Unit 7 - Evolution
Big Ideas of the Unit
· How do the organisms of life respond to changing conditions over generational time?
· How can evolutionary changes in a population lead to the production of a new species?
o Especially if selective pressure favors two different phenotypes
· How are living things connected by lines of common descent?
Vocabulary
· Evolution (n): The change in a population over generations at a genetic level, which we can see as a change in phenotypes over time
· Adaptation (n): A trait that increases the probability of successful reproduction in a specific environment
· Adapt (v): Be careful with this word! Do you mean individual acclimatizes to conditions over its lifetime? Or do you mean a population adapts genetically (evolves an adaptation)? Be very careful with this word (avoid it if possible).
Natural Selection
· Pressures
o Civil war made humans hunt elephant tusks for ivory
o Poaching by humans
o Selective pressure among males for tusks (for survival)
· Changes in traits
o Tusks in elephants have been decreasing (specifically in females)
§ Tusks are an inheritable trait
§ Tuskless elephants have become a favorable trait in recent years
Evolution by Natural Selection
· In every generation there is variety in the traits observed in a population
o Much of this variation is heritable
o Example: tuskless females are favorable
· More individuals are born each year than will survive and reproduce
· For natural selection, the trait must be an adaptation
· Individuals with traits that increase their probability of surviving and reproducing will leave more offspring on average
o Offspring may carry the heritable, advantageous trait (example: tuskless)
· Therefore, over time advantageous traits will increase in frequency in the population from one generation to the next
· Natural selection is the mechanism for evolving adaptations
Rapid Changes to Environment
· Global warming (temperature change)
· Rising sea levels
· Ocean acidification
· Deforestation
· Droughts
· Glaciers melting
· Changes in farming practice
Understanding Evolution (18.1 of Textbook)
· Natural selection is the survival of the fittest (the only mechanism known for adaptive evolution)
· More offspring are produced than are able to survive
· Genetic diversity comes from mutations and sexual reproduction
· Mutations
o Source of new alleles and genetic variation in any population
· Adaptations help increase an organism’s “fitness” (chance of survival)
· Divergent evolution
o When two species evolve in diverse directions from a common point
· Convergent evolution
o When similar traits evolve independently in two species that do not share a recent common ancestry
· Evidence of evolution
o Fossils show the gradual evolutionary changes over time (over millions of years)
o Anatomy allows us to see the similarities between different species’ bone structures to trace evolution through change in bone structure (synonymous parts/anatomy across multiple species are called “homologous structures”)
§ Unused structures are called vestigial structures
§ When a structure and embryonic origin is completely different, they are called “analogous structures”
o Embryology allows us to trace an organism’s development to its adult form
§ There are some structures that are part of the organism while it is developing (like tail buds in humans) but are lost when the organism is born, we can trace evolution trees through this
o Biogeography is the geographic distribution of organisms on the planet
§ We can use ancient supercontinents to follow species who thrived together, but then split apart when the supercontinents broke apart. This allows us to trace evolutionary trees back to a common origin of the species before its great migrations
o Molecular biology allows us to use the molecular structures of life to reflect common descent with modification in those structures
§ DNA reflects a common ancestor for all life
§ Using membrane structures and genetic code, we can trace back different lineages of species to see how they became so diverse in present day
Formation of New Species (18.2 of Textbook)
· A hybrid is a cross between two species
o Some hybrids can end up being fertile to reproduce, some will not
· Organisms must be able to reproduce with each other to pass on new traits to offspring
· Speciation: biological definition of species
o A group of actual or potential interbreeding individuals
o For speciation to occur, two new populations must form one original population and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed
· Allopatric speciation: geographic separation of populations from a parent species and subsequent evolution
o Examples
§ The development of a branch of a river, erosion creating a new valley, a group of organisms traveling to a new location without the ability to return, or seeds floating over the ocean to an island
o Dispersal group: when a few members of a species move to a new geographical area
o Vicariance group: when a natural situation arises to physically divide organisms
o The further the distance the two species are, the more likely it is that speciation will occur
o Adaptive radiation: where many adaptations evolve from a single point of origin; thus, causing the species to radiate into several new ones
§ Island archipelagos provide an ideal context for adaptive radiation
· Sympatric speciation: speciation occurring withing a parent species remaining in one location
o Polyploidy: the unequal division of chromosomes during meiosis
o Autopolyploidy: when all the of the chromosomes move into one new cell instead of separating due to an error in meiosis
o Allopolyploid: when gametes from two different species combine (notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results)
§ Most common in plants
§ Some scientists believe that this takes place more as an adaptation than as an error
· Reproductive isolation: the ability to interbreed
o Prezygotic barrier is a mechanism that blocks reproduction from taking place
§ Includes barriers that prevent fertilization when organisms attempt reproduction
o Postzygotic barrier occurs after zygote formation
§ Includes organisms that don’t survive the embryonic stage and those that are born sterile
o Temporal isolation acts as a form of reproductive isolation (differences in breeding schedules)
o Habitat isolation is when populations of a species move to a new place that no longer overlaps with the same species’ other populations
o Behavioral isolation occurs when the presence or absence of a specific behavior prevents reproduction
o Gametic barrier occurs when differences in a species gamete cells (eggs and sperm) prevent fertilization from taking place
o Hybrid inviability is when hybrid individuals cannot form normally in the womb and simply do not survive past the embryonic stages
o Hybrid sterility is when reproduction leads to hybrid birth that is sterile
Reconnection and Speciation Rates (18.3 of Textbook)
· Reconnection
o Two species may recombine or even continue interacting indefinitely
o Hybrid zone is where two closely relates species continue to interact and reproduce, forming hybrids
§ Hybrids can either be more fit, less fit, or about at the same level of fitness as the parents (they typically tend to be less fit)
· Varying rates of speciation
o Models
§ Gradual speciation model is where species diverge gradually over time in small steps
§ Punctuated equilibrium model is where a new species undergoes changes quickly from the parent species, and then remains largely unchanged for long periods of time afterwards
Theory of Natural Selection
· Overproduction
o Every species tends to produce more individuals than can survive to maturity
· Variation
o The individuals of a population have many characteristics that differ
· Selection
o Some individuals survive longer and reproduce more than others do
· Adaptation
o The traits of those individuals that survive and reproduce will become more common in a population
Time Scaling
· Microevolution: The change in the genetic structure of a population from one generation to another
o May be reflected in a change in phenotypes
o May be driven by selection or another mechanism
· Macroevolution: Large scale change in species over paleontological time
o For example, the appearance of new species (…and genera, and families, and orders, etc.)
Natural Selection = VISTA
· Variation: The substrate for natural selection
o Occurs randomly through mutations and sexual reproduction
· Inheritance: Traits must be inheritable
· Selection: The impact of the environment on survival and reproduction for different phenotypes
o Non-random aspect of natural selection
· Time: The amount of time it takes for the population to change is a function of the strength of the selective pressure
· Adaptation: The beneficial trait becomes more common in the population
Variation and Inheritance
· Phenotypic variation comes from genes, the environment, and the interaction between them
· Natural selection occurs on aspects of phenotypic variation that have a genetic component
· Phenotypic variation reflects genetic variation
o Genetic variation is caused by different DNA sequences
Sources of Genetic Variation
· Formation of new alleles
o A mutation is a change in the nucleotide sequence of DNA (can include environmental causes)
o New nucleotide sequences can result in alternative phenotypes
o Only mutations in cells that produce gametes can be passed to offspring
o Remember: Mutations are random, natural selection is not
Two Types of Traits
· Monogenic traits
o Caused by different alleles in a single gene
o Discrete traits (option A or option B)
o Selection on a monogenic trait
· Polygenic traits
o Caused by many genes that all may have different alleles
o Continuous traits
o Selection on a polygenic trait
Sexual Selection
· The selection of specific traits that enhance mating success (specifically for males)
· There are two types of sexual selection
o Intrasexual selection: competition among males
§ Can lead to brave, strong, fierce males
o Intersexual selection: females choose males based on desired characteristics
§ Females choose the best-looking males (most attractive with ornaments, etc.)
· Intrasexual selection
o Males compete for access to females or prevent other males from mating
o Even whose sperm gets to fertilize eggs can be an arena for competition
§ For example, male damselflies scrub rival sperm out of the female reproductive tract when mating
· Intersexual selection
o Females choose which males to mate with
o Females can even choose whose sperm will fertilize her eggs
§ Some females can eject sperm from an undesirable mate
Chi-Square Analysis
· For a chi-square analysis, you must be able to divide your population into discrete categories
· We will compare the data we collect to the expected data (what you do vs. what you expect)
· Null hypothesis (H0)
o There is no difference between the two different groupings, any difference is due to chance alone
· Alternate hypothesis (HA)
o There is a difference between the two groupings that is being caused by something
· Rejecting or failing to reject the null hypothesis
o In statistics, the tests are designed to allow us to reject the null hypothesis (and by implication, support the alternative hypothesis) or fail to reject the null hypothesis
o “Rejecting the null hypothesis” means that it is statistically unlikely to see the amount of difference that you see by random chance alone
o “Failing to reject the null hypothesis” means the difference you see in your results, compared to the expected results, is likely due to chance alone
· Statistical significance
o When the difference between observed and expected is unlikely to occur by chance alone we say that there is a “statistically significant” difference between the observed and expected frequencies in the population
o By convention, if the difference would only occur by random chance 5% (p-value = 0.05) of the time or less, than we reject the null hypothesis, and the difference between our observed and expected values is said to be statistically significant
· P-values
o The p-value is statistics describes how likely you are to see an event based on random chance alone
§ P-value = 0.05 means that I would see that event by random chance by 5% of the time. 95% of the time your results are being caused by some outside factor
o If the p-value is less than 0.05, by convention we reject the null hypothesis. There is a less than 5% chance that random chance caused your results
o Anytime on the AP test, if the chi-square value calculated is less than the critical value (at 0.05), we fail to reject the null hypothesis (not a statistical significant difference)
o Anytime on the AP test, if the chi-square value calculated is greater than the critical value (at 0.05), we reject the null hypothesis (a statistical significant difference)
· Reporting your data
o The chi-square value that you calculated
o The degrees of freedom (d.f.)
o The comparison to the critical value (this is not always necessary)
o The p-value (is it greater or less than 0.05?)
o An interpretation of the p-value
· Limitations of chi-square test
o This test only works in categorical data. If you measure something or take an average, this isn’t the right test for you
o Your observed data must be counting data (whole numbers)
o The total number of individuals in the observed and expected categories must be the same
o The smallest number of expected observations for any category should be at least 5 (ideally the expected observations should be greater than 30)
o Larger samples mean more work, but also more power for the test
· Using the right language
o The data may or may not show that there is a “statistically significant difference,” but all data is “significant” or meaningful even if it does not show the difference that you wanted to see
o When the p-value < 0.05, we:
§ “Reject the null hypothesis”
§ “Support the alternate hypothesis”
o We do not:
§ “Prove the alternate hypothesis.” Unlikely events happen every day
o We also never:
§ “Accept the null hypothesis.” We just fail to reject it
o Statistics provide evidence or support, they do not provide proof
· Chi-square table
o
o d.f. = # categories - 1
Genetic Drift
· Definition: random fluctuations in the frequency of alleles in a population due to random events
· Random, not selective, not adaptive
· Founder effect: New population is founded by a small group previously part of a larger population
o This new group may not carry with them the same distribution of alleles found in the larger population
· Bottleneck effect: The occurrence of a catastrophic event that results in a major decline in population size
o Remaining individuals do not display the same distribution of alleles found in the larger population
Graph Examples
