Social Darwinism

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Chapter 2

The Genetic and Evolutionary Roots of Behavior

Gleitman • Gross • Reisberg

Psychology

EIGHTH EDITION

Henrik Spohler/laif/Redux

Chapter Topics

Genetics and DNA
Evolution by Natural Selection
The Genetics and Evolution of Behavior
Some Final Thoughts: The Strengths and the Limits of Evolutionary Theorizing
Summary

(left) Andrew Syred/Photo Researchers, (middle) Visuals Unlimited/Corbis, (right) University of Edinburgh,Wellcome Image

2.1 Cell types (A) Red blood cells contain large amounts of a protein called hemoglobin. Hemoglobin picks up oxygen when red blood cells pass through the lungs and then drops off the oxygen when these cells pass through the body’s other tissues. (B) Goblet cells, found in the lining of the small intestine, manufacture granules of a protein called mucigen. When a goblet cell releases mucigen granules into the intestine, they combine with water to make mucin, a component of the mucus that lubricates the intestine. (C) Rod cells, which serve as light detectors in the eye, contain a protein called rhodopsin that absorbs light.

Genetics and DNA

The nucleus of each biological cell contains chromosomes, which each contain a single molecule of DNA.
Within this molecule, genes govern the cell’s functioning by providing detailed instructions for making proteins.

Biological influences on behavior are governed by genes: what are genes and how does genetic inheritance operate?
Genes are sections of DNA molecules, stored within chromosomes in every cell, that describe the structure and expression of specific proteins.

(left) Jim Zuckerman/Corbis; (right) Biophoto Associates

2.2 The cell’s control center Cells contain nuclei, which contain chromosomes, which contain DNA.

(photo) Science Photo Library

2.3 A DNA molecule The production of proteins is governed by the sequence of subunits on the DNA molecule. The sequence (for example, CAGGTC or TCCA) determines the structure of the protein that will be produced as well as when, where, and in what amount the protein will be made.

Genome

Humans have 23 pairs of chromosomes and roughly 25,000 protein-coding genes.
These are collectively called the person’s genome.

Leonard Lessin, Photo Researchers, Inc.

2.4 Human chromosomes The human genome consists of 23 pairs of chromosomes; these together contain roughly 25,000 genes. One pair of chromosomes (the so-called sex chromosomes) is either XX or XY; this pair determines whether the person is genetically female or male. The other 22 pairs are called autosomes.

Gene Expression

In each cell, some genes are expressed at any point in time and others are not.
Gene expression is controlled by the biochemical environment inside the cell, which is influenced by the organism’s:
overall environment.
experience.
behavior.

Gene Expression

An organism’s genome therefore specifies only its genotype.
The overt traits and behaviors of the organism define its phenotype.
the product of the genotype and experience, which are in continual interaction

- How genes are transcribed into actual proteins, or expressed, in any given cell or individual is controlled by biology and timing, but also by experience and the social and physical environment.

- Thus, an organism’s specific sequence of genes (its genotype) plays only an indirect role in determining its observable features and behaviors (its phenotype).

2.5 Parents’ chromosome pairs combine to create a child’s Cells in the human body generally contain 46 chromosomes— that is, 23 pairs. When females produce an egg, however, the egg contains just 23 chromosomes—one from each of the mother’s pairs. The same is true when males produce sperm cells—they contain just one chromosome from each of the father’s pairs. When egg and sperm combine, the baby ends up with the appropriate number of chromosomes.

Interactions among Genes

Most characteristics are influenced by the action of many genes.
Each gene is paired with another gene.
The pairs are located at corresponding positions on pairs of chromosomes.

Bettmann/Corbis

2.6 Gregor Mendel (1822–1884) Our modern understanding of genetics grows out of Mendel’s pioneering work 150 years ago.

Dominant vs. Recessive

The genes may or may not be the same allele.
If they are different:
one gene may be dominant and the other recessive,
or the genes may be codominant,
or one may be incompletely dominant.

- Genes are inherited from an organism’s parents, and the laws of Mendelian inheritance control how traits shaped by single genes are passed along; for example, a person who inherits a gene for dimples (a dominant trait) from a single parent will have dimples, while a person needs two copies of the gene for PKU (a recessive trait) to have the disorder.

- Most genetic variation, though, is not so simple.

Even single-gene variations can be incompletely Mendelian (as in the serotonin transporter).
More importantly, the vast majority of an organism’s traits, from physical variations like height to specific disorders like schizophrenia, are influenced by many genes. These traits are still influenced by biology, but in much more complicated ways.

2.7 Inheritance of dimples Dimples are one of the few traits largely determined by variation in just one gene. If someone inherits the allele that favors dimples from both parents (meaning the person’s genotype is DD), the person will almost certainly have dimples. If someone inherits this allele from neither parent (genotype dd), then dimples are unlikely. What if the person is heterozygous? The allele favoring dimples is dominant, and so the genotype Dd usually leads to dimples.

Darwin and Evolution

Charles Darwin hypothesized that all modern organisms:
are descended from a small set of shared ancestors.
have emerged over time through the process of evolution.
An enormous amount of evidence has confirmed these proposals.

If most traits and behaviors are influenced by patterns of many genes, how have these complicated patterns arisen and been maintained over generations? This is a question about why biological forces would have shaped a particular behavior—the “ultimate causes” of behavior.
Darwin addressed these questions with the principles of evolution: that all life had a common origin, and that the process of natural selection had shaped variations among organisms in both their physical traits and their behavior.

Wikimedia Commons

2.8 Charles Darwin (1809–1882) Darwin’s theory of evolution by natural selection has dramatically changed the way scientists think about life on Earth. Here Darwin is shown at about 30 years old, in a watercolor by George Richmond.

Darwin and Evolution

The key mechanism is natural selection.
If individuals with certain traits are more likely to survive and reproduce,
their genes will be better represented in the next generation.
And if the genes gave rise to the advantageous traits,
those traits will be more common in the next generation.

Natural selection occurs when a heritable variation in a species leads to reproductive advantage (but not necessarily survival) in a specific environment, and thus becomes “favored,” or more common, as that variation is passed on to offspring.
This process, repeated over generations, results in organisms that are increasingly adapted to their specific environment (but not necessarily, as the text notes, “better” in any meaningful way).

The New Yorker Collection, 2006, Robert Leighton/cartoonbank.com

2.9 Naturalistic fallacy

Naturalistic Fallacy

It is important to avoid the naturalistic fallacy, however—it does not follow that evolution somehow improves organisms or that anything natural is good.

Genes and Evolution

Darwin’s three principles:
There must be variation among individuals within a population.
Certain of the variants must survive and reproduce at higher rates than others.
The traits associated with this advantage must be passed from parents to offspring.

Genes and Evolution

Both the variation and the transmission of traits depend on the organism’s genome.

The emphasis is on the survival of genes!
This explains behaviors in which organisms endanger their own survival to protect their offspring or relatives.
What matters is the survival of genes, NOT the survival of individuals.

Although Darwin knew nothing of genes, we now know that genes are the mechanism by which favored variations are passed down to offspring.
Sexual reproduction (and occasionally mutations) helps introduce variations into the population, which can be selected for over time.

Biosphoto/Cordier Sylvain/Peter Arnold

2.10 The broken-wing display When a predator approaches the plover’s nest, the mother flies a short distance away and drags a wing, as if she were injured. The predator is likely to turn away from the nest and pursue the (apparently vulnerable) mother. This behavior is easily understood in Darwinian terms—but only if we emphasize the survival of the mother’s genes, and not the mother herself.

Evidence for Evolution

The evidence for modern evolutionary theory comes from many sources, including:
the fossil record.
examination of the resemblance between genomes of various organisms.

- Modern biological research, from the fossil record to molecular investigations of the genome, has produced overwhelming evidence for evolution; understanding how closely all life is related and how much of the genome is shared between species provides the basis for learning about human psychology by studying animals.

2.11 Public acceptance of evolution Despite overwhelming evidence for the theory of evolution, many Americans remain skeptical about it. In a 2005 survey (see figure), only in Turkey did a larger group of people express reservations about the statement that “human beings, as we know them, developed from earlier species of animals.” This statement—almost universally endorsed by scientists—was much more favorably received in most European countries and in Japan.

Wikimedia Commons

2.12 Fossilized remains supporting evolution The four-winged dinosaur (Microraptor gui) has flight feathers on its arms and its legs. This is compelling evidence that this dinosaur is one of the ancient ancestors of modern birds.

Evidence for Evolution

We can also document the unfolding of evolution in some modern organisms.

The unity of life

Shared ancestry for many organisms is evident.

(photos) scienceblogs.com

2.13 Natural selection in the modern world In the 1950s, the murky waters of Lake Washington hid the stickleback from the view of predators. Now that the water has cleared, trout are more easily finding and eating the sticklebacks. The result has been a striking increase in the number of sticklebacks with bony plates on their sides—plates that serve as armor, protecting the fish from the trout.

Anthony Chan, Yerkes National Primate Research Center

2.14 The unity of life These baby monkeys make green fluorescent protein because they have been implanted with genetic instructions borrowed from a jellyfish. This result is an extraordinary confirmation that the biochemical mechanisms inside these animals’ cells are quite similar.

Genetics and Behavior

Evolution by natural selection has shaped behaviors just as much as physical traits.
Natural selection favored behavioral flexibility.
Organisms have evolved mechanisms so they can:
alter their responses.
learn new skills.

Heinrich Van Denberg/ WWI/Still/Peter Arnold

2.15 Evolutionary influence on behavior In some cases, evolution has guided organisms toward specific, well-defined behaviors—like the weaver bird’s specific style of nest building.

Say Cheese!

The behavior of smiling seems to be species general for humans, but is not species specific.
Smiling is also evident in individuals blind since birth.
It does not depend on a history of learning.

Smiling is a universal human behavior (as shown by evidence from babies, from blind athletes, and from widely separated cultures).
It is also shared across species; many closely related primates smile in specific social situations.

What Kind of Smile Is That?

There are at least two types of smiles.
One type is expressive of an individual’s inner state.
produced even if no other people are around
The other type is more social.
functions as a greeting or a means of defusing tense situations

Primates, including humans, typically show at least two kinds of smiles, one in response to pleasant stimuli and a distinctly different one used as a social signal.
Analyzing the role smiling plays in animal behavior can thus help us understand how it is used in human interactions today and how the behavior was promoted and preserved by natural selection across species.

Andersen Ross/cultura/Corbis

2.16 The expressive smile The expressive smile involves both changes in mouth shape and a shift in the muscles surrounding the eyes. The shift creates the pattern known as “crows’ feet,” radiating outward from the eyes.

Corbis

2.17 The polite smile This form of smile involves lips drawn back, revealing the teeth—but with the teeth plainly closed. This smile may be a gesture of submission or a means of avoiding conflict.

What Kind of Smile Is That?

These points provide powerful indications that smiles have ancient roots.
selected by evolution as a means of communication, allowing others to read our inner states and intentions

Genetics and Intelligence

An individual’s level of intelligence is influenced by genetic factors.
Identical twins are a good example: their levels of intelligence are similar, even if they grew up separately.

People differ in their abilities to learn, solve problems, and adapt; what psychologists call intelligence is a general version of this capacity.
Measurements of intelligence have a large environmental component, but they interact with a significant genetic component to intelligence, as suggested by twin studies and studies of adopted children.

2.18 Monozygotic and dizygotic twins Sometimes a woman releases two eggs in the same month and both are fertilized. The result is dizygotic twins—conceived at the same time, born on the same day, but with only 50 percent overlap in their genotypes (the same overlap as for ordinary siblings). Sometimes a woman releases a single egg that is fertilized and then splits into two. This sequence results in monozygotic (identical) twins with 100 percent overlap in their genotypes.

2.19 Genetic relatedness and intelligence Monozygotic twins tend to resemble each other in intelligence, whether the twins are raised together or not; dizygotic twins show less resemblance, although they resemble each other more than do randomly selected individuals.

Heritability Ratio

Intelligence is also influenced by environmental factors.
heritability = genetic variance ∕ total phenotypic variance
The heritability ratio is a summary of the effect of genetic differences within a given population and environment.

Heritability Ratio

The value of the heritability ratio depends on the group being examined.
In groups with lower socioeconomic status (SES), the heritability may be zero.
Heritability also increases with a person’s age.
This may be because people choose environments that amplify their genetic potential.

Heritability estimates measure how much of the variation in a trait is due to genetics; for intelligence, estimates of heritability are often around 60 percent, but they vary significantly depending on the group that is studied (for example, middle-class vs. low socioeconomic status).
One possibility is that genetic inheritance provides a potential for developing intelligence, but this potential must be nurtured in the right environment to be achieved.
The evolutionary perspective on intelligence suggests that intelligence has a genetic component because natural selection favored the ability to adapt to different environments.

Wikimedia Commons

IQ and Natural Selection

Human intelligence was favored by natural selection.
Our ancestors had a reproductive advantage if they could communicate, solve problems, and draw conclusions
Why do humans vary in their intelligence?
It’s unclear—not all inherited characteristics are the direct result of natural selection.

(left) © Andersen Ross/cultura/Corbis, (right) John-Francis Bourke/Corbis

2.21 Not all heritable traits have functions Some people have earlobes that dangle off the bottom of their external ears; some people have earlobes that do not dangle but rather are attached to the upper neck. This trait is highly heritable; in fact, it’s almost entirely determined by a single gene. But the trait has no apparent function, and thus serves as a reminder that some phenotypic differences are merely the results of random variation in the human genome.

Evolution of Mating

Most mammals are polygynous; this is easily understood in evolutionary terms.
To maximize their reproductive success,
males should mate with as many females as possible.
females mate just a few times during their lives but try to ensure the well-being of each of their progeny.

In mammals, mothers are far more critical to an infant’s physical needs after conception (in pregnancy and following birth) than fathers are, which results in different selection pressures between the sexes.
Males should be under pressure to mate with as many females as possible to increase reproductive success, while females should be under pressure to ensure the success of each child. This argument explains mating patterns in many mammalian species; evolutionary psychology argues it also can address key differences in how human men and women approach mating.

WILDLIFE/Peter Arnold Inc.

2.22 The cost of reproduction For mammalian females, reproduction has serious costs. A doe must devote an entire breeding season to carrying the young and then nursing them or the offspring will not survive.

Human Mating Habits

This logic helps explain
why human males express a desire for multiple partners,
and why it’s the female, in most species, who makes the choice about mating.
By the same logic
Natural selection explains why males take the major role in courtship.

Biosphoto/Vernay Pierre/Peter Arnold, Inc.

2.23 Males, females, and courtship In most species, it’s the female who decides whether to mate, which causes the male to do some self-promotion.

Mars versus Venus

Men seem to care more about appearance.
Women seem to put greater weight on social status.
Natural selection likely favored these tendencies.
appearance = health and fertility
status = resources needed to raise young

Buss’ cross-cultural studies have identified widespread patterns in how women and men choose mates.
Among humans, females appear to seek mates who demonstrate the ability to commit resources to raising offspring; thus, women are attracted to mates with wealth or high status.
By contrast, men are under pressure to mate with partners who demonstrate youth and health (and thus maximize their future reproductive success); men thus place more emphasis on physical attractiveness and youth.
However, virtually all men and women desire a long-term, mutually exclusive sexual relationship, which suggests that something more than the pressures for maximal reproductive success are at play.

Stuart Freedman/Panos Pictures

2.24 Male preference for younger women In a wide range of cultures, men prefer partners who are younger than they are; women prefer high-status, slightly older men.

Evolution and Infidelity

An evolutionary perspective also leads to the expectation that men will be more distressed by sexual infidelity in their partners than by emotional infidelity, which indeed is reported.
Women should show the reverse pattern, as is also reported.

Other pressures can account for how men and women vary in concerns about jealousy and paternity; for example, unlike men, women can be sure their children are their genetic relations.
Differences that men and women report in jealousy—that men are more threatened by sexual infidelity and women by emotional infidelity—are consistent with an evolutionary perspective based on these different pressures; so are data that men tend to prefer children that look like them.
These data and their interpretation, though, have been the subject of much debate, highlighting the difficulty of examining human behavior from a purely biological and evolutionary perspective.

Michael Newman/ Photo Edit

2.25 DNA testing Modern biotechnology now makes it easy to compare the DNA of two individuals—and thus to establish the identity of an infant's father.

Behavior: It’s Complicated

Biology versus environment
Either-or arguments are increasingly viewed as irrelevant as the constant interaction between the two has become clearer.
Does evolution produce “good” traits?
Natural selection can favor traits that used to be valuable but no longer are.
example: storing body fat or responding to threat

Concept Quiz

Most of an organism’s traits are not controlled by a single gene, but rather are influenced by a large number of gene pairs. This pattern is called:

gene dominance.

genotype.

polygenic inheritance.

natural selection.

Answer: C

Type: vocabulary

Concept Quiz

An evolutionary perspective on jealousy predicts that males will be more upset by their partner’s _________________ than females will be.

imagined infidelity

emotional infidelity

sexual infidelity

serial infidelity

Answer: C

Type: conceptual

Video Clips

Scared Slow (1:32)

After a car accident or other frightening experience, many people report feeling like the event happened in slow motion. One neuroscientist set out to test whether our brains really perceive things slowed down in response to fear. This ScienCentral News story shows why he had to scare people in order to do the experiment.

Featuring: David Eagleman, Baylor College of Medicine

Red Sways Men (1:30)

The way to a man’s heart on Valentine’s Day? Wear red. This ScienCentral News video explains that there is scientific evidence for the attractive power of red.

Featuring: Daniela Niesta, University of Rochester

Threat and Politics: Are Political Views Rooted in Biology? (1:36)

If you’re ever been in an argument with someone who had different political views, chances are neither of you won. Maybe it’s not just stubbornness. New research has found that people with strong opposing political views might also have very different physical responses to threat.

Featuring: John Hibbing, University of Nebraska-Lincoln

Baby Scent and Dads (1:32)

Even the toughest dads can be tender and nurturing when it comes to their children. Now researchers studying monkeys have found that’s not just an attitude, it’s a physical response to the mere scent of their infant. This ScienCentral News video explains.

Featuring: Toni Ziegler, Wisconsin National Primate Research Center

This concludes the presentation slides for Chapter 2

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