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
The Malaysian orchid mantis (Hymenopus coronatus)
closely resembles the flower on which it rests, waiting for unwary prey to come within it’s reach orchid mantis Evolution: Descent with modifications Evolution refers to the process by which species accumulate differences from their ancestors as they adapt to different environments over time Evolution studies populations individuals NOT
Tree of life describes the relationship between organisms, both
living and extinct, over time University of California Museum of Paleontology’s Understanding Evolution
Evolution
- PATTERN
- Is revealed by scientific data showing that life has evolved over time
- PROCESS
- Consists of MECHANISMS that cause the pattern of change
4 Mechanisms of Evolutionary Change
- Mutation
- Migration
- Genetic drift
- Natural Selection
descent with modification ancestor is caused by evolution occurs through natural selection* current species *and other mechanisms
Let’s first consider how it’s possible to get diversity of forms through artificial selection
Dogs: perfect example of artificial selection Dog breeds vary substantially 7 All dog breeds are descended from wolves 8 ICA Q1: How to make a chihuahua-sized dog? start with wolves end with chihuahuas
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)
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)
ICA Q2: Which of the following scenarios would NOT result in evolution by natural selection? (Select all that apply) A. A population of butterflies with variation in wing color, where wing color is determined entirely by temperature during development B. A population of bacteria where some individuals have a mutation for antibiotic resistance C. A population of deer where all individuals have identical running speeds D. A population of flowers where red flowers attract more pollinators than white flowers, and color is genetically determined"
Natural selection vs. selective breeding
Humans select for traits we want Natural selection selects for traits that give a reproductive advantage in the current environment
How natural selection occurs
if there’s no “selection pressure” then there would be no advantage to having dark or light fur but there IS: white sand + predators Di re Heritability Differential Reproduction in this case, the dark fur mice have fewer offspring because they are getting eaten dark allele poor reproduction 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 ancestor variation (organisms with certain traits reproduce more successfully) is caused by evolution occurs through natural selection* current species *and other mechanisms can be diverse because of acts on heritable traits more fit, leading to interact with less fit environment extinction? (at least they don’t adapt as well to this environment) differential reproductive success ICA Q3 a. What trait is advantageous? A Generation 1 X X X X D Reproduction and undirected mutation E Generation 3 Non random survival Non random survival F Many generations of mutation and natural selection X XX B Reproduction and undirected mutation Generation 2 Generation N b. Is the pattern of survival and reproduction random? c. C is more diverse than B. How? C G d. Why might G go extinct if conditions change?
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
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 3 SEPTEMBER 2004 VOL 305 SCIENCE
Bmp4 and Morphological Variation of Beaks in Darwin’s Finches
Arhat Abzhanov,1 Meredith Protas,1 B. Rosemary Grant,2 Peter R. Grant,2 Clifford J. Tabin1*
- manipulated gene expression of Bmp4 to approximate large beak finches
- chicken embryos
- changed beak shape!
American cliff swallows: a case study
Where has all the road kill gone?
Collected data for nearly 30 years! Brown and Brown, Current Biology. 23: R233-234
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
ICA Q4: What could be happening? Come up with 2 potential models/hypotheses.
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
Q8. Draw a prediction of wing length of live birds over time if wing length is under natural selection
If wing length is under natural selection, birds that live will have shorter wings over time
Wing length of cliff swallows over time
GRAPH
Q9: Did the data presented by Brown & Brown address all the requirements for natural selection to occur?
- Variation in trait
- Heritability of trait
- Differential reproductive success
Determine which results support these requirements. Make sure to refer to specific data that establishes that this requirement is met. If a requirement is not sufficiently supported by the data you have seen, describe what additional data you want to see to the convinced that the requirement has been met.
Q10. Is it still reasonable to conclude that wing length is under natural selection?
Fitness
- Fitness= 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- A drab bird that can avoid predators is likely to live longer and leave more offspring than a brightly colored bird
- Fitness is relative and changeable: depends on the environment
We’ve seen many different examples of traits under natural selection that result in 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
- sometimes, 2 (or more) traits cannot be optimal at the same time
-
eggs vs egg size
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
- The optimum characteristic for the trait may be different for the two different functions
- Natural selection could result in a compromise (medium intensity or both bright and drab flowers in the population)
-
- 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) Eglar (1990)
- Fruit fly species show a pattern of a trade-off between egg number and egg size (p<0.001) Berrigan (1991)
ICA Q11: Consider the trade-off between egg number and egg size shown in the data.
- Explain why a fish that produces medium-sized eggs (instead of very large or very small eggs) might have the highest fitness.
- If food became scarce for baby fish after hatching, which would be more advantageous: larger eggs or smaller eggs? Explain your reasoning.
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