Natural Selection Flashcards
4.2.4 Recognise natural selection occurs when the pressures of environmnetal selection confer a selective advantage on a specific phenotype to enhance its survival (viability) and reproduction (fecundity).
What are the two concepts of the theory of evolution?
Mutations cause variability in the phenotypes of a population.
Natural selection acts on the phenotypic variability of the population.
4.2.4 Recognise natural selection occurs when the pressures of environmnetal selection confer a selective advantage on a specific phenotype to enhance its survival (viability) and reproduction (fecundity).
What two factors does the process of natural selection rely on?
Viability- the more viable an organism, the greater chance there is that it can survive and reproduce
Fecundity- organisms that are ‘fit’ for an environment can produce a greater number of viable offspring (enhanced reproduction)
Therefore natural selection requires:
- Varitaion (due to genetic differences between individuals in a population)
- Selective pressure (for specific traits that cinfers an advantage to the organism)
4.2.4 Recognise natural selection occurs when the pressures of environmnetal selection confer a selective advantage on a specific phenotype to enhance its survival (viability) and reproduction (fecundity).
Summarise the factors that influence allele frequency in gene pool.
- Variation-genetic differences between individuals in a population
- Viability-chances of survival of offspring dependent on environmental factors
- Reproduction - organisms reproduce and pass on alleles to next generations (offspring are similar - sexual OR genetially identical - asexual)
- Fecundity- the rate of repoduction and number of offspring produced influences selection
- Environmental selection pressures- some phenotypes are better suited to particular environmnetal conditions and give the individual a survival advantage
4.2.4 Recognise natural selection occurs when the pressures of environmnetal selection confer a selective advantage on a specific phenotype to enhance its survival (viability) and reproduction (fecundity).
Explain and provide examples of environmental selection pressures.
Natural selection is the influence of environmental pressures on the allele frequency in a population. They affect the survival and reproduction of an organism by removing individuals through death or reducing reproductive rates.
Examples of environmental selection pressures include:
Climate conditions (eg. drough, temperature changes)
Competition for resources (eg. food, water)
Competition for shelter (from extreme conditions)
Mate availability (when mates are rare, reproduction rates drop, doesn’t affect asexual reproduction)
Predator abundance (more predators is equal to increased survival pressures)
- 2.5 Idenitify that the selection of allele frequency in a gene pool can be positive or negative
- 2.6. Interpret data and describe the three main typesof phenotypic selection: stabilising, directional and disruptive
What does it mean if a population is not evolving.
If there is no change in allele frequency of the gene pool of a population over time then the population is NOT evolving. This would happen if there was:
No mutation. No new alleles are generated by mutation, nor are genes duplicated or deleted.
Random mating. Organisms mate randomly with each other, with no preference for particular genotypes.
No gene flow. Neither individuals nor their gametes (e.g., windborne pollen) enter or exit the population.
Very large population size. The population should be effectively infinite in size.
No natural selection. All alleles confer equal fitness (make organisms equally likely to survive and reproduce)
- 2.5 Idenitify that the selection of allele frequency in a gene pool can be positive or negative
- 2.6. Interpret data and describe the three main typesof phenotypic selection: stabilising, directional and disruptive
Recall the five mechanisms of evolution.
Evolution is the cumulative change in the heritable characteristics of a population across successive generations. This requires that allele frequencies change within the gene pool (the combination of all the genes (including alleles) present in a reproducing population or species) of the population to reflect these evolving characteristics. Therefore for evolution — small population, non-random mating, mutations, gene flow, adaptation — that impact evolution (ie. the changes in the gene pool of a population from generation to generation) are required.
- 2.5 Idenitify that the selection of allele frequency in a gene pool can be positive or negative
- 2.6. Interpret data and describe the three main typesof phenotypic selection: stabilising, directional and disruptive
Explain natural selection.
Natural selection is the process through which populations of living organisms adapt and change. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others.
4.1.5. Idenitify that the selection of allele frequency in a gene pool can be positive or negative.
If there is no change in the allele frequency in the gene pool of a population, then the population is not evolving. However, if there are changes in the frequency of alleles, then it can be said that the population is evolving. If an allele’s frequency increases, selection is positive. If an allele’s frequency decreases, selection is negative.
4.2.6. Interpret data and describe the three main types of phenotypic selection: stabilising, directional and disruptive
Describe the three types of phenotypic selection.
Natural selection can act on populations of organisms and affect the distribution of phenotypes in three different ways:
Stabilising selection-In phenotype based stabilizing selection, the mean value of a phenotype is selected for, resulting a decrease in the phenotypic variation found in a population. This creates a normal distrubition or a bell curve for the pehnotypic possibilities.
Directional selection- occurs when individuals with traits on one side of the mean in their population survive better or reproduce more than those on the other. It causes the highest phenotype from the originial population to shift either left or right after one is selected against by the environment.
Disruptive selection- a mode of natural selection in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups thus generating a binomial distribution.
4.2.5 Explain microevolutionary change through the main processes of mutations, gene flow and genetic drift.
Explain gene flow.
Gene flow is the movemnet of alleles from one population to another. Such movement may be due to migration of individual organisms that reproduce in their new populations, or to the movement of gametes (e.g., as a consequence of pollen transfer among plants). It increases the variation and keeps differing populations similar.
4.2.5 Explain microevolutionary change through the main processes of mutations, gene flow and genetic drift.
Explain genetic drift.
Genetic drift is a mechanism of evolution in which allele frequencies of a population change over generations due to chance (rather than being selected due to selection pressures). It occurs in all populations, but its effects are strongest in small populations. It may result in the loss of some alleles (including beneficial ones) and the fixation, or rise to 100% frequency, of other alleles. Therefore genetic drift can have major effects when a population is sharply reduced in size by a natural disater (bottleneck effect) or when a small group splits from the main opopulation to find a colony (founder effect). The founder effect is an extreme example of genetic drift, one that occurs when a small group breaks off from a larger population to establish a colony. The new colony is isolated, and the founding individuals may not have the same allele frequency as the original population.Gene flow is the movemnet of alleles from one population to another. It increases the variation and keeps differing populations similar.
4.2.5 Explain microevolutionary change through the main processes of mutations, gene flow and genetic drift.
Microevolution is defined as changes in the frequency of a gene in a population. These are subtle changes that can occur in very short periods of time, and may not be visible to a casual observer. Gene Flow (Migration):The transfer of alleles that results from emgration and immigration of individuals between populations. Mutation: when an advantageous mutation spontaneously arises in an organism, this mutated gene can increase in frequency over generations if it conveys an advantage over those who do not have it. If a neutral mutation (one that is neither beneficial nor harmful) arises in a population, it can increase in a population by genetic drift. While recombination during meiosis can shuffle genes into new combinations, mutation is the only source of new genes. Genetic Drift (Bottlenecks, Founder Effects): In a small population, gene frequencies can change rapidly due to random events. For example, in a population of only 10 individuals, each one carries 10% of the genes. In a population of 100 individuals, each one carries only 1% of the genes. In a population of 1000 individuals, each one carries only 0.1% of the genes. If an accident happens to one individual in each of these populations, it is more likely to significantly change gene frequencies in the small population.
4.2.5 Explain microevolutionary change through the main processes of mutations, gene flow and genetic drift.
Provide an example of microevolution.
Microevolution is a result of either positive or negative selection. For example, a mutation in the beta-goblin gene produces an allele Hb5. People with genotype HbSHbS have sickle cell anaemia. This causes the red blood cells to have a sickle shape ehich reduces the cell’s ability to carry oxygen around the body. Those individuals affected often do not survive or pass on the alleles (negative selection).
4.2.6. Interpret data and describe the three main types of phenotypic selection: stabilising, directional and disruptive
Provide examples on the three types of selections.
Stabilising selection: Classic examples of traits that resulted from stabilizing selection include human birth weight, number of offspring, camouflage coat color, and cactus spine density
Directional selection: An example of directional selection is fossil records that show that the size of the black bears in Europe decreased during interglacial periods of the ice ages, but increased during each glacial period. Another example is the beak size in a population of finches.
Disruptive selection: Light-colored oysters would blend into the rocks in the shallows, and the darkest would blend better into the shadows. The ones in the intermediate range would show up against either backdrop, offering those oysters no advantage and make them easier prey.
