Chapter 10.doc

14 views3 pages
25 Nov 2012
of 3
Chapter 10- Studying Adaptation: Evolutionary Analysis of Form and
Explanation of organismal design is among the triumphs of the theory of evolution by natural
Individuals in previous generations varied in their design, and the ones with the best designs
passed on their genes in greater numbers
A trait, or integrated suite of traits, that increases the fitness of its possessor is called an
adaptation and is said to be adaptive
In order to prove that a trait is an adaptation, we need first to determine what a trait is for and
then show that individuals possessing the trait contribute more genes to future generations than
individuals lacking it
10.1 All Hypotheses Must Be Tested: Oxpeckers Reconsidered
Read Example on Oxpeckers Pg. 364-366.
Oxpeckers are vampires and eaters of earwax
Even when they do eat ticks, Oxpeckers prefer adult females that have already engorged
themselves with blood- that is, ticks that have already done their damage to the host
When studying adaptations, there are some key things to keep in mind:
oDifferences among populations or species are not always adaptive. There are two species of
oxpecker; one has red bills, the other yellow. It is possible that each color is adaptive for the
species that wears it. But it is also possible that the difference is not adaptive at all.
Mutations causing different colors may have become fixed in the two oxpeckers by genetic
drift. At the molecular level, much of the variation among individuals, populations, and
species may be selectively neutral.
oNot every trait of an organism, or every use of a trait by an organism, is an adaptation. While
feeding on large mammals, oxpeckers may sometimes meet a potential mate. This does not
necessarily mean that feeding on large mammals evolved because it creates mating
oNot every adaptation is perfect. Feeding on the blood and earwax of large mammals may
provide oxpeckers with high-quality meals. But because many large mammals migrate long
distances, it may also expose oxpeckers to the risk of an unpredictable food supply.
Three methods used to test hypotheses about the adaptive significance of traits:
oObservational studies
oComparative method
10.2 Experiments
Experiments are the most powerful method for testing hypotheses. A good experiment restricts the
difference between study groups to a single variable.
Read Experiment on jumping spiders and flies Pg. 367- 371
Jumping spiders tended to retreat from flies that gave the wing-waving display with marked wings,
but attacked flies that lacked either wing markings, wing waving, or both.
Important points about experimental design:
oDefining and testing effective control groups is critical
oAll of the treatments (controls and experimental) must be handled exactly alike
oRandomization is a key technique for equalizing other, miscellaneous effects among control
and experimental groups. In essence, it is another way to avoid bias.
oRepeating the test on many individuals is essential. Larger sample sizes are better.
Replicated experiments or observations do two things:
oThey reduce the amount of distortion in the estimate caused by unusual individuals or
oReplicated experiments allow researchers to understand how precise their estimate is by
measuring the amount of variation in the data. Knowing how precise the data are allows the
use of statistical tests. Statistical tests, in turn, allow us to quantify the probability that the
result we observed was simply due to chance.
Large sample sizes are better but researchers have to trade off the costs and benefits of collecting
ever more data.
10.3- Observational Studies
When an experiment is impractical, a careful observational study may be the next best method for
evaluating a hypothesis.
Behavioral Thermoregulation
Vast majority of organisms are ectothermic, means that their body temperatures are determined
by the temperatures of their environments
Body temperature has a profound effect on an ectotherm’s physiological performance
Desert iguanas can survive short exposures to body temperatures as low as 0ºC and as high as
about 47ºC
oThey can only function between about 15ºC - 45ºC
Within this range, cold iguanas run and digest slowly, tire quickly, and hear poorly
As they get warmer, they run and digest more quickly, tire more slowly, and hear
more keenly
The relationship between physiological performance and temperature is called a thermal
performance curve
Given the sensitivity of physiological function to temperature, we can predict that ectotherms will
exhibit behavioural thermoregulation
Ectotherms should move around in the environment so as to maintain themselves at or near the
temperature at which they perform the best
As temp. Of their environment changes, iguanas must regulate their body temperature by moving
into the sun to warm up or into the shade to cool off
oIguanas prefer to keep their body temp. In the high 30s; this is the center of the range of
temperatures at which the iguanas perform best
To prove behavioural thermoregulation, we must show:
oThat the animal in question is choosing particular temperatures more often than it would
encounter those temperatures if it simply moved at random through its environment
oThat its choice of temperatures is adaptive
Do Garter Snakes Make Adaptive Choices When Looking for a Nighttime Retreat?
Snakes have many options for thermoregulation during the daytime, as long as they avoid thin
rocks or direct sun in the afternoon
At night, however, it appears that the best place for a snake to be is under a rock of medium
Many garter snakes do retreat under rocks at night
Under the hypothesis of behavioural thermoregulation, snakes would choose their nighttime
retreats adaptively
oThat is snakes would preferentially select rocks of medium thickness
oAlmost always, garter snakes are almost always found under medium rocks or thick rocks
oFact that snakes avoid thin rocks is good evidence that the snakes are active behavioural
10.4- The Comparative Method
Why Do Some Bats Have Bigger Testes than Others?
Males in some bat species have larger testes for their body size than others
It was hypothesized that large testes are an adaptation for sperm competition
Sperm competition occurs when a female mates with two or more males during a single estrus
cycle, and the sperm from the different males are in a race to the egg
One way a male can increase his reproductive success in the face of sperm competition is to
produce larger ejaculates
By entering more sperm into the race, he increases his odds of winning.
oTo produce larger ejaculates is to have larger testes
Females living in larger groups would have more opportunities for multiple matings, and that males
living in larger groups would thus experience greater sperm competition
10.6 Trade-Offs and Constraints
It is impossible to build a perfect organism. Organismal design reflects a compromise among
competing demands.
Large testes help bats win at sperm competition but appear to impose metabolic costs that lead to
the evolution of smaller and less energetically demanding brains
Female Flower Size in a Begonia: A Trade-Off
A tropical plant Begonia involucrate is moneoecious- that is, there are separate male and female
flowers on the same plant
Flowers are pollinated by bees
As bees travel among male flowers gathering pollen, they sometimes also transfer pollen from
male flowers to female flowers
Male flowers offer the bees a reward in the form of the pollen itself
Female flowers offer nothing; instead they get pollinated by deceit
Not surprisingly, bees make more and longer visits to male flowers than to female flowers
Read experiments on bees and flowers Pg. 383-388
The larger the flower, the more bees that approaches and visits it attracted. Selection by bees on
female flowers is strongly directional.
Selection by bees favours larger flowers, yet female flowers are no bigger than male flowers
oWhy aren’t they huge?
the species B. Involucrate simply lacks genetic variation for female flowers that are
substantially larger than male flowers
female flower size in B. Involucrate has been determined, at least in part, by two opposing factors:
directional selection for larger flower and the trade-off between flower size and number
Flower Color Change in a Fuchsia: A Constraint
Read Pages 386-388. Can’t make notes on this shit, just an experiment. Could be tested
Host Shifts in an Herbivorous Beetle: Constrained by Lack of Genetic Variation?
Genetic variation is the raw material for evolution by natural selection
Since natural selection is the process that produces adaptations, genetic variation is also the raw
material from which adaptations are molded
Sometimes, populations of organisms may be prevented from evolving particular adaptations
simply because they lack the necessary genetic variation to do so
Host Shifts in Feather Lice: Constrained by Dispersal Ability
Read Pg. 390-391
It is important that you read these pages and understand the experiments...or else on
the exam you’re FUCKED!