Behavioral Genetics

Genes and Evolution

Basic Principles of Genetics (Chromosomes Genes and Alleles) Review: chromosomes, locus, homologous,

alleles, homozygous, heterozygous, dominant, recessive, structural genes, regulatory genes, genotype, phenotype.

Your text says that human beings have an estimated 100,000 genes. The human genome project showed that there are only 30 to 40,000 genes in human beings. Read about mutation as a source of genetic variation.

Fitness and Adaptation Fitness, in an evolutionary sense, is a measure of

reproductive success. Those most fit are those who leave the most offspring.

The relative fitness of organisms is a function of their adaptations. Adaptations, in this sense, are evolved features that enable organisms to survive and reproduce in their environment.

Realize that a feature may be adaptive in one environment (e.g. a thick white furry coat in the arctic snow of winter) and detrimental in another (the same coat in the tropics).

Fitness and Adaptation Realize that a feature may be adaptive in one

environment (e.g. a thick white furry coat in the arctic snow of winter) and detrimental in another (the same coat in the tropics).

Your text makes the point well that fitness should be measured as a probability.   When thought of this way the concept leads to testable predictions as to which features are adaptive.  

Adaptive value.  Another way to think of fitness.

Evolutionary Forces Forces which cause changes in gene

frequencies over time, i.e. cause evolution. Natural selection Genetic drift

Founder effect Bottleneck

Mutation Gene flow

Hardy Weinberg Formula If two alleles at a locus (A and a) then there

are three possible genotypes for an individual to possess at that locus; AA, Aa or aa.  

If p = the probability of A and q = the probability of a, then (p + q) = 1.  True because there are no other possibilities, a gene at this locus must be either A or a.

Hardy Weinberg Formula

Use the product rule of probability which states that two independent events occurring together have a joint probability equal to the product of their separate probabilities.  Probability of AA = p2; Probability of aa = q2;  Probability of Aa = 2pq (because there are two ways to get pq).

Hardy Weinberg Formula So, when two individuals combine gametes to

produce an offspring the genotypes will be distributed as (p + q)(p + q) =1 = (p + q)2 = (p2 + 2pq + q2) = 1.

This formula applies to a population in equilibrium; i.e. one in which none of the evolutionary forces are active.  Such a population is not evolving.  When we find that the observed frequencies of alleles are different from the predictions of the formula, we can look to see which one of the forces is/are involved.  (See examples, pp 47-38. and solve question 3 on p. 51).

Polymorphic loci Loci with two or more alleles.  If one of a pair of alleles provides a  selective

advantage to its possessor, why doesn't the "inferior" gene disappear through natural selection? Lost in heterozygote and drifts. Stage of development.  Good in juvenile, not so valuable in

adult. Sex differences. Heterozygote advantage. Preference in mating for rare phenotypes (fruit flies).

Units of selection Individuals. Darwin envisioned selection

acting upon individuals and so do I for the most part.

Groups. Difficult to see how this can work. Wade showed that some groups survive better than other groups even though members of both are originally chosen randomly. Still debated hotly.

Units of Selection Genes. Since fitness is the result of the entire

constellation of genes possessed by an organism, it is difficult to see how genes can be the unit of selection. However consider Dawkins’ selfish gene concept.

Genes don't reproduce, individuals do.  But they do so preferentially, depending upon the genes they have.  Genotype of an individual is result of entire gene complement he or she possesses, not of only one allelic pair.  Nevertheless if one allele confers a reproductive advantage to its possessor, that gene should increase over time.

Units of selection Kin selection. Explains how altruism can

evolve. Altruistic acts are preferentially received by the actor’s close relatives. In this way the altruist’s genes are passed on even if the act prevents the altruist from reproducing.

Fitness becomes a measure of inclusive fitness.

Altruism is sometimes considered to be a subset of group selection.

Adaptive Behavioral Explanations (problems)

Need to be careful when applying Darwinian natural selection to behavioral traits. Adaptations are evolved and, therefore, inherited, but a behavioral trait may be learned.

Other evolutionary forces such as genetic drift can be important explanations of behavior.

Selection pressures change over time. What is good now may be harmful at another time. This may obscure the way in which selection has operated to produce a trait.

Adaptive Behavioral Explanations (problems) If evolution proceeds in a directional path,

with favorable genes leading to reproductive success, over many generations organisms should become homozygous for favorable traits.

Not that simple. Gene interactions (epistasis), heterozygous advantage, pleiotropy (genes that affect more than one trait), and location of genes on chromosomes, can also be important.

Evolutionary Stable Strategies A "best" strategy used by most of the members

of a species.  Use models from game theory and economics

to predict behavior patterns.  Grasshopper and redwing blackbird examples.

Proximate and Ultimate Questions Proximate questions are concerned with the

way in which genes control behavior. Difficult problem. How can DNA make RNA

which results in proteins that can eventually be responsible for the production of complex behavioral responses? See diagram page 53.

Proximate and Ultimate Questions To trace the evolutionary history of behavioral

patterns, we want to know the extent to which the pattern is genetic.

Genes never act independent of the environment in which they find themselves. This includes both the micro and macro environment. Behavioral geneticists try to determine the extent to which the behavior is genetic vs. environmental.

Mendelian Techniques (selective breeding)

American foulbrood (AFB) = bacillus which infects honey bee larvae and kills pupae after cells have been capped. Bacteria make way thru the wax into adjacent cells, spreading the infection. Disease can sweep like an epidemic thru a hive. Spreads from hive to hive. Treat by burning or burying infected hive.

Mendelian Techniques (selective breeding)

They found some bees that were resistant to the disease. Is resistance due to differences in physiology or behavior? Rothenbuhler set out to discover which.

He found that resistant colonies could uncap a cell and remove a dead pupa soon after it died. They were called "hygienic". Susceptible colonies did not ("unhygienic").

Rothenbuhler’s bees (continued) Cross of two strains --> all unhygienic

(hygienic trait recessive) Backcross to hygienic strain --> 1/4 hygienic,

1/4 uncapped, but failed to remove larvae, 1/4 removed larvae if they were already uncapped, but would not uncap themselves, 1/4 hopelessly unhygienic (neither uncapped nor removed).

These results best explained with 2 gene pair model.  

Rothenbuhler’s bees (continued) These results best explained with 2 gene pair

model.   U = won't uncap; u = uncaps.  R = won't remove; r

= removes        unhygienic = U_R_               hygienic = uurr  uurr x UURR --> UuRr (unhygienic)         Backcross:  UuRr x uurr -->  1/4 UuRr (unhygienic, neither uncaps nor

removes) 1/4 Uurr (removes, but won't uncap so unhygenic) 1/4 uuRr (uncaps, but won't remove so unhygenic) 1/4 uurr (uncaps and removes, so hygenic)

Rothenbuhler’s bees (continued) The situation is, however, not as simple as

the two gene explanation. There are environmental influences affecting

the behavior as well. Sometimes unhygienic bees will uncap and remove and sometimes hygienic base will not.

Rothenbuhler’s results show that at least two gene pairs are involved. Probably several controlling genes turn other groups of genes on and off.

Hybridization Experiments Cross 2 species which are related closely enough to

interbreed, but show species-specific behavioral differences.

Observe hybrids for clues re what genes doing and how.

Ideally do backcross to parents. Get clues re how many genes responsible for behavioral trait(s) and what they do.

Problem because hybrids usually sterile, behavioral differences involve several genes and matings require situations in which normal barriers to mating between species can be overcome. Can't cross the animals that would provide the most information.

Hybridization Experiments (classic example-Dilger’s lovebirds) He used to species of lovebirds, Agapornis

roseicollis and A. fischeri . A. roseicollis tuck several strips into feathers

of lower back and rump and carry them back to the nest. A. fischeri carry strips in bills, one at a time.

These birds were closely enough related that he could get them to interbreed and produce hybrids.

Hybridization Experiments (classic example-Dilger’s lovebirds) Hybrids are confused. Try to tuck, but hold

strips in the wrong place, fail to let go, begin to preen (displacement?) and drop strips or fail to entrap them by smoothing the feathers. Finally will carry strips in bills, but still try to tuck. Greatly improve carrying with experience, but never able to dispense with fruitless tucking.

Hybridization Experiments (classic example-Dilger’s lovebirds) Is improvement result of the A. fischeri

behavior pattern in hybrid, or from some innate plasticity in the A. roseicollis behavior?

To isolate source of improvement it would be nice to be able to design an experiment where A. roseicollis would be made as incapable of tucking as the hybrids and see if they could learn to carry. Can't do this.

Mendelian Techniques (crossing races) Avoids all the problems of hybrid infertility. Use just one species and cross animals with

naturally occurring behavioral differences. Differences will be smaller, but breeding

easier. Fairly common in animals which experience a range of environments and a certain degree of reproductive isolation.

Mendelian Techniques (crossing races) In 1800s an Austrian monk wanted to

combine the superior gentleness of Italian honey bees with the greater industriousness of the German strain. Got colonies of stormily aggressive hybrid bees.

Monk, Gregor Mendel, decided to switch to peas.

Mendelian Techniques (crossing races) More recently - "killer" bees have gained

attention. = central African race of honey bees which moved north from Brazil and are now in the southern United States.

Need to quantify aggressiveness to detect and evaluate differences resulting from crosses.

Killer Bees To quantify the aggressiveness a 2 cm black leather

ball is suspended 5 cm in front of a colony. The hive is then "stimulated" (i.e. kicked) and the ball bobbed up and down for 60 sec. Then the ball is slowly carried away.

The measured parameters are latency to first sting, number of stings in the ball (count stingers), number of stings in the gloves of the person conducting the test, and distance from the hive at which the last sting is delivered.

Killer Bees African honey bees are often highly aggressive.

Mean latency is 4 sec (12 sec for Italians). No. of stings in the ball is a problem because more bees would like to sting the ball than can find room.

Also, beekeeper conducting test rarely holds out for the full 60 sec, and usually runs away instead of walking slowly as he is supposed to.  Italian colonies rarely pursue an intruder more than 20 m, but African colonies regularly keep up the chase for 1000 m.

Crossing killer bees with Italian bees. Aggressiveness is mostly dominant, hybrids show

latency of 6 sec. (African bees 4 sec, Italian bees 12 sec).

Backcrosses suggest 4 gene system. Differences between the two races can be accounted for by four genes, even though aggression undoubtedly involves many more genes which the 2 groups share.

Practical difficulties of breeding, maintaining and testing bees make more detailed analyses extremely difficult even in this case of a very prominent intraspecific behavioral difference combined with an apparently finite no. of genes.

Mutations When a mutation causes a behavioral

change in an animal, researchers can compare the behavior of the mutant animal with normal animals and try to assess the function of the mutated gene.

Can create mutations using ultraviolet light etc., sort them to determine if they alter a behavior pattern and then analyze one by one the specific actions of each mutated gene.

Not as simple as it sounds

Reasons for Difficulty

Most gene mutations are recessive and masked by dominant normal alleles.

Blockage of a behavior may be lethal. Gene may be involved in other processes as

well (pleiotropic effects). Effect of knocking out two quite different

genes may be the same

Knockout genes Instead of generating random mutations, insert a

mutant gene into an embryo. breed animals that are homozygous for the mutation

and in which the normal gene is "knocked out". See what behavioral effect the mutation has.  Better

than above because know what gene has been inserted, but still has problems.  

When knock out a functional gene in an embryo can produce developmental effects.  Interaction between knocked out gene and other genes in the animal may be interfered with.  Need to use caution.

Knockout genes (nurturing behavior in mice) A mutation in a gene in mice called fosB caused

mothers to neither retrieve their babies nor keep them warm.

Researchers found that normal mice nurture their young because of changes in the amount of hormone they produce and in response to exposure to their babies over time.

Knockoutfor mice have normal hormone levels, so the mutation is affecting the response to the babies.

Drosophila mutations

Only four chromosome pairs so males are hemizygous for 1/4 of their genes (on X) and can create mutations which show up.

Have been able to create mutations in one or more genes controlling phototaxis, mating, chemoreception, visual development, circadian rhythms, sight, learning and memory.

Caenorhabditis elegans This roundworm has only 302 cells in its

entire nervous system and is the only animal for which a complete wiring diagram of the entire nervous system has been completely described.

Recently the entire genome of this worm has been completed.

A “Yahoo” search for” Caenorhabditis elegans behavior” resulted in 86,200 hits. Check it out. Check it out.

Heritability measures Attempts are frequently made to determine

how much of the phenotypic variance in a population is due to genetic variance, how much to environmental variance, and how much to variability due to interaction between the two.

This is called broad sense heritability

Heritability measures Narrow sense heritability is more useful. It

measures the proportion of total phenotypic variance differently, by removing some factors that can be obscuring the situation.

Example in text page 60. Hedrick measured trill length in crickets and found a high correlation between the trill length of fathers and that of their sons. In the population studied to your heritability for this trait was high.

Heritability measures It is extremely important to realize that

heritability measures answer a particular type of question. They do not answer the question, “how much of the trait is due to genetics and how much to the environment?”

They answer questions about variance. How much is the variability in a trait due to

variability in the environment and how much to variability in the genes.

Heritability measures Heritability measures are population dependent.

They apply only to the particular population in the particular environment from which they were measured.

Using such measures in the wrong way have led to gross social injustices.

Heritability measures on IQ, for example, have demonstrated that the variability within populations swamps variability between populations and cannot be used to say that one group has a lower IQ than another.