Lecture 38—Evolution of Love
The word love has many meanings. The primary concern of this lecture is interpersonal love: a strong positive emotional attachment of an individual toward another individual of the same species. In keeping with theme of evolution, we look at the different types of intense commitments that individuals have for members of their own species and see what the selective advantages might be for those types of loving behaviors.
1. Self-love—all organisms can be said to display attention to their health and safety.
2. Sexual love—Most eukaryotes reproduce by sexual- the fusion of two gametes’ DNA (fertilization) forming a new combination of genetic material. (Note that gamete formation necessitates meiosis when the diploid cell condition is cut in half to form haploid gametes.) How did such a system evolve and to what purpose, given that prokaryotes and some eukaryotes do not indulge. Asexual reproduction seems to have many selective advantages.
Asexual reproduction includes binary fission of prokaryotes, vegetative reproduction by plants, unisexual reproduction by some lizards, and parthenogenesis by bees and ants.
In asexual reproduction every offspring is a clone of the parent. Each parent sends a complete set of its genes to each offspring. And all parents are effectively females. But in sexual reproduction, 2 individuals (female and male) only give ½ of their genes to the new offspring. So asexual reproduction is twice as efficient as sexual reproduction at sending one’s genes into the next generation. Asexual populations should rapidly outproduce sexual populations in good environments. This has been called the Intrinsic Cost of Sex.
Sexual reproduction has a clear selective advantage when environmental conditions are unstable or harsh; the mixing of genes will produce new combinations each generation. Thus, in harsh conditions with sex there is much better chance than some individuals will survive than with asexual reproduction.
3. Parental Love and the attention that parents lavish on their offspring brings a clear selective advantage to those species which practice it. Most invertebrate species, fish, amphibians and reptiles lack any parental care. Sperm and eggs are released into the water at the same time where fertilization occurs and the larvae fend for themselves. In a few cases sprinkled throughout the family tree there are cases of insects, fish, amphibians and reptiles where parental care has evolved. But such evidence of parental “love” is the rule in birds and mammals. Species which care for young have much higher reproductive success and can reduce their gamete production accordingly.
4. Offspring Love for Parents—In care-giving species the young animals typically display behavior that elicits caregiving from the parents. Examples are food begging and crying.
5. Sibling love—Some species routinely have brothers/sisters/cousins who bond and assist one another. This is an example of kin selection in action. They all have genes in common and by sharing the risks they improve the chances that their genes will be passed on. (e.g. lion brothers take over a pride together). This is the Inclusive Fittness argument.
6. Love between unrelated individuals—This is common among individuals of social species which have good memories and live a long time. Coalitions of friends bond together knowing that they can count on one another for assistance and this improves their chances of survival.
There is evidence that both learning and genetics are involved, the degree depends on the species and particular behavior.
Evidence for genetic involvement in human expression of love.
1. Chimpanzees etc. have several behaviors that are similar to humans—smiling, laughing. Bonobos have face to face copulations.
2. All human populations have the same behaviors to show pleasure and joy—smiling, laughing, pupil dilation and similar appeasement gestures: crying, bowing, crawling, averting eyes.
3. Particular brain centers are activated with pleasure (hypothalamus/pituitary gland) and specific hormones are released:
Three neural circuits seem to exist that are associated with romantic love:
l Lust (with increased levels of testosterone and estrogen)
l Attraction, romanticized commitment to an individual (with increased pheromones, dopamine, norepinephrine, and serotonin— produce reactions like amphetamines stimulating the brain’s pleasure center)
l Attachment= Pair bonding/ maternal behavior/promotes trust and generosity (oxytocin & vasopressin). Drug MDMA (“Ecstasy”) increases feeling of love, empathy, and connection to others stimulates oxytocin activity.
Terms/Concepts to Define:
Love
Sexual reproduction
Asexual reproduction
Binary fission
Budding
Vegetative reproduction
Parthenogenesis
Intrinsic cost of sex
Kin selection
Inclusive fitness
Reciprocal altruism
Can you answer these questions?
1. There are large numbers of asexual species. Please give two reasons why they may have a selective advantage over sexual reproduction.
2. Give three example of asexual reproduction.
3. Why is a process like meiosis necessary for sexual reproduction?
4. Here is a quote by Rolf Hoekstra. Explain in detail why he makes such a claim.
“In a world without sex there would be no males and females. No flowers, no insects specialized in pollinating them, no extravagant colour and form like the peacock’s tail; and much animal behaviour would not exist.”
5. August Weisman asserted that sexual reproduction increased genetic variation, in what way was he correct? How is such variation produced?
6. The experiments of Curtis Lively in New Zealand demonstrated that when parasites were present in ponds that there were 2.5 times as many sexually reproducing snails as in ponds where parasitism was low. What does this suggest about the advantages of asexual vs. sexual reproduction?
7. Most frogs they lay their eggs and leave them to develop in water. There are a few exceptions where parental care is involved. What predictions might you make about the number of eggs laid and the potential reproductive success in the two situations?
8. What possible advantage might there be for young mammals to have large eyes?
9. Nepotism is common in humans, how could such a behavior have evolved?
10. Would you expect that reciprocal altruism would occur in fruit flies? Explain.
11. What value, if any, do social signals have like smiling or crying?
12. Various recreational drugs mimic many of the emotions that we associate with love. How do we explain this?
13. There are many advantages to increased expressions of love for humans and other animals. If such expressions are largely genetically determined and thus subject to evolutionary forces, why hasn’t this type of behavior replaced any aggressive tendencies?