Natural Selection Components

The Components of Natural Selection

• Natural selection requires the meeting of three conditions: variation, inheritance, and differential reproductive success.
• Variation means individuals in a population differ from one another in traits.
• Inheritance means that some differences can be passed from parents to offspring.
• Differential reproductive success means some individuals survive and reproduce better than others because of their traits.
• When all three conditions are present, natural selection occurs, leading to evolutionary change.

Key Considerations in Natural Selection

• Mutations occur randomly and are a major source of variation.
• Mutations can be beneficial and favored by selection, but they aren't directed by an organism's needs.
• Evolutionary biologists study how specific traits change over time.
• Traits can be physical such as bird plumage, behavioral such as lyrebird dance, or genetic such as gene sequences.
• Natural selection acts on individuals, but populations change over time.
• Evolutionary studies focus on how traits spread or disappear within populations.
• Selection acts on phenotypic differences, not directly on genes.
• A genotype doesn't determine a single trait, but interacts with environmental factors.

Yarrow Plant Experiment

• Different genotypes of Achillea millefolium grow to different heights at different elevations.
• A genotype's response to environmental variation is known as the norm of reaction.
• A single genotype can produce different phenotypes depending on environmental conditions, visualized as a curve.
• Natural selection is a simple but powerful process where variation, inheritance, and selection drive evolution.
• Mutations create the raw material for selection, occurring randomly.
• Natural selection gradually shifts trait distributions over generations, causing populations to evolve, not individuals.
• Genes don't directly dictate traits, but influence how they develop under different conditions due to genotype-environment interactions.

Natural Selection and Coat Color in the Oldfield Mouse

• Coat color, an adaptive trait influenced by predation, was the focus of studies on natural selection in Peromyscus polionotus by Hopi Hoekstra and colleagues
• The species studied was Peromyscus polionotus, the oldfield mouse.
• The predators were owls and other visual hunters.
• Inland populations have dark coats, while coastal dune populations have light coats.
• Natural selection favors coat color that matches the environment.
• Light-colored mice are better camouflaged on sandy beaches.
• Dark-colored mice are better camouflaged in vegetated inland areas.

Key Steps in Natural Selection

• Natural selection requires variation in the trait under selection.
• Mice within and across populations exhibit phenotypic variation in coat colors.
• Genetic variation, linked to coat color differences in Mc1R and Agouti genes, was seen
• Traits must be heritable for natural selection to act.
• Coat color is influenced by genes like Mc1R and Agouti, not just environmental factors.
• The Mc1R gene controls pigment type (eumelanin for dark, phaeomelanin for light).
• The Agouti gene regulates pigment distribution, contributing to lighter coat color.
• Coat color is genetically variable among populations and heritable through Mc1R and Agouti genes.
• Studies confirmed that these genetic variants are inherited from parents.
• A trait must affect survival and reproduction to be subject to selection and affect fitness.
• Fitness is defined as reproductive success relative to others in the population.

Experimental Evidence (Kaufman, 1974)

• Mice with coats that matched their background were more likely to escape predators.
• The first mouse caught in a cage experiment with an owl was the one that stood out against the background.
• Mice with better camouflage have a fitness advantage, surviving longer and reproducing more.
• Natural selection favors coat colors that improve camouflage and predator avoidance.
• Camouflage and predator avoidance leads to adaptive divergence between inland and beach populations.
• Color is most helpful with better camouflage and predator avoidance.
• Mice that blend into their environment are less likely to be caught by predators.
• Natural selection favors coat colors that provide camouflage.
• Mice that blend into the the environment have increased chances of survival and reproduction.

Kaufman's 1974 Experiment

• Released pairs of mice, one dark and one light, were released into a cage with an owl.
• The mice were tested in three environments: dark soil with sparse vegetation, light soil with sparse vegetation, and light soil with dense vegetation.
• Typically, the first mouse captured contrasted with the background.
• Matching coat color provides a survival advantage.

Hoekstra's Silicone Model Experiment (2010)

• Painted silicone models of mice were used to eliminate behavioral differences as a factor.
• 125 models of each type (light and dark) were placed in natural habitats.
• Predator attacks were measured by missing models and teeth, talon, or beak marks on recovered models.
• Models that matched the environment were attacked less often.
• There was strong evidence of selection for camouflage.

Barrett et al. Deer Mouse Experiment (2019)

• Large enclosures (50m × 50m) were used in two habitats: sand dunes and vegetated areas.
• Mismatched mice (dark mice in dunes, light mice in vegetation) suffered high predation rates.
• The same pattern was observed in deer mice as in oldfield mice, reinforcing natural selection's role.
• Small differences in fitness lead to major genetic changes over time.
• If camouflaged mice have just 1% higher reproductive success, the advantageous allele doubles every 70 generations, and a single beneficial gene copy can reach 100% frequency in a few thousand generations in a population of 10,000.

Conclusion: Coat Color as an Adaptation

• Natural selection strongly favors coat color that provides camouflage.
• Predation drives the evolution of adaptive coloration in mice.
• Over generations, selection leads to populations well-matched to their environments.

Adaptations: How Organisms Match Their Environments

• An adaptation is an inherited trait that enhances an organism's fitness in both abiotic (nonliving) and biotic (living) environments.
• Adaptations must have evolved through natural selection for their current function.

How Adaptations Help Organisms Survive

• The saguaro cactus in the Sonoran Desert serves as an example.
• The saguaro cactus contains waxy coating that reduces water loss, a shallow root system that quickly absorbs water from rare rainfall, and a low surface-area-to-volume ratio that conserves water in extreme heat, as abiotic adaptations.
• Flowers attract pollinators for reproduction, and spines protect against herbivores trying to steal water as biotic adaptations.
• Exaptations are traits that originally evolved for one function but are now used for another.
• Feathers, which are essential for flight today, may have evolved for temperature regulation or display, not flight.

Testing Adaptation Hypotheses

• Evolutionary biologists study natural selection by observing traits in different environments, testing hypotheses about their function, and collecting data to determine if traits increase fitness.
• Natural selection shapes molecular, morphological, behavioral, and physiological traits in wild populations.

Natural Selection in the Field

• Guppies in Trinidad and Tobago live both upstream and downstream of waterfalls.
• Guppies are separated by waterfalls, creating distinct environments with varying predation pressures.
• Upstream environments have low predation (small fish Rivulus hartii), while downstream environments have high predation (larger fish like the pike cichlid Crenicichla alta).
• Guppies in Upstream environments have larger but fewer offspring.
• Guppies in Downstream environments have many smaller offspring.
• Upstream environments: Larger offspring grow quickly and escape predation.
• Downstream: Smaller offspring "flood the market," increasing survival chances.
• When transplanted guppies from high-predation (downstream) sites were transplanted to low-predation (upstream) sites.
• Over 12 years, the transplanted population evolved to produce fewer but larger offspring, like natural upstream populations, and lab studies confirmed that these differences were genetically inherited, not just environmental.
• Natural selection rapidly shaped guppy life history traits based on predation pressure.
• Evolution can occur in a few thousand years or even decades in response to environmental changes.

Natural Selection on Lizards in Urban Heat Islands

• Cities are warmer than surrounding areas due to heat-absorbing materials like asphalt and brick.
• Scientists are studying how species evolve in urban environments (urban evolution).
• Crested Anole Lizards (Anolis cristatellus) in Puerto Rico were studied.
• Compared anoles from urban vs. forest environments in Puerto Rico
• Urban lizards had higher body temperatures and lived in hotter microhabitats.
• Heat lizards were heated until they could no longer flip themselves over in a thermal tolerance test, which found lizards had higher heat tolerance (CTmax) than forest lizards.
• Urban lizards had a higher frequency of a specific genotype (C/C) at the RARS locus in a genetic analysis
• Analysis suggests natural selection favored heat-tolerant lizards in cities as this gene helps prevent protein damage from heat stress.
• Urban environments create new selection pressures.
• Lizards evolved higher heat tolerance in response to city life.
• Genetic evidence supports rapid adaptation to human-altered environments.
• Natural selection can be observed and tested in the wild.
• Environmental pressures such as predation and heat shape species over time.
• Rapid evolution can happen in decades, not just millions of years; urban environments are creating new evolutionary pressures

Natural Selection in the Laboratory

• Laboratory experiments offer precise control over environmental conditions, allowing scientists to manipulate evolution in real time.
• Wild populations allow biologists to observe natural selection.
• Advantages of Laboratory Studies include observing evolution over thousands of generations, controlling factors like nutrients, temperature competitors, running parallel experiments to compare evolutionary paths, and storing organisms and "reviving" ancestors to compete with their descendants.
• The LTEE has been studying 75,000+ generations of Escherichia coli since 1988.
• It involves 12 parallel bacterial populations evolving separately under identical conditions.
• Samples are stored in a "time machine" freezer.

Key Findings of studying E. Coli: Long-Term Evolution

• E. coli cells became larger within 2,000-3,000 generations.
• All 12 populations evolved larger cells but at different rates.
• Fitness increased steadily over time.
• Some populations found "better" evolutionary paths than others.
• All populations adapted in a similar direction, showing evolution is predictable.
• The specific mutations and rate of adaptation varied between populations, demonstrating evolution is random.

Why Is LTEE Important?

• Evolution can be restarted from any point.
• By running evolution multiple times, they test whether the same traits evolve again and again or if random mutations create different outcomes.
• Even if evolution produces similar phenotypes (larger cells), the mutations driving these changes can differ.
• Multiple genetic solutions can lead to the same evolutionary outcome.
• Fitness significantly increased after just 500 generations.
• Natural selection can act quickly, even in controlled environments,
• Evolution, which studies in the lab, often leads to predictable changes, but random mutations influence the details
• Parallel experiments reveal how different evolutionary paths can lead to similar adaptations, and evolutionary history is shaped by both selection and chance in the lab.

Thermal Adaptation and Antagonistic Pleiotropy in E. coli

• When a bacterial population evolves under constant temperature conditions, fitness in that environment increases.
• E. coli has already experienced billions of generations in host guts around 37°C, so adaptation improving fitness at 37°C come with trade-offs at other temperatures.
• E. coli encounters temperature fluctuations in natural environments meaning optimizing for a single temperature might reduce survival under different conditions.
• E. coli growth was tested at nine temperatures (20°C to 42°C) at generations 2,000, 5,000, 10,000, 15,000, and 20,000.
• Optimal growth shifted from 40°C to 37°C over time.
• Growth rate declined at extreme temperatures (20°C and 42°C) in most populations.
• Mutation accumulation was the proposed theory, but rejected on studies
• If bacteria lost performance at 20°C and 42°C simply due to random mutations, then mutator strains should have shown a greater decline, but decline was not observed.
• Antagonistic pleiotropy explains that some genes that increase fitness at 37°C also reduce fitness at other temperatures, it was therefor supported.
• A single gene may have positive effects at 37°C but negative effects at 20°C and 42°C, and the trade-off evolved quickly, mostly within the first 5,000 generations.

Follow-Up Experiment: Convergent Evolution in Response to Temperature

• Split a single population (grown at 37°C for 2,000 generations) into 30 new populations.
• Each evolved for another 2,000 generations under different temperature regimes: constant 20°C, 32°C, 37°C, or 42°C and alternating 32°C and 42°C.
• Populations evolved at the same temperature shared more mutations than those evolved at different temperatures.
• Shared mutations were a minority, even among same-temperature populations.
• Multiple genetic pathways can lead to similar adaptations.
• Researchers identified candidate genes likely involved in temperature adaptation.
• Evolution optimizes organisms for specific environments, but at a cost.
• Antagonistic pleiotropy explains why adaptation to one condition can reduce fitness in others.
• Temperature adaptation is partly predictable, but multiple genetic solutions exist.
• Trade-offs shape how organisms evolve in response to environmental pressures.