Answer;
Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes. In frequency-dependant selection, the fitness of a phenotype declines it it becomes too common in the population.
Explanation;
The heterozygotes have both alleles, which is needed for genetic variation. Heterozygotes are essential for there to be genetic variation in a population. In other words it is when When a single copy of a disease allele doesn't result in a disease but instead is good for the person or organism that carries it, we say that allele has a heterozygote advantage. For example a sickle cell trait, which protects against malaria in heterozygotes, but causes a deadly disease in homozygotes.
-Frequency-dependent selection occurs when the fitness of a genotype depends on its frequency. It is possible for the fitness of a genotype to increase (positively frequency-dependent) or decrease (negatively frequency-dependent) as the genotype frequency in the population increases.
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Because as we evolve .....Some adaptation occurs according to the environment.....so the parts which are not right according to the environment and evolution become vestigial !!!
Answer:
Diffusion is a passive process involving the movement of molecules from a region of a higher concentration to one of a lower concentration (the term passive means that the process does not require an input of energy to take place).
Diffusion can occur across partialy permeable membranes, such as those surrounding cells. Therefore, diffusion is involved in the movement of important molecules into and out of cells. It is important for the uptake of substances needed by cells, and also the removal of waste products produced by the cells.
In animals:
Respiration - Oxygen and glucose react to form carbon dioxide and water along with ATP (a source of energy) in the process of aerobic respiration. Therefore, oxygen and glucose must be taken up by the cell, and typically the concentration of these molecules outside the cell is greater than inside. Therefore, the overall net movement of these molecules will be down the concentration gradient, and they will move into the cell via diffusion. Similarly, the carbon dioxide produced is a waste product and moves out of the cell, again via diffusion down its concentration gradient.
In Plants:
Mineral uptake - Useful minerals and ions need to be taken up from soil into plants via root hair cells. These cells are adapted (through a large surface area and large number) to maximise the rate of diffusion. Therefore, the useful molecules in the soil move down a concentration gradient and into the roots to be taken up by the plant. Many molecules found in the soil are essential for the growth and survival of plants, making diffusion a very important process.
The temperate forest biome covers latitudes ranging approximately from the southern United States to southern Canada, while the taiga biome, also known as boreal forest, extends from the latitude of southern Canada to about 60 degrees north latitude. (see References 1, References 3) Thus, these two biomes are adjacent, which explains the many similarities between taiga and northern temperate forests. Both biomes have four distinct seasons, but the temperate forest climates cover a much wider range of temperatures and precipitation patterns. Taiga, in contrast, is reliably cold: most of the precipitation falls as snow, winters are severe and the growing season is short -- about 130 days compared to 140 to 200 days for temperate forests.