BEHAVIORAL CHANGE
(Part 1: Natural Selection)
A very important part of animal behavior is understanding how behavior changes over time. This process of change can occur in three very different ways.
Platforms of Change
1.Natural Selection (slow, can take thousands of years and changes are passed forward to future generations by heredity).
2. _____________________ (rapid, occurring within an individual's lifetime. Such changes die with the individual).
3. Cultural Transmission (Relatively rapid; Animals learn new behaviors which are then passed forward through social learning to their offspring and succeeding generations; transmission can be more rapid than natural selection)
Today we will focus on the first platform - natural selection
I. How Does Natural Selection Work?
Questions about the function of behavior (one of the four biologically relevant questions) require us to understand the process of natural selection. What differentiates the "function" question from the others is that we seek to determine how behavior might promote reproductive success.
A. Darwin's View of Evolution
1. _________________ - members of a particular species differ with respect to their characteristics
2. Heredity - parents pass on their particular characteristics to their offspring
3. Differential Reproduction - Some of these distinctive characteristics favor offspring survival and ultimately populations become dominated by the characteristics of successful reproducers.
4. See Videoclip of African Wild Dogs. African wild dogs hunt in packs. Imagine in early evolutionary history that most individuals hunted alone whereas a few individuals tended to pair up with others. Assume that this difference was controlled by one gene. If hunting together meant more meat for an individual and allowed it to produce more offspring, then group hunting would become fixed in the population. Even a 1% advantage would produce this effect. See the Table below and fill in the missing data.
Based on the formula where # generations = 2/the benefit of group housing X the natural log of population size (from Carroll, 2007 as modified by Dugatkin, 2009).
Benefit of Group Hunting Population Size Generations needed for Group Hunting to reach 100% of the population
1% 100 - 5% 100 - 10% 100 - 20% 100 -
6. Thus, even a small advantage can have very potent effects over thousands of years - and that is generally the scale of time in which natural selection operates.
B. Requirements of Natural Selection
To understand how Darwin's scheme of variation, differential reproduction, and heredity work, we will study response to novelty. Many species show individual differences in response to novelty.
see videoclip of young hyenas just to observe individual differences in action
However, we will focus on rats and novel stimuli as our example. Reaction to novelty can be determined by how quickly an individual rat approaches a novel object. Some will hesitate whereas others will respond fairly quickly. The measure is # of seconds it takes for a rat to contact the stimulus.
1. There must be genetic variation in the trait
a. Animals must differ in their reaction to novel stimuli; indeed as noted above rats vary considerably in their reaction to novel stimuli (see hypothetical times for 3 rats from slide).
b. However, individual variation in contact scores can also result from nongenetic factors (such as watching others respond to novelty and learning from them). Our focus here is on genetic variation, so we will assume that the individual variation is under genetic control. .
c. Causes of genetic variation
1. mutation (any change in genetic structure; e.g., addition or deletion mutations occur when a single nucleotide is added or deleted from a section of DNA)
2. genetic recombination (in sexually reproducing organisms; when pairs of chromosomes come together, sections of one chromosome may cross over and swap positions with sections of the other chromosome).
3.__________________
2. There must be fitness consequences (i.e., differential reproduction and survival).
a. Variation in contact scores must be related to differences in reproductive success.
b. scenario 1: contact scores range from 20 secs to 80 secs, and rats with contact scores 0f 20-40 secs produce more offspring that in turn reproduce than those with contact scores of 60-80 secs. Does this example meets both the requirement of variation and the requirement of fitness consequences? ______________
c. scenario 2: 40% of the population has a contact score of 45 secs whereas 60% has a contact score of 80 secs. Does this example meet both requirements?
3. There must be a mode of inheritance (ability to pass traits to offspring). Heritability analysis measures the proportion of variance in a trait that can be attributed to genetic variation upon which natural selection acts. It can be studied in different ways. Two examples are described below.
a. truncation selection experiment (we will use the previous rat example; after testing the reaction of 100 rats to a novel object, we can generate the following graph)
1. Draw the graph of the contact score distribution
2. The graph shows how contact scores are distributed in a population of 100 3-month-old rats. X(0) is the mean which equals 30 secs.
3. We now truncate the contact scores by allowing only those animals with a mean contact score of 60 sec or higher to breed (draw the line through the distribution). We then measure the contact scores of this first generation group of rats, yielding a mean of 70 secs = X(1)
4. This information allows us to calculate S (the selection differential) = X(1) - X(0) = 40 secs. (difference between the mean trait in the population as a whole compared to the mean for the selected parents)
5. We then breed the first generation rats and measure the contact scores of ALL second generation offspring, yielding a mean of 50 sec which is represented by X(2).
5. This information allows us to calculate R (effect of selection) = X(2) - X(0) = 50 - 30 = 20 secs. (becomes the numerator) This is a measure of how the 2nd generation offspring changed in comparison to the original population.
6. Finally, Heritability is measured as R/S = 20/40 = 0.50 (thus, 1/2 or 50% of the variance in contact scores is due to genetic factors). What about the remaining 50%? (list other variables below)
b. Parent offspring regression (use the work by Charles and Mary Brown on cliff swallows).
1. Cliff swallows nest in groups that vary in size from one population to the next.
2. Is this variation hereditary?
3. Offspring of parents live in group sizes that are similar to their parents, regardless of whether they breed at the same site or emigrate to a new site. Thus group size is not just a function of a particular site. But this fact alone does not provide definitive support to the role of heredity - it just rules out nesting site as a variable of importance.
4. Two possible explanations:
---a. Cliff swallows acquire a preference for group size by learning it from their parents.
---b. Controlled by genetic factors
4. Cross fostering enabled the Browns to take nestlings from parents residing in large groups and give them to foster parents residing in small groups and vice versa. If experience with parents mattered, then offspring should prefer group sizes that coincided with their foster parents.
5. What happened?
IV. MODERN THEORY OF NATURAL SELECTION
A. Types of Selection (understanding distributions)
1. We often think of selection as unidirectional, most often acting to increase some variable (e.g., brain size in primates). However, there are many ways that selection can act on variability. To understand how selection acts, one must understand distributions.
2. Below is bell-shaped curve for the possible distribution of leg length in chickens. The curve shows both the mean (height of the curve) and the variance (distribution of scores around the mean). The upper and lower ends of the distributions are called the tails. This a hypothetical example not actual data. We will assume that differences in leg length are associated with genetic differences. We will then show how this distribution (mean and variance) can be modified by natural selection.
3. Stabilizing Selection: Unlike the common view that selection is unidirectional, the most common form of selection is one that reduces variability. Draw the associated graph and indicate what is selected here?
3. Directional Selection (draw the associated graph showing that one of the tails (upper or lower of the distributions is selected)
4. Disruptive Selection (draw the associated graph showing that both tails of the distribution are selected; this type of selection is thoght to be relatively rare)
5. Frequency Dependent Selection (we have to switch to another example here as leg length in chickens is probably not subject to FDS).
a. success of a genotype depends on its frequency in the population; having different fitness when it is common as opposed to when it is rare
b. negative FDS which is _________________
c. the net result is a stable equilibrium
d. Rare male effect: an example of negative FDS
1. occurs in fruit flies where females prefer to mate with unusual or rare males
2. imagine a population of mostly red-eyed male fruit flies with a few males having white eyes.
3. females will preferentially mate with white eyed males
4. When this rarer phenotype becomes more common what happens?
B. Fitness (estimating reproductive success)
1. Initially biologists thought that one could simply estimate reproductive success of individual animals by counting those offspring that survived to reproduce.
a. However, this simple counting proved inadequate because Hamilton (1964) showed that some animals appeared to forego reproduction to help others raise offspring. (see scrub jay video)
b. This finding seemed at odds with the theory of natural selection because those who give up breeding should not pass their genes on and this trait should be lost.
c. However, this trait might persist if helpers were helping relatives. One can propagate one's genes even though more slowly by helping relatives.
d. see scrub jay video: Why do scrub jay offspring forego reproduction for up to 7 years and remain at their parents' nest to help rear younger brothers and sisters?
2. Calculating fitness now requires that the degree of relatedness also be included
a. direct fitness (the number of one's own offspring) multiplied by the coefficient of relatedness)
b. indirect fitness (the number of offspring you helped rear that would not have survived without your efforts multiplied by the coefficient of relatedness) must differentiate between the number of offspring that an individual could raise on its own from the "additional" offspring produced through helping (why is this so?)
c.inclusive fitness (how is this calculated?) _____________________________________________
d. Turn to the handout
3. pied kingfisher (helpers can be either relatives or nonrelatives - how can that be?)
Turn to the handout on the pied kingfisher
Questions to think about
1) You plan to examine the heritability of leg length in chickens. The original population yields a mean leg length of 4.2 inches = X(0). You truncate the population and breed animals that have 6.5 inch legs or longer. The resulting Gen 1 population has a mean of 7.2 inches = X(1). You then breed the Gen 1 population and the resulting Gen 2 population has a mean leg length of 6.2 inches = X(2). What is the heritability of this trait? (you don't need a calculator for this).
2) Imagine a situation in which raccoon females typically rear 3 offspring by themselves. Two raccoon sisters are living nearby, and sister A visits sister B regularly providing assistance and help with B's 3 offspring. However, sister A is only rearing 2 of her own offspring. How many extra offspring would sister A have to help sister B rear in order to do better than rearing 3 offspring on her own? How many total offspring would sister B have?