bio lecture 5: natural selectoin + adaptations

what is fitness how is it scaled

reproductive success. how well an org does compared to another

scaled from 0-1 (most fit = 1)

what is frequency dependent selection (positive and negative)

1. positive: the more common a genotype is, the more fit it becomes. selection rapidly drives the most common trait to become the only one left (fixation)

• ex: where it happens: evolution of signaling traits/communication (flower colors to attract pollinators, bright colors on butterflies)

  If a butterfly has a common pattern, predators recognize it and leave it alone.

  If a butterfly has a rare pattern, predators don't recognize it yet and are more likely to attack it.

2. Negative: fitness declines as the genotype becomes more common. helps to maintain genetic diversity.

• When a specific flower color becomes too common, pollinators learn to avoid it.

  pollinators then switch to visiting the rarer color morph that they haven't learned to avoid yet, giving the rare morph higher reproductive success.

  
  

discrete vs. continuous traits

• Discrete (Polymorphisms/mendelian traits): Controlled by single/few genes (e.g., human eye/hair color or ant queen social structures). distinct categories of traits. no in betweens

• Continuous: Polygenic traits influenced by environment (e.g., human height or milk yield in livestock). a wide range of values. controlled by many genes.

what is a selective advantage

the amount by which individuals are better adapted to their environment b/c of their genotype

what is an agent of selection

any external factor that influences the fitness of individuals with a specific trait

• Could be abiotic, biotic (predators), and anthropogenic (human-caused)

what are the 3 types of natural selection on alleles

Natural selection on alleles:

• Positive (directional) selection

  • In a population the allele is spread through across generations until it's the only one

  • This is called "fixation on the adaptive allele"

• Negative (purifying) selection

• Neutral: selection to maintain variation (balancing)

what is a polygenic trait

trait controlled by multiple genes

what kind of natural selection is this

selection for continuous traits

what are the 3 types of selection for quantitative traits. does it inc or dec variation. give an example

1. stabilizing selection

• intermediate traits favored

• decreases variation

• ex: human birth weight

2. disruptive selection

• favors both extremes, creates bimodal populations

• increases variation

• ex: either very big and small seeds with no medium sized seeds so the birds with big/small beaks are more fit

3. directional selection

• favors one extreme

• change in the mean of variation

• ex: darwin’s finches beak size when drought happened and the finches with big beaks survived b/c big seeds

what kind of selection is this

stabilizing selection

what kind of selection is this

directional selection

what kind of selection is this

disruptive selection

what are the 3 ways to study adaptation by natural selection

1. Look for an external force in nature (an agent of selection) and see how the organism responds to it

   • Look for correlations across multiple different populations

   • Look for correlations in environment and traits across time (ex. Darwin's finches)

2. Apply genetic tools

   • Test for distinctive patterns in genes targeted by selection (at the nucleotide level)

   • dN/dS ratio

3. Conduct experiments in field and in lab (hardest to do but most powerful)

why is there a struggle to determine agents of selection

• Lacking concrete documentation of the relationship between an agent of selection and the selection in nature

  • Ex: super easy to go study rabbits for a couple generations and conclude that bigger body size is favors, but HARD to determine the REASON for this selection

  • Recognize that evolution is happening but what is the cause?

Hard to play god and change factors in environmental selective pressures.

what are strategies to combat the struggle to determine agents of selection

correlation-based or genetics-based experiments in nature used to represent an agent of selection (like human pollution and see if pops adapt to it)

correlation-based:

• evolution of pepper moth color (light to dark)

• heavy metal tolerance in plants near mines

genetics-based:

• evolution of vision in stickleback fish

• Fish have SWS2 allele that affects vision

• If there is a mutation, they can see better

• When clear water fish come into the black lake, the allele should increase in frequency

• Nearby the locus for SWS2, decrease diversity. Further away from locus, is more normal

what are hitchhiker alleles

when a good allele increases in frequency due to natural selection, and nearby alleles that are physically linked to it get carried along and also increase in frequency