Darwin’s Second Idea – Natural Selection

    Evolution: controversial?

    Natural selection

   Darwin’s postulates


   Two examples of NS


Darwin’s Ideas

1.       Species evolve over time: “descent with modification”

        Darwin compiled abundant data

        Accepted over 100 years ago

Darwin’s Ideas

“a naturalists…might come to the conclusion that each species … had descended…from other species.  Nevertheless, such a conclusion… would be unsatisfactory until it could be shown how the innumerable species inhabiting this world have been modified….”

Darwin’s Ideas

1.        Species evolve over time: “descent with modification”

2.       Adaptations arise by natural selection

        Major evolutionary mechanism.

        Controversial until Modern Synthesis (1940s).



NS explains:

Natural selection: 4 Postulates

1. Variability





   Size, color



2. Heritability


Genetic basis:

    Single locus:

   discrete variation


   continuous variation


    The proportion of phenotypic variation due to alleles inherited from parents

3. “Competition”

     More individuals are born than can survive

    “Struggle for existence”

     Differential reproductive success between individuals



4. Fitness nonrandom

     Fitness varies among phenotypes

     Relationship between phenotype and fitness

Still controversial? (after all these years)

    Given 4 postulates, natural selection is inevitable

    Accepted since the Modern Synthesis of genetics and evolutionary theory


Still controversial? (after all these years)

Darwin’s Finches

    Galapagos Islands

    15 species

    Differences in beak size, shape

Darwin’s Finches

Can natural selection account for adaptation of beak size?

     Peter and Rosemary Grant

     Medium ground finch

     Study began in 1973


1.       Variability: beak depth

        Continuous variation, a quantitative trait, many loci


    2. Heritability

   Parent-offspring resemblance


3. “Competition”: during drought

   Fewer seeds, larger seeds

   Not all survive


Key Point 1

    Natural selection: not forward looking

    Many selective deaths: lacked the right bill for the drought conditions

4. Non-random success


Key Point 2

    Natural selection is not a random, chance, undirected process

    Reproductive success is associated a non-random set of trait values