Natural Selection and Adaptation Original Population Final Population

Natural Selection The process of organisms with the best adaptations surviving to reproduce and pass on their genes is known as natural selection.

Natural Selection = Survival of the Fittest Natural selection occurs because environmental resources, both biotic and abiotic, are limited. Organisms that can acquire these resources the best are most likely to survive and reproduce. Natural Selection = Survival of the Fittest

Well camouflaged insects avoid being eaten by predators. Natural Selection When organisms have traits that help them acquire resources and reproduce, they are well adapted to the environment. An adaptation is a trait that helps an organism survive and reproduce. Photo by P. Kuczynski Photo by Theuerkauf Long, sharp fangs allow the tiger to catch and kill large prey more easily. Well camouflaged insects avoid being eaten by predators. The shape of this bird’s beak allows it to crack open seeds more easily.

Mutated gene results in brown allele Natural Selection Different versions of a gene, called alleles, can make organisms more adapted to the environment. However, new alleles are created randomly through mutations. 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.

Mutated gene results in brown allele Natural Selection Random Mutation sperm Gamete Cells egg Mutated gene results in brown allele Remember, only mutations that occur in gamete cells are passed on to the organism’s offspring.

Natural Selection If a new allele helps an organism to survive longer and reproduce more offspring, the frequency of the allele will increase in the population. This means that more organisms will have the new, better allele than before.

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!

Natural Selection Changes in the climate or migrating to a new environment can direct natural selection. Species must adapt to the new conditions, and if they cannot, they may become extinct.

Peppered Moths The peppered moth (Biston betularia) is an insect that lives in Britain and Ireland. A mutation caused some of the moths to be completely black instead of the peppered version you see on the left. Moth Photos by Olaf Leillinger

Camouflaged Pepper Moth Peppered Moths Originally, the mutation caused the black moths to be eaten more rapidly. This meant that the normal moths lived longer and produced more offspring. Camouflaged Pepper Moth Photo by Olaf Leillinger

In the 1800s, the Industrial Revolution created large amounts of pollution. This killed the lichens growing on the trees that hid the peppered moths. This environmental change caused the black moths to be more adapted. Photo from Kettlewell, 1959

Peppered Moths Over time, the frequency of the allele for dark color increased, and the population became mostly dark instead of mostly peppered. Individuals did not become darker, the population became darker. 13

Peppered Moths This also illustrates the importance of diversity. When a population contains variations of a gene, it is more likely to adapt to new conditions. The dark color was not beneficial at first, but eventually it helped the species survive! Moth Photos by Olaf Leillinger

Resistance Resistance to pesticides is another example of natural selection. When a pesticide is used, most of the insects are killed. However, some insects have genes (alleles) that help them survive.

Resistance The alleles that allow these insects to survive and reproduce are passed on to their offspring. These alleles also help the offspring survive the pesticide. Natural selection causes the frequency of this allele to increase in the population. 16

Resistance Eventually the population changes until the pesticide is no longer effective at controlling the insect population. When this happens, the population has become resistant to the chemical. Original Population Final Population

Resistance Bacteria can also become resistant to antibiotics. Some bacteria have genes that allow them to survive antibiotic treatments. These bacteria reproduce and pass the genes on, and eventually the entire population is resistant to the drug. The growth of different strains of bacteria (yellow streaks) around disks soaked in antibiotics. No bacterial growth Antibiotic does NOT inhibit growth Methicillin-resistant Staphylococcus aureus