Macroevolution and Population Genetics
Week 12: Chapter 7.3pt2, 7.4
Focuses on concepts of macroevolution and population genetics.
7.3 Macroevolution: Evidence
There are three main lines of evidence that suggest existing species came from a common ancestor:
Fossils
The anatomical record
The molecular record (includes DNA and protein)
7.3 Macroevolution: Fossils
Fossils are considered the most direct evidence of evolution.
Fossilization process:
Bones are resistant to degradation by microorganisms; under specific conditions, minerals can infiltrate and create rock-like fossils.
Common fossilized materials include bones, feathers, scales, hooves, horns, teeth, shells, and structural impressions.
Soft tissues are infrequently fossilized, though rare occurrences exist.
Preservation in amber: If an organism becomes encased in fossilized tree sap (amber), preservation is possible.
Challenges of fossilization: Fossilization is rare and requires perfect conditions; therefore, the fossil record is incomplete.
Example: Spider attacking prey in amber, dated to be approximately 100 million years old.
Location alert: Rock Glen, a park approximately 45 minutes from London, allows fossil collection under specific rules.
Human fossil ancestry:
Similarities among modern primate skulls (including chimpanzees and gorillas) suggest a common ancestor.
Homo sapiens have existed for roughly 200,000 years, sharing significant time with Homo neanderthalensis.
Note: The diagram referenced shows present human species distribution without indicating direct lines of descent.
Timeline of hominid evolution:
Anatomically modern Homo sapiens have existed for about 50,000 years of the 200,000 years since their emergence.
Multiple ancestors within the Homo genus, including Australopithecus, Ardipithecus, and Paranthropus, co-existed at various points in history.
7.3 The Anatomical Record: Homologous Structures
Homologous structures:
Anatomical structures with shared origins or structures (like bones) indicating descent from a common ancestor.
Homologous structures noted in vertebrate embryos:
All vertebrate embryos possess pharyngeal slits, which develop into gills in fish but either disappear or morph into a larynx in mammals.
All exhibit a bony tail that vanishes in many species during development.
Homologous bones examples: Observed in the forelimbs of several mammals, all sharing a similar basic bone structure (despite differing sizes and shapes), including:
Humerus (light blue)
Radius (medium blue)
Ulna (green)
Carpals (purple)
Metacarpals/Phalanges (yellow)
7.3 Molecular Record/Homologous Molecules
Shared similarities in macromolecules among species, primarily in proteins and DNA sequences:
Eukaryotes share an actin cytoskeleton; all organisms use the same four bases in their nuclear genetic code.
Numerous organisms possess common genes, with related species generally exhibiting more closely related genetic similarities.
Example of Homologous Molecules: Mice share nearly identical gene sequences and proteins with humans, despite sequence differences.
Technology in evolution study: DNA sequencing technology serves as a vital tool for understanding evolutionary patterns and establishing species relatedness.
Example with Cytochrome C:
A comparison between the human and mouse cytochrome C sequences reveals some DNA sequence differences (green) that do not alter amino acids, versus others (red) that do change amino acids, signifying relationships.
7.3 Molecular Record: Hominids
A cladogram illustrates the evolutionary relationships among several living great apes, indicating common ancestry at each branch point.
Humans and chimpanzees: We are most closely related to this genus (Pan).
Clarification on evolution of hominids: This process does not represent a linear trajectory toward 'human-ness'.
7.3 Microevolution Example
Guppies in home aquariums: Selective breeding promotes colorful guppies due to the absence of predators.
In nature, guppy coloration tends to be dull, influenced by the presence and type of predators in various habitats.
Generation time: The short generational span of 3-4 months facilitates rapid observation of evolutionary selection effects.
7.3 Microevolution Example: Trinidad Guppies
Trinidad guppies inhabit two distinct environments, separated by waterfalls:
High-Predation Pools:
Larger predators, including adults, consume guppies actively.
Low-Predation Pools:
Smaller predators target juvenile guppies predominantly.
Movement dynamics: Guppies primarily stay within their home pools but may migrate across waterfalls during flooding events.
Effects of environmental shifts:
Guppies migrating from high to low predation pools, or from no-predation laboratories, demonstrate rapid phenotypic changes (within years in wild; within a year in lab).
In high-predation environments:
Males exhibit duller colors (to evade pike).
Maturity occurs sooner, enhancing mating opportunities before predation.
Higher numbers of smaller offspring increase juvenile survival rates.
In low-predation environments:
Males display brighter colors (to attract mates).
Maturity occurs later, ensuring safety before reproduction.
Fewer, larger offspring are produced.
Aquarium effects allow these transformations to occur more rapidly, with selective pressures based mainly on mate attraction.
These observed differences serve as evidence for natural selection, demonstrating a collection of traits influenced by environmental pressures.
7.3 Microevolution Example: Gene Flow
Guppy movement between pools exemplifies gene flow, crucial for genetic mixing between different environmental pressures.
Genetic isolation is essential for speciation; thus, limited environmental interactions can delay this process.
Over extended periods, if these populations remain isolated, they may develop differences due to mutations and selection pressure, leading to the formation of distinct species.
7.3 Macroevolution: Agents of Evolution
Evolution is influenced by changes in allele frequencies in populations over time, driven by several key factors:
Different types of natural selection
Mutation
Non-random mating
Migration
Genetic drift
7.3 Macroevolution: Types of Selection
Examples of three types of natural selection include:
Directional selection: One extreme phenotype is selected against, causing an evolutionary shift toward the opposite extreme.
Example: antibiotic resistance in bacteria.
Stabilizing selection: Both extreme phenotypes are selected against, prompting a shift toward the average phenotype.
Example: human birth weight.
Disruptive selection: The average phenotype is selected against, leading to increased diversity of traits.
Example: Darwin’s finches.
7.3 Macroevolution: Mutation
Mutation: Refers to a change in the nucleotide sequence of DNA.
It serves as the ultimate source of genetic variation in populations over extended time scales.
While most mutations can be deleterious, some confer neutral phenotypes (neutral mutations) that do not adversely impact survival.
Example: ABO blood type variation resulted from a mutation that inactivated the A gene creating O-type blood.
For mutations to contribute to evolution, they must occur in germ-line cells, ensuring they are passed to offspring during:
Gamete production (gametogenesis) or early zygote development.
7.3 Macroevolution: Migration
Migration: The movement of individuals among populations facilitates gene flow.
Gene flow: Involves allele transmission from one population to another, influenced by:
The number of migrants moving into a population.
The difference in allele frequencies between incoming migrants and the established population.
Consequences of migration:
Occasional migration can enhance population variation.
Extensive gene flow can also restrict evolutionary opportunities (for example, continuous guppy migration between environments).
Migration may restrict variation in the leaving group if it consists of a limited number of different alleles: this scenario is termed the founder effect.
7.3 Macroevolution: Non-random Mating
Random mating: Occurs when no genes are favored for mate selection; is typical in plants (e.g., pollination).
In animals, random mating rarely exists due to mate selection behaviors linked to genotypes.
Examples of animal mating behaviors:
The blue-footed booby employs foot coloration and dance to attract females.
Peacocks with elongated tails receive preferential treatment from peahens based on tail characteristics.
7.3 Macroevolution: Genetic Drift
Genetic drift: Refers to random fluctuations in allele frequencies within a population due to chance rather than selection.
Not driven by the organism’s fitness but often instigated by significant environmental events.
Bottleneck effect: A type of genetic drift occurring when population size drastically decreases (e.g., due to a volcanic eruption), leaving a survivor gene pool that may differ in alleles from the original population, irrespective of their fitness.
7.4 Population Genetics
Population genetics: The analysis of allele frequencies within populations.
Hardy-Weinberg Principle: Developed around 50 years after Darwin, this principle examines gene pool frequencies.
The gene pool encompasses all genes and their variants within a population.
In a stable population, dominant and recessive allele frequencies remain constant, meaning dominant alleles do not eliminate recessive alleles.
If allele proportions remain unchanged from generation to generation, the population is in Hardy-Weinberg equilibrium, indicating a lack of evolutionary change.
7.4 Population Genetics: Assumptions
Hardy-Weinberg equilibrium necessitates:
A large population size to minimize random effects.
Random mating behavior.
The absence of mutations, as infrequent mutation rates do not significantly alter equilibrium within one or two generations.
No migration; no new alleles should enter or leave the population.
No natural selection, eliminating pressures on specific traits.
No environmental changes that could randomly alter allele frequencies, avoiding genetic drift.
7.4 Population Genetics: Justification of Assumptions
It is acknowledged that the number of assumptions underlying Hardy-Weinberg equilibrium is extensive.
Significance of the model: In complex systems like population genetics, simplified models with numerous assumptions can yield preliminary insights that inform real-world observations.
If the model results closely align with nature, refinement follows; if significantly deviated, a new model might be required.
Hardy-Weinberg equilibrium is refined over time through practical applications, proving useful as a foundational model in introductory biology.
7.4 Population Genetics: Examples
Population genetics explores allele, genotype, and phenotype frequencies:
Example: In a population of 1000 mice, with 840 having black fur and 160 white:
Black mouse phenotype frequency: rac{840}{1000} = 0.84
White mouse phenotype frequency: rac{160}{1000} = 0.16
To determine allele and genotype frequencies using Hardy-Weinberg equations:
Alleles: Represented as dominant allele p, and recessive allele q.
Concluded equations:
p + q = 1 (sum of allele frequencies equals 1).
Genotype breakdown in the population for black/white fur mice example:
Defined in combinations: BB, Bb, bb (similar to Punnett squares).
Assembled into the Hardy-Weinberg format: p^2 + 2pq + q^2 = 1
7.4 Population Genetics: Calculating Genotypes
In the black/white fur example:
Given black mouse frequency of 0.84, white mouse of 0.16, and black being dominant:
For white mice (homozygous recessive), determined by q^2 = 0.16, yielding:
q = 0.4 (since p + q = 1, therefore p = 0.6).
With genotype equations applied:
Calculate heterozygotes: 2pq = 2(0.6)(0.4) = 0.48
Phenotype to frequency contrast: Notably, the frequency of the black phenotype (0.84) diverges from the frequency of the black allele (0.6), prompting questions about population structure and allele inheritance.
7.4 Population Genetics: Summary of Mouse Example
Summary results from population genetics concerning mice:
Black mouse phenotype: 84% (frequency = 0.84)
Black fur allele frequency: 0.6
Black allele distribution involves both homozygous dominant (p^2) and heterozygous (2pq) individuals.
White mouse phenotype: 16% (frequency = 0.16)
White fur allele frequency: 0.4; solely homozygous recessive (q^2).
The distribution clarifies that black alleles are split between homozygotes and heterozygotes, while white alleles revert to recessive forms.
7.4 Population Genetics: Importance of Equations
Equation layouts:
p + q = 1 for allele frequency determination in populations.
p^2 + 2pq + q^2 = 1 for genotype frequency assessment.
Understanding phenotype frequencies serves as a gateway to gaining insights into genotype frequencies.
During Hardy-Weinberg analysis, discern the context of phenotype, genotype, or allele information to navigate calculations properly.
7.4 Population Genetics: Practice Questions
For a population of lizards with red eyes (dominant) vs green (recessive), with red allele frequency at 0.7, determine:
Proportion with green eyes (recessive phenotype).
Proportion of heterozygous individuals.
In a cow population with spots (dominant) vs solid fur, with spotted allele frequency at 0.8, establish:
Proportion of solid fur individuals.
Proportion of spotted individuals.
Proportion of heterozygotes.
For rabbits with floppy ears (recessive) vs straight (dominant) with floppy allele frequency of 0.3, calculate:
Amount of heterozygous rabbits.
Homozygous straight ear population.
With a purple pea flower allele frequency at 0.5, and purple as dominant, assess the number of white pea plants in the population.
Determine the frequency of the recessive allele given that 16% of a population has a recessive phenotype.