Adaptation
&
Limits to Natural Selection
The complexity of living organisms
demands an explanation
William Paley
(1743-1805)
• Natural Theology: or,
Evidences of the Existence
and Attributes of the Deity,
Collected from the
Appearances of Nature.
(1802)
• “the watch must have had a
maker”
Natural Theology:
A Circular Argument
• Wanted to explain the origin of complex
entities that are highly adapted/designed to
fit their environment
• A designer?
• But the designer must also be a complex
entity, so the question remains
Design and Adaptation
• Natural theology argued that the perfect
design of organisms was evidence of a
designer (i.e. God).
• The adaptive explanation put forward by
Darwin needed no designer
• For Darwin, the watchmaker was blind...
In fact, Nature is not very
perfect
• Appendix
• Wisdom tooth
• Tough for natural theology as it implied that
either God was not perfect or deliberately
created such imperfect beings
The only scientific explanation
for adaptive evolution is
Darwinian natural selection
The Importance of Variation
• Random, heritable variation drives
evolution
• Deleterious mutations are eradicated by
natural selection
• Advantageous mutations are favored by
natural selection => adaptation
Adaptation
Feature of an organism enabling it to survive
and reproduce in its natural environment better
than if it lacked the feature
Adaptionist program:identifying traits and
demonstrating that is a solution to a problem
encountered by the organism
Problems with the Adaptionist Program
Other tests
• Observational – snakes at night
• Comparative – bat testes
–
–
–
–
Sperm competition – go for the gold!
Hypothesis?
Independent points
Phylogenetically independent contrasts
Do ectotherms regulate their body
temperatures?
Garter snakes appear
to chose environments
to maintain their body
temperature
• Distribution of rocks available to snakes vs.
rocks chosen by snakes
Thin
Medium
Thick
Rocks available
32.4%
to snakes
34.6%
33%
Rocks chosen
by snakes
61.5%
30.8%
7.7%
Comparative Method Example:
Bat species that live in larger groups seem to
have larger testes
Problems
–Independent points
–Phylogenetically independent
contrasts
An imperfect world
• Organisms are not perfectly adapted to their
environment
• If they were, they would live forever, escape
all predators, lay eggs at an infinite rate etc.
• Often organisms are not even very close to
being perfect (e.g. wisdom tooth)
Why?
Evolution by Natural Selection explains
the complex adaptations seen in nature
and also the imperfections
• Natural selection can only act on variations
present in a population
• The result is adaptations that are contrivances,
structures modified and used for functions quite
different than the ancestral functions of those
same structures.
Evolutionary Constraints
• Historical constraints
– Contrivances
– Contingencies
• Trade-offs
• Physiological constraints
• Genetic constraints
– Selection constraints
– Lack of variation
Historic/Current Functions
• All traits have evolved from previously
existing traits
• In the beginning, an organ may have had the
same function as it does now
• or it may have had a different function
• Adaptations are the best solution possible
given these constraints, but they may not be
the “optimal” solution
Brachiating Apes
•
•
•
•
•
•
Swinging in trees
Straightened spine
Legs parallel to spine
Preadaptation to erect posture?
Apes are knuckle-walkers
Bipedalism is unique to
humans and our human-like
ancestors
A comparison of ape and hominid skeletons
Side effects of bipedalism
• Necessitated curvature of the spine & restructured pelvis
• New pelvis had smaller opening (birth problems)
• Necessitated life history changes
-birth earlier in development
-Extensive parental care
Ø Other problems:
• Shorter jaw: wisdom teeth (no room)
• Back problems (curvature, pressure on lower spine)
• Hemmorhoids (pressure of intestinal system on lower
parts, cuts off blood flow)
Blind spot in the human eye
Draw a similar picture on your paper:
Contingency
• Chance may lead some groups to solve
problems in one way, while other groups find
different though not necessarily worse
solutions
• For example, kangaroos and gazelles have
different but equally (?) successful modes of
locomotion
Trade-offs
• Organs may be adapted for more than one
function
• For example, mouths are used for breathing
and eating
• Amphibians must hold their breath while
chewing
• Mammals can chew and breath at the same
time due to the secondary palate separating
nose and mouth
A second trade-off for humans: our larynx is
lower in our throats which allows us to talk and
communicate more effectively, but also makes
us more susceptible to choking
Trade-off: Example 2
Why do females
flowers resemble
male flowers?
Why aren’t all begonia flowers
bigger?
Physiological Constraint
Fuchsia excorticata
Why do Fuchsias keep old flowers rather than
drop them after pollination?
Days since pollination 1
2
3
4
% of flowers with
pollen tubes in ovary
20%
100%
100%
0
Given the plant is constrained to maintain flowers
for at least 3 days after pollination, selection may
have favored individuals offering cues to their
pollinators to help distinguish between receptive
and unreceptive flowers
Genetic constraints
• Selection may be unable to get rid of
disadvantageous alleles:
– Heterozygous advantage: the sickle cell anemia
allele is maintain despite its negative effects
– Selection can only act on phenotypes: When AA
and Aa have the same phenotype, selection can’t
detect heterozygotes
• Selection requires genetic variation
– Selection will stop until mutation creates new
variation
Limits to what selection can accomplish
Selection for increased speed in racehorses no longer effective
Example 2: Can any host shift evolve?
Genetic Variation for Ability to Shift Hosts
Beetle tested for feeding
on a plant that is…
…in the same tribe as
actual host
…in a different tribe as
actual host
Genetic Variation?
YES
7
NO
1
14
17
Lagniappe
• Natural selection is not ‘progressive’
• It does not inevitably lead to an harmonious
and stable state
• It does not optimize
• It merely suffices
• It is myopic (shortsighted)- species can be
‘perfected’ to extinction!
“Our textbooks like to illustrate evolution with
examples of optimal design—nearly perfect mimicry of
a dead leaf by a butterfly or of a poisonous species by a
[tasty] relative. But ideal design is a lousy argument for
evolution.... Odd arrangements and funny solutions are
the proof of evolution—paths that a sensible God
would never tread but that a natural process,
constrained by history, follows…”
-Stephen Jay Gould, The Panda’s Thumb