Microevolution — change in allele frequencies in a population over generations

• Mechanisms that cause allele frequency change:

  • Natural selection

  • genetic drift

  • gene flow

Only natural selection causes adaptive evolution

Genetic variation among individs. is caused by differences in genes or DNA

In order for evolution to occur there needs to be variation in heritable traits

Phenotype is the combination between inherited genotypes and environmental influences

Natural selection can only act on variation w/ a genetic component

Average heterozygoisty — what percent of individ/genes are likely to have both dominant and recessive

Geographic variation — differences between gene pools of separate populations

• Ex: cline — change in a trait geographically

  • Fish vary in a cold-adaptive allele along a temperature gradient

    • Due to natural selection

New genes & alleles due to mutations

• Only mutations in reproduction cells (gametes) can be passed to offspring

  • Changes in somatic cells will not be passed along

Mutation rates are low in animals & plants, higher in viruses

Sexual reproduction can shuffle existing alleles into new combinations

• Recombination

  

  • Makes adaptation possible

Population — localized group of individuals capable of interbreeding & producing fertile offspring

Gene pool — all of the alleles for all loci in a population

• Ex: all the lanternflies at GHS!

If every individual in the population are the same, meaning the are homozygous for every same allele —> change is not possible — locus is fixed

Calculating Allele Frequencies using Hardy-Weinberg:

• The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for every heterozygous individual; the same logic applies for recessive alleles

• p+q = 1

  • p = frequency of the dominant allele, q = frequency of the recessive allele

• Hardy-Weinberg principle — describes population that is not evolving

  • If it doesn’t meet the criteria, it is evolving

  • p²+2pq+q²=1

    • p² and q² represent the frequencies of the homozygous genotypes & 2pq represents the frequency of the heterozygous genotype

      • p² is the frequency of the homozygous dominant genotype

      • q² is the frequency of the homozygous recessive genotype

  • In real populations, allele & genotype frequencies do change over time. This represents a hypothetical population that is not evolving

• Conditions that must be met in order for Hardy-Weinberg equilibrium to hold true:

  1. No mutations

  2. Random mating — no sexual selection

  3. No natural selection

  4. Extremely large population size —> no genetric drift

  5. No gene flow —> no migration

• You can have some loci be in Hardy-Weinberg equilibrium while others are not

What causes changes in allele frequencies?

• Natural selection

• Genetic drift

  • Seen much more often in smaller populations

  • Ex: 50 Question test & you miss 1 vs. 5 question test & you miss 1 —> much larger affect

  • Random changes reduce genetic variation bc of losses of alleles

• Gene flow —> migration/immigration

Founder effect — when some individuals become isolated from a larger population, so sometimes the allele frequencies in the small founder population can be different from those in the larger parent population

• Ex: some individuals of the population get separated from the parent population mb because of a natural disaster, and they aren’t able to connect with them. They evolve separately and adapt to their new environment. If you were to combine them again they would likely look different

Bottleneck effect — sudden reduction in population size due to a change in the environment

• Resulting gene pool may no longer be reflective of the original population’s gene pool

• Ex: original population had red, blue, yellow, and green bugs. Bottleneck effect happens/natural disaster —> only red and blue bugs remain, which isn’t reflective of the original population with the other color bugs too. Then the species evolves with red, blue, and maybe purple bugs, but the yellow & green bugs die off.

• By chance

• Humans can have an impact with bottleneck effect —> understanding how we impact other species

Effects of genetic drift:

1. Significant in small populations

2. Causes allele frequencies to change at random

3. Can lead to a loss of genetic variation within populations

4. Can cause harmful alleles to become fixed

Gene flow — migration/immigration

• Alleles can be transferred through the movement of fertile individuals or gametes

• Gene flow tends to reduce variation among populations over time & can reduce the fitness of the population

Evolution by natural selection involves both chance and “sorting”

• New genetic variations arise by chance

• Beneficial alleles are “sorted” and favored by natural selection

• Only natural selection consistently results in adaptive evolution

Relative fitness — contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals

• “survival of the fittest” is misleading bc it implies direct competition among individs. —> reproduction is usually more subtle and depends on many factors

• Selection favors certain genotypes by acting on the phenotypes of certain organisms

Three modes of selection:

• Directional selection: favoring one end of the phenotypic range

• Disruptive selection: favors both extremes of the phenotypic range

• Stabilizing selection: favoring the outlier

Natural selection increases the frequencies of alleles that enhance survival and reproduction

Bc the environment can change, adaptive evolution is a continuous process

Genetic drift & gene flow do not consistently lead to adaptive evolution bc they can increase or decrease the match between an organism & its environment

Selection selection:

• Can result in sexual dimorphism — physical differences between genders

• Intrasexual selection — competition among one sex (usually males) for mates of the opposite sex

• Intersexual selection — mate choice, when individuals of one sex (usually females) are choosy in selecting their mates

• Male showiness due to mate choice can increase a male’s chance of attracting a female while decreasing his chances of survival

• Female preferences evolve over time, but the
  good genes” are related to male health —> both the male trait & female preference for that trait should increase in frequency

Neutral variation — genetic variation that does not have an advantage or disadvantage

Diploidy — genetic variation in the form of hidden recessive alleles

• Heterozygotes can carry recessive alleles that are hidden from the effects of selection

Balancing selection — natural selection results in more than 2 phenotypic forms in a population

• Includes heterozygote advantage — when heterozygotes have a higher fitness than do both homozygotes

  • Ex: malaria resistance

  • Natural selection tends to maintain 2+ alleles at the locus

• Frequency-dependent selection: fitness of the phenotype declines if it becomes too common in the population

  • Selection can favor whichever phenotype is less common in a population

Natural selection can’t make perfect organisms

1. Selection can act only on existing variations

2. Evolution is limited by what's already present

3. Adaptations are compromises

4. Nothing is static — chance, natural selection, & the environment all work together