Chapter 3: Natural Selection, Speciation, and Extinction Flashcards
Four different patterns of natural selection?
Directional, stabilizing, balancing, and disruptive
Directional Selection? How can it occur? Example?
Directional Selection: favors individuals at one extreme of a phenotypic distribution that have greater reproductive success in a particular environment.
Occur? There is a rise in a new allele that may be introduced through mutation, and be beneficial for the population increasing their fitness
the graph quite literally moves from the left to the right
Example? Galapagos Island - medium ground finch, and other type of beak finch show that beah depth is a heritable trait, so the population desired trait allow for a shift or direction
Stabilizing Selection? Example? What did David Lack suggest?
Favors the survival of individuals with intermediate phenotypes. The extreme traits are favored against.
ex: Bird lay a preferred amount of eggs
Lack- those who lay too many or too few eggs per next have lower fitness values than do those that lay an intermediate number of eggs. (why? bc laying too many eggs is disadvantageous bc many chicks will die to an inadequate supply of food. survival of parents may also reduce bc they will have/need a large amount of food to feed their children, putting them at risk; while, having too few offsprings results in the contribution of a small amount of individuals that are need for the next generation)
Balancing Selection? What are the two pathways for this?
type of natural selection that maintains genetic diversity in a population
in a balancing selection, two or more alleles are kept in balance and maintained throughout the course of many generations (one allele is not favored)
Two pathways
1) heterozygote advantage
2) frequency-dependent selection
Example of Heterozygote advantage?
Ex: sickle-cell allele of the human B-globin gene. A homozygous individual with two copies of this allele has sickle-cell disease, a hereditary disease that damages blood cells
sickle cell homozygotes= lower fitness than a homozygote with two copies of the normal or more common B-globin gene. HOWEVER, in areas where Malaria is present (endemic), the heterozygote increases the chance of survival (10%-15%) when infect by Plasmodium falciparum. It is for that reason that the sickle-cell allele is maintained in populations where Malaria is prevalent (even though the allele is detrimental in the homozygous state)
Frequency-dependent selection? Negative frequency-dependent.. Example? Positive frequency dependent selection?
Another mechanism that causes balancing selection–> describes when the fitness of one phenotype is dependent on its frequency relative to other phenotypes in the population
Negative frequency-dependent: rare phenotypes are favored over common phenotypes
Ex: invertebrates have different colors and are thus more identifiable for the predator. This is bad.
Positive frequency dependent: common phenotypes have an advantage
Ex: prey are warningly colored to advertise bad taste or toxicity, the prey gain the most benefit where many individuals have warning coloration and the predator is deterred.
Disruptive selection? Example?
Favors the survival of individuals at both extremes of a range, rather than an intermediate. (favors the survival of two phenotypes)
ex: the normal wild-type colonial bentgrass, agrostis tenuis, is intolerant to metals in the soil. A mutation creates a metal-tolerant variety, which can grow in soils contaminated with metals from mining operations.
Speciation? What can it be caused by?
Occurs where genetically distinct groups separate into species; can be caused by disruptive selection
Speciation concepts? How many?
4 speciation concept:
1) biological: species are separate if they are unable to interbreed and produce fertile offspring
2) phylogenetic: differences in physical characteristics (morphology) or molecular characteristics are used to distinguish species
3) evolutionary: phylogenetic trees and analyses of ancestry serve to differentiate species
4) ecological: species separate based on their use of different ecological niches and their presence in different habitats and environments
Biological species? Example? Disadvantages?
Biological species: groups of populations that can actually or potentially exchange genes with one another and that are reproductively isolated from other such groups
–> has allowed to distinguish northern leopard frog (rana pipiens) and the southern leopard frod, (R. Utricularia)
disadvantages: 1) many species with widely separate ranges, we have no idea if the reproductive isolation is by distance only or whether there is some species-isolating mechanism
2) plants, individuals called hybrids, often form when parents from two diff species are crossed with each other (blurs species distinction) Example: oaks form reproductively viable hybrid populations that produce with oaks… why consider them different if they don’t see a difference
3) can not be applied to asecually reproducing species, such as bacteria and some plant/fungi, or extinct fungi
Phylogenetic species concept? disadvantage?
-based on physical (morphological) characteristics and molecular features such as DNA
disadvantage: trying to figure out how much of a difference is needed to consider something a new species
Example: coluber constrictor- they’re different colored snakes, so they have subspecies
Peromyscus gossypinus, exists as six formal subspecies in the southeastern U.S
Evolutionary species concept? What are some problems associated with this concept?
Proposed by George Gaylord Simpson whereby species is distinct from other lineages if it has its own evolutionary tendencies and historical fate
Considered the best way to categorize species, but its also the hardest thing to do (examining lineages are difficult to trace); there are incomplete fossil records and lack of transitional forms making lineage difficult to trace
- molecular scientists compare DNA sequences of particular organisms
Ecological species concept? Useful? Why and how?
Proposed by American biologist Leigh Van Valen, that each species occupies a distinct ecological niche, a unique set of habitat requirements. This will reduce competition between species
Useful to explain and distinguish asexually reproducing and morphologically similar species such as bacteria
What are the two processes that have been proposed to explain the process of speciation? Definition and example.
1) allopatric speciation: involves spatial separation of populations by a geographical barrier
ex: nonswimming populations separated by a river may gadully diverge because there is no gene flow between them OR aquatic animals were separated by land may go through allopatric speciation (habitat fragmentation could promote this, but less likely to do since population would be too small and it would just die off)
2) sympatric speciation (alternative to allopatric speciation) when members of a species that initially occupied the same habitat within the same range diverge into two or more different species
ex: fish: areas where fish may have been isolated (bodies of water) but somehow now there’ a bunch of different species in the same lake
Continental drift
the slow movement of the Earth’s surface plates, helped understand why some organisms were similar or in the same place in different areas of the world
