Unit 3

Evolution and Biodiversity

What explains how life has changed over time?

According to scientific evidence the major force of adaptation to changes in environmental conditions is biological evolution

Charles Darwin and Alfred Wallace proposed the concept of natural selection as the mechanism for biological evolution

Natural Selection and Evolution

Biological evolution by natural selection involves the change in a population’s genetic makeup through successive generations.

Three conditions are necessary for biological evolution:Genetic variabilityTrait must be heritableTrait must lead to differential reproduction. (More

viable offspring)

Gene Pools• Genetic variation may

arise through micro-evolution of a population’s gene pool.

• The gene pool consists of all the alleles (Copies of genes) in all the individuals making up a population which acts as the reservoir from which the next generation draws its genes

Mechanisms of Microevolution• There are four processes that can change the

composition of a gene pool– Mutation (change in DNA)– Natural Selection (3 major types)– Gene flow (immigration, emmigration)– Genetic Drift

Mutation• Random changes in the genetic structure or

number of DNA molecules in a cell is the ultimate source of genetic variability in a population.

Natural Selection• When some individuals of a population have

genetically based traits that cause them to survive and produce more offspring than other individuals

• There are three types of selection that will shift a populations gene frequencies.– Directional– Disruptive– Stabilizing

Three Types of Selection• Directional-Changes in the environment cause

allele frequencies to shift so that individuals with traits at one end of the normal range become more common

Three Types of Selection• Disruptive-Environmental conditions favor the

individuals at the extreme ends of genetic spectrum

Three Types of Selection• Stabilizing-Conditions favor the average genetic

makeup

Genetic Drift• Another way gene pools can change is due to

genetic drift• Genetic drift is the change of a small population

due to chance not specific selection• Two situations can shrink populations down to a

size where genetic drift has a greater impact– Bottleneck Effect-Disasters, such as earthquakes,

floods, droughts and fires reduce the size of a population

– The Founder effect- When a few individuals colonize an isolated island, lake, or some new habitat

Limits on Adaptation through Natural Selection

A population’s ability to adapt to new environmental conditions through natural selection is limited by its gene pool and how fast it can reproduce.Humans have a relatively slow generation time

(decades) and output (# of young) versus some other species.

Common Myths about Evolution through Natural Selection

Evolution through natural selection is about the most descendants.Organisms do not develop certain traits because they

need them.There is no such thing as genetic perfection.

Biodiversity• One important renewable resource is

biodiversity: The different life forms and life sustaining processes

• Biodiversity has four main components– Species Diversity– Genetic Diversity– Ecological Diversity– Functional Diversity

Species Diversity

• The number and abundance of species present in different communities

Genetic Diversity• The variety of genetic material within a

species or population

Ecological Diversity

• The variety of ecosystems found on an area or on earth

Functional Diversity• The biological and chemical processes such as

energy flow and matter recycling needed for the survival of living things.

Why Should We Care About Biodiversity?

• Biodiversity provides us with:–Natural Resources (food water, wood,

energy, and medicines)–Natural Services (air and water

purification, soil fertility, waste disposal, pest control)

–Aesthetic pleasure

Biodiversity of the Past

Fossil Evidence, chemical analysis, Ice Core samples, and DNA analysis demonstrate that the biodiversity today is different from the biodiversity in the past

Figure 4-4