Chapter 3 – Lecture 1
Darwinian Natural Selection
3.1 Artificial Selection: Domestic Animals and Plants
- Breeders scrutinize their flocks and select the individuals with the most desirable traits.
- When they reproduce, the next generation consisting of the progeny of only the selected birds,
will show the desirable traits in a higher proportion than existed in last year’s flock
3.2 Evolution by Natural Selection
- Darwin realized that a process much like artificial selection happens in nature. his Theory of
evolution by Natural Selection hold that descent with modification is the logical outcome of four
1. individuals within populations are variable
2. the variations among individuals are, at least in part, passed from parents to offspring
3. in every generation, some individuals are more successful at surviving and reproducing than
4. the survival and reproduction of individuals are not random; instead they are tied to the
variation among individuals. The individuals with the most favorable variations, those who are
better at surviving and reproducing, are naturally selected.
* Darwinian evolution: gradual change in populations over time.
- if there are differences among the individuals in a population that can be passed on to
offspring, and if there is differential success among those individuals in surviving and/or
reproducing, then some traits will be passed on more frequently than others. As a result, the
characteristics of the population will change slightly with each succeeding generation.
- Darwinian fitness – is the ability of an individual to survive and reproduce in its environment
- Adaptation – a trait or characteristic of an organism, like a modified form of reverse
transcriptase, the increases its fitness relative to individuals without the trait
3.3 The Evolution of Flower Color in an Experimental Snapdragon Population
* Bumblebees could influence the evolution of a floral trait controlled by alleles of a single gene.
- Postulate 1: there is variation among individual
- postulate 2: some of the variation is heritable - testing postulate 3: do individuals vary in their success at surviving or reproducing
- testing postulate 4: is reproduction non-random? – it was not random
**ALL 4 POSTULATES WERE TRUE **
Testing Darwin’s Prediction: Did the Population Evolve
- The snapdragon population evolved as predicted, the next generation of snapdragon
had a higher proportion of white flowers. With this population evolving rate, it would
not take many years for white flowers to all take over.
3.4 The Evolution of Beak Shape in Galapagos Finches
- Close examination of the evolutionary tree reveals that all of these species are closely
related (birds). Making them a single species
- They only have a remarkable variation in the size and shape of their beaks
- They ran the tests on the medium sized/ground finches.
- Birds with bigger beaks eat larger seeds, and birds with smaller beaks eat smaller
Testing Postulate 1: Is the Finch Population Variable?
- Variation among the individuals within populations is virtually universal
Testing Postulate 2: Is Some of the Variation among individuals Heritable?
- Heritability – used to determine whether at least part of the variation among finch
beaks is genetically based, and thus passed from parents to offsprings.
- The heritability of a trait is defined as the proportion of the variation observed in a
population that is due to variation in genes.
- If the differences among individuals are due to differences in the alleles they have
inherited, then offspring will resemble their parents.
- Darwin’s second postulate is true: a substantial fraction of the variation in beak size
is due to variation in genotype.
- Focused on growth factors known to be active during embryonic development.
Among them was bone morphogenic protein 4, or BMP4, a signalling molecule that
helps sculpt the shape of bird beaks. They earlier they make the protein in their age
the bigger the bird gets.
- A reasonable hypothesis would be that a similar genetic mechanism is responsible for
some of the variation among individuals in the medium ground finch population on
Testing Postulate 3: Do individuals Vary in Their success at Surviving or Reproducing?
- In every natural population studied, more offspring are produced each generation
than survive to breed. - Data show that in most populations some of the individuals that survive to breed are
more successful at mating and producing offspring than others. Just as variation in
survival does, variation in reproductive success represents selection.
- Darwin’s third postulate is universally true.
Testing Postulate 4: Are Survival and Reproduction Nonrandom?
- Individuals who survive and go on to reproduce, or who reproduce the most, are those
with certain, favorable variations.
- The average survivors of the drought had a deeper beak than the average nonsurvivor.
Because deep beaks and large body sizes are positively correlate, and because large
birds ten to win fights over food, the average survivor had a larger body size too.
- After when there was a wet season: larger birds were favored in drought condition,
but smaller birds were favored in wet years.
- Natural selection is dynamic
Testing Darwin’s Prediction: Did the Population Evolve?
- Darwin’s theory predicts a change in the composition of the population from one
generation to the next.
- The population evolved.
- Darwin’s mechanism of evolution can be documented in natural populations. When
all four postulates are true in a population, the population evolves.
- The study also shows that small evolutionary changes over short time spans can
accumulate into larger changes over longer time spans.
3.5 The Nature of Natural Selection
- Some key points about how selection does and does not operate.
Natural Selection Acts on Individuals, but its Consequences Occur in Populations
- The effort of cracking Tribulus seeds did not make finch beaks become deeper and their
bodies larger, and the effort of transcribing RNA in the presence of AZT did not change
the amino acid composition of the reverse transcriptase active site. Instead, the average
beak depth and body size in the finch population increased because more smaller finches
died than larger ones, and the average active site sequence in reverse transcriptase
changed because certain mutants did a better job of making new virions.
Natural Selection Acts on Phenotypes, but Evolution Consists of Changes in Allele
- Only when survivors of selection pass their successful phenotypes to their offspring, via
genotypes that help determine phenotypes, does natural selection cause populations to
change from one generation to the next.
- On Daphne Major, the variation in finch phenotypes that selection acted on had a genetic
basis. As a result, the new phenotypic distribution seen among the survivors persisted
into the next generation. Natural Selection is Not Forward Looking
- Each generation is descended from the survivors of selection by the environmental
conditions that prevailed in the generation before.
- If the environment changed again during the lifetime of these offspring, however, they
might not be adapted to the new conditions.
- There is a common misconception that organisms can be adapted to future conditions, or
that selection can look ahead in the sense of anticipation environmental changes during
future generations. This is impossible. Evolution is always a generation behind any
changes in the environment.
New Traits Can Evolve, Even Though Natural Selection Acts on Existing Traits.
- Differences in survival or reproduction—that is, natural selection – can only occur among
variants that already ex