Natural Selection

Natural Selection All of the members of a species make up its total population. In order for a species to survive and not become extinct, members of its population must successfully reproduce new offspring. Photo by Salimfadhley

Natural Selection There is always a limited amount of resources in any ecosystem. Eventually a population reaches its carrying capacity which means there are not enough resources for the population to grow any larger and have all of its members survive. Water is a limited abiotic resource in desert ecosystems.

Natural Selection However, populations often produce more offspring than will be able to survive based on the amount of resources. When this occurs, competition for resources causes organisms with the best adaptations to survive. Photo by Salimfadhley

Phenotypes are the physical Natural Selection Within a population, organisms have different versions of genes called alleles. These different alleles cause organisms to have different phenotypes. Phenotypes are the physical expression of genes Photo from Kettlewell, 1959

Natural Selection If an allele causes an organism to be more adapted to its environment, it is more likely to survive longer and reproduce more offspring. Photo from Kettlewell, 1959

Natural Selection When these organisms with better adaptations reproduce, they pass the better genes to their offspring. These alleles make the offspring more likely to survive and reproduce.

Natural Selection Over time, the frequency of these alleles in the population increases. This means that more and more members of the future generations have these well adapted alleles.

Natural Selection The process of organisms with the best adaptations surviving to pass their genes to their offspring is known as natural selection. Photo by John O’Neill Faster rabbits can avoid predators and survive to reproduce. Falcons with better vision can see and capture more prey. Storing fat in concentrated humps prevents overheating and helps camels survive.

Natural Selection Populations may migrate into new territories with different resources. They can also become isolated by the formation of canyons or mountains or because of human expansion.

Natural Selection Changes in the environment can also drive natural selection. During periods of drought or climate change, previous adaptations may no longer help populations survive.

Natural Selection When this occurs, the population is forced to adapt to these new environmental conditions. If the members of a species cannot adapt, then the species will most likely become extinct.

Natural Selection The greater the diversity within a species, the more likely it is that it will be able to adapt to new conditions. However, new alleles do not appear because they would be useful. New alleles are only created through random mutations. Image by Debivort

Natural Selection sperm egg Only mutations that occur in gamete cells are passed on to the organism’s offspring.

Mutated gene results in brown allele Natural Selection If an allele is beneficial allowing the organism to survive and reproduce, then the frequency of the allele will increase in the population. This natural selection of beneficial traits will help the population become more adapted to its environment. Random Mutation Mutated gene results in brown allele Brown insects are more likely to survive, so the frequency of this allele increases in the population.

Natural selection changes populations, The allele frequency will keep increasing in future generations until most of the population has the allele and the new adaptation. Natural selection changes populations, not individuals!