Animal Behavior/Nature v Nurture

Nature vs. Nurture

Behaviors exist on a continuum from being to a large extent contingent on genetic factors (i.e., instinctive), to others which are mostly acquired through experience. Instinctive components are those that are exhibited without the benefit of, or need for, prior experience. They come to the fore when, for instance, newly hatched, inexperienced individuals of a particular species of spider construct their webs with little variation, despite the lack of a tutor. Learned behaviors are those that are acquired through interactions with the surrounding world.

Nature vs. Nurture describes a debate about the degree to which individual differences in physical and behavioral traits are the product of an individual's innate qualities ("nature") versus those shaped by personal experiences ("nurture"). This dichotomy aims to delineate how much of the human (psychological) make-up is the result of environmental influences and how much is a result of phylogenetic heritage. In today's view there are multiple problems with phrasing the issue in this either-or form. Most significant of all, as the two are NOT mutually exclusive concepts. It is now clear that in every instance both influences play significant roles as all development represents a complex interaction between inherited factors and environmental conditions.

Essentially, an individual’s phenotype is a result of the genetic and environmental factors that contribute to its development. A genotype is the entire set of genetic material an individual inherited from its parents. A phenotype is the observable characteristics or traits of an individual’s genotype, which include morphologic, physiologic, and behavioral traits. The environment encompasses every other aspect, besides genes, of an individual and its surroundings (hormone levels, diet, family life, etc.).

The Four Primary Relations of Developmental interactions

In order to better understand and study how this complex network of interactions between an individual’s genotype, phenotype and environment work in concert to affect the development of an individual, it is useful to break down these interactions into four primary relations: (1) how the parent’s genotype contributes to the individual’s genotype, (2) how the individual’s genotype affects its phenotype, (3) How the individual’s environment affects its phenotype, and (4) how the individual’s phenotype affects its environment.

Parent’s Genotype → Offspring’s Genotype

Parents each contribute 50% of their genetic material to comprise the entire genome of their offspring. The genes that are coded for within an individual’s DNA affect development and behavior. Although an individual’s entire genome is inherited from its parents, because there are mechanisms that contribute to genetic diversity (such as mutation and recombination) no two individuals will have the exact same genotype (with the exception of identical twins).

Individual’s Genotype → Individual’s Phenotype

An individual’s genotype contains all of the genes that were parentally inherited, but not all of those genes are expressed in the individual’s phenotype. This occurs through several mechanisms, which include developmental changes and dominance patterns.

Developmental changes: In any given cell at any given time in an organism’s body, some genes will be active (turned on) while others will be inactive (turned off). Regulator genes primarily control when genes will be turned on or off, both for changes that occur only during critical times in development as well as for matters of day-to-day operation. Genes are typically used in concert with other genes, forming a complex network of genetic expression, and are rarely used in isolation.

Dominance patterns: Many genes have more than one allele (a different form or expression of the same gene). Due to Mendelian inheritance patterns, such as dominant-recessive or co-dominant patterns, many genes are never expressed or are only partially expressed. Therefore, all of the different alleles of a gene that influence the expression of a particular trait or characteristic (e.g. eye color) may result in a different phenotypic expression (e. g. brown, green, blue, hazel). However, the phenotypic expression of most traits or characteristics are the resulting summation of multiple genes (polygenic inheritance), and some genes can affect multiple traits.

Individual’s Environment → Individual’s Phenotype

As stated earlier, an individual’s phenotype is the result of the interactions of both their genes and the environment. Therefore, any given genotype will develop differently under different environmental conditions; this is called phenotypic plasticity. The concept of the Norm of Reaction describes this potential variability in the phenotypic expression of an individual genotype: “all the phenotypes that can theoretically result from a given genotype in relation to all the environments in which it can survive and develop.”

An extreme example of phenotypic plasticity is the recessive autosomal metabolic genetic disorder, Phenylketonuria (PKU). This disorder is due to a mutation in a gene on chromosome 12 which codes for the enzyme phenylalanine hydroxylase. Because the mutation renders the enzyme nonfunctional, people who possess two copies of this recessive gene cannot metabolize phenylalanine, an amino acid found in many foods. If individuals with PKU eat a normal diet, the accumulation of phenylalanine builds up in the bloodstream and leads to impaired brain development, intellectual disability, seizures and other health issues. However, if individuals with PKU are placed on a strict phenylalanine-free diet shortly after birth they can develop and live healthily. Therefore, individuals with PKU (The genotypic trait of interest), under different environmental conditions (the presence or absence of Phenylalanine in the diet), will develop drastically different behavioral phenotypes (impaired brain development vs. unimpaired brain development).

Parents also contribute greatly to their offspring’s environment. Parents have a great influence over what experiences their children are exposed to, including the quality of parental care and nutrition that they receive. It is important to note that parent’s behavior toward their offspring is influenced partially by their genetics as well as by their previous developmental experiences, which all contributes to their particular parenting style. Therefore, an individual’s environment is also greatly affected by the phenotype and genotype of its parents.

For example, mice with a mutation that inactivates the fosB gene are significantly deficient in their nurturing ability towards their young. This suggests that nurturing ability in mammals has a genetically induced component.

Individual’s Phenotype → Individual’s Environment

The genetic predispositions in behavior an individual has and exhibits will naturally evoke certain responses from those around it. For instance, active kittens who do not like to be held will receive significantly less snuggling than those who enjoy to sleep curled up on a human’s lap. Therefore, an individual also has a significant influence on its own environment which in turn influences its phenotype.

Additionally, an individual actively shapes its own environment by seeking the experiences and surroundings that suit its interests, temperament, and abilities. For instance, children who enjoy reading will tend to read more than children who don’t enjoy to read. The more they read, the better they will become at reading which will allow them to read more challenging books and lead them to develop a more advanced vocabulary. With this more developed skill-set and general knowledge that they have acquired though pursuing their interests, children who enjoy reading will most likely experience greater success in school.

Studying The Four Primary Relations

These four primary relations, typically summarized by the term gene-environment interactions, form the basic perspectives through which the complex web of interactions that influence development can be understood. These various gene-environment interactions are comprised of many time-sensitive and precisely-incorporated changes that work together to produce every trait. Therefore it would be foolish to say that a complex phenotypic trait, such as behavior, was purely genetically or environmentally determined. Likewise, neither can it be claimed that either an individual’s genotype or its environment are more significant in the development of its phenotype. Therefore the Nature vs. Nurture Debate is a fallacy, however the debate did frame many questions which are still of great interest. The field of behavioral genetics is concerned with studying how these relationships between the genetic and environmental factors that result in the variation observed in the behavioral and developmental phenotype

Behavioral geneticists study this through two main premises: the extent to which a behavioral or phenotypic trait is influenced by genetic factors, and the extent to which a behavioral or phenotypic trait is influenced by environmental factors. Twin Studies: An extremely valuable source for research in the heritability of behavioral traits in humans are adoptive twin studies, because scientists cannot control and manipulate human pedigrees or developmental environment. These studies compare monozygotic (identical) twins who were raised together with those who were separated at birth and raised apart. Because identical twins share 100% of their genome, differences between the twins can be assumed to be due to environmental factors.