Evolution: Processes and Genetic Variation

• Chapter 19 focuses on the processes of evolution, including natural selection, other evolutionary mechanisms, types of selection, and the maintenance of genetic variation in populations.
• The chapter aims to define evolution, describe evolutionary processes, outline reproductive strategies related to sexual selection, explain types of selection, and describe mechanisms maintaining genetic variation.

Evolution

• Evolution involves changes in a population's gene pool over time, influencing their genetic makeup and leading to either the extinction or origin of species.
• Evolutionary processes include natural selection, mutation, gene flow, genetic drift, non-random mating, and constraints/trade-offs.
• A population is the unit of evolution, while individuals undergo natural selection and evolutionary processes.
• Evidence from geological, morphological, behavioral, and molecular data supports the evolutionary theory.
• Evolutionary changes can be observed in labs and natural populations, like antibiotic or pesticide resistance.

Charles Darwin and Alfred Wallace

• Charles Darwin and Alfred Wallace proposed evolution by natural selection.
• Species change through time, share a common ancestor, and diverge over time, explained by natural selection.
• Darwin's work in the Victorian era influenced evolutionary science, which was originally biased, influenced by the Victorian gendered social context, leading to androcentrism and assumptions about sex roles.

Natural Selection

• Natural selection increases the relative individual fitness of certain phenotypes within a population, leading to the favored phenotypes spreading.
• Natural selection results in traits that are heritable and tied to ones' fitness and therefore increase chances for evolution.
• Variation in traits within a population happens to mutations.
• Adaptation refers to a trait and the process by which it spreads through a population due to natural selection.
• Artificial selection involves humans selectively breeding organisms, which also results in evolution, like with pigeons, cauliflower, pecans etc.

Evolutionary Processes

• Mutation generates genetic variation, is the raw material for natural selection.
• Mutation events are rare at the individual level, but become common at the population level.
• Natural selection favors the spread of beneficial mutations, without increasing the original frequency of mutations.
• Gene flow is the movement of genes in or out of a population through emigration and immigration, which changes allele frequencies.
• Genetic drift is a random evolutionary process with the most impact on small populations, causing random changes in allele frequencies.
• Bottleneck effects and founder effects are special cases of genetic drift.
• The effects of genetic drift can be detrimental, especially if positive alleles might be lost.
• Genetic drift can impact large populations as well, specifically with a population bottleneck.
• Bottleneck effects include decrease in egg hatching rates and have been observed in the greater prairie chicken population in Illinois.
• Founder effects result in lower genetic variation.
• Nonrandom mating changes the genotype of allele frequencies based on individual preference over mates; this leads to no evolution.
• Sexual selection happens in species with sexual reproduction and two or more sexes.
• Intersexual selection involves individuals of one sex choosing mates based on certain phenotypes.
• Intrasexual selection involves the competition of individuals to mate.

Reproductive Strategies

• Reproductive strategies are more diverse.
• Asexual reproduction happens in the absence of fertilization.
• Sexual reproduction happens in the meiosis of germ cells through fertilization is what drives population differences.
• Vegetative reproduction is reproduction using non-sexual tissues, which creates clones with now increase in genetic variation.
• Parthenogenesis is reproduction in germ cells, creating clones that are genetically variable.
• Isogamy has production of similar gametes in both parents.
• Anisogamy has a system of different gametes.
• Hermaphroditism is simultaneous, and sometimes self-fertilizing, or is sequential.
• Possible sexual dimorphisms have two or more sexes, decided by sex determination systems involving sex chromosomes, physical environment, etc.
• Gametic sex doesn't always translate into a clear sexual dimorphism.
• Many species have more than 2 sexes, binary male and female is an assumption.
• Some species have reversed sex roles.
• There is same-sex sexual behavior in over 1,500 species Evolutionary Constraints
• Evolution relates to trade-offs between history and the result.
• Preexisting traits can limit the outcome of selection.
• Physical/chemical constraints limit evolution.
• Evolutionary innovations come from prior existing structures.
• Lack of the specific genetic variation restricts creating certain traits.
• Trade-offs balance the costs and benefits in adaptation evolution; as such the trait needs to be positive overall
  • E.g. A peacock's tail is attractive to female, but costly in terms of predation. Types of Selection
• There are three types of selection that can happen, each type must reflect the underlying genetic variation
  • Stabilizing
  • Directional
  • Disruptive Traits and Selection
• Natural selecting works directly with phenotype and indirectly on the related genotype.
• Each phenotype has relative fitness within population.
  • This is relative rate of survival and reproduction of people that use and have that phenotype. Qualitative Traits and Selection
• Traits affected by one locus are qualitive
• Discrete variation
• Ex Blood type Quantitative Traits and Selection
• Traits affected by more than one locus are quantitative
• Continuous variety
• Ex height There are three types of "quantitative " selection

"Stabilizing" Selection

• "Stabilizing" selection: mean phenotype is chosen.
• Although there is genetic distribution, some variation in lost in the process.
• Individuals around the mean do the most in terms of reproducing
• Birth weight is an example of "stabilizing" selection

"Directional" Selection

• "Directional" selection: a phenotype differently impacts the meaning selection (favored by natural process )
• It moves but genetic variation doesn't Change .
• The mean changes, but variation, in principle, is not lost.
• " Texas "longhorn is the example of" directional," resulting in introduced cattle for Europe

"Disruptive" selection

• "Disruptive" selection: phenotype's that vary in both directions, and this mean are favored by natural selection
• Mean doesn't change, increase and genetic variation increase.
• "Black"-bellied sedcrackers result in disruptive Maintenance and Genetic variation in People
• If several evolutionary processes can "Erase" genetic variation maintain in population

1. Frequency-dependent selection
2. A advantage of heterozygotes
3. Geographic variation

"Frequency-Dependent" Selection

• "Frequency-dependent" selection; maintains genetic variation within populations.
• Negative frequency dependent
• "Genetic" Variation (normally) resulting in having or more phenotype variation's within (the same population).

Heterozygote Advantages

• Heterozygote outperform Homozygote due to environment
• Overlap beta-ween malariat sickle cell, anemia

Geographic Variations

"Genetic" variation within species by populations.

• Local selection pressure's ("Different" environment.)