ENV107 - Lecture 12

1. Biological diversity
• Biological evolution
– Mutation
– Natural selection
– Genetic drift
– Migration, geographical and reproductive
isolation
• Extinction
• Some major factors that increase and decrease
biological diversity
1Lec 12; MrL

2. Natural selection
• When there is variation within a species, some
individuals may be better suited to the environment than
others.
• Organisms whose biological characteristics make them
better able to survive and reproduce in their
environment leave more offspring than others. Their
descendants form a larger proportion of the next
generation and are more “fit” for the environment. This
process of increasing the proportion of offspring is called
natural selection.
2
Under certain circumstances natural selection can
lead to an entirely new species. This process is called
speciation. In this process, two species arise from
one. Lec 12; MrL

3. Genetic drift
• Genetic drift refers to changes in the
frequency of a gene in a population due not to
mutation, selection, or migration, but simply
to chance.
3Lec 12; MrL

4. Geographic isolation
It occurs when two populations of a species or two groups of the
same population become physically separated into areas with
different environmental conditions.
Example: Part of a population may migrate in search of food and then begin
living in another area with different environmental conditions.
4Lec 12; MrL

5. Geographic isolation
• Populations may also become separated
– by a physical barrier (such as a mountain range, stream,
lake, or road),
– by a change such as a volcanic eruption or earthquake, or
– when a few individuals are carried to a new area by wind
or water.
• A biome is a kind of ecosystem, such as a desert, a tropical rain
forest, or a grassland.
• The same biome can occur on different continents because
similar environments provide similar opportunities for life and
similar constraints.
• As a result, similar environments lead to the evolution of
organisms similar in form and function.
5Lec 12; MrL

6. Reproductive isolation
It occurs as mutation and natural selection operate
independently in two geographically isolated
populations and change the gene characteristics.
If this change continues long enough, members of the
geographically and reproductively isolated populations
may become different.
They become so different in genetic make up that they
can’t interbreed-or if they do, they can’t produce live,
and speciation can occur – one species becomes two.
6Lec 12; MrL

7. Extinction
After evolution, the second process affecting
the number and types of species on the earth is
extinction.
When environmental conditions change, a
species may either evolve (become better
adapted) or cease to exist (become extinct).
7Lec 12; MrL

8. Factors affect patterns of speciation and extinction:
– Large-scale movements of the continents (continental drifts)
over millions of years.
– Gradual climate changes caused by continental drift and slight
shifts in the earth’s orbit around the sun
– Rapid climate change caused by catastrophic events (such as
large volcanic eruptions, and global warming)
– Humans also have become a major force in the premature
extinction of species. As the human population and resource
consumption increase over the next 50 years, and we take more
and more of the planet’s surface, we may cause the extinction
of upto a quarter of the earth’s current species.
– Speciation - extinction = biodiversity
8Lec 12; MrL

9. SOME MAJOR FACTORS THAT INCREASE AND DECREASE
BIOLOGICAL DIVERSITY
FACTORS THAT TEND TO INCREASE DIVERSITY
• 1. A physically diverse habitat
• 2. Moderate amounts of disturbance (such as fire or storm in a
forest or a sudden flow of water from a storm into a pond).
• 3. A small variation in environmental conditions (temperature,
precipitation, nutrient supply, etc.).
• 4. High diversity at one trophic level increases the diversity at
another trophic level. (Many kinds of trees provide habitats for
many kinds of birds and insects.)
• 5. An environment highly modified by life (e.g., a rich organic soil).
• 6. Middle stages of succession.
• 7. Evolution.
9Lec 12; MrL

10. FACTORS THAT TEND TO DECREASE DIVERSITY
• 1. Environmental stress.
• 2. Extreme environments (conditions near the limit of
what living things can withstand).
• 3. A severe limitation in the supply of an essential
resource.
• 4. Extreme amounts of disturbance.
• 5. Recent introduction of exotic species (species from
other areas).
• 6. Geographic isolation (being on a real or ecological
island).
10Lec 12; MrL