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CHAPTER 3 NOTES
The Development of Evolutionary Theory
Darwin argued that, over time, organisms originate and become adapted to their
environments by biological means. His concept of biological evolution is that changes
take place in the genetic and physical characteristics of a population or group of organisms
over time – stands as the primary explanation of the origin of life.
Adaptive significance is the effectiveness of behaviour in aiding organisms to adapt to
changing environmental conditions.
Ultimate causes are evolutionary conditions that have slowly shaped the behaviour of a
species over generations.
Proximate causes are immediate environmental events and conditions that affect behaviour.
To understand the present, we must understand the past – the history of the individual and
history of our species. We behave as we do because we are members of the human species –
an ultimate cause – and because we have learned to act in special ways – proximate cause.
Culture is the sum of socially transmitted knowledge, customs, and behaviour patterns
common to a particular group of people.
No theory of behaviour can be complete without considering the role of evolution.
Alfred Russell Wallace devised the theory of natural selection at about the same time
Darwin did. Yet we only remember Darwin for natural selection.
Doctrine of essentialism – a view dating back to Plato that all living things belong to a fixed
class or “kind”, defined by an essence that characterizes it alone.
Artificial selection is the procedure in which particular animals are deliberately mated to
produce offspring that possess especially desirable characteristics.
Natural selection – the consequence of the fact that, because there are physical and
behavioural differences among organisms, they reproduce differentially. Within a given
population, some animals – the survivors – will produce more offspring than will other
Darwin came to the realization of natural selection in 1838 but did not publish his theory
until 20 years later.
The book by Malthus inspired both Darwin and Wallace.
Darwin`s immense contribution to modern thinking about evolution can be traced to four
insights: that species are not fixed, but rather change over time; that evolution is a
branching process, implying all species descend from a single common ancestor; that
evolution is continuous, with gradual changes; and that evolution is based on natural
Natural selection is based on two premises. First, individuals within a population
show variability in heritable behavioural and physical characteristics. Second, the capacity
of the environment to sustain a population of any species is limited, producing competition.
Darwin and Wallace realized that these two factors meant that those individuals within the
population with characteristics that compete better are most likely to survive and
reproduce. Behavioural adaptations were especially important to survival and therefore an
important part of evolution.
Reproductive Success – the number of viable offspring an individual produces relative to
the number of viable offspring by other members of the same species.
Variation refers to the differences among members of a species, including physical
characteristics such as size, strength, or physiology, and behavioural characteristics such as
intelligence or sociability. Genotype is an organism’s genetic makeup and it differs for all
organisms except for identical twins. As a result to these genetic differences, an individual
organism’s physical characteristics and behaviour, or its phenotype, also vary from
individual to individual.
It is important to recognize that every individual’s phenotype is produced by the
interaction of its genotype with the environment. In essence, the genotype
determines how much the environment can influence an organism’s development and
Although evolution occurs over the long run, natural selection can produce important
changes in the short run- in the space of only a few years. Phenotypic variation can produce
important selective advantages that affect survival.
Competition is the second premise underlying the concept of natural selection. Because
individuals of a given species share the same environment, competition within a species for
food, mates, and territory is inevitable. Competition also occurs between species when
members of different species vie for similar ecological resources, such as food and territory.
Natural selection works because the members of any species have different phenotypes.
Because these phenotypes are caused by different genotypes, successful individuals will
pass on their genes to the next generation. Over time, competition for food and other
resources will allow on the best adapted phenotypes go survive, thereby producing
Proximate causes of behaviour - how animals adapt to the environmental changes through
learning. Ultimate causes of behaviour – historical events and conditions in the evolution of
a species that have shaped behaviour. The primary element of biological evolution is
natural selection: the tendency of some members of a species to produce more offspring
than other members do. Members of a species vary genetically. If any of these traits gives
an animal a competitive advantage over other members of the species, then that animal is
also more likely to have greater reproductive success. Its offspring will then carry its genes
into future generations.
Heredity and Genetics
Genetic s is the study of the genetic makeup of organisms and how it influences their
physical and behavioural characteristics. Heredity is the sum of the traits and tendencies
inherited from a person’s parents and other biological ancestors.
Although Darwin had a strong case for natural selection, he could not explain a key tenet of
this theory – inheritance. Gregor Mendel uncovered the basic principles of heredity.
Basic Principles of Genetics - Heredity is determined by genetic material called DNA –
strands of sugar and phosphate that are connected by nucleotide molecules of adenine,
thymine, cytosine, and guanine. James Watson and Francis Crick discovered that DNA is
configured like a twisted ladder: the sugar and phosphate form the sides and the four
nucleotides form the rungs. The location of a particular sequence of nucleotides along the
DNA molecule is known as a gene. The particular sequence, long or short, directs the
synthesis of protein molecules that regulate the biological and physical development of the
body and its organs. The total set of genetic material is known as the genome. The human
genome comprises 24 different DNA molecules in women and 25 different DNA molecules in
men. There appear to be 30 000 and 40 000 genes within the human genome.
Genes as “Recipes” for Protein Synthesis – Genes influence our physical and
behavioural development in only one way: through protein synthesis. Proteins are strings of
amino acids, arranged in a chain and whose order is specified by the nucleotides. A
sequence of three nucleotides corresponds to a particular amino acid. Strictly speaking,
there are no genes for behaviour, only for the physical structures and physiological
processes that are related to behaviour. Genes also direct the synthesis of enzymes –
proteins that regulate the structure of bodily cells and the processes occurring within those