Most of our offspring survive because they have inherited characteristics that are adaptive. Units of heredity are called genes and these genes indirectly control the expression of the adaptive characteristics. We know that physical characteristics, such as height, bone structure, hair and eye colour are inherited. Behavioural geneticists are interested in the degree to which psychological characteristics, such as ability, temperament and emotional stability are transmitted from parent to offspring. Some characteristics are determined by single genes, such as having curly hair rather than straight hair but other characteristics are determined by a number of different genes, i.e. they are polygenic.
& Reading: Pennington pages 24 - 28
Genotype is the genetic make-up of the individual and phenotype is the characteristics of the individual as they actually develop.
In some cases the influence of genetic endowment is easy to trace, as in the case of eye colour. Genes function in pairs to influence hereditary characteristics, one from each parent. Individual members of a pair of genes are known as alleles and the relationship between alleles regulates the occurrence of certain discrete characteristics. In some pairs, one allele is dominant and the other is said to be recessive. Thus, the brown-eye gene is dominant while the blue-eye gene is recessive.
If both members of a gene pair are dominant, the child will receive that trait. If one gene is dominant and the other recessive, the child will show the form of trait determined by the dominant gene, but will also carry the recessive gene that may be expressed in a different way as a trait in his/her offspring. A recessive form of the trait will be expressed only if the genes contributed by both parents are recessive.
Parents transmit genes to their offspring in chromosomes, which are found inside the nucleus of each body cell, including the fertilised egg. In humans, all body cells, except sperm and unfertilised eggs, contain 23 pairs of chromosomes. The sperm and the eggs contain half sets which combine on fertilisation to create 23 chromosome pairs (46 altogether). At conception, the offspring receives 23 chromosomes from the father’s sperm and 23 from the mother’s ovum. Thus, a child receives only half of each parent’s genes. Because the total number of genes is thought to be in the thousands, it is unlikely that two human beings would be genetically identical – to have the same genotype.
The one exception is identical (monozygotic) twins, who, having developed from the same fertilised egg, have exactly the same chromosomes and genes. Non–identical (dizygotic) twins develop from two fertilised eggs and are no more genetically similar than ordinary siblings. As we shall see below, identical twins are a very important resource for researchers into the genetics of behaviour.
Darwin maintained that all living things originated from the earliest, very simple, life-forms which changed over millions of years by means of evolution. This is a gradual process (although some theorists believe that it happens in ‘bursts’ very slowly), where one species emerges from the other species by a series of inter-linked steps. Darwin’s theory states that the better adapted to its environment an animal is, the more likely it is to survive and produce offspring. An animal is said to be well adapted if it is well-suited to its environment — this precise environment to which it is adapted is called its ecological niche. Evolution depends on the fact that when there is competition for resources, those animals that are best suited will survive longer than those who are less well suited. This is the process known as natural selection.
Darwin spent many years observing animals, especially those on the Galapagos islands. He noted that on some of these islands, finches had thick, heavy beaks, whereas on other islands their beaks were elongated. This difference could be related to differences in diet. On the first set of islands the finches ate hard-shelled seeds, on other islands they fed off insects hiding under rocks. Darwin proposed that the physical differences evolved due to natural selection.
Farmers use artificial selection to breed animals, for example choosing the meatiest bull to mate with the cows to produce better beef cattle. In other words they select certain desirable characteristics and breed only from individuals exhibiting these characteristics.
The question of whether a characteristic is advantageous or disadvantageous is measured in terms of survival. If the characteristic promotes the individual’s survival chances then we are likely to see progressively more members of the species with that characteristic, whereas if the opposite is true we will see fewer individuals.
It isn’t just that a characteristic promotes the individual’s survival but in doing so it enhances their successful reproduction. An individual who is more likely to survive and reproduce is described as fitter. Their physical (or behavioural) characteristics are better adapted to their environmental (or ecological) niche. Fitness is measured in terms of the number of offspring. This is only an approximate measure since not all progeny are viable (i.e. capable of surviving).
Beware of explanations which sound as if an individual animal has decided to behave in a more adaptive way or which make selection sound like an active process. Any changes occur naturally and are selected because individuals with new characteristics reproduce and survive, whereas others don’t. Evolution is a passive process.
To summarise, Darwin’s theory suggests that natural variation over the course of time produces new characteristics, some of which enable the possessor of the characteristics to be better adapted to its environmental niche, i.e. fitter. Such characteristics are naturally selected. Through this process, all living animals and plants have evolved from a common ancestor.
Ideas from evolution are becoming increasingly popular as explanations of human behaviour (‘evolutionary psychology’). In fact, such explanations have been used ever since Darwin’s time. It does not matter whether we are talking about the colour of a moth’s wings, or the behaviour of mothers toward their offspring; the raw material of evolution is genetic variation. Therefore any explanation (whether of behaviour or anything else) which relies on genetic determination can be related to the principles and pressures of evolution.
& Reading: Gross pages 30 - 33 and 627-628
The following are examples of human behaviour that have been explained in evolutionary terms by sociobiologists:
Sexual behaviour: Sociobiologists see gender roles as part of a wider adaptation of humans to the environment. Males and females have evolved instincts and physical attributes which equip them for hunting and childcare. (Gross page 627.)
Sleep: according to some theorists, this may have an important evolutionary survival function to conserve energy when food gathering has been completed and to avoid damage from nocturnal predators or accidents by remaining motionless. (Gross pages 118-119)
Language: Pinker (1994) argues that language is far too complex to be learned; rather there must be some innate ‘programme’ that enables development of speech. (Gross page 31.)
Altruism: humans may have evolved kin altruistic behaviour and delayed reciprocal altruism. (Gross pages 522.)
Aggression: controversially, Robert Ardrey (1967) suggested that humans show similar territorial behaviour to animals and the same gender differences (males are more aggressive because they have to compete for females).
 This section includes contributions by Cara Flanagan.