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Introduction to Biological Anthropology: Notes 4
Kinds of variation, cumulative change, local optima, and rates of evolution
 Copyright Bruce Owen 2011
− Announcements
− The first set of self-study questions is posted, both blank and with answers
− The in-class evolution quiz is next class: Thursday, Sept. 8
− so review the readings, notes, and slides up through today’s material
− 5% of total course points; short written answers
− suggestion: work on the self-study problems posted on the class website.
− answers are also posted; naturally, I don’t expect you to use the exact words I do, except
when they are necessary technical terms
− the self-study questions are similar to the format of the quiz, midterm, and final exam
− On Thursday, all you will need is something to write with.
− You will get a printed quiz that looks a bit like the self-study problems.
− No scantron or blue book is needed.
− Natural selection and the evolution it can cause do not necessarily “improve” individuals or the
population
− evolution is just the accumulation of whatever features the most reproductively successful
parents have had
− natural selection can favor traits that are harmful to the group as a whole
− natural selection can even drive the whole group to extinction
− Let’s consider a variation on Boyd and Silk’s hypothetical example of high-fecundity vs.
low-fecundity females
− (fecundity is the ability to produce offspring)
− Imagine a small population of bugs
− food supply is limited to enough to support 100 adult bugs
− in order to keep the slides simple, I will divide all the numbers by ten
− and show only the females
− if each female produces two offspring, and 50% of them die, the population remains
constant
− but what if ten of the females produce four offspring each?
− the next generation starts off a bit larger
− but there is still food only for the same number to survive, so more of the offspring die
− mortality rises from 50% to 58%
− this change is clearly harmful for individuals, who are now more likely to die before
adulthood
− nevertheless, the high-fecundity females produced more offspring, so more of their
offspring survive into the next generation
− the next generation then has even more high-fecundity females
− so even more offspring are created in the next generation
− the food supply is still the same
− so even more of the offspring die off
− mortality rises to 64%...
Intro to Biological Anthro F 2011 / Owen: Variation, cumulative change, local optima, rates of evolution
p. 2
− This is harmful not only to individuals, but also to the population
− all those additional infants will eat some food before they die
− wasting it, and leaving less for the rest
− eventually, with the additional food consumption of all those doomed infants, the food
will be enough for only fewer than 100 to survive to adulthood
− the population of adults begins to decline
− higher fecundity is clearly harmful for the population, too
− if food is scarce, it might be better for the group to produce fewer offspring
− more would survive overall if fewer offspring had been born in the first place
− but natural selection won’t favor low-fecundity females
− not to benefit the survival of individuals
− nor for the overall good of the group
− all that matters is that the high-fecundity females produce more offspring
− if all offspring die at the same rate, then females who produced more to start with will
have more surviving offspring in the end
− the high-fecundity females will have produced a larger share of the next generation
− so there will be more high-fecundity females in the next generation, and the average
fecundity will keep rising
− even though the increasing competition for food allows fewer and fewer offspring
overall to survive to adulthood
− so the population gets smaller and smaller, maybe until it becomes vulnerable to
disease, predators, or other bad luck and gets wiped out completely
− There are real examples of this happening, although the data are not good enough to prove it
positively
− Extinct Irish elk (based on bones and fossils)
− the males developed ever larger bodies and outrageously large antlers
− various arguments suggest that the antlers were primarily involved in attracting females
− the larger the antlers, the greater the reproductive success for the males
− large body size was essentially a byproduct of selection for large antlers
− but the huge antlers and the large bodies associated with them required enormous amounts
of calcium and other nutrients to grow and maintain
− as climate changed around the end of the last Ice Age and the available foods shifted, the
elk went extinct
− apparently, the males with the biggest antlers kept having the most offspring
− so instead of adapting to the changing environment, the elk kept evolving larger and larger
bodies and antlers
− until the entire species went extinct around 8000 years ago
− so natural selection does not necessarily lead to “better” organisms
− it just perpetuates whatever variants produce the most surviving offspring at that moment
− the end result might not be what a designer would have aimed for
− A useful clarification: There are two kinds of variation that selection can act on
− Continuous variation: variation in the magnitude of a characteristic, like height or the
depth of birds’ beaks
Intro to Biological Anthro F 2011 / Owen: Variation, cumulative change, local optima, rates of evolution
p. 3
− a smooth range of variation without gaps between different types
− evolution can affect a continuously variable trait by changing its average value
− for example, the average height of Americans might increase from one generation to the
next
− Discontinuous variation: variation in which there are a few discrete types, like mottled vs.
black moths
− there are no intermediate types
− there is not a range of coloration from mottled through darker variants to black, but just
two (in this case) sharply defined types
− evolution can affect a discontinuously variable trait by shifting the frequency of the types
− so one type might start off rare, but become more common from one generation to the
next
− Another useful concept: very complex adaptations can arise from the accumulation of simpler
changes that each increase reproductive success
− natural selection does not have long-term goals
− it just favors whatever variant results in more surviving offspring at the time
− but these changes that each increase reproductive success can accumulate into very
complex features
− example: people who doubt that evolution is possible often point to the human eye
− it has many parts that all have to be there in order to work as a normal human eye does
− the lens has to be just the right shape and position to cast a sharp image on the retina, etc.
− just a lens, or just a retina, would not permit sight
− so how could such a complex device evolve gradually?
− it must have been created all together, by a grand designer!
− but this is looking at the process backwards, assuming that the full, camera-like eye was the
goal
− natural selection was not aiming to produce a human eye
− it was simply favoring certain variants every generation
− each step was advantageous at the time
− these favored variants eventually accumulated to produce the eye
− a hypothetical scenario, with each step based on real organisms alive today
− imagine starting with an organism that could not detect light
− if some individuals happened to have slightly light-sensitive skin, they might have an
advantage
− since plant foods grow better where there is light, they would know when they were
likely to be near food, and when they should move to improve their odds of finding food
− there could be many other advantages to simply distinguishing light from dark
− so light-sensitive skin would become more common
− if some had more light sensitivity on one part of their body than another, by moving
around they could tell which direction was lighter
− and thus determine which way to move to go towards the light
− this would be advantageous, and would become more common
Intro to Biological Anthro F 2011 / Owen: Variation, cumulative change, local optima, rates of evolution
p. 4
− if the light-sensitive skin was concentrated in a concave part of the body, that would
restrict its “view” and improve the organism’s ability to tell which in direction the light (or
dark) was
− that would be advantageous, and would become more common
− the deeper the concave area and the smaller the opening, the better the directionality…
− the ability to distinguish which part of the light-sensitive lining of the pit was being
illuminated would also improve directionality
− improving that ability would allow detection of motion, as in the shadow of a predator
moving from one side to the other…
− a transparent layer over the skin might protect it…
− a thicker transparent layer with some curvature might concentrate the light…
− a thicker, curved layer might cast a very fuzzy image on the light-sensitive skin, improving
the ability to detect direction and motion…
− and so on…
− the point is that selection simply favors the most successful variant each generation
− and these successively better variations will keep accumulating
− the end result may be very complex
− but was never designed or intended
− it was reached “blindly”, by taking the best step at each generation
− this lack of planning severely limits what features evolution can develop
− if something cannot be reached by a succession of changes that each increase the number
of surviving offspring that individuals leave, natural selection will never produce it
− there are some interesting exceptions in which a small genetic change can cause a large
change in the organism, but we will stick to the more typical process here
− this is probably why living things generally do not have wheels or screws, among other
seemingly useful features
− because organisms “can’t get there from here”
− there is no way to alter the existing organism to produce that feature without going
through stages that would reduce reproductive success
− this partially answers the question I posed earlier: Why are organisms so strangely, even
badly, designed?
− because the only designs that ever develop are those that can be reached by small
improvements on a previous design
− this gradual tinkering approach can lead to local optima, but not necessarily global optima
− local optimum: the adaptation with the highest fitness of all the ones similar to it
− but not necessarily better than one that would be very different
− global optimum: the best of the various local optima
− Natural selection should push organisms to evolve towards local optima
− but not necessarily the global optimum
− it depends on the starting point, or rather, on the previous history of evolutionary change
− Say you are in the woods and have no map, but you prefer the view from high places to
being down in the valley
− each step, you choose the direction that is most uphill
− after a while, you may be high up on a mountain
Intro to Biological Anthro F 2011 / Owen: Variation, cumulative change, local optima, rates of evolution
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p. 5
− you got there without ever having any idea of where you were going
− but you will never get up the even higher mountain on the other side of the valley…
example of camera-type eyes (like ours) versus compound eyes (like insects’)
− compound eyes are essentially bundles of mirrored tubes with light-sensitive spots at the
bottom; a large number of these tubes are mounted on part of a sphere, facing outwards
− they produce an image made up of one dot from each receptor
− compound eyes are poorer than camera-type eyes
− they have a limited, fairly poor resolution (fewer “pixels” in the image)
− they are less efficient at gathering light, so they don’t work as well in low light
− so why aren’t compound eyes replaced by camera-type eyes?
− because there is no gradual path to convert one to the other that goes through everimproving variants
− the two are results of different variants that happened to be selected for early in the
evolution of the eye
− as early eye-spots were evolving, in some lineages, variants that had larger spots were
favored
− in other lineages, variants that more, but smaller, spots were favored
− both approaches improved the early eyes’ sensitivity to light
− the larger, single spots could be gradually improved by becoming concave eye to
improve directionality
− leading to pinhole eyes, and eventually camera eyes
− the multiple-spot approach could be gradually improved by becoming convex to
improve directionality
− leading to compound eyes
− once the first few steps were taken, progressive improvements to each design led in
completely different directions
− there is no way to improve a compound eye by moving it towards the concave shape
of a camera eye
if there were a divine “designer”, she might pick the right early step so that gradual
improvements would lead to the best possible adaptation
but in the absence of such foresight, evolution can only work with what it has at any given
moment, and can only go in a direction that immediately produces better results
so the outcome will often be the best that could be done by improving the earlier design,
not the best that could have been done by designing from scratch or by starting with a
different design.
− that is, evolution cannot work towards a long-term goal
− it only selects for whatever leaves more offspring right now, one generation at a time
− Rates of evolution
− Gradualism: a view of evolution which supposes that changes accumulate at a steady pace
over time
− the pace is generally imagined to be slow
− implies that populations respond slowly to natural selection
Intro to Biological Anthro F 2011 / Owen: Variation, cumulative change, local optima, rates of evolution
p. 6
− so in this view, most populations are not yet well adapted, but are still being pushed
slowly towards the optimal form for that environment
− mya = million years ago
− Punctuated equilibrium: a view of evolution which supposes that populations experience
long periods of equilibrium (or “stasis”) in which little or no change occurs, occasionally
“punctuated” by spurts of rapid change leading to a new, different equilibrium
− supposes that populations sometimes evolve rapidly
− this would be most likely in very small, isolated populations
− implies that most populations are well adapted and are kept the same by stabilizing
selection
− while occasional splinter populations may evolve rapidly to some new configuration
− if the changed population later mixes with the original one, the new, more successful
variants quickly become common in the original population, too
− or the changed population may be so different that it is a new species altogether
− the punctuated equilibrium model has a big plus: it explains the scarcity of transitional
forms in the fossil record
− we would expect to find lots of fossils of the species from its long periods of
equilibrium
− and few or none from the brief punctuation events in which a small sub-population
rapidly evolved into a new variant or new species, because so few individuals were
involved in the brief period of change
− A modern view of rates of evolution - the “evolutionary jitters”, or the “evolutionary hair
trigger”
− the fossil record seems to show very slow rates of evolution
− J.B.S. Haldane invented a unit to discuss this: a 1% change in a characteristic per 1 million
years = 1 darwin
− typical rates of evolution based on fossils are around one darwin
− but if we look at the beak depth of the finches on Daphne Major during the drought, which
changed 4% in 2 years, the rate is 2 million darwins!
− this is a pattern that recurs in many cases:
− studies of long time spans based on fossils find slow rates of evolution,
− while studies of short time spans based on field observations or lab studies find rapid rates
of evolution
− how can this be?
− the fossil record is very incomplete
− it gives us widely separated snapshots, rather than a continuous record
− fossil studies usually assume that the change between snapshots was a single, constant,
one-way shift
− simply because we have no information that shows a more complex pattern
− but we now know that populations shift back and forth a lot
− beak depth evolves up and down on a several-year basis in Darwin’s finches
− so the real rate of change is rapid, but it keeps reversing itself, so the net change over a
long period is little or none
Intro to Biological Anthro F 2011 / Owen: Variation, cumulative change, local optima, rates of evolution
p. 7
− so say fossils finches from one layer show birds with small beaks, and in a layer a
million years later, they have larger beaks
− the beak size may have gone up and down hundreds of times in that million years
− we just happened to get some fossils from a wet period (small beaks) in one layer, and
some fossils from a dry period (large beaks) in the other
− so we infer very slow directional change, when it has actually been rapid but backand-forth with no long-term trend
− what does this mean?
− populations are not lumbering, slow-changing things
− instead, they are constantly being bumped back and forth by minor variations in selection
pressures
− the reason that species seem relatively stable is because they are mostly already well
adapted to their environments,
− and environments tend to vary around long-term averages that (usually) change slowly
− the implications:
− if there is ever a big change in the environment that changes the selection pressures,
− then populations are able to respond very rapidly, and quickly evolve off in new
directions
− or if a new, advantageous variant somehow arises
− it can become common very rapidly
− far from being fixed or slow to change, populations are on an “evolutionary hair trigger”
− this explains why the fossil record seems to show the punctuated equilibrium pattern
− But even this newly recognized capacity for “rapid” evolution probably isn’t fast enough to
respond well to changes on the scale of a human lifetime
− that is, trees and large mammals, which take many years for each generation to grow to
maturity, may not be able to evolve fast enough to cope with rapid changes like global
warming, which is happening over just a century or two.
− but organisms with very short generations may be able to evolve that fast
− like rodent and insect pests
− and disease organisms like Staphylococcus and other bacteria, that have evolved resistance
to antibiotics