Evolutionary Evidence, Population Genetics, and Speciation: Study Notes

Evidence for Evolution

Types of Evolutionary Evidence

Biologists use several lines of evidence to support the theory of evolution, including anatomical, embryological, molecular, and fossil evidence.

• Embryological Evidence: Similarities in embryonic development among different species suggest common ancestry. For example, chick, human, and cat embryos all have gill pouches at certain stages, indicating descent from a common ancestor with this feature.

• Anatomical (Morphological) Evidence: Shared physical traits, such as tails in both cats and dogs and their ancestors, are examples of homology—traits inherited from a common ancestor.

Example: The presence of gill pouches in vertebrate embryos is embryological evidence for evolution. The presence of tails in both cats and dogs is anatomical evidence for evolution.

Natural Selection and Heritability

Conditions for Natural Selection

For a trait to be subject to natural selection, it must meet specific criteria:

• Variation: The trait must vary among individuals in the population.

• Heritability: The trait must be genetically inherited.

• Differential Reproductive Success: Individuals with certain trait variants must have higher reproductive success than others.

Example: To determine if the tufts of long white hair in cotton-topped tamarins are under natural selection, the trait must be variable, heritable, and affect reproductive success.

Misconceptions in Evolutionary Thinking

Teleological Thinking

It is incorrect to say that a species evolved a trait "in order to" achieve a specific function. Evolution does not have foresight or goals; traits arise by random mutation and are favored if they confer a reproductive advantage.

• Correct View: Fish with teeth capable of breaking clams survived and reproduced more successfully, leading to the prevalence of this trait.

Example: Carnivorous fish did not evolve teeth "to eat clams"; rather, those with advantageous teeth were naturally selected.

Population Genetics

Hardy-Weinberg Principle

The Hardy-Weinberg equilibrium describes allele and genotype frequencies in a non-evolving population. The equations are:

• Allele frequencies: $p + q = 1$

• Genotype frequencies: $p^2 + 2pq + q^2 = 1$

Where $p$ and $q$ are the frequencies of alleles A and B, respectively.

Example Calculation

• Given: $p = 0.55$, $q = 0.45$, population size = 300

• Heterozygotes ($2pq$): $2 \times 0.55 \times 0.45 = 0.495$

• Number of heterozygotes: $0.495 \times 300 = 148.5 \approx 149$

• Homozygotes AA ($p^2$): $0.55^2 = 0.3025 \rightarrow 0.3025 \times 300 = 90.75 \approx 91$

• Homozygotes BB ($q^2$): $0.45^2 = 0.2025 \rightarrow 0.2025 \times 300 = 60.75 \approx 61$

Allele and Genotype Frequencies in a Population

Given observed genotype counts, calculate allele frequencies and expected genotype frequencies under Hardy-Weinberg equilibrium.

Genotype	Observed Count
TT	560
Tt	280
tt	160

• Total alleles: $2 \times 1000 = 2000$

• T alleles: $2 \times 560 + 280 = 1400$

• t alleles: $2 \times 160 + 280 = 600$

• Frequency of T: $1400/2000 = 0.7$

• Frequency of t: $600/2000 = 0.3$

• Expected genotype frequencies: TT: $0.7^2 = 0.49$, Tt: $2 \times 0.7 \times 0.3 = 0.42$, tt: $0.3^2 = 0.09$

• Expected counts: TT: $0.49 \times 1000 = 490$, Tt: $420$, tt: $90$

Compare observed and expected counts to determine if the population is evolving (i.e., not in Hardy-Weinberg equilibrium).

Speciation and Reproductive Isolation

Barriers to Gene Flow

Populations may diverge into separate species due to barriers that prevent gene flow:

• Geographic Isolation: Physical separation prevents interbreeding.

• Reproductive Isolation: Biological factors prevent gene flow even when populations are in contact.

Example: Two bird populations on opposite sides of a peninsula remain separate due to unsuitable habitat between them (geographic isolation).

Types of Reproductive Isolation

• Prezygotic Isolation: Prevents mating or fertilization (e.g., temporal, habitat, behavioral, mechanical, or gametic isolation).

• Postzygotic Isolation: Offspring are inviable or sterile.

Example: Rocky Mountain juniper and one-seeded juniper are separated by gametic isolation—pollen grains cannot fertilize ovules of the other species.

Hybrid Zones and Reinforcement

In hybrid zones where reinforcement is occurring, natural selection strengthens reproductive barriers, reducing gene flow between populations.

• Expected Observation: Decreased gene flow as hybrids are less fit or selected against.

Phylogenetics and Evolutionary Relationships

Interpreting Phylogenetic Trees

Phylogenetic trees depict evolutionary relationships among species.

• Closest Relatives: Determined by the most recent common ancestor.

• Comparative Relationships: Turtles and birds may be more closely related to each other than to crocodiles, depending on their branching points.

• Common Ancestors: The node where two lineages diverge represents their most recent common ancestor.

Example: On a tree, the closest relative to iguanian lizards is the group that shares the most recent node with them.

Homology vs. Convergent Evolution

Definitions

• Homology: Similarity due to shared ancestry (e.g., similar proteins in fruit flies and humans due to descent from a common ancestor).

• Convergent Evolution: Similarity due to independent evolution in different lineages (e.g., wings in bats and birds).

Example: If 65% of amino acids in Acat1 protein are identical in fruit flies and humans, this is likely homology, reflecting shared ancestry.