Darwin’s Second Idea – Natural Selection

• Evolution: controversial?

• Natural selection

– Darwin’s postulates

– Controversial?

– 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).

Adaptation

NS explains:

Natural selection: 4 Postulates

1. Variability

• Discrete

• Continuous

Variability

• Discrete

– Size, color

• Continuous

– Size

2. Heritability

Heritability

Genetic basis:

• Single locus:

– discrete variation

• Polygenic:

– continuous variation

Heritability

• 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

– Survival,reproduction

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