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Introduction to Biological Anthropology: Notes 3
Why evolution happens
 Copyright Bruce Owen 2011
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− Recall that Darwin's observations on the Beagle convinced him of the fact of evolution
− that is, that populations of organisms change over time
− The problem was to explain how evolution could occur.
− There was already a theory for this, originally proposed by Jean Baptiste Lamarck in several
volumes from 1815-1822, when Darwin was still a child
− called Lamarck’s theory of inheritance of acquired characteristics
− Like Darwin and some others, Lamarck also believed that evolution occurred
− he explained it by proposing that organisms changed over their lifetimes
− body parts that they used a lot developed more, and ones that they used less developed less
− for example, a giraffe that spent its life straining to reach high leaves would develop a
longer neck than it would have it if had grazed on grass
− a longer neck due to use during life would be an “acquired characteristic”
− since the individual was not born with the characteristic
− but acquired it due to events during its life
− Lamarck then suggested that these acquired characteristics would be inherited by the
animal's offspring
− so that the next generation of giraffes would start with a slightly longer neck to begin with,
and would develop it even further if they kept reaching for high leaves
− this sounds reasonable, but there was no evidence of acquired characteristics actually being
inherited
− you can observe that tanned parents have kids who are born just as pale as the kids of their
un-tanned relatives
− children of people who work out and become strong or fast are not born with any greater
strength or speed than children of parents who never exercised
− animal breeders did not succeed in creating
− faster horses by vigorously training them, generation after generation,
− or fatter cows by overfeeding them over several generations
− it just didn't work
− Darwin (like many people at the time) was aware of how animal breeders actually did develop
new variants
− by controlling mating
− breeders would select the individual animals that had the desired characteristics
− and allow them to mate, and not the others
− then pick the offspring with the features they wanted
− and allow those to mate, and not the others
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− and so on, generation after generation
− this method was well known to actually produce new variants
− pigeons with larger tails, flatter faces, odd flying patterns, etc.
− dogs that were larger or smaller, more or less aggressive, different kinds of hair, etc.
− that is, what actually worked for animal breeders was selecting which individuals would
have offspring, and which would not
− not, as Lamarck suggested, creating acquired characteristics by training, special feeds, etc.
− this method is sometimes called artificial selection
− “selecting” which individuals would mate, and which would not
− But artificial selection obviously did not happen in nature…
− In 1838, two years after returning from the voyage of the Beagle, and at the ripe old age of 28,
Darwin read An Essay on the Principle of Population, by Thomas Malthus
− Malthus's book had been published 40 years before, in 1798
− Malthus argued that human populations grow geometrically (they double every so many
years), while food production grows more slowly
− so population eventually outstrips food supply
− and some correction inevitably occurs: famine, disease, or war, which reduces the population
back down to what the food supply can support
− Darwin realized that the same thing happens with populations of plants and animals
− more individuals are born than survive to adulthood
− and some adults have more offspring than others
− the ones that are best suited to survive and to reproduce are the ones that leave the most
offspring
− this natural process selects the parents that are most suited to survive and to have offspring
− that is, natural selection of parents does the job of the animal breeder
− the characteristics of the ones that reproduced would become more common in the
following generations
− just as if an animal breeder had artificially selected them
− The theory of adaptation by natural selection was born.
− Darwin was a perfectionist, so he spent the next twenty years working on his collections and
refining the evidence to support his arguments
− When Darwin learned that a younger naturalist named Alfred Russel Wallace had come to the
same conclusions that he had, and was planning to present a paper about them, he finally had
to go public or become a forgotten footnote in history
− Fortunately, both Darwin and Wallace were honest, civil people, and they agreed to both
present papers at the same conference in 1858.
− Darwin then quickly finished the book that outlined and defended his theory, and published it
in 1859.
− The title was On the Origin of Species by Means of Natural Selection or the Preservation of
Favoured Races in the Struggle for Life.
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− The Origin was an instant bestseller. It sold out on the first day, and it eventually went
through five more editions, plus countless reprints since.
− It was aimed both at specialists and the educated public
− It laid out a logical, scientifically supported theory of evolution, and popularized it at the
same time
− it was controversial
− some people accepted it, and others blasted it
− Today, many readers find the Origin a bit too detailed and the writing style a bit oldfashioned, but it is clear and understandable. If you can read Charles Dickens, you can read
Charles Darwin.
− if evolution interests you and you have some patience, I highly recommend it
− There are lots of copies around in used bookstores
− The class web page has a link to the full text online
− Darwin got it right
− Although some important details could not be filled in until almost a century later,
− Darwin’s basic theory is still the core the model of evolution that virtually all biologists
believe explains the nature of life on earth – and other things, too.
− Darwin's theory of adaptation by natural selection
− Adaptation has at least two meanings:
− Adaptation as a feature:
− An adaptation is a physical or behavioral feature of an organism that helps the individual
to survive and reproduce in its environment
− examples of adaptations in cats: good vision, sharp teeth, and speed that make them good
hunters, able to get food
− Adaptation as a process:
− Adaptation is a process of biological change by which a population of organisms becomes
adjusted (“adapted”) to its environment over many generations.
− as in “The species adapted to its new environment.”
− “adaptation” refers only to heritable change over generations (that is, evolution)
− “adaptation” does not refer to the adjustments that an individual organism makes during
its lifetime
− like developing stronger muscles or a tan
− “adaptation” refers only to features that help the organism survive and reproduce
− that is, that make the organism better suited to its environment
− Not all evolution results in better adaptations!
− some evolutionary change may be neutral, or even harmful to survival and reproduction
− the development of adaptations is only a subset of evolution
− although it is a very important one
− and the easiest to understand
− and the one the Darwin focused on
− Darwin’s theory rests on three simple postulates:
− (Postulate: an assumed condition or fact
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− in this case, all three are observed to be true in effectively all real circumstances)
− 1. Organisms produce many more offspring than can survive and reproduce
− This is easy to observe, even in human populations.
− Prior to modern medicine, a large percentage of infants did not make it to adulthood
− In a litter of kittens, unless people intervene, usually only some survive
− in the California sierra, pine trees are often surrounded by hundreds or thousands of
cones that were once full of seeds, of which almost none actually grow into trees
− Darwin called this “the struggle for existence”
− 2. Individuals vary in ways that affect how successful they are at surviving and reproducing
− All individuals are different
− Some of these differences affect how successful they are at surviving and reproducing
− This is sometimes expressed as: “There is variation in fitness” or “there is variation in
reproductive success”
− fitness is best measured by reproductive success: how many offspring the individual has
that survive to reproductive age
− note that “fitness” does not mean strength, intelligence, etc. -- it means only the ability to
survive and successfully reproduce
− if stronger or smarter individuals leave more offspring, they have greater fitness
− but if being stronger requires spending enough more time eating and less time mating
that the individual leaves fewer offspring, then stronger individuals have lower fitness
− 3. Offspring tend to resemble their parents
− not in all ways, of course, but many traits do tend to be inherited
− Short, blonde parents are likely to have short, blonde children
− relatively more “fit” individuals will tend to have relatively more “fit” offspring
− This is sometimes expressed as: “Some variation is inherited”
− The logical, necessary result is natural selection
− Some parents leave more offspring than others
− those offspring tend to be similar to their parents
− so the traits of the most successful parents are common in the next generation
− that is, natural selection occurs
− the three postulates mean that natural process will automatically do what animal
breeders do in artificial selection
− But instead of a animal breeder who intentionally picks the desired traits, in nature it is
the “struggle for existence” that does the selecting
− many individuals don’t survive, or if they do, don't reproduce as much as others
− The variants that are most “fit” survive and reproduce most
− The traits they have are passed on to the next generation
− over time, the most “successful” traits become more and more common, and the least
“successful” traits become rare and disappear
− Note: this is NOT simply “survival of the fittest”!
− surviving counts for nothing if you don't reproduce
− the bottom line is leaving the most offspring (that themselves survive and reproduce)
− reproducing, not just surviving, is what affects evolution
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− So where is “evolution” in this picture?
− natural selection may cause evolution
− say the parents that leave the most offspring are taller than average for the population
− and tallness tends to be inherited
− then there will be more tall offspring in the next generation
− so the average height will be greater
− evolution will occur
− this is directional selection: selection that causes change in some direction
− this change is called evolution
− but natural selection does not always cause evolution
− say the parents that leave the most offspring are the height that is already average in the
population
− then they will leave lots of offspring with that same average height
− and the average height of the population will remain unchanged
− the average height is already optimal, so there is no selective advantage for those who are
taller or shorter
− in this case, natural selection is maintaining the status quo, and no evolution occurs
− this is stabilizing selection: selection that keeps the features stable
− remaining the same is called stasis
− So, let's go back to the watchmaker analogy
− when you see an intricate, functioning watch, you assume there was a watchmaker
− when you see an intricate, functioning organism, what is the difference?
− living things reproduce themselves
− but they reproduce themselves with some variation
− this makes living, reproducing things a whole different ballgame
− it makes natural selection possible
− in fact, it makes natural selection inevitable: it has to happen
− so it makes evolution possible
− in fact, it makes evolution inevitable: it has to happen!
− now, consider that we are beginning to make artificial things that reproduce themselves
− computer viruses
− genetically engineered organisms
− perhaps, eventually, machines
− if they reproduce themselves with some variation, and they vary in the number of “offspring”
that they produce, then these things will evolve too…
− Let’s look more closely at a well-documented example of evolution: Darwin’s finches
− The same case I mentioned last time
− This is simply one very clear and well-documented example case
− that shows that Darwin’s model actually does fit real life
− there is nothing special about these finches except than that Peter and Rosemary Grant
went to the trouble to document them thoroughly
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− and that they are historically associated with Darwin, since he observed these birds on the
voyage of the Beagle
− Other cases have been studied (although few so thoroughly) with similar results
− presumably the same kinds of things have happened around the world and throughout
time, even when no scientists were looking.
The finch study
− was conducted on one of the Galapagos islands, Daphne Major, in the Pacific far off the
coast of Ecuador
− The subjects are medium ground finches (Geospiza fortis), which mostly eat seeds
− The Grants captured, banded, and recorded measurements and other information about
almost all the birds on the island over a number of years
− and also recorded other information about the environment, especially the availability of
different kinds of seeds
Early in the study, the island received a lot of rain, but later on, it suffered a severe drought
− This provided a great opportunity to watch natural selection in action
− The Grants were able to show that all three of Darwin’s postulates were actually
happening
− and that natural selection and evolution did occur, just as Darwin (and logic) predicted
During the drought, the plants on the island produced fewer of the small, soft varieties of
seeds that the finches prefer
− The finches quickly ate most of the small, soft types of seeds
− This left mostly larger, harder seeds, which many of the birds found difficult to eat
− The food shortage caused many birds to starve to death
Postulate 1: Darwin’s first postulate was met: more birds were born than survived to
reproduce
− The finch population declined from 1200 birds to 180 birds in just two years of drought
− This was obviously a severe case, but not that unusual in the real world
Postulate 2: The Grants found that there was variation in the depth (height) of the finches
beaks, and it affected their survival (and thus reproduction)
− beaks ranged smoothly from relatively shallow beaks to relatively deep ones
− birds with different beak depths preferred different kinds of seeds
− birds with shallow beaks preferred smaller, softer seeds
− probably because their beaks were not strong enough to crack the larger, harder seeds
− birds with deeper beaks preferred larger, harder seeds
− probably because there were more of them around, and these birds were able to crack
them with their stronger (deeper) beaks
− So, the variation in beak depth caused a variation in fitness when small seeds were scarce
− birds with shallow beaks had trouble finding enough food
− birds with deeper beaks were more likely to find enough food
− so birds with deeper beaks were more likely to survive during the drought
− So, Darwin’s second postulate was also met: the birds varied in how well they survived,
apparently because they varied in how well they could get food during the drought
− The deeper the beak, the more likely the bird was to survive during the drought years
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− Postulate 3: Finally, the Grants showed that Darwin’s third postulate was also working in
nature: offspring tended to resemble their parents
− They showed that shallow-beaked birds tended to produce shallow-beaked offspring, and
deep-beaked birds tended to produce deep-beaked offspring
− This is no surprise, of course
− but it is absolutely necessary for the theory to work
− otherwise, the offspring would not perpetuate the traits of their successful parents
− So, the three postulates of Darwin’s theory were all observed to actually be working
− selection was actually occurring
− naturally, without human intervention
− Result: the expected result was also observed to happen: the bird population evolved
− many birds with shallow beaks died, while only a few of the deep-beaked birds died
− the distribution of beak depths shifted to a deeper average after a year of drought
− this left a population with relatively more deep-beaked birds
− who went on to have offspring that also averaged deeper beaks than the previous year
− and the process repeated during the second year of drought
− After two years of drought, the new generation of adult finches had beaks that averaged 4%
deeper than the adults of just two years earlier
− This was evolution caught in the act!
− The Grants' study had two main points
− 1. natural selection really did occur, just as in Darwin's theory
− we often observe evolution, in the sense of a change in a population
− but usually we just assume that the cause was directional selection
− in this case, we know that for sure that it happened in the way Darwin’s theory predicts
− all the details are documented
− all three of Darwin’s postulates held true
− more were born than survived and reproduced
− they varied in ways that affected survival (and therefore reproduction)
− that variation was heritable
− and evolution really occurred, just as the theory predicts
− 2. the other main point is that in severe natural conditions, evolution can be very rapid
− they observed 4% change in two generations
− this is not the slow, gradual evolution we usually think of
− this could add up to big changes, very fast
− Imagine if you found one island with small-beaked finches, and another with finches
with beaks 50% larger
− you would probably call them separate species
− You would identify two distinct kinds of birds, even though in fact they might both
have been descended from the same group of parents just twenty years earlier…
− This is exactly what Darwin thought had happened on the Galapagos islands
− but he had no idea it could be this rapid
− that is, he thought one small population of birds had probably gotten blown out to the
Galapagos chain
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these then managed to populate most or all of the islands
but were relatively isolated on each one
so all the birds descended from the same original population
but the birds on each island evolved in different directions to suit the conditions on
each island
− this model still seems to be right, over 150 years later.
You might well ask: If large beaks are so much better, why hadn’t the finches already
evolved deep beaks?
− Because of stabilizing selection…
as long as there is variation in fitness among individuals, selection always occurs
− but it does not have to be directional, which causes evolution
− it can also be stabilizing, which inhibits change
If selection favors the type that is already most common
− there is stabilizing selection
− and no change occurs: stasis
− which is what was presumably happening before the drought
− Before the drought, the average beak size remained the same due to stabilizing selection
− birds with smaller-than-average beaks were less likely to survive and reproduce
− because they were less likely to find enough food, because they could eat only the
smallest, softest seeds
− in normal, non-drought years, birds with larger-than-average beaks were also less likely
to survive and reproduce
− true, they could eat larger, harder seeds as well as the small ones
− but there are costs, as well as benefits, to deep beaks
− deep-beaked birds more often die as juveniles
− the Grants documented this with their bird census data
− we don’t really know why
− it may be because growing a deeper, bigger beak requires more food
− so these birds may be slightly more likely to be undernourished as juveniles, to
get sick, to be less able to avoid predators, etc.
− during the normal, wet years, there was a balance between
− the higher mortality of the shallowest-beaked birds due to their greater difficulty
finding food as adults
− and the higher mortality of the deepest-beaked birds due to their difficulty finding
enough food as juveniles
− birds with medium-sized beaks were the ones most likely to survive and reproduce
− so the result of natural selection was to stabilize the population
− to keep the most common type the most common
− this balance of opposing forces is called an “equilibrium”
− and the absence of change due to equilibrium is, as I mentioned before, called stasis
If selection favors a type that is less common
− there is directional selection
− and the population average shifts in some direction: evolution occurs
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− this is what happened during the drought
− During the drought, the benefit of having a deep beak increased, but the cost apparently
did not increase as much
− so having a deeper-than-average beak became advantageous
− and the selection ceased to be “stabilizing”
− instead, it became “directional”, favoring birds with deeper beaks
− Stabilizing selection and directional selection work in the same way
− the only difference is whether the most successful type is already the most common, or not
− as long as there is variation among individuals, selection is always going on
− Boyd and Silk point out that stasis (staying the same) is no more “natural” than change
− both require explanation
− and both are explained by natural selection
− Notice that even the direction of the evolution changed during the study
− during the drought, selection favored deeper-beaked birds
− and the population evolved to have a deeper average beak depth
− but after the drought, the advantage of having a big beak was less, since more of the small,
soft seeds were around
− while the cost of the bigger beak remained high (juvenile mortality is higher for biggerbeaked individuals)
− so selection favored birds with smaller beaks than the average during the drought
− the direction of the selection reversed
− and the population evolved to have a lower average beak depth
− selection and evolution do not necessarily push in a constant direction
− evolution does not necessarily move in one direction towards a goal
− instead, it simply tracks whatever characteristics happen to help individuals leave the
most offspring at the time
− if conditions push in the same direction over time, this can lead to long-term change
− but if conditions swing back and forth within a range, like from wetter to drier years on
Daphne Major, selection will cause evolution that just jiggles back and forth over time,
without causing any long-term change
− Finally, let’s be careful to separate our thinking about individuals and populations
− natural selection acts on individuals, not the populations
− Individual organisms live or die, reproduce or don’t
− but natural selection’s effect is on the population, not the individual
− as certain types of individuals leave more offspring than others, the population’s average
type changes
− Silk and Boyd point out that what is good for individual survival and reproduction is not
necessarily good for the population as a whole
− for example, selection may favor individuals who have more offspring
− even if the many doomed offspring eat so much food that the whole population declines
− Natural selection has no goal, and it is not always for the best…