13.7, 13.12, 13.13 Flashcards
Note 1 —-»
An unsettling example of natural selection in action is the evolution of pesticide resistance in hundreds of insect species. Pesticides control insects and prevent them from eating crops or transmitting diseases. The few survivors of the first pesticide wave are individuals that are genetically resistant, carrying an allele (alternative form of a gene, colored red in the figure) that somehow enables them to survive the chemical attack. So the poison kills most members of the population, leaving the resistant survivors to reproduce and pass the alleles for pesticide resistance to their offspring. The proportion of pesticide-resistant individuals thus increases in each generation.
In what sense is natural selection more an editing process than a creative process?
Natural selection cannot create beneficial traits on demand but instead “edits” variation in a population by selecting for individuals with those traits that are best suited to the current environment.
Note 2 —-»
The condition for Hardy-Weinberg equilibrium that there be no natural selection—that all individuals in a population be equal in ability to reproduce—is probably never met in nature. Populations consist of varied individuals, and some variants leave more offspring than others. In our imaginary iguana population, individuals with webbed feet (genotype ww) might survive better and produce more offspring because they are more efficient at swimming and catching food than individuals that lack webbed feet. Genetic equilibrium would be disturbed as the frequency of the w allele increased in the gene pool from one generation to the next.
Genetic Drift
A change in the gene pool of a population due to chance.
Note 3 —-»
The smaller the population, the more impact genetic drift is likely to have. In fact, an allele can be lost from a small population by such chance fluctuations. Two situations in which genetic drift can have a significant impact on a population are those that produce the bottleneck effect and the founder effect.
Bottleneck Effect
Genetic drift resulting from a drastic reduction in population size.
Founder Effect
Genetic Drift that occurs when a few individuals become isolated from a new population, whose gene pool is not reflective of that of the original population.
Gene Flow
The transfer of alleles from one population to another as a result of the movement of individuals or their gametes.
How might gene flow between populations living in different habitats actually interfere with each population’s adaptation to its local environment?
The introduction of alleles that may not be beneficial in a particular habitat prevents the population living there from becoming fully adapted to its local conditions.
Note 4 —-»
Genetic drift, gene flow, and even mutation can cause microevolution. But only by chance could these events result in improving a population’s fit to its environment. In natural selection, on the other hand, only the events that produce genetic variation (mutation and sexual reproduction) are random. The process of natural selection, in which better-adapted individuals are more likely to survive and reproduce, is not random. Consequently, only natural selection consistently leads to adaptive evolution—evolution that results in a better fit between organisms and their environment.
Note 5 —-»
By consistently favoring some alleles over others, natural selection improves the match between organisms and their environment. However, the environment may change over time. As a result, what constitutes a “good match” between an organism and its environment is a moving target, making adaptive evolution a continuous, dynamic process.
Relative Fitness
The contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals in the population.
Explain how the phrase “survival of the fittest” differs from the biological definition of relative fitness.
Survival alone does not guarantee reproductive success. An organism’s relative fitness is determined by its number of fertile offspring and thus its relative contribution to the gene pool of the next generation.
