Natural Selection

Natural Selection The forces of nature create a selective pressure so that traits which are better adapted in a particular environment enable the organisms within a population to survive to reproductive age. The ability to reproduce is called fitness. Alfred Wallace coined the term “survival of the fittest”.

Natural Selection Natural Selection is a process based on the following: Organisms exhibit variations that can be passed from one generation to the next (heritable variation) Organisms compete for available resources. Individuals within a population differ in terms of their reproductive success. Organisms become adapted to conditions as the environment changes.

5 observations and 3 inferences that have been made about natural selection All species have such great potential fertility that their population size would increase exponentially if all individuals that are born reproduced successfully. Most populations are normally stable in size, except for seasonal fluctuations. Natural resources are limited. Inference #1: Production of more individuals than that environment can support leads to struggle for existence among individuals of a population, with only a fraction of offspring surviving each generation. Individuals of a population vary extensively in their characteristics; no two individuals are exactly alike. Much of this variation is heritable. Inference #2: Survival in the struggle for existence is not random, but depends in part on the hereditary constitution of the surviving individuals. Those individuals whose inherited characteristics fit them best to their environment are likely to leave more offspring than less fit individuals. Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over the generations.

Types of Natural Selection There are 3 general types of natural selection: Stabilizing Selection Directional Selection Disruptive Selection

Stabilizing Selection This occurs when an intermediate phenotype provides an advantage to the population. With this type of selection, extreme phenotypes are selected against. Example of this is human birth rate. Based on hospital data, infants born with an intermediate birth weight have a better chance of survival than those at either extreme. Those will a low birth rate may not be fully functioning and those with high birth weights may experience a difficult delivery.

Stabilizing Selection

Directional Selection This type of selection occurs when an extreme phenotype provides an advantage and therefore is selected for. Over time, the average phenotype of the population shifts. This type of selection generally happens when a population adapts to a changing environment. Example of this is the peppered moth. In London around the beginning of the 1800’s black moths became more favorable because their coloring blended in better with the black soot from coal being burned.

Directional Selection

Disruptive Selection This type of selection occurs when the extreme phenotypes are selected for over the intermediate. This leads to a wide range in phenotypes of a population. Example of this type is the British land snail. The snails either have a light banded shell or a dark shell depending on the habitat they live in, the forested area or the grassland area.

Disruptive Selection

Disruptive Selection

Sexual Selection Adaptive changes in males and females that lead to an increased ability to secure a mate. For males it may result in an increased ability to compete and females select a male with the best ability to produce surviving offspring, or fitness. Many consider sexual selection a form of natural selection because of it affects fitness.

Sexual Selection Female Choice Females produce less eggs compared to males sperm production, therefore their choice in mate is important. There are 2 hypothesis being tested regarding female choice: Good Gene Hypothesis: females chose a mate based on traits that improve survival Runaway Hypothesis: Females chose a mate based on traits that improve the males appearance. In studies involving Satin Bowerbirds and Raggiana Bird of Paradise neither hypothesis could be fully supported or rejected.

Sexual Selection Male Competition Cost-benefit analyses have been done to determine if the benefit of access to mating is worth the cost of competing with other males. Ex: Baboons compete with other males to gain dominance. Dominant males have more access to females when they are most fertile. The cost of dominance is requiring more resources because of larger size and possibility of getting hurt.

Maintenance of Diversity Diversity is maintained by a number of reasons: Mutation Sexual Reproduction recombines alleles Genetic Drift Natural Selection produces imperfect adaptations. Evolution is constrained by the available diversity. The environment plays a role in the diversity and evolution of a population.

Maintenance of Diversity Heterozygote Advantage This occurs when the heterozygote is favored over the homozygotes. This assists the maintenance of genetic diversity in future generations. Examples: Sickle cell disease and Cystic Fibrosis