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Friday, March 19, 2010

Evolution and Genes -2

Types of Genetic Studies

Researchers do different kinds of studies to see whether and to what extent a characteristic might be genetically transmitted.

Family Studies 

In family studies, researchers look at similarities among members of a family with respect to a particular trait. If the trait is genetically inherited, it should be similar in blood relatives. The closer the blood relationship, the more similar people should be.
Family studies alone don’t reveal whether a trait is genetically inherited. A family shares genes, but they also share similar environments. When researchers find trait similarity in a family study, their findings may suggest that the trait is genetically inherited, but the study can’t prove it.

Twin Studies

Compared to family studies, twin studies give researchers more solid evidence about whether a trait is inherited. In twin studies, researchers compare pairs of identical twins to fraternal, or nonidentical, twins. When doing these studies, researchers assume that identical twin pairs share the same environment, just as fraternal twin pairs do. However, identical twins share all of their genes with each other, while fraternal twins share only half of their genes. When a trait shows more similarity between identical twins than between fraternal twins, the greater similarity probably comes from shared genes, not shared environment.
One problem with this type of study is that identical twins may not in fact share an identical environment while fraternal twins do. People tend to treat identical twins in unusual ways. For example, people may treat identical twins as if they are similar in every respect, or they might focus intensely on differences between them.
Studies of Separated Twins
In order to avoid uncertain environmental factors, researchers sometimes study separated twins. Twins who are separated when they are very young and brought up in different families have different environmental influences but identical genes. Trait similarities between separated twins result mostly from genes.
However, separated twin studies can also be problematic. The environments of separated twins may not actually be that different from each other for the following reasons:
  • The twins shared a similar prenatal environment before they were born.
  • Adoption agencies may tend to place twins in similar households.
  • Since they are similar in appearance and in genetically inherited abilities, the twins may evoke similar responses from people around them.
As in other types of studies, trait similarities in separated twins may be due to both similar genes and similar environments.

Adoption Studies

In adoption studies, researchers compare adopted children to their biological parents and to their adoptive parents. Adopted children share more genes with their biological parents. The children’s living environments, however, more closely resemble the environments of their adoptive parents. When adoptive children resemble their biological parents more than their adoptive parents with respect to a certain trait, researchers can hypothesize that the trait has a genetic basis.

Interaction of Genes and Environment

In conducting all these types of studies, researchers have found that while genes influence psychological traits, they don’t act alone. Highly influential environmental factors also play a major role. These factors include:
  • Prenatal influences
  • Child-rearing and other parental influences
  • Nutrition
  • Experiences throughout life
  • Peer influences
  • Culture
Genes and environment interact in complex ways. People usually inherit a vulnerability or predisposition to having a particular psychological trait, and the environment in which those people live shapes the development of that trait. The opposite is also true: people’s psychological traits influence their environments. People don’t just live in environments—they also shape their worlds by exerting their traits.
Example: Suppose there are two nonidentical twins, Ben and Tom. Ben is calm by nature, while Tom has always been fussy. Mom and Dad will be more taxed by Tom, so they may be less responsive and patient with him than they are with Ben. Therefore, Tom and Ben experience different parental influences, which may make Tom less trusting than Ben as they grow up. Genes and environment influence Tom’s personality, but the interaction between genes and environment also plays a role.

Evolution and Natural Selection

Evolution is a change in the frequency of genes in a population over time. Evolutionary psychologists try to explain universal behaviors. They study how natural selection has encouraged certain behavior patterns to develop.

The Theory of Natural Selection

Charles Darwin (1809–1882) was a British naturalist who is best known for his contributions to evolutionary theory. Although others had noted that species evolved over time, Darwin first put forward the theory of natural selection to explain the process of evolution.
According to this theory, certain inherited characteristics give an organism a survival or reproductive advantage. Organisms pass on these characteristics more often than they pass on other inherited traits.
Example: The species of primates called mandrills have evolved to have bright blue rear ends, because brightly colored rumps help them attract mates and give them a reproductive advantage. Porcupines evolved to have quills, because quills help them to avoid predators and reproduce. This gives porcupines a survival advantage.
A characteristic that gives a reproductive advantage helps an organism to mate successfully and pass on its genes to the next generation. A characteristic that gives a survival advantage helps an organism to live long enough to reproduce and pass on its genes.

Reproduction of the Fittest

People often use the phrase “survival of the fittest” instead of “reproduction of the fittest,” but according to evolutionary theory, survival alone isn’t enough. Creatures need to survive long enough to reproduce. Reproductive success is measured by how many offspring a creature produces.

Inclusive Fitness

Another concept related to reproductive success is inclusive fitness, described by W. D. Hamilton in the 1960s. Inclusive fitness is the reproductive fitness of an individual organism plus any effect the organism has on increasing reproductive fitness in related organisms. Some researchers believe that the concept of inclusive fitness explains why certain organisms sacrifice themselves to save others in the species. According to this theory, people might risk their lives to save their children or close relatives, but not to save distant relatives or unrelated people. Because people share more genes with close relatives, saving them has more payoff in terms of passing on genes to the next generation.

Adaptations

An adaptation is an inherited characteristic that becomes prevalent in a population because it provides a survival or reproductive advantage. Because evolution occurs over a long period, an adaptation can remain in a population even after it has stopped being useful.
Example: Human beings have a genetic preference for fatty foods, which explains why fried chicken, french fries, and buttery popcorn are so popular. Evolutionary psychologists say that the preference for fatty foods derives from the days when people hunted and gathered and food was scarce. Eating high-fat foods was important because fat gave people the calories they needed. In other words, the preference for fat was adaptive. Today, in wealthy countries with abundant food and sedentary lifestyles, the preference for fat remains, despite the fact that it no longer has beneficial effects. In fact, consuming fatty foods can lead to health problems.

Mutations

Evolution relies on mutations, or small changes in genes. Mutations happen because of two events that can occur during the formation of egg and sperm cells:
  • An error during copying of DNA
  • Random rearrangement of small pieces of DNA in a chromosome pair
Sometimes, a mutation results in a new trait. If the individual with the mutation reproduces successfully, the mutation will be passed on. If the new trait proves advantageous, the mutated gene that caused the trait will increase in the population over a long period and thus propel evolution.

 

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