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Biological diversity

• Biological evolution– Mutation

–Natural selection–Genetic drift–Migration, geographical and reproductive

isolation• Extinction• Some major factors that increase and decrease

biological diversityLec 12; MrL

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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.

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

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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.

Lec 12; MrL

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.

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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.

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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.

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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).

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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

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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.

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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).

Lec 12; MrL