Learning Objectives
- Define evolution as descent with modification.
- Identify the three requirements for natural selection: variation, heredity, and differential reproductive success.
- Contrast artificial selection (humans choose traits) with natural selection (environment "chooses" traits).
- Explain how natural selection increases the frequency of advantageous traits in populations.
- Recognize that trade-offs limit how well-adapted organisms can be.
Terminology
- Pattern
- Is revealed by scientific data showing that life has evolved over time.
- Process
- Consists of mechanisms that cause the pattern of change.
Evolution
- Evolution
- Descent with modifications.
- Evolution studies populations NOT individuals.
- Refers to the process by which species accumulate differences from their ancestors as they adapt to different environments over time.
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Tree of life describes the relationship between organisms, both living and extinct, over time
4 Mechanisms of Evolutionary Change
- Mutation
- Migration
- Genetic drift
- Natural Selection
3 Basic Requirements for Natural Selection
- Heredity
- A trait must be passed from parent to offspring through alleles
- Variation
- There must be more than one version of a trait in the population
- Differential Reproduction
- An individual possessing the trait must have more offspring than other individuals (and those offspring must be fertile)
Natural Selection vs. Selective Breeding
- Selective Breeding Artificial Selection
- Humans select for traits we want.
- Natural Selection
- Natural selection selects for traits that give a reproductive advantage in the current environment.
How Natural Selection Occurs
- Variation
- There is variation among the individuals' colors.
- Heritability
- The variation in color is inherited.
If there’s no “selection pressure” then there would be no advantage to having dark or light fur, but there IS: white sand + predators.
- Differential Reproduction
- In this case, the dark fur mice have fewer offspring because they are getting eaten.
dark color allele != poor reproduction
Thus, there are more light fur alleles in the population’s gene pool.
What changes as a result of natural selection?
Populations change, not individuals.
Descent with Modification
Speciation is caused by evolution, occurs through natural selection (and other mechanisms), which acts on heritable traits (which can be diverse because of variation), that interacts with the environment, leading to (more fit) differential reproductive success. Organisms with certain traits reproduce more successfully. Less fit individuals don't adapt to the environment as well and die off over time.
Natural selection → Adaptive evolution
What is an adaptation?
Inherited characteristic that enhances an organism’s survival and reproduction in a specific environment
Snake jaws are an adaptation that allow them to swallow large prey
What’s not an adaptation?
Vestigial eyes in blind cavefish (live in dark caves so their loss of sight has no impact on their reproductive success)
Natural selection → Adaptive evolution
Charles Darwin and Alfred Russel Wallace proposed their theories of descent by modification (evolution)
- made observations of the diversity of forms (and its heritability)
- realized that these forms often suited environmental conditions
- Inference: new species could arise from ancestral forms through gradual accumulations of adaptations
Fitness
- Fitness
- Relative and changeable: depends on the environment.
- How good individuals with a particular genotype are at leaving offspring compared to the other individuals (with different genotypes)
- Reproductive success is often the result of subtle differences- e.g. a barnacle that is more efficient at food collection can invest more energy into egg production.
- Reproductive success can be indirect:
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- A drab bird that can avoid predators is likely to live longer and leave more offspring than a brightly colored bird
Adaptations which make the population “fitter”
- Camouflage in the mantis to mimic flowers
- Fur color on mice that blends with the environment
- Moveable bones the snake jaws that allow them to eat large prey
- Beak shapes that allow finches to eat specific foods
- Shorter wings that allow swallows to avoid getting killed on roads
Natural selection enables traits that increase reproductive success to become common
- are organisms alive today at their peak reproductive ability?
- and for that matter, does this apply to all traits?
Trade-Offs
Trade-offs: situation where one trait cannot increase without a decrease in another
- Energy invested in one thing cannot be invested in another.
- Think: Money spent on rent cannot be spent on food
Bat species in which males grow larger testicles also have smaller brains
Two ways to think about trade-offs
- Resources are Limited
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Sometimes, 2 (or more) traits cannot be optimal at the same time
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eggs vs egg size
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Natural Selection could result in a Compromise when there is a Tradeoff
- More Eggs Opposing of Big Eggs
- increased fitness (more are likely to survive)
- Big Eggs Opposing of More Eggs
- increased fitness (more are likely to be healthy)
- A Trait may have 2 (or more) Functions
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The optimum characteristic for the trait may be different for the two different functions
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Natural selection could result in a compromise (medium intensity or both bright and drab flowers in the population)
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- Colored flowers (recognized by predators (bright colors might be worse) / recognized by pollinators (bright colors might be better)
Example of a trade-off: Egg size and number of eggs
- Twenty six fish families show a pattern of a trade-off between egg number and egg size (p<0.001)
- Fruit fly species show a pattern of a trade-off between egg number and egg size (p<0.001)
The Importance of Genetic Diversity
- All populations have harmful recessive alleles but this is not a big problem when there are a lot of heterozygotes.
- Harmful recessive alleles become a problem when genetic diversity is reduced- often seen in inbred population- e.g. high prevalence of hemophilia in old European monarchies.
More diversity -> more resilience to change.
Genetic Diversity is Important for Conservation and Agriculture
- Zoos and conservation groups maintain careful breeding programs to maintain genetic diversity in endangered species while increasing population size
- Farm groups also need to maintain genetically diverse breeding programs so that crops and livestock do not become overly susceptible to changes in weather, pests, diseases etc
Bison population hunted down from >60 million to only 541 individuals
- Evolution
- Exons can be shuffled between genes
- Creates new protein combinations
- "Modular" protein evolution
Examples
Artificial Selection
- Dogs
- Perfect example of artificial selection
- Dog breeds vary substantially
- All dog breeds are descended from wolves
How to make a Chihuahua-sized dog
- Start with a population of wolves that have varying sizes, shapes, and temperaments.
- Select the wolves that are smallest.
- Breed these smaller dogs with each other.
- Continue until you have a Chihuahua-sized dog.
What MUST be true?
- There must be genetic variation in size
- Size must be a heritable trait
- There must be a reproductive non-random advantage (only small dogs selected to breed)
American Cliff Swallows: A Case Study
Where has all the road kill gone?
Collected data for nearly 30 years!
The American Cliff Swallow (Petrochelidon pyrrhonota)
- Average life span: 5-8 years
- Aerial insectivores (eat insects while flying)
- Agile flyers
- Build nests on cliffs, but also increasingly under man-made structures like overpasses or bridges
- Have a behavior where they rest on the ground and then launch into flight for feeding, then return to nests
Total Swallow Road Kills Over Time
Roadkill are decreasing
Is this difference enough to cause evolution over time?
- ANY consistent difference in reproductive success associated with a trait will lead to change in the population over time
- The bigger the difference, the faster the trait will change in the population
Finches
Galapagos Finches: Different Species that have Adapted the Ability to Consume Different Foods
- Gene involved in beak shape
- Common ancestor to the grassquit
- Natural selection
- Each have different mutations that have driven adaptive evolution based on food supply
Bmp4 and Morphological Variation of Beaks in Darwin’s Finches
- Manipulated gene expression of Bmp4 to approximate large beak finches in chicken embryos; changed beak shape!