Evolution - Units of selection

"In 1859 I obtained a very fertile descendant with large, tasty seeds from a first generation hybrid. Since in the following year, its progeny retained the desirable characteristics and were uniform, the variety was cultivated in our vegetable garden, and many plants were raised every year up to 1865" -- Gregor Mendel in a letter to Carl Nägeli,1867

Evolution is a process by which the characteristics of living organisms change over many generations as traits are passed from one generation to the next. It attempts to explain how living things first appeared on earth and how they have since diversified and changed. It involves change of gene frequency within populations, which ultimately result in behavioral and phenotypic changes.

Natural Selection is the mechanism that leads to changes in gene frequency (think selfish gene) or a (box of genes)

It will be universally admitted that instincts are as important as corporeal structures for the welfare of each species, under its present conditions of life. Under changed conditions of life, it is at least possible that slight modifications of instinct might be profitable to a species, and if it can be shown that instincts do vary ever so little, then I can see no difficulty in Natural Selection preserving and continually accumulating variations in instinct to any extent that may be profitable. It is thus, I believe, that all the most complex and wonderful instincts have originated.

C. Darwin, 1859, The Origin of the Species

1.15 An evolved response to the risk of infanticide

1.11 In an assassin bug, a predatory insect, males stand guard over clutches of eggs that females (usually their mates) have laid on leaves. The males protect the clutch against parasitic wasps that attempt to lay their eggs in assassin bug eggs. From time to time, a male stabs one of the eggs in his clutch with his beak, injecting it with digestive enzymes before drawing out the fluid. Needless to say, this act destroys the egg and so constitutes a kind of infanticide. If infanticide of this sort has evolved through sexual competition for quicker reproduction, as in Hanuman langurs and lions, how should males treat clutches laid by their copulatory partners versus those that the males “adopted” when they forced another male to leave an egg mass he had fertilized, something that happens occasionally? Once you have a prediction in hand, consider these two findings: (1) the parentage of the eggs has no effect on the number eaten, which increases steadily as the period of egg guarding lengthens, and (2) males do not lose weight while egg guarding, even though they cannot feed elsewhere. Are these findings relevant for the quicker reproduction hypothesis? Do they support another hypothesis for the tendency of males to eat some of the eggs under their care?

The prediction is that males should tend to destroy the eggs fertilized by other individuals rather than consuming the eggs they have fertilized themselves.

However, the failure of males to discriminate against eggs fertilized by other males suggests that sexual competition is not responsible for the fact that males destroy some eggs under their care. Instead, it appears that males eat some eggs in order to maintain body condition during the long period when they are forced to remain close to the brood they are guarding.

1.7 Imagine that a mutation occurs this year in the gene that codes for the V1-a receptor protein in prairie voles. The altered protein increases the tendency of a male to form social bonds with a mate, in effect making him somewhat more monogamous than the typical male prairie vole. What is required if enhanced monogamy is to become more common in this species over time? If monogamous males have 3.7 pups on average during their lifetime while enhanced monogamous males have 4.1 on average, will the allele associated with enhanced monogamy necessarily make up an increasing proportion of the population? (“No” is the correct answer. Why? Hint: How does natural selection “measure” individual reproductive success?) Now imagine two types of prairie voles, one type that usually lives for 1.5 years and another for 0.8 years. Would it ever be possible for the one that died sooner to replace the longer-lived type over time?

Any change in monogamous behavior will spread only if individuals carrying the altered gene that influences the development of their behavior have more surviving offspring than others with a different hereditary basis for their behavior. The number of babies individuals have is not necessarily an indicator of the number of surviving offspring they will have, which is why males with 4.1 babies on average will not necessarily leave more copies of their alleles than males with 3.7 babies on average.

Male Hanuman langurs apparently use a rule of thumb when deciding which infants to kill: they attack youngsters of females that they did not copulate with prior to the birth of those offspring. In light of this finding, some observers have an explanation for why it is that after a takeover pregnant females may mate with newcomers. These researchers have suggested that the females’ behavior may create confusion about the paternity of their babies, the better to limit male infanticidal behavior. If females also engage in paternity confusion when their band contains several adult males, what predictions can you make about (1) the duration of female receptivity each reproductive cycle, (2) the relationship between the period when females are in heat and the timing of ovulation, and (3) the occurrence of copulations by subordinate males as well as the dominant male langur, who attempts to monopolize sexual access to the female?

(1) If the paternity confusion hypothesis is correct, female receptivity should last longer than the period in which egg fertilization can occur.

(2) Ovulation and estrus need not be closely correlated.

(3) Subordinate males should succeed in copulating with females, although not necessarily when ovulation is actually taking place.

"It takes a very long period of observing to become really familiar with an animal and to

attain a deeper understanding of its behaviour; and without love for the animal itself, no

observer, however patient, could ever look at it long enough to make valuable observations on

its behaviour" -- Konrad Z. Lorenz

• Scientific Method • Initial Observation(s) • Phrase a Question • Phrase Null and Alternate Hypotheses • Plan Experiment that distinguishes between the

two hypotheses • Conduct Experiment in Double-Blind Fashion• Un-blind samples • Analyze Data • Draw Conclusions • Communicate Results

"When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind: it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the state of science" -- William Thomson Kelvin

Statistical Analyses involve two competing hypotheses: a null hypothesis and an alternative hypothesis.

A null hypothesis is an explicit claim about the value of a specific parameter. What would you predict if the decision to turn left/right on the T-maze is random? In our example, the probability to turn right P(R) must equal the probability of the subject turning left P(L). Because subjects have two mutually exclusive choices at the decision point we know that the probability of turning either left or right P(R or L) = P(R) + P(L) = 1, or P(R) = P(L) = 0.5. Thus, we can make an explicit claim about the probability that a subject turns right or left at the decision point. Our null hypothesis is that P(R) = 0.5 (or, equivalently, that P(L) = 0.5).

Our alternative hypothesis simply states that our null hypothesis is not true, or P(R) 0.5 (or, equivalently, P(L) 0.5). Every statistical test involves the computation of a test statistic. A comparison of the test statistic to a critical value allows us to either accept our null hypothesis or reject our null hypothesis in favor of the alternative hypothesis. We accept the null hypothesis if our test statistic does not exceed the critical value and we reject the null hypothesis in favor of the alternative if the test statistic exceeds the critical value.

Food for thought:

"Many people assume that the absence of evidence is evidence of absence. This is bad scientific thinking." — Fouts

Proximate Causes explain how a

behavior is elicited and coordinated –

What are the anatomical & physiological

mechanisms underlying the behavior?

Search for physiological mechanisms

Environmental conditions needed

Hormones and Behavior

Behavioral Genetics

Survival Value - Ultimate Causes explain why a behavior has evolved INTO ITS PRESENT FORM–

Why is the behavior used? How does it help the animal survive and reproduce?

Search for functional significance

Evolutionary history, adaptiveness of behavior

Danger!!! Don’t assume all mechanisms are adaptive –

Need to be tested carefully!!

EXAMPLE 1 — Tinbergen’s gull experiments. Tinbergen observed that ground breeding gulls removed the empty egg shells from the nest right after hatching while a closely related species, which breeds in inaccessible rocky outcrops, does not.

Why do gulls remove eggshells from the nest

immediately after the chick hatches?

• Tinbergen phrased it and said it better:

• What might be the adaptive value of egg shell removal?

• Adaptive value ===== Survival value

Comparative Studies Ecology and social behavior of three species of marmots

Woodchucks -inhabit fields at low elevations - reproduce every year - solitary - aggressive towards

conspecifics - - juveniles disperse at

early age

Yellow-bellied marmots -li ve at intermediate elevations - reproduce every year

(but sometimes skip a year)

- colonial - moderately aggressive

towards conspecifics - juveniles disperse at

two years of age

Olympic marmots -inhabit meadows at high elevation - reproduce every other

year - colonial - highly tolerant of

conspecifics - juveniles disperse at

age three

EXAMPLE 2 —Three species of marmots – discuss each species

differences in reproductive behavior

Law of Parsimony – foundation of animal behavior

“In no case should we interpret an action as the outcome of the exercise of a higher psychical faculty if it can be interpreted as the outcome of the exercise of one which stands lower in the psychological scale”

Must seek out the simplest explanation of the observed facts. Reduce complex hypotheses to the simplest terms to facilitate the clearest understanding of the mechanisms that control behavior.

Concepts in Animal Behavior

• Behavior contains genetically based, heritable components • Instinct: capacity of an animal to complete a complex behavior in response

to a first-time encounter with a given situation. • Instinctive behavior can usually be elicited by a limited combination of

complex environmental cues.• Motor patterns: Fixed Action Pattern (FAP) different individuals produce

identical behavioral response to specific sign stimuli, once initiated FAPs continue until completion.

• Ethogram: comprehensive compilation of the behavioral repertory of a species, a catalogue of actions as complete and precise as possible.

• Imprinting: capacity to learn specific types of information at certain critical periods in development.

• Behavioral Ecology - the study of how animals use behavior to interact with their environments.

John Krebs - acoustic communication in birds and crickets, optimal foraging theory

N. B. Davies - the economics of behavioral strategies

• Sociobiology - the study of social behavior from an adaptive evolutionary perspective, behavior patterns come into being, are modified, and even disappear through the process of natural selection. --- E. O. Wilson - social behavior of ants.

• Neuroethology - the study of the proximate mechanisms of behavior. It concerns itself with mechanisms that underlie the performance and control of behavior. How does the nervous system gain a view of the outside world? How does it pattern and regulate movement? How is information derived from experience stored in the brain? What are the physiological mechanisms underlying motivational states?

• Walter Heiligenberg - social behavior and communication via electric signals in fish

• Eric Kandel - learning and memory in Aplysia• Mark Konishi - auditory sensory map in prey-capture of owls • Franz Huber - acoustic signaling in courtship and aggression of

crickets

• Peter Marler - Why do birds sing?

Proximate and Ultimate Causes of Bird Song (Chapter 2)

Summary Causes of any behavior can be understood in terms of four different levels of analysis; 1) how the behavior develops; 2) how the physiological mechanisms make behavior possible; 3) how the behavior promotes the animal’ reproductive success and 4) how behavior originated and has changed over evolutionary time.

Four levels can be condensed into two (proximate and ultimate causes of behavior).

Proximate and ultimate causes are interrelated. Genes present in animal today reflect adaptations over time and exert proximate influences over course of individual’s development allowing it to do certain things.

Proximate and ultimate questions are tackled the same way: Begin with a question about what causes an animal to do or exhibit a particular behavior; we devise a testable null hypothesis to answer the question; we design an experiment and collect data; if results differ from the null hypothesis we reject Ho and we accept the alternative hypothesis. Further testing of the alternative hypothesis is performed.

Summary continued

Our theories are shaped by the working hypotheses we generate based on how a given behavior might advance the reproductive success of individualsNatural selection trumps group selection (law of parsimony holds here). Selection at level of individual is more powerful than group selection in causing evolutionary changeUse logic and evidence (scientific method) to evaluate the validity of competing theories and alternative hypotheses