Module 2 Flashcards
(13 cards)
Types of natural selection?
The three main types of natural selection are disruptive, stabilizing, and directional.
Natural selection consistently increases the frequencies of alleles that provide a fertility advantage and leads to adaptive evolution. The relative fitness and certain traits in each environment increase fitness, and selection act more directly on the phenotype and genotype.
Regardless of selection mode, selection favors the trait that provides the highest fertility compared to other traits. A new trait can appear that is more appropriate to the food – directional, predators – stabilizing, or if the medium phenotype dies – disruptive.
Directional?
Conditions favors one extreme on the phenotypic range and shifts the populations frequency curve for the phenotypic character in one direction or the other. Maybe the environmental changes or members of a population migrate to a new habitat. One extreme of a trait to be selected over the other extreme. This causes the distribution pattern of the trait to shift in the direction favored by natural selection. Notice how the height of the distribution and the width do not change because of directional selection. no effect on the amount of genetic variation
Disruptive?
Favors both extremes of the phenotypic range over individuals with intermediate phenotypes. Lower fitness for the intermediate phenotypic trait. The disruptive selection will cause organism with intermediate traits to reproduce less and will allow those organisms with extreme traits to reproduce more. This causes the alleles for the extreme traits to increase in frequency. Increases the genetic variation.
Stabilizing?
Favors intermediate variants. Leads to lower variation and tends to maintain the status quo for a phenotypic character. Stabilizing selection results in a decrease of a population ‘s genetic variance when natural selection favors an average phenotype and selects against extreme variations. Reduces genetic variation.
Frequency-dependent?
The fitness of a phenotype depends on how common or rare it is compared to other phenotypes in a population. In general, the more familiar, the higher fitness. Usually, resulting in a balance in the abundance of alternative behaviors or characteristics. An example may favor non-poisonous butterflies that have the same pattern as poisonous butterflies. It can be negative or positive and is usually caused by interactions with other species - predation, parasitism, competition, or within-species – competitive or symbiotic.
Describe sexual selection. How do they work?
Individuals with a certain trait are more likely to obtain mates that results in sexual dimorphism – a difference in secondary sexual characteristics between males and females in the same species. Size, color, and behavior.
Intrasexual selection: Within the same sex compete for the opposite sex.
Intersexual selection: Mate choice – as females are choosy in the choice of partner. Based on the showiness of the male’s appearance and behavior.
Traits correlate to good genes (hypothesis) and traits indicative of a male’s overall genetic quality or overall male health. Alleles can be indicative of a robust immune system. The balance between fitness cost and gains – predation or sexy characteristics, trade-offs. The sexy son’s hypothesis, suggesting that the traits will be more favorable in the next generation.
Sexual selection often results in the development of secondary sexual characteristics, which help to maximize a species ‘reproductive success, but do not provide any survival benefits.
The handicap principle states that only the best males survive the risks from traits that may be detrimental to a species; therefore, they are more fit as mating partners.
In the good gene’s hypothesis, females will choose males that show off impressive traits to ensure they pass on genetic superiority to their offspring
Sexual selection is a special kind of natural selection in which mating preferences influence the traits of the organism. Sometimes natural selection and sexual selection are in direct opposition to one another. For example, populations of wild guppies that live in streams in Trinidad in the Caribbean have different color patterns depending on where they live. Male guppies living in streams that have a crayfish predator are drab green in color, but males living in streams that lack the crayfish predator have bright red tails. Apparently, females prefer to mate with males with red tails, but in streams with a predator (the crayfish) that has good color vision, not enough red-tailed males survive to reproduce. The contradiction is probably in the affects to the survival, and a lower viability will decrease fitness and thus natural selection will not choose the fittest organism, with a disadvantageous trait. The overall effect though, is that natural selection will still act on the population, and favoring those individuals remaining in the environment.
Why and how selection decreases variation in a population?
Selection tends to decrease the genetic variation among individuals in a population. However, in most populations, the genetic variation remains high for most phenotypic characteristic – How?
Mechanisms that preserve genotypic or phenotypic variation – introducing: mutations and recombination’s – delaying: flat fitness profile – conserving: heterozygote advantage or frequency-dependent selection. Selection leads to individuals becoming better adapted on average – as high fitness spreads genes at a high rate, the next generation will be better adapted on average. A higher fitness of individuals of a population to the local environment because of selection. Natural selection ó Adaption. Natural selection leads to slowly but continuously a higher degree of adaptation in all organisms.
Adaptations can arise gradually over time as natural selection increases the frequency of alleles with higher fitness, and species become more suited to the environment. – adaptive evolution. If environment changes, a new adaptive evolution continuous – dynamic process, and different alleles will be favored in different environments. That leads to populations of species that differ genetically.
The fundamental theorem of natural selection?
“The rate of increase in the average fitness of a population is equal to the genetic variance of fitness of that population.”
If fitness increases, it correspond to the genetic variance, and the same if fitness decreases. The view can be a linear relationship in a graph.
Natural selection increases the average fitness of a population at a rate equal to the genetic variance in fitness. Thus, the average fitness of a population is a nondecreasing quantity. An exact, realistic models show that average fitness does not always increase. For example, showed that average fitness can decrease when selection acts on two linked loci that have epistatic effects on fitness, no linkage disequilibrium and no frequency dependence, among a variety of other conditions about mating and nonadditive genetic interactions.
Why evolution leads to increasing adaptation?
Alternative to adaptive explanation: Time-lag – trait today, resulting from selection and evolution yesterday, more time before trait is high.
Constraints – traits cannot develop because the genetic is lacking.
Trade-offs – change in trait values of a trait, which can affect other characteristics negatively.
Low selection value – to change a trait takes time since it is not common in a population.
Which characteristics are adaptive? The mistakes are usually to observe, assume adaptation, and thus, a logical explanation. An adaptive hypothesis can be tested by experiment, observations, and comparative studies
Artificial selection?
Radical transformation in few genes. A selection of the preferred genes instead of natural selection by man, that led towards the understanding of natural selection.
Fitness?
Fitness: The fertility and viability of an individual, natural selection acts on the phenotypic traits in a population with the higher fitness.
Phenotypic distribution?
How a trait or characteristic is distributed among individuals in a population, as seen in disruptive, directional, and stabilizing.
Heritability?
Is a statistic used in the fields of breeding and genetics that estimates the degree of variation in a phenotypic trait in a population that is due to genetic variation
between individuals in that population. Mendels and Darwin – Modern synthesis.
