BIOL 112 Lecture 5

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Chapter 23: The Evolution of Populations

Population

• heritable variation
• competition among individuals over resources (Malthus' observation)

Both give rise to differential reproductive success

best-suited individuals produce the most progeny in the next generation ("survival of the fittest")

→ adaptation (evolution) of population

Alleles

Alleles encode variants of a characteristic (e.g. Flower color in plants)

Mendel's laws of inheritance for diploids (See BIOL 111 Chapter 14→)

1. Variants of a characteristic are encoded by different alleles
   • Dominant – show phenotype
   • Recessive – don't show
2. Diploids inherit 2 alleles (1 per chromosome)
3. Laws of segregation: alleles segregate randomly into gametes (egg/sperm) and are randomly recombined

Types of Variation

1. Discrete
   • dominant and recessive alleles
   • no intermediates (purple or white flowers)
   • single locus on chromosome defines trait
2. Quantitative
   • Continuum of various forms (e.g. human height)
   • Final phenotype defined by sum of alleles in multiple loci
   • Dominant/recessive, semidominant/recessive, large pool of variants (more than 2 types of alleles, but diploids only get 2)

Variation Between Populations

Example: cold hardy Ldh(b) (lactate dehydrogenase isoenzyme B; works well in cold climates) allele

Samples taken from many fish populations over geographical range from Maine to Georgia:

fish in colder climates (near Maine) had a larger frequency of Ldh(b) than those in warmer climates (near Georgia)

Clinal variation: graded change in a character over a geographical transect (range)

Sources of Variation

Sexual reproduction:

• Major source of redistribution and changes in frequency
• recombination, random association of gametes, and random mating provide fresh combinations of traits.

Genetic mutation

• Minor source of variation
• Only source of new alleles
• Types
  1. point mutation: change in DNA sequence
     • missense change amino acid (neutral, positive/adaptive, or negative/deleterious effect)
     • nonsense puts a STOP codon which terminates the protein
  2. gene duplication ("play around" with copy)