & Audesirk, 1993
1079 in 16 years. Source: Dodson, 1960. 1079 is the estimated number
of electrons in the visible universe
Testing Postulate 4: Are survival and reproduction nonrandom?
2.9.- After the severe drought of 1977 the team of Grant found that a distinct subset of
the population had survived best: those with the deepest beaks (Fig. 3.6). Because the average
survivor had a deeper beak than the average nonsurvivor, the average bill size in the
population changed.
Figure 3.6.: Beak depth before and after natural selection
2.10.- We can link ecological cause with evolutionary effect. After the drought both the
number and type of seeds changed. The large and hard fruits of an annual plant called
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Tribulus cistoides became a key food item after the drought. These seeds are largely ignored
in normal years, but during the drought the supply of small, soft seeds quickly became
exhausted. Only large birds with deep narrow beaks can crack and eat Tribulus fruits
successfully. In addition large birds defend food sources more successfully during
conflicts. Because large size and deep beaks are positively correlated, the two traits
responded to selection together. See point 3.6 for further discussion on selection for body
size and for beak depth and width
Did evolution occur?
2.11.- The answer is yes: As the data in Table 3.2 show, the offspring of birds surviving the
1977 drought were significantly larger, on average, than the population than existed before
the drought
Table 3.2.- Evolutionary response to selection. Sample size before selection: 634 individuals.
Sample size after selection: 135 individuals
Trait
Weight
Wing
length
Tarsus
length
Bill
length
Bill
depth
Bill
width
Mean before Standard error Mean after Standard error
selection
before selection selection
after selection
(1976)
(1976)
(1978)
(1978)
16.06
0.06
17.13
0.13
Differences
between
generations
+1.07
67.88
0.10
68.87
0.20
+0.99
19.08
0.03
19.29
0.07
+0.21
10.63
0.03
10.95
0.06
+0.32
9.21
0.03
9.70
0.06
+0.49
8.58
0.02
8.83
0.05
+0.25
3.- The nature of Natural Selection
There are some key points about how selection does and does not operate, in this section we
will cover these key points
3.1.- Natural selection acts on individuals, but its consequences occur in populations.
When HIV strains (see chapter 1) were selected by exposure to AZT, or finch populations
were selected by changes in seed availability, none of the selected individuals
(virions or finches) changed in any way. They simply lived through the selection event
while others died, or reproduced more than competing virions or birds. What changed
after the selection process were the characteristics of the populations of virions and
finches, NOT the affected individuals themselves. Specifically, a higher frequency
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of HIV virions in the population were able to replicate in the presence of AZT, and
a higher proportion of finches had deep beaks. To state this point another way,
the effort of cracking Tribulus seeds did not make finch beaks become deeper
and their bodies larger. Instead he average beak depth and body size in the finch
population increased because more smaller finches died than larger ones.
In the example of the medium ground finch from the Isla Daphne Major, after selection a
higher proportion of finches had deep beaks.
3.2.- Natural selection acts on phenotypes, but evolution ONLY consists of changes in
population allele frequencies for a paticular population.
Later we will expand more about the concept of evolution at population level.
Finches with large bodies and deep beaks would have been favored during
the drought even if all the variation in the population had been environmental in origin (that
is heritabilities had been zero). In that case no evolution would have been
occurred because there has not been selection to particular genotypes, the same genotypes
would have existed before and after selection occurred
3.3.- Natural selection is backward looking, not forward looking. Each generation
is a product of selection by the environmental conditions that prevailed in the
generation before. The offspring of finches that underwent natural selection are better
adapted to environments dominated by drought than their parent's generation was.
If the environment changed again during the lifetime of these offspring, however, they
would not necessarily be adapted to the new conditions. There is a common
misconception that organisms can be adapted to future conditions, or that selection
can lok ahead in the sense of anticipating environmental changes during future
generations. This IS IMPOSSIBLE. Evolution is always a generation behind any
changes in the environment
3.4.- Natural selection can produce new traits, even though it acts on existing traits.
Selection cannot, for example, instantly, create a new and optimal beak for cracking
Tribulus fruits. It only selects from the range of beaks already present in the population.
Over time, however, the evolution of new traits is possible because, in each generation,
mutations produce new variants, and thus a new suite of existing traits for selection to act
upon to. Many examples of this process can be found in artificial selection experiments.
A research team conducting these experiments with corn started with 163 ears of corn with
oil content ranged from 4-6%. They selected the 24 ears with the highest oil content to
be the parents and raised the offspring, tested their kernels for oil content, and again selected
the individuals with the highest oil content. By continuing this selection regime for 60 years,
the researchers succeeded in producing corn plants whose kernels had an oil content of
about 16% (Fig. 3.8). There is no overlap between the distribution of oil content in the
ancestral and descendant population. Evolution produced a new trait value
Figure 3.8.: Continued selection can result in dramatic changes
in traits
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3.5.- Natural selection can also lead to novel characteristics. This is possible because
selection is able to "repurpose" existing behaviors, structures, or genes for new functions.
The giant panda's thumb is a good example. Pandas use this structure like a sixth finger
when eating their favorite food, bamboo. But this sixth digit is not a true thumb at all.
Anatomically the bone that forms the "thumb" is a highly modified radial sesamoid, which
in closely species is a component of the wrist. A trait that is used in a novel way and
is eventually elaborated by selection into a complete new structure, like the radial sesamoid
of the ancestral panda, is known as a preadaptation
Figure 3.9.: The panda's 'thumb". Giant pandas strip the leaves from
bamboo by passing the stalk through their hands. The panda's
"thumb" forms a slot for bamboo stalks to pass through
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3.6.- Natural selection is not perfect. Evolution does not result in "perfect" traits.
For example when Boag and Grant (1981) analyzed their data on survival of Galapagos
ground finches after a drought which took place in 1977. As we indicated in point 2.10, it
was noticed that large individuals with relatively deep beaks survived better. The same
authors also notice that But narrow beaks also survive better as they are more
effective at cracking Tribulus fruits. But beak width is positively correlated with beak
depth and large body size. As a result large birds with deep beaks also tend to have
wide beaks, which are less effeective at cracking Tribulus fruits. The final outcome is
that selection for larger size and deeper beaks resulted in wider beaks, evem when
narrower beaks should have been favored. This is a very important point. Because of the genetic
correlation among characters, natural selection did not optimize all of the traits involved.
Natural selection DOES NOT OPTIMIZE ALL OF THE TRAITS involved. Natural
Selection results in overall adaptation, not perfection.
3.7.- Natural selection is nonrandom, but it is not progressive. Evolution by natural
selection is nonrandom because it increases adaptation to the environment. Although it is true
that evolution has tended to increase the complexity, degree of organization, and
specialization of organisms over time, it is not progressive in the sense of leading toward
some predetermined goal. Evolution makes organisms "better" only in the sense of
increasing their adaptation to their environment. There is no inexorable trend toward
more advanced form of life. For example contemporary tapeworms have no digestive
system, and have actually evolved to be simpler than their ancestors. Snakes evolved
from ancestors that had limbs. The earliest birds in the fossil record had teeth. It is
true that some organisms are the descendants of more recent lineages, but all organisms
in the fossil record and those living today were adapted to their environments. They
are all able to survive and reproduce. None is "higher" or "lower" than any other
3.8.- Natural selection acts on individuals, not groups. One misconception about natural
selection is that individual organisms will perform actions for the good of the species.
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Altruistic acts occur in nature [i.e., Prairie dogs give alarm calls when predators approach
(this draws attention to themselves), lion mothers sometimes nurse cubs that are not
their own.
3.9.- However selection CANNOT (!!!) favor traits unless they increase the bearer's
fitness relative to competing individuals. If an allele existed that produce a truly
altruistic behavior - that is, a behavior that reduced the bearer's fitness and increased
the fitness of others - it would be strongly selected against. Every case of altruistic
behavior that has been studied in detail has been found to increase the altruist's fitness
II: The evolution of Darwinism
4.- Main initial problems of Darwinism
4.1.- The Theory of Evolution by Natural Selection was not universally accepted by
biologists until some 70 years after it was initially proposed. There were serious problems
with the theory; as originally formulated by Darwin, that had to be resolved. The two most
important problems are:
4.1.- Because Darwin knew nothing about mutation, he had no idea how variability
was generated in populations. It was not until the early 1900s, when geneticists such
as Thomas Hunt Morgan began experimenting with fruit flies, that biologists began
to appreciate the continuous and nature of mutation. Morgan and colleagues showed
that mutations occur in every generation and in every trait
4.2.- Because Darwin knew nothing about genetics, he had no idea how variations
are passed on to offspring. It was not until Mendel's' experiments with peas were
rediscovered and verified, 35 years after their original publications (1865), that
biologists understood how parental traits are passed on to offspring.
5.- The modern synthesis
5.1.- Between 1932 and 1953 a series of landmark books was published that successfully
integrated genetics with Darwin's four postulates and led to a reformulation of the
Theory of Evolution. This restatement known as the Modern Synthesis or Evolutionary
Synthesis, was a consensus grounded in two propositions:
i.- Gradual evolution results from small genetic changes that are acted upon
by natural selection
ii.- The origin of species and higher taxa, or macroevolution, can be explained
in terms of natural selection acting on individuals, or microevolution
5.2.- With the synthesis, Darwin's original four postulates - and their outcome - could be
restated along the following lines:
i.- As a result of mutation creating new alleles, and segregation and independent
assortment shuffling alleles into new combinations, individuals within
populations are variable for nearly all traits
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ii.- Individuals pass their alleles on to their offspring intact
iii.- In most generations, more offspring are produced than can survive
iv.- The individuals that survive and go on to reproduce, or who reproduce
the most, are those with the alleles and allelic combinations that best
adapt them to their environment
5.3.- Other processes can cause evolutionary change in addition to natural selection (we
will explore these process in detail in other lectures of this course). However the Darwin
view of life, as a competition between individuals with varying abilities to survive and
reproduce, has proven correct in almost every detail
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