Evolutionary Selection of Behavior

Evolution, Learning, and Behavior

What do we mean by evolution?

Biological evolution is change in the heritable characteristics of populations across generations. It can affect structure, physiology, and behavior. Paleontology Professor John Horner says in Dinosaur Lives: "When you flip through the pages of the family album you're witnessing evolution at work."

Living organisms share an evolutionary history. The broadly shared genetic code and similarities in DNA support common ancestry. Humans and other primates descend from common ancestors; evolution is a branching process rather than a ladder from primitive to advanced forms.

Mutation and natural selection

Mutations cause variations

Mutations are changes in DNA. They can arise from copying errors or other processes. A change in the genes’ "spelling" may alter a protein, affect when a gene is used, or have no noticeable effect. In the simplified example, a new instruction might be "build Peter with a different eye-color." Mutations do not occur because an organism needs a particular adaptation, though researchers can sometimes identify which genes are involved.

Natural selection

I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection. – Charles Darwin (British naturalist, 1809–1882)

Charles Darwin, and independently the British naturalist Alfred Russel Wallace, discovered the theory of evolution by natural selection. Darwin called his work On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.

Darwin was inspired by the Reverend Thomas Malthus's An Essay on the Principle of Population in which Malthus wrote: "Human population grows exponentially, like compound interest in a bank account, but farm output rises at a slower, arithmetic rate; the result, human population will inevitably and repeatedly outstrip its food supply." Malthus argued that population can outstrip resources but is held in check by diseases, war, predators, and by limited resources like food.

Darwin made the following three observations:

1. Competition and environmental change. In most species (a species is a group of individuals capable of producing fertile offspring; like snakes, lion, humans) there are always more offspring born than can survive to adulthood and reproduce. Darwin saw two reasons for this: (1) Since there is a limited amount of resources (like food, space, mates) there is competition between individuals for these resources, and (2) Since the environment changes over time and from one region to another, there are threats (predators, change in climate, isolation, diseases, change in the physical environment) to the children's survival and their reproductive success.

2. Individual variability. Within a species, there is an enormous amount of individual variation. No two individuals of the same species are alike in their anatomical structure, physiology or behavior (we're not an exact copy of our parents). Individuals vary in their cell structure, fighting ability, and social skills. Variations make every individual unique and that variation must in some way be heritable otherwise children wouldn't resemble their parents more than they resemble other individuals.

3. The world is not fixed but evolving. Species change, new ones arrive and others go extinct.

Darwin called his principle natural selection. Any slight variation in traits that gives an individual an advantage in competing with other individuals of the same or different species or in adapting to changes in their environment increases the chance that the individual will survive, reproduce, and pass along its characteristics to the next generation. Maybe they have greater resistance to disease, or can run faster, or survive climate changes better.

Darwin used the word "selection," but nature doesn't care who gets selected for survival. Evolution has no goal. Another way to describe natural selection is as a process of elimination. Certain individuals survive because they have structural, physiological, behavioral or other characteristics that prevent them from being eliminated. Individuals without those characteristics may leave fewer descendants in that environment. Heredity enhances the likelihood that the non-eliminated or "selected" variations are preserved. Darwin didn't know about genetics. Therefore he couldn't know that these characteristics were caused by mutations and that they could be passed on through the genes.

A trait’s advantage depends on the environment

If the new trait is helpful, the mutated individual is more likely to survive, reproduce and pass the new trait to his children. Take a poisonous spider as an example. Assume that a population (a group of individuals belonging to one species that occupy the same geographic/ecological niche at the same time) of black widows differs in how toxic their venom is. If some spiders (mutants) are born with more toxins than others, two things could happen over time. If more toxins give an advantage in the spiders' environment, more toxins might be "selected" for and the "more-toxins" characteristic might be passed to children. If the more toxic black widow spiders survive and reproduce better than the less toxic ones, then black widow spiders will, over time, evolve more toxic venom. The frequency of the "more-toxic" spiders in the population increases over time. A trait with no selective advantage need not disappear. Its frequency may also change through genetic drift or its association with other inherited traits.

When organisms undergo selection, some characteristic may be carried along that wasn't selected. Even if some trait didn't provide an advantage it could still be carried along as long as it isn't harmful, i.e., doesn't negatively influence survival and reproduction. But a situation may arise in the future when that trait can become useful.

What happens when the environment changes?

Since environments change over time and with geography, different variants are "selected" under different conditions. Characteristics that are successful in one environment may be unsuccessful in another. This is well expressed by the late American paleontologist Stephen Jay Gould in Wonderful Life: "Even if fishes hone their adaptations to peaks of aquatic perfection, they will all die if the ponds dry up."

Organisms respond differently to environmental stress. Cockroaches illustrate considerable adaptability, though they cannot survive independently of all environmental conditions. Writer Richard Schweid puts his admiration more vividly in The Cockroach Papers: "If there is a God that made all life forms, a particularly rich blessing was bestowed on the roach, because it got the best design of all."

The account points to cockroach relatives in fossil records dating back 325 million years and gives 45 days without food as an example of their endurance. Cockroaches can exploit varied foods and habitats, but survival limits and reproductive traits differ among species and conditions. An ancient lineage does not mean that modern cockroaches have remained unchanged.

Often a new trait or a change in an individual doesn't occur through a single step but through a gradual accumulation of small mutations being selected over a long period of time. By dating meteorites, most scientists say that the earth is about 4.6 billion years old. The oldest bacteria fossils go back 3.5 billion years. Given enough variation and time, even such a complicated thing as the eye has gradually developed. Evolutionary changes can act fast. Studies of fruit flies show that differences in wing size could take place in as little time as a decade. A change in a species' territory (e.g. climatic) can also bring on rapid structural changes (variation). Other studies show that a change that involves few genes could come about faster (and cause large behavioral changes) than one that depends on small changes in many genes.

Mutation is not the only source of genetic variation.

Mutation creates new genetic variants, and recombination reshuffles inherited variants. Genetic drift changes their frequencies through chance, especially in small populations. Gene flow moves variants between populations through interbreeding; cultivated plants and their wild relatives provide an example. Gene duplication is another kind of mutation. Close associations between species can also produce new inherited capabilities, while co-evolution describes reciprocal evolutionary change. These processes differ from natural selection, which favors heritable variants according to their effects on reproductive success.

The evidence for evolution

There is fossil, anatomical and molecular evidence of evolution. The fossil record shows how morphology was modified. Similarities of organs in related organisms show common ancestry. Comparing DNA sequences also helps researchers assess relatedness and reconstruct shared ancestry.

One example of evolution is industrial melanism. Before the Industrial Revolution in England, the color of the peppered moth was mainly light. When there was no industrial pollution, the darker moth cropped up by mutation. But since the darker moths were easier to spot against the tree bark, hungry birds snapped them up. Only when the environment changed, when soot from new factories covered the tree trunks, did darkness become an advantage. Selection began to favor the darker moth. Darker moths were better disguised on tree trunks covered by soot. The lighter moths were eaten and the darker ones increased in numbers. Around 1950 the environment started changing again. A decrease in the use of coal and better filtering equipment in the factories produced a cleaner environment and the peppered moth is in the process of returning to its lighter color.

Why aren't antibiotics as effective against dangerous bacteria as before?

Evolution is at work today. Evidence of evolution is seen in pesticide resistance among insects and the antibiotic resistance of bacteria.

Bacteria can reproduce rapidly. A population may contain resistant variants, whether arising through mutation or acquired from other bacteria. When an antibiotic removes susceptible bacteria, resistant survivors may have more opportunities to reproduce and spread. This changes the population over generations; it does not require bacteria to anticipate or intentionally solve the problem.

Antibiotic exposure can select for resistance, and patterns of use affect how resistance spreads. This does not mean that every exposure inevitably produces resistance or that every control method must fail. Outcomes depend on the organisms, the treatment, and the conditions.

Just as we can't blame an animal for eating another animal to survive, we can't blame bacteria for giving us an infection. They have no intention of harming us. Bacteria do what comes natural to all of us – survive and reproduce.

Are bacteria always bad for us?

No, they are important for our digestion and immune system. They are also vital for life on earth. Without them we wouldn't exist. In order to generate energy, we need oxygen. This oxygen is assumed to have been generated by a group of bacteria called cyanobacteria (or blue-green algae), the light-harnessing microbes that live primarily in seawater.

The key source of energy for nearly all life is sunlight. This light energy is transformed into chemical energy in plants, algae and certain bacteria by photosynthesis. For example, plants make their food – usually glucose – from carbon dioxide (through leaves) and water (mainly through roots). Sunlight provides the energy needed to run the biochemical process that yields sugar and the by-product oxygen (that water molecules contain) which is released into the atmosphere. When we eat plants (or animals that eat plants), we take in their stored energy.

In plants, photosynthesis is handled by chloroplasts. They contain the molecule chlorophyll, allowing plants to absorb the energy from the sun. Chloroplasts may have evolved from cyanobacteria that became incorporated into ancestral plant cells a long time ago. Fossil evidence shows that there were cyanobacteria-like microbes on earth 3.5 billion years ago. It is also believed that mitochondria (the non-bacterial cell structures where oxygen is used to burn food for energy) evolved through an association between different kinds of cells.

Some cyanobacteria fix atmospheric nitrogen. Other bacteria do so as well: bacteria associated with the roots of legumes such as beans and peas convert nitrogen into compounds the plants can use.

Why isn't the world covered with dead plants and animals?

Partly because bacteria break down the tissue of dead plants and animals into nutrients like carbon and nitrogen that are then released back into the environment.

Go out and look at a recently dead bird. Then observe how it decomposes over the following month. How much remains will depend on the conditions and on other decomposers and scavengers.

Thus, evolution selected the behavior that made our ancestors survive and reproduce. What guidance system has evolution selected to help us make better decisions for survival and reproduction?

Guidance through values and life experiences

Human beings are pulled forward toward and by nature seek pleasure, whereas they flee from and reject pain. – Epicurus (Greek philosopher, 341–270 BC)

What drives us?

The 17th Century English philosopher John Locke said: "Good and evil, reward and punishment, are the only motives to a rational creature: these are the spur and reins whereby all mankind are set on work, and guided." Reward and aversion help guide behavior. Eating can relieve hunger and feel pleasurable, while starvation causes distress. Such systems reflect evolutionary history, but pleasure is not a guarantee that a behavior is beneficial, nor does pain always identify something harmful in the present setting.

Harm avoidance first

Pain can command attention, and negatively arousing experiences can leave strong memories. Neurology Professor Antonio Damasio says in Descartes' Error that "it is the pain related signal that steers us away from impending trouble." Avoiding damage can aid survival. Psychology Professor Randolph Nesse and Biology Professor George Williams say in Why We Get Sick: "Pain is the signal that tissue is being damaged. It has to be aversive to motivate us to set aside other activities to do whatever is necessary to stop the damage."

That we are sensitive to events or stimuli that have painful implications for us, explains why we have such an aversion to loss. Richard Dawkins says in The Blind Watchmaker: "However many ways there may be of being alive, it is certain there are vastly more ways of being dead, or rather not alive." The fear of loss is much greater than the desire to gain. Research shows that we feel more pain from losing than we feel pleasure from gaining something of equal value and that we work harder to avoid losing than we do to win. That we pay more attention to possible losses than gains makes sense. In Steven Pinker's book How The Mind Works, social psychologist Timothy Ketelaar says, "As things get better, increases in fitness show diminishing returns: more food is better, but only up to a point. But as things get worse, decreases in fitness can take you out of the game: not enough food and you're dead."

Our aversion to pain also encourages a certain human behavior: to take the most rewarding view of events. We interpret choices and events in ways that make us feel better. We often prefer to hear supporting reasons for our beliefs; think of ourselves as more talented than others, and make the best of bad situations.

How are certain connections strengthened?

Learning can strengthen some patterns of neural activity and weaken others. Whether a connection changes depends on how it is activated and on the feedback and context, not simply on how often an event occurs.

Inherited preferences and experiences both influence what we seek and avoid. Reward can reinforce behavior within an individual’s life. Natural selection, by contrast, changes populations across generations; these are related but distinct processes.

For example, light is preferred to darkness, eating certain food is better than not eating, etc. When we drank our mother's milk, our brains told us that "eating" was pleasurable. Our chance of survival increased. If we didn't eat after we were born, the feedback from our brain would be that "not eating" was painful. The chances are that we ate in the future. Since the feedback from eating was pleasurable, certain neural connections were strengthened. In the future, when we were exposed to the same stimuli, this group of neurons reacted stronger. Reward can make a behavior more likely to recur, though the effect depends on the circumstances.

Neural connections change with experience. Repeated practice and useful feedback can support learning, while some repeated stimuli produce habituation rather than stronger responses. We draw on past experience when responding to new situations, but a response that once worked may not fit the present circumstances.

Does the brain work like a computer – systematically and logically?

The comparison in this account emphasizes the brain’s ability to recognize patterns. Dr. Ralph Greenspan expressed that view in the following historical discussion of chess and computers:

In no sense does the brain work like a computer. Computers record, and computers have things stored in specific places that are stable. Our brains do none of that. When the great chess master Gary Kasparov lost to Big Blue everybody said "Aha, this machine can think!" Big Blue was not thinking. Big Blue was simply replaying the entire history of chess. That's not the way that Gary Kasparov or any human being plays chess. We do pattern recognition. Even though we are capable of logic, our brain does not operate by the principles of logic. It operates by selection of pattern recognition. It's a dynamic network. It's not an "if-then" logic machine.

Chess programs can combine search with evaluation of positions; they do not need to explore every possible move. Skilled human players also recognize familiar configurations and use them to narrow the options worth considering. The following historical comparison emphasizes that selective judgment. Warren Buffett follows up with:

There was a great article in the New Yorker magazine ... when the Fischer/Spassky chess matches were going on. And it got into this speculation of whether or not humans would ever be able to take on computers in chess. Here were these computers doing hundreds of thousands of calculations a second. And the article asked, "When all you're really looking at is the results from various moves in the future, how can a human mind deal with a computer that's thinking at speeds that are so unbelievable?"... Well, it turns out a mind like... that of a Fischer or a Spassky essentially was eliminating about 99.99% of the possibilities without even thinking about 'em. So it wasn't that they could outthink the computer in terms of speed, but they had this ability of what you might call "grouping" or "exclusion", where essentially they just got right down to the few possibilities out of these zillions of possibilities that really had any chance of success.

What part of the value system is called "human nature?"

So far we've learned that connections between neurons determine how we think and behave. Our genes provide us with the framework for neural development and our life experiences and our environment shape our brain.

Since the brain is shaped by experience and people can learn from trial and error, learning helps us adjust behavior during our lives. Natural selection also shaped inherited tendencies in the environments our ancestors encountered. The question then becomes: What was the operating environment in which the human brain evolved?

The hunter-gatherer environment has formed our basic nature

The account places the beginning of the human lineage about 4 to 7 million years ago and the appearance of a "modern" human brain some 150,000 to 200,000 years ago. These are approximate historical framing figures, not a claim that the brain appeared fully formed at a single moment. Hunting and gathering supported human populations through the last Ice Age, ending around 13,000 years ago. Agriculture developed some 10,000 years ago, with different timings across regions; hunting and gathering also continued afterward.

On this broad framing, humans spent more than 99% of their evolutionary history in hunter-gatherer settings. As a rough illustration, compressing 4 million years into 24 hours places agriculture near 23 hours and 55 minutes. The point is the relative brevity of agricultural life, rather than an exact biological timetable.

To examine how ancestral conditions may have influenced behavior, we need to ask about resources, social groups, climate, predators, and disease. We cannot observe the distant past directly, but archaeological evidence and comparisons among populations can inform the inquiry. Hunter-gatherer environments were varied; a single account of small groups competing for food and mates cannot capture all of them.

One proposed division of labor assigns hunting and defense mainly to men, and gathering, food preparation, and childcare mainly to women. Treat this as a broad model to examine, rather than a fixed arrangement shared by every population or an explanation of every present-day difference.

If this were the environment, what would be appropriate behavior to increase the likelihood for survival and reproduction? What behavior has been natural during 99% of our history?