6.
6 Evolution
6.6.1 Darwin-Wallace Theory of Evolution
The original theory that natural selection might be a mechanism by which
evolution could occur was put forward independently by both Charles Darwin and
Alfred Russel Wallace in 1856
They realised the significance of variation. Their observations and deductions can
be summarised briefly as follows:
Observation 1: Organisms produce more offspring than are needed to replace the
parents.
Observation 2: Natural populations tend to remain stable in size over long periods
Deduction 1: There is competition for survival
Observation 3: There is variation among the individuals of a given species
Deduction 2 : The best adapted variants will be selected for by the natural
conditions operating at the time
6.6.2 Species & Speciation
‘Specie’ refers to a group of organisms, with similar morphological, physiological,
biochemical and behavioural features, which can interbreed to produce fertile
offspring, and are reproductively isolated from other species
Speciation is caused by evolution
New species are formed from pre-existing ones due to changes in gene pools
from generation to generation
Genetic isolation between the new population and the pre-existing species
population is necessary for speciation
There are two different situations when speciation can take place:
Two groups of a species are separated by a geographic barrier
Two groups of species are reproductively isolated but still living in the same area,
experiencing similar environmental selection pressures
6.6.3 Molecular Evidence of Evolution
DNA that is found in the nucleus, mitochondria and chloroplasts of cells can be
sequenced and used to show evidence of evolutionary relationships between
species
The differences between the nucleotide sequences (DNA) of different species
shows that:
More similar the sequence, more closely related the species are
Two groups of organisms with similar DNA will have separated into separate
species more recently than two groups with less similarity in their DNA sequences
DNA sequence analysis and comparison can also be used to create family trees
that show the evolutionary relationships between species
6.6.4 Allopatric Speciation
Allopatric speciation occurs due to geographical isolation and is the most
common type of speciation
A population can be split into one or more groups when they become separated
from each other by geographical barriers. This creates two populations of the
same species who are isolated from each other resulting in no genetic exchange
to occur between them
If there is sufficient selection pressure or genetic drift acting to change the gene
pools within both populations then eventually these populations will diverge and
form separate species
The changes in the alleles of each population will affect the phenotypes present in
both population
Over time, the two populations may begin to differ physiologically, behaviourally
and morphologically
6.6.5 Sympatric Speciation
Sympatric speciation occurs without a geographical barrier
A group of the same species could be living in the same place but in order for
speciation to take place there must exist two populations within that group and
no gene flow occurs between them
Something has to happen that splits or separates the population:
Ecological separation: Populations are separated because they live in different
environments within the same area
Behavioural separation: Populations are separated because they have different
behaviours
Points to Note
Phenotypic variation may be continuous (as in the height or mass of an organism)
or discontinuous (as in the human ABO blood groups)
The genotype of an organism gives it the potential to show a particular
characteristic. In many cases, the degree to which this characteristic is shown is
also influenced by the organism’s environment
Environmental factors come into play to limit population growth. Such factors
decrease the rate of reproduction or increase the rate of mortality so that many
individuals die before reaching reproductive age
Within a population, certain alleles may increase the chance of an individual to
survive long enough to be able to reproduce successfully. These alleles are more
likely to be passed on to the next generation than others. This is known as natural
selection
Natural selection that keeps allele frequencies as they are; this is stabilising
selection. If environmental factors that exert selection pressures change, or if new
alleles appear in a population, then natural selection may cause a change in the
frequencies of alleles; this is directional selection. Directional selection may
produce large changes in allele frequencies. This is how evolution occurs
Allele frequencies in a small population may change due to a random process
called genetic drift. The allele frequencies and the proportions of genotypes of a
particular gene in a population can be calculated using the Hardy–Weinberg
principle
A species can be defined as a group of organisms with similar morphology,
behaviour, physiology and biochemistry that are capable of interbreeding to
produce fertile offspring
Artificial selection involves the choice by humans of which organisms to allow to
breed together, in order to bring about a desirable change in characteristics.
Artificial selection, like natural selection, can affect allele frequencies in a
population
New species arise by a process called speciation. In allopatric speciation, two
populations become isolated from one another, perhaps by some geographical
feature, and then evolve along different lines until they become so different that
they can no longer interbreed. In sympatric speciation, new species may arise
through polyploidy
