Darwin's Theory of Evolution
13.1 A sea voyage helped Darwin frame his theory of evolution
Darwin is best known for On the Origin of Species by Means of Natural Selection, which launched evolutionary biology.
The cultural and scientific context of Darwin’s time:
Aristotle’s view: species are fixed, permanent forms that do not evolve.
Judeo-Christian worldview: literal interpretation of Genesis; each form of life created in present-day form.
Age of Earth:
Earlier scholars estimated Earth’s age at about 6,000 years, contributing to the belief that all living species were unchanging and relatively recent.
Darwin’s voyage on HMS Beagle (age 22):
Served as ship’s naturalist on a long expedition charting South American coast.
On shore, he collected thousands of fossils, living plants, and animals; kept detailed journals of observations.
Darwin’s developing idea:
Observations and later analysis led him to propose that present-day species are descendants of ancient ancestors.
Descendants spread into various habitats over millions of years, accumulating diverse modifications (adaptations) to fit their environments.
Influence from Lyell:
Charles Lyell’s Principles of Geology argued for an ancient Earth shaped by gradual processes that continue today.
Timeline of development:
By early 1840s: Darwin wrote a long essay outlining major features of evolution by natural selection.
Delayed publication to gather more evidence, and later published after learning of Alfred Wallace’s nearly identical hypothesis.
Origin of Species (1859):
Presented a strong, logical case with extensive evidence from biology, geology, and paleontology.
Predictions from the hypothesis of evolution have been tested and supported for more than 150 years.
Conceptual takeaway:
Darwin’s theory is a broadly applicable explanatory framework, more than a mere hypothesis, supported by a large body of evidence.
13.2 The study of fossils provides strong evidence for evolution
Fossils defined:
Imprints or remains of organisms from the past.
Document differences between past and present organisms; reveal extinctions.
Example:
Fossil skull of Homo erectus shows a specimen about 1.5 million years old from Africa.
Fossil record as a historical chronology:
The sequence of fossils in sedimentary strata records life’s history.
The fossil record is the chronicle of evolution across geologic time, as reflected in rock layers.
Incompleteness of the fossil record:
Many organisms did not fossilize due to environment or biological factors.
Fossils formed may be in rocks that later distorted or destroyed by geologic processes.
Not all preserved fossils are accessible to paleontologists.
13.3 Scientific thinking: Fossils of transitional forms support Darwin’s theory of evolution
Transitional fossils link extinct species with modern species.
Examples of transitional sequences:
Gradual modification of jaws and teeth in mammals from a reptilian ancestor.
Evolution of whales from a land-dwelling mammalian ancestor.
13.4 Homologies provide strong evidence for evolution
Core concept: evolution = descent with modification.
Mechanism:
Ancestral traits are modified over time through natural selection as descendants face different environments.
Homology:
Similar underlying anatomical features across diverse organisms reflect common ancestry and can be repurposed for different functions (remodeling).
Example: vertebrate forelimbs
Humans, cats, whales, and bats have forelimbs built from the same skeletal elements, but they serve different functions.
These structures are homologous.
13.4 (continued) Molecular evidence for shared ancestry
Molecular comparisons across diverse organisms support the idea that all life is related.
All life uses the same genetic language (DNA and RNA).
The genetic code is essentially universal: RNA triplets code for amino acids in a nearly universal way.
How homology helps explain development:
Early development across diverse animals shows similarities not visible in adults (e.g., vertebrate embryos share features such as tails and pharyngeal pouches).
13.4 Embryology and vestigial structures
Embryos of different vertebrates show shared developmental features:
Tail posterior to the anus and pharyngeal (throat) pouches appear in embryos.
Vestigial structures:
Leftover, nonfunctional or marginally functional features in modern organisms that served important roles in ancestors.
13.5 Homologies indicate patterns of descent that can be shown on an evolutionary tree
Darwin’s insight: life history resembles a tree with multiple branches from a common trunk.
Modern representation:
Evolutionary trees showing relatedness between lineages.
Visual example (illustrative): relationships among lungfishes, amphibians, reptiles, birds, and mammals, with common ancestors at branch points.
Homologous characters are shared by groups to the right of a hatch mark on the tree.
13.6 Darwin proposed natural selection as the mechanism of evolution
Darwin’s contribution:
Explained how evolution occurs, especially given gradual change over long time periods.
Observational basis:
Direct observation of artificial selection demonstrates how selection can modify species over comparatively short timescales.
Inspiration from natural processes:
Artificial selection shows how human breeders modify traits; natural conditions can do the same.
Pigeon breeding as an example (illustrated in Figure 13.6-0):
Fantail, Frillback, Rock pigeon, Old Dutch Capuchine, Trumpeter, etc. (diverse varieties arise from selective breeding).
Mendel’s inheritance:
Gregor Mendel’s work on pea plants laid the foundation for understanding genetic variation.
Rediscovered in 1900, enabling the genetic basis of variation to be understood alongside Darwin’s theory.
Variation in populations:
Individuals in natural populations exhibit small but measurable differences.
Role of Malthus:
Thomas Malthus argued that resources (food, etc.) limit population growth, leading to competition and struggle for existence.
13.6 Mechanism of natural selection
Core deduction:
Overproduction of offspring + limited resources lead to a struggle for existence; only some survive.
Unequal reproduction:
Individuals with traits that improve food acquisition, predator avoidance, or tolerance to conditions are more likely to survive and reproduce, passing traits to offspring.
Implication:
If artificial selection can cause rapid change, natural selection can produce substantial changes over hundreds or thousands of generations.
13.6 Key points about natural selection
Important clarifications:
Evolution occurs in populations, not individuals.
Natural selection can only amplify or diminish heritable traits.
Evolution is not goal-directed and does not produce perfectly adapted organisms.
13.7 Scientists can observe natural selection in action
Nature vs artificial selection:
Natural selection acts as an editing process rather than a creative mechanism.
Contingency of selection:
Outcomes depend on time and place; advantages are environment-specific.
Summary idea:
Natural selection sorts existing variation and can lead to adaptive evolution when traits are heritable and advantageous in a given environment.
13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible
Variation among individuals:
Darwin could not explain the source of variation or its heritability in his time.
Mendel’s contribution:
-heritance in peas established the genetic basis for variation; rediscovery in 1900 connected genetics with evolution.Individual genomes:
Each person has a unique genome, reflected in phenotypic variation.
Mutations:
Changes in the nucleotide sequence of DNA; the ultimate source of new alleles and genetic variation for evolution.
Phenotypic impact:
A single nucleotide change in a protein-coding gene can significantly affect phenotype (e.g., sickle-cell disease).
Genetic variation in sexually reproducing populations:
Most variation results from unique allele combinations inherited by individuals.
13.9 Evolution occurs within populations
Population definition:
A group of individuals of the same species living in the same area that interbreed.
Measuring evolution:
Evolution is a change in the prevalence of heritable traits in a population across generations.
13.9 Population structure and isolation
Gene flow and population isolation:
Geographically isolated populations may have little or no genetic exchange.
Isolation is common in populations confined to different lakes, for example.
13.10 The Hardy-Weinberg equation can test whether a population is evolving
Gene pool concept:
All copies of every allele at every locus in all individuals.
Hardy-Weinberg principle (assumes no evolution):
Allele frequencies remain constant across generations unless acted upon by other factors.
Genotype frequencies in a non-evolving population can be predicted by p^2, 2pq, q^2 where p+q=1 and p, q are the frequencies of the two alleles.
Practical use:
If a population is in Hardy-Weinberg equilibrium, allele and genotype frequencies remain constant.
The principle helps detect when evolutionary forces are at work.
13.10 Conditions for Hardy-Weinberg equilibrium
Five conditions required for equilibrium:
Very large population
No gene flow between populations
No mutations
Random mating
No natural selection
Reality:
Rarely are all five conditions met; allele and genotype frequencies often change.
Application:
The Hardy-Weinberg equation can test whether evolution is occurring in a population.
MECHANISMS OF MICROEVOLUTION
13.12 Natural selection, genetic drift, and gene flow can cause microevolution
If the Hardy-Weinberg conditions are not met, allele frequencies may change.
But mutations are rare and typically have small effects; nonrandom mating may alter genotype frequencies but often little effect on allele frequencies.
The three main causes of evolutionary change:
Natural selection
Genetic drift
Gene flow
13.12 Natural selection, genetic drift, and gene flow can cause microevolution (examples)
Natural selection:
If survival and reproductive success differ among individuals, allele frequencies shift.
Example: imaginary iguana population where individuals with webbed feet (genotype ww) survive better and reproduce more due to swimming efficiency, increasing w allele frequency.
Genetic drift:
Random fluctuations in allele frequencies due to chance events.
More pronounced in small populations.
Bottleneck effect:
Catastrophes (hurricanes, floods, fires) drastically reduce population size, causing loss of genetic diversity.
Surviving population may have different allele frequencies than the original population.
Exact analogy with shaking a few marbles through a bottleneck: some alleles overrepresented, others underrepresented or lost.
Founder effect:
When a few individuals colonize a new habitat, their gene pool may not reflect the source population.
Explains higher frequency of certain inherited disorders in some human populations founded by small groups.
Importance for real populations:
Bottlenecks can reduce genetic diversity; founder events can alter genetic makeup and disease risk.
13.12-13.13 Gene flow, genetic drift, and natural selection in action
Gene flow:
Allele frequencies in a population can change when fertile individuals move between populations or when gametes (e.g., pollen) move.
Gene flow tends to reduce differences between populations.
Practical example:
Illinois greater prairie chickens had 271 birds introduced from neighboring states to counteract lack of genetic diversity.
Result: new alleles entered the population and egg-hatching rate improved to over 90%.
Gene flow versus adaptation:
Gene flow can hamper adaptation by introducing maladaptive alleles but can also increase genetic diversity and adaptive potential.
13.13 Natural selection is the only mechanism that consistently leads to adaptive evolution
Distinguishing microevolutionary processes:
Genetic drift, gene flow, and mutations can cause microevolution, but only by chance can these changes improve fit to the environment.
Natural selection is non-random and consistently sorts existing variation to improve fitness in a given environment.
Conclusion:
Among the mechanisms, natural selection is the only one that reliably produces adaptive evolution.
13.13-13.14 Natural selection in more detail
Clarifying terms:
Struggle for existence and survival of the fittest are often misinterpreted as direct competition; in reality, reproductive success is more nuanced and often passive.
Relative fitness:
The contribution of an individual to the next generation’s gene pool relative to others’ contributions.
Three modes of selection on phenotype distributions:
Stabilizing selection favors intermediates.
Directional selection shifts the population by acting against one extreme.
Disruptive selection favors extremes when environmental variation favors both ends over intermediates.
Illustration (Figure 13.14): original population vs evolved populations under stabilizing, directional, and disruptive selection.
13.14 Summary of how natural selection shapes variation
Natural selection acts on existing variation and changes phenotype distributions in populations over generations.
The outcome depends on environmental context and time; it is not a directed process aiming for perfection.
This framework explains how adaptation occurs and why different species exhibit different traits in different habitats.
Key formulas and concepts to remember
Hardy-Weinberg equilibrium:
Allele frequencies: p + q = 1
Genotype frequencies (assuming random mating): p^2, 2pq, q^2 corresponding to genotypes AA, Aa, aa respectively.
Relative fitness and selection:
Relative fitness assigns reproductive success to genotypes relative to the most fit genotype; affects allele frequency changes across generations.
Evolutionary mechanisms:
Natural selection, genetic drift, gene flow, mutations (as a source of variation) contribute to microevolution; natural selection uniquely yields adaptive evolution in changing environments.
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