Evolutionary Theory & Genetics 1.1

1. Explain evolution by natural selection. How does natural selection work?

Natural selection is a process where individuals with traits better suited to their environment survive and reproduce more successfully. Over generations, these traits become more common in a population. It works through:

• Variation: Individuals differ in traits.

• Differential Survival & Reproduction: Some traits provide advantages, leading to higher survival and reproductive success.

• Heritability: Traits are passed to offspring.

• Time: These small changes accumulate over generations, leading to evolution.

2. What are Darwin’s principles that underlie the general principle of evolution by natural selection?

Darwin proposed four main principles:

• Variation: Organisms within a species vary.

• Inheritance: Traits are passed from parents to offspring.

• Differential Survival & Reproduction: Some traits confer advantages, leading to greater reproductive success.

• Adaptation Over Time: Over generations, advantageous traits become more common.

3. What is sexual selection? What are the main processes involved, and which is more important for which sex? What are some typical consequences of sexual selection?

Sexual selection is a type of natural selection where traits increase an organism’s ability to attract mates. It involves:

• Intrasexual Selection: Competition among individuals of the same sex (e.g., male-male competition).

• Intersexual Selection: Mate choice (e.g., female preference for elaborate traits).

Consequences:

• Sexual Dimorphism (males and females look different).

• Elaborate Displays or Ornaments (e.g., peacock tails).

• Risky Traits (e.g., large antlers make males more vulnerable to predators).

4. How can sexual selection explain sexual dimorphism in primates? Why might indicator mechanisms be less important for female choice in primates than in many other animals?

• Males often compete for access to females, leading to traits like larger body size or large canines.

• Females may select mates based on behavior rather than physical indicators, as primates have complex social interactions.

• Indicator mechanisms (e.g., bright colors, elaborate calls) are less critical in primates than in birds or fish because primates can assess mate quality through long-term social interactions.

5. What is the relevance of Mendel’s work to inheritance and variation? How does it complement Darwin’s theory of evolution by natural selection?

• Mendel showed that traits are inherited as discrete units (genes), not blended.

• This explained how variation persists in populations, which Darwin couldn’t fully explain.

• When combined with understanding of continuous traits (polygenic inheritance), Mendel’s work supported Darwin’s ideas by providing a mechanism for heritable variation.

6. What is the genetic code? How does it work? Why is it redundant?

• The genetic code is a set of rules that translates DNA sequences into proteins.

• It works via codons (triplets of nucleotides) that correspond to amino acids.

• It’s redundant because multiple codons can code for the same amino acid, reducing the impact of mutations.

7. Why is "junk DNA" a misnomer? What are coding and non-coding DNA? Why are regulatory genes and different types of RNA important?

• "Junk DNA" is misleading because non-coding DNA has important regulatory functions.

• Coding DNA contains genes that produce proteins.

• Non-coding DNA includes regulatory sequences, non-coding RNAs (tRNA, rRNA, microRNA), and elements that influence gene expression.

• Regulatory genes and alternative splicing allow organisms to produce multiple proteins from the same gene, increasing complexity.

8. Importance of gene duplications in human evolution (AMY1 and SRGAP2).

• AMY1 (salivary amylase) gene duplication enhances starch digestion, which was advantageous in agricultural societies.

• SRGAP2 gene duplication affects brain development, contributing to cognitive differences between humans and other primates.

9. What is a single nucleotide polymorphism (SNP)? Why is HbS an SNP, and how does it relate to sickle cell anemia and malaria?

• An SNP is a single base-pair change in DNA.

• The HbS allele, which alters the beta-globin gene in hemoglobin, is an SNP.

• HbS in homozygous individuals causes sickle cell anemia.

• In heterozygous individuals, HbS provides resistance to malaria, illustrating balancing selection.

10. What is genetic drift? Why are its effects inversely related to population size?

• Genetic drift is random changes in allele frequencies.

• It has a stronger effect in small populations because chance events (like a storm wiping out individuals) can drastically change allele frequencies.

11. What is an adaptive radiation? How do Darwin’s finches illustrate this? What about guenons, Miocene apes, and hominins?

• Adaptive radiation is when one species rapidly diversifies to fill different ecological niches.

• Darwin’s finches evolved different beak shapes for different food sources, showing microevolution and speciation.

• Guenons diversified into many species with different facial patterns.

• Miocene apes adapted to various environments, leading to modern primates.

• Hominins in the Pliocene-Pleistocene evolved new locomotion and dietary adaptations.

12. Where does genetic variation come from? Why is the genetic code important for variation? Examples of human functional variation.

• Variation arises from mutations, recombination, and genetic drift.

• The genetic code ensures that mutations can produce new traits while maintaining redundancy.

• Examples of human variation: lactose tolerance, skin pigmentation, sickle cell trait.

13. What is Hardy-Weinberg equilibrium? Why is it important? How do we calculate expected gene frequencies?

• Hardy-Weinberg equilibrium (HWE) describes a non-evolving population.

• If a population is in HWE, allele frequencies remain constant.

• Equation: p² + 2pq + q² = 1 (where p and q are allele frequencies).

• Deviation from HWE suggests forces like natural selection, genetic drift, or mutation are acting.

14. What is the biological species concept? What are some problems with applying it in nature?

• The biological species concept defines species as groups that can interbreed and produce fertile offspring.

• Problems:

  • Doesn’t apply to asexual species.

  • Difficult to test in fossils.

  • Hybridization blurs species boundaries.

15. Why do we think in terms of tradeoffs, compromises, and constraints instead of perfection when explaining evolution?

• Evolution doesn’t produce perfect organisms, just ones that are good enough to survive and reproduce.

• Tradeoffs: Traits that improve reproduction may reduce survival (e.g., peacock tails).

• Constraints: Evolution builds on existing traits rather than creating new ones from scratch.

• Local optima: Natural selection favors improvements within existing structures rather than finding a global ‘best’ solution.