The Theory of Natural Selection (College Board AP Biology)

Charles Darwin & Natural Selection

• Charles Darwin proposed the theory of evolution by natural selection
• His theory culminated as a result of observations on a round-the-world expedition, backed by years of experimentation and discussion and linked to developing knowledge of geology and fossils
• Natural selection is said to be a major driving force of evolution
• Evolution can be defined as the change in the frequency of a phenotype in a population over many generations
• Darwin’s theory, very simply, is:
  • Individuals in a species compete for limited resources
  • The population shows a wide range of variation caused by differences in genes
  • Individuals with characteristics more favorable phenotypes have a higher chance of survival and more chances to reproduce
  • Therefore these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival
  • Over many generations, these beneficial characteristics become more common in the population and the species changes (the species evolves)
• This idea of natural selection became known as ‘survival of the fittest’
• Darwin published his ideas in his famous book, On the Origin of Species (1859)

Natural Selection & Populations

• Every individual within a species population has the potential to reproduce and have offspring which contribute to population growth
• If the offspring for every individual survived to adulthood and reproduced then the population would experience exponential growth
  • This type of growth only happens when there are no environmental factors or population checks acting on the population (for example, when there are plentiful resources and no disease)
  • One well known but rare example of exponential growth in a population is the introduction of 24 European rabbits into Australia in the 1800s. The rabbits had an abundance of resources, little or no competition and no natural predators. This meant the population increased rapidly and they became a major pest
• In reality, there are several environmental factors that prevent every individual in a population making it to adulthood and reproducing

Exponential Growth in the Absence of Environmental Factors

Exponential growth in a population of rabbits that have no environmental checks

Environmental factors

• Environmental factors limit population sizes by reducing the rate of population growth whenever a population reaches a certain size
• Environmental factors can be biotic or abiotic
• Biotic factors involve other living organisms
  • This includes things like predation, competition for resources and disease
• Abiotic factors involve the nonliving parts of an environment
  • Examples of abiotic factors include light availability, water supply and soil pH
• When biotic and abiotic factors come into play not all individuals within a population will survive
  • For example, if a food source is limited some animals within a population will not get enough to eat and will starve to death
• For most populations in the wild, the number of offspring produced is much higher than the number of individuals that make it adulthood

Population Limitation by Environmental Factors

• For African lions living in the wild there are several environmental factors that limit their population growth rate:
  • 1. Competition for food
    • There is a limited supply of prey: other lions and carnivores will also be hunting the same prey. If a lion is not able to hunt and feed then they will die from starvation
  • 2. Competition for a reproductive mate
    • Male lions compete with each other to mate with females; when one male is in a contest with another male one (or both) could be injured or killed
    • Whoever loses the contest won’t be able to mate with the females in a pride and so won’t pass on his genes to any offspring
  • 3. Supply of water
    • African habitats can be very arid during the dry season. The water sources that the lions drink from can be miles apart. If a lake or source of water dries up then they can die due to dehydration
  • 4. Temperature
    • The extreme heat experienced in the lion’s African habitat can cause them to overheat and die. It can also prevent them from hunting for long periods during the day, meaning they are less likely to get the food they need to survive
• The combined effect of all these environmental factors leads to a decrease in population growth as fewer individuals survive to adulthood and reproduce

Population Growth in the Presence of Environmental Factors

The different effects of environmental factors on the population growth of African lions in the wild

Evolutionary Fitness

• The stability of an environment affects the rate and direction of evolution
• If environmental conditions are fluctuating, different traits can be selected for each generation
• The individuals with the favored phenotypes are described as having a higher fitness
  • The fitness of an organism is defined as its ability to survive and pass on its alleles to offspring
  • Organisms with higher fitness possess adaptations that make them better suited to their environment