Chapter 24Evolution by Natural Selection

Key Concepts

• Populations and species evolve, meaning that their heritable characteristics change through time. Evolution is change in allele frequencies over time.

• Evolution by natural selection occurs when individuals with certain alleles produce the most surviving offspring in a population. An adaptation is a genetically based trait that increases an individual’s ability to produce offspring in a particular environment.

Key Concepts, cont’d

• Evolution by natural selection is not progressive, and it does not change the characteristics of the population. Animals do not do things for the good of the species, and not all traits are adaptive. All adaptations are constrained by trade-offs and genetic and historical factors.

The Theory of Evolution

• Evolution: o Change of allele frequency (heritable traits

change over time) Testable

o All species are related by common ancestry Theoretical

Evidence of Evolution - Relatedness

• Homology (same)o Genetic: comparing similar DNA and/or Amino

Acid Sequenceo Developmental: comparing similar embryoso Structural: comparing similar anatomy

• Geography: o Comparing two similar species with similar

traits o Cause- separation of the species in different

environments produce different traits

Evidence of Evolution - Static (change)

• Fossils

• Transitional featureso In the fossil record, intermediate traits an

organism has that links the traits or two organisms

o Example- Lizard -> Lizard with feathers -> Bird

• Vestigial traitso Traits that are present that have no useo Example- human tailbone & human

goosebumps

Natural Selection

• Survival of the Fittest

• How does a species survive?o Adaptation:

There is a variety of different traits (genetic variation) in a population

The environment changes Increased genetic variation helps the

population survive in the new environment

Evolutionary Limitations

• Fitness Trade-offs: an adaptive trait can both help and hurt an organismo Example- big organisms can get food easier but need more

food

• Genetic Constraints: alleles that affect more than one gene which can both help and hurt the organism

• Historical Constraints: traits passed down which can help or hurt the organism