Lecture 4: Theory of Natural Selection

Question 1

Two major hypothesis in the origin of species

Answer 1

1. Descent with modification (evolution)
2. Natural selection is the process of adaptation

Question 2

Conditions for Evolution by natural selection (3)

Answer 2

1. Potential for evolution
   -Individuals differ from one another in phenotype
   -These differences are (at least in part) heritable
2. Variation in fitness
   -some individuals are more successful in survival and reproduction than others
3. Phenotype affects fitness
   -Successful individuals succeed because of the variable trait they have inherited

Question 3

Malthus and Darwin

Answer 3

Thomas Malthus’s ideas about population growth influenced Charles Darwin’s theory of natural selection. Malthus believed that human population growth would outpace food supply, leading to famine and misery. Darwin applied Malthus’s ideas to the natural world, concluding that only the fittest individuals would survive and reproduce.

Question 4

The Struggle for survival (2)

Answer 4

-Every organism produces more gametes than offspring, and more offspring than survivors
-Small variations in phenotype may affect survival and reproduction

Question 5

Darwinian fitness (3)

Answer 5

-Two major components: Survival (viability) and number of offspring (fecundity)
-Fitness = viability X Fecundity
- Fitness = the lifetime contribution of genes to the next generation

Question 6

Fitness is a relative measure because….

Answer 6

The score you get can be deceiving! Imagine you got a fitness score of 29 in a population of 30 vs a fitness score of 29 in a population of 100. 29/30 is a lot better than 29/100!

Question 7

Fitness sub components (5)

Answer 7

1. Survival to maturity
2. Adult survival
3. Mating Success
4. Gamete number and quality
5. Fertilization success

Question 8

Polymorphic traits (definition + example)

Answer 8

A polymorphic trait is a trait that is controlled by multiple genes, resulting in multiple different forms of the trait. An example is fur color in rats that can be white, grey or black.

Question 9

Flower color in Snap dragons (4)

Answer 9

-Snap dragon flowers can either be yellow (ss) or white (SS / Ss) (phenotypic variation)

-Snap dragons are pollinated by bees

-The bees favor the yellow flowers = fitness associated by phenotype

-Natural selection shifts the frequency distribution of flower color, increasing s allele frequency

Question 10

Continuous traits (definition + example)

Answer 10

A continuous trait is a trait that can vary along a spectrum rather than having distinct types. Examples are height and weight

Question 11

Humans and natural selection (4)

Answer 11

-Researchers took DNA from 250,000 ppl analyzing 8 million SNPs (single nucleotide polymorphisms)

-Looked to see if there were alleles that changed in frequency with age

-What they found was that alleles with linkage to Alzheimer’s and smoking habits sharply decreased with age , meaning those who had those alleles had a increased mortality rate

-Does this mean people who live longer are more likely to pass on their genes? No! This is because the people who die from these alleles most likely have ALREADY reproduced

Question 12

Properties of natural selection (6)

Answer 12

1. Acts on individuals, affects populations
2. Acts on phenotypes, changes allele frequencies
3. Is not random, but not progressive (ie no “Higher forms” due to natural selection)
4. Existing variations + new mutations = novel phenotypes
5. Not perfect
6. Acts on individuals, not for the “good of the species”

Question 13

Conceptual changes that came with Darwinism (4)

Answer 13

-Tree thinking in terms of evolution (think cladogram)

-Variation = different opportunity (not just random noise)

-Small changes accumulate into large scale patterns

-Speciation is a gradual process (variants -> subspecies -> species)