AP bio - Unit 7 (Natural Selection)

Natural Selection

Process by which organisms having adaptations suited for a particular environmnet have a greater chance of survival and reproduction — pass their genes to subsequent generations (Biological Fitness)

Biological Fitness

Reproductive Sucess where individuals survive and can pass their genes to next geneerations

Evolution

Change of genetic makeup of a population over time.

Several conditions must be met for evolution by natural selection

1) Competition

There are competition between limited resources (food, mates, space, etc..)

• Differences in phennotypes will determine how competitive an organism is

• Darwin’s theory of Natural Selection: Competition for limited resources results in differential survival among organisms

2) Favorable Phenotypes

Favorable phenotypes improve chances of survival

• Mutations/sexual reproduction increase variation within populations

3) Evolutionary Fitness

Evolutionary fitness is measured by reproductive success

• Due to selection, traits of individiuals that have more reproductive success will become common in populations

  • Reproductive success/heritabillity of adaptations contribute to evolutionary fitness

4) Ecosystem Stability

Ecosystem stability determines the rate and direction of evolution

• Biotic (living e.g. plant, animals, bacteria) and abiotic (sunlight, water, soil, temperature) environments can remain more or less stable

• Populations are less likely to evolve in stable environments

• Unstable evironment = faster rate of evolution

  • Genetic variation can depend on how much the environment flucturates.

Genetic Variation

Genotypic and phenotypic differences between individuals in a population

• Leads to different adaptations among organisms

• Natural selection acts on phenotypic variation in populations

Due to natural selection, organisms with…

Due to natural selection, organisms with more favorable traits are likely to survive and reproduce

• Changes in the environment (selective pressure) can lead to some phenotypes with increased or decreased fitness

What increases the probability of survival among populations of organisms in a changing environment?

Genetic Variation increases the probability a populaion of organisms will survive in the changing environment

• One of the phenotypes may be better suited for the changed condition

Changing environments introduce…

Changing environments introduce different selective pressures on populations

• Individual’s fitness is relative to environmental condition

• Phenotypes selected for can be selected against when environmental conditions change and vice versa.

  • Any phenotypes that decrease or increases the chances of survival and reproduction negatively or positively impact fitness.

Example of fitness: DDT resistance in insects

Due to natural variation in populations, some insects are resistant to DDT. Resistant insects are not killed by DDT, survive, and pass down their resistant genes to their offsprings. Overtime, the entire population of insects become resistant to DDT.

Variations can also be evident in…

Variations can also be evident in cellular and molecular level including molecular structures and types of moleclues —- Increases fitness in an organism

• E.g. Chlorophyll moleclues vary within plant cells — Different types of chlorphyll capture light energy of different wavelengths (greater flexibility to absorb incoming wavelengths of light used for photosynthesis).

Artificial Selection

Process by which humans select desirable traits in other species and selectively breed organisms with the desired traits

• Results in unatural phenotypes

• Can contribute to genetic diversity or less of it

• Humans affect diversity within the population through (artificial selection) breeding plants and animals and selecting desirable traits.

Example 1 of Artificial Selection: Dog Breed

Hundreds of dogs have been bred from a single wild species (e.g. wolf) as a result of selective breeding

Example 2: Brassica Plants

Different parts of the Brassica Plants have been developed by human selection to produce at least six diverse vegetables: Brocoli, cauliflower, cabbage, kale, etc..

Depending on which traits are selected for and how often they’ve selected…

Depending on which traits are selected for and how often they’ve selected, the genetic diversity of the population can change over time