There is variation among individuals
of the same species,
and therefore differential survival,
with survival of only the fittest.
• Human variation
E.g. darker skin at the equator is a function
of increased melanin in the skin for
protection against the sun
Pale skin is an environmental adaptation to
reduced sunlight at higher latitudes.
Less melanin allows the sun in because sunlight
promotes the production of vitamin D.
E.g. Oriental eyes are genetic, due to a
relatively isolated gene pool (breeding
population) in the past 120 000 years in the
Lions Panthera leo
Lions (Panthera leo) only mate with lions, and that makes
them one species. They do not mate with other cats like
tigers, leopards and panthers.
Tiger Panthera tigris Leopard Panthera pardus Panther Puma concolor
Equus zebra (mountain zebra)
Equus quagga (common zebra)
Equus africanus (wild ass)
All of the horse type, thus the genus Equus,
but different enough to be divided into species.
Equus zebra Equus quagga Equus africanus
Zebras do not mate with race horses.
Different species cannot produce viable
offspring if mated.
a mule is a cross between
a donkey (species) and a
A mule is always sterile
and cannot reproduce.
2. This infers a struggle for survival
and competition between
individuals of the same species.
4. There is a natural selection for
individuals with traits favoured in a
5. Traits are in the
genes, so are
(male and female
• New genetic combinations allow for
variation, giving more chances to
adapt to a changing environment.
6. Offspring carry the
advantageous feature and
pass it on to their offspring.
The result is a gradual
genetic shift (evolutionary
change) in the population
as the trait is naturally
e.g. better thermo-regulation in some lions
living in extremely hot desert environments
Genes are not the only factor that
determine an organism’s appearance.
• Environmental influences: A person
with a dominant gene for height could
end up short due to poor diet and living
• Mutation: random change in an
E.g. of a useful mutation:
A lamb born with short, bent legs that
prevented it from jumping fences.
Used in breeding to establish short-legged sheep.