7 Natural Selection

Describe the causes of natural selection.

• Natural selection is a major mechanism of evolution.
• According to Darwin’s theory of natural selection, competition for limited resources results in differential survival. Individuals with more favorable phenotypes are more likely to survive and produce more offspring, thus passing on those favorable traits to subsequent generations.

Explain how natural selection affects populations.

• Evolutionary fitness is measured by reproductive success.
• Biotic and abiotic environments can fluctuate, affecting the rate and direction of evolution. Different genetic variations can be selected in each generation.

Describe the importance of phenotypic variation in a population.

• Natural selection acts on phenotypic variations in populations.
• Environments change and apply selective pressures to populations.
• Some phenotypic variations can increase or decrease the fitness of an organism in particular environments.

Explain how variation in molecules within cells connects to the fitness of an organism.

• Variation in the number and types of molecules within cells can provide populations a greater ability to survive and reproduce in different environments.

Explain how humans can affect diversity within a population.

• Through artificial selection, humans affect variation in other species.

Explain how random occurrences affect the genetic makeup of a population.

• Evolution is also driven by random occurrences.
  • i. Mutation is a random process that adds new genetic variation t o a population.
  • ii. Genetic drift is a change in allele frequencies attributable to a nons elective process occurring in small populations. iii.The bottleneck effect is a type of genetic drift that occur s when a population size is reduced to a small number of individuals for at least one generation. iv.The founder effect is a type of genetic drift that occur s when a population is separated from other members of the population. The frequency of genes and traits will shift based on the genes in this new founder population.
  • v. Migration can result in gene flow (the addition or remov al of alleles from a population).

Describe the role of random processes in the evolution of specific populations.

• Random processes can lead to changes in allele frequencies in a population.
  • i. Mutations result in genetic variation, which provides phenotypes on which natural selection acts.
  • ii. Genetic drift can allow a small population to diverge from other populations of the same species.
  • iii. Gene flow between two populations prevents them from diverging into separate species.

Describe the change in the genetic makeup of a population over time.

• Changes in allele frequencies provide evidence for the occurrence of evolution in a population.

Describe the conditions under which allele and genotype frequencies will change in populations.

• The Hardy–Weinberg Equilibrium is a model for describing and predicting allele frequencies in a non-evolving population. Conditions for a population or an allele to be in Hardy–Weinberg equilibrium are:
  • i. A large population size
  • ii. No migration
  • iii. No new mutations
  • iv. Random mating
  • v. No natural selection These conditions are never met, but they provide a valuable null hypothesis.
• Allele frequencies in a nonevolving population can be calculated from genotype frequencies. RELEV ANT EQUATIONS Hardy–Weinberg Equation— ++ =pp qq2 122 pq 1+= , where: p = frequency of allele 1 in the population q = frequency of allele 2 in the population

Describe the types of data that provide evidence for evolution.

• Evolution is supported by scientific evidence from many disciplines (geographical, geological, physical, biochemical, and mathematical data).

Explain how morphological, biochemical, and geological data provide evidence that organisms have changed over time.

• Molecular, morphological, and genetic evidence from extant and extinct organisms adds to our understanding of evolution.
  • i. Fossils can be dated by a variety of methods. These include 1) the age of the r ocks where a fossil is found; 2) the rate of decay of isotopes including carbon-14; and 3) geographical data.
  • ii. Morphological homologies, including vestigial structur es, provide evidence of common ancestry.
• A comparison of DNA nucleotide sequences and protein amino acid sequences provides evidence for evolution and common ancestry.

Describe structural and functional evidence on cellular and molecular levels that provides evidence for the common ancestry of all eukaryotes.

• Structural and functional evidence indicates common ancestry of all eukaryotes. This evidence includes:
  • i. Membrane-bound organelles
  • ii. Linear chromosomes
  • iii. Genes that contain introns

Explain how evolution is an ongoing process in all living organisms.

• All species have evolved and continue to evolve. Examples include:
  • i. Genomic changes over time
  • ii. Continuous change in the fossil record
  • iii. Evolution of resistance to antibiotics, pesticides, herbicides, or chemotherapy drugs iv . Pathogens evolving and causing emergent diseas es

Describe the types of evidence that can be used to infer an evolutionary relationship.

• Phylogenetic trees and cladograms show hypothetical evolutionary relationships among lineages that can be tested.
• Phylogenetic trees show the amount of change over time calibrated by fossils or a molecular clock, whereas cladograms do not show time scale or the evolutionary difference between groups.
• T raits that are either gained or lost during evolution can be used to construct phylogenetic trees and cladograms. The out-group represents the lineage that is least closely related to the remainder of the organisms in the phylogenetic tree or cladogram.
  • i. Shared derived characters can be present in more than one lineage and indicat e common ancestry. These are informative for the construction of phylogenetic trees and cladograms.
  • ii. Molecular data typically provide more accurat e and reliable evidence than morphological traits in the construction of phylogenetic trees or cladograms.

Explain how phylogenetic trees and cladograms can be used to infer evolutionary relatedness.

• Phylogenetic trees and cladograms can be used to illustrate speciation that has occurred. The nodes on a tree represent the most recent common ancestor of any two groups or lineages.
• Phylogenetic trees and cladograms can be constructed from morphological similarities of living or fossil species and from DNA and protein sequence similarities.
• Phylogenetic trees and cladograms represent hypotheses that are constantly being revised based on evidence.

Describe the conditions under which new species may arise.

• Speciation occurs when two populations become reproductively isolated from each other.
• The biological species concept provides a commonly used definition of a species for sexually reproducing organisms. It states that species can be defined as a group capable of interbreeding and exchanging genetic information to produce viable, fertile offspring.

Describe the rate of evolution and speciation under different ecological conditions.

• Punctuated equilibrium is when evolution occurs rapidly after a long period of stasis. Gradualism is when evolution occurs slowly over hundreds of thousands or millions of years.
• Divergent evolution occurs when adaptation to new habitats results in phenotypic diversification. Speciation rates can be especially rapid during times of adaptive radiation as new habitats become available.
• Convergent evolution occurs when similar selective pressures result in similar phenotypic adaptations in different populations or species.

Explain the processes and mechanisms that drive speciation.

• Sympatric speciation occurs in populations with geographic overlap. Allopatric speciation occurs in populations that are geographically isolated.
• Various pre-zygotic and post-zygotic mechanisms can maintain reproductive isolation and prevent gene flow between populations.

Explain how the genetic diversity of a species or population affects its ability to withstand environmental pressures.

• The level of variation in a population affects population dynamics.
  • i. The ability of a population to respond to changes in the envir onment is influenced by genetic diversity. Species and populations with little genetic diversity are at risk of decline or extinction.
  • ii. Genetically diverse populations are more resilient t o environmental perturbation because they are more likely to contain individuals that can withstand the environmental pressure.
  • iii. Alleles that are adaptive in one envir onmental condition may be deleterious in another because of different selective pressures.

Describe the scientific evidence that supports models of the origin of life on Earth.

• The origin of life on Earth is supported by scientific evidence.
  • i. Geological evidence reinforces models of the origin of life on Ear th.
  • ii. Earth formed approximately 4.6 billion year s ago (bya). The environment was too hostile for life until about 3.9 bya, and the earliest fossil evidence for life dates to 3.5 bya. T aken together, this evidence provides a plausible range of dates for the origin of life.
• The RNA world hypothesis proposes that RNA could have been the earliest genetic material. There are three assumptions:
  • i. At some point in time, genetic continuity was assured b y the replication of RNA.
  • ii. Base-pairing is necessary for replication.
  • iii. Genetically encoded proteins were not inv olved as catalysts.