Adaptation &

Limits to Natural Selection

The complexity of living organismsdemands 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 ThickRocks available to snakes 32.4% 34.6% 33%

Rocks chosen by snakes 7.7% 61.5% 30.8%

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

0 20% 100% 100%

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 accomplishSelection 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…

Genetic Variation?

YES NO…in the same tribe as actual host

7 1

…in a different tribe as actual host

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