D4.1 Natural Selection

the foundational mechanism of evolution where organisms better adapted to their environment tend to survive, reproduce more, and pass advantageous genetic traits to offspring

Observations:
• Individuals in a population show variation in traits.
• Species produce more offspring than the environment can support.
• Resources are limited, leading to competition.

Inferences:
• Individuals with advantageous traits are more likely to survive and reproduce.
• These traits are passed to offspring.
• Over generations, favourable traits increase in frequency in the population.

Inductive reasoning involves forming general conclusions from specific observations. Darwin observed patterns of variation, inheritance, and competition in many species and inferred the general theory of natural selection to explain evolutionary change.

• Correspondence: The theory matches observations from nature such as fossil records and biodiversity.
• Coherence: The theory is consistent with other scientific knowledge such as genetics and ecology.
• Pragmatic: The theory successfully explains and predicts biological patterns such as adaptation and speciation.

Natural selection requires variation because individuals in a population must differ in traits. Selective pressures act on these differences, some variations provide a survival or reproductive advantage, so individuals with those traits are more likely to survive and reproduce, passing the advantageous alleles to offspring.

Without variation, all individuals would be equally affected by selective pressures and no evolutionary change would occur.

Genetic variation arises from:
• Mutation, which creates new alleles.
• Meiosis, including crossing over and independent assortment.
• Sexual reproduction, which combines genetic material from two parents.

Mutations create new alleles by altering DNA sequences.

Sexual reproduction does not create new alleles but generates new combinations of existing alleles through recombination and random fertilization.

Overproduction occurs when species produce more offspring than the environment can support. Because resources such as food, water, and space are limited, individuals must compete for these resources, and not all offspring survive to reproduce.

Carrying capacity is the maximum population size that an environment can sustainably support based on available resources.

• food
• water
• space or territory
• nesting sites
• mates

Direct competition involves individuals physically interacting or competing for the same resource at the same time.
Indirect competition occurs when individuals use the same limited resource but do not directly interact.

Because more offspring are produced than can survive, individuals compete for limited resources. Those with advantageous traits are more likely to survive and reproduce, passing these traits to offspring. Over generations, these advantageous traits increase in frequency in the population.

Selective pressure is an environmental factor that influences survival and reproduction of individuals with certain traits.

A density-independent factor affects populations regardless of population density, such as temperature or natural disasters.

Biotic pressures:
• predation
• disease
• competition

Abiotic pressures:
• temperature
• drought
• salinity

Selective pressures favour individuals with advantageous traits that increase survival or reproduction. These individuals reproduce more successfully and pass the traits to offspring. Over generations, the frequency of these traits increases within the population.

Adaptation: a heritable trait that increases an organism’s ability to survive and reproduce in a particular environment.

Fitness: the reproductive success of an individual relative to others in the population.

Individuals with advantageous adaptations are more likely to survive and reproduce, while less-adapted individuals are more likely to die before reproducing or produce fewer offspring.

Random genetic variation produces different traits in individuals. If a trait increases survival or reproduction in a given environment, individuals with that trait are more likely to reproduce and pass the trait to offspring. Over generations, the frequency of the advantageous trait increases, resulting in adaptation.

Heritable characteristics are traits determined by genes and passed from parents to offspring.

Acquired characteristics develop during an organism’s lifetime due to environmental influences and are not inherited.

Natural selection acts on traits that affect survival and reproduction, but evolutionary change occurs only if those traits are passed to offspring. Acquired traits are not encoded in DNA and therefore cannot be inherited, so they do not contribute to evolution.

Sexual selection is a form of natural selection where traits increase an individual’s ability to obtain mates and reproduce.

• Intersexual selection: mate choice, usually where one sex selects mates based on traits such as coloration or displays.
• Intrasexual selection: competition between members of the same sex for access to mates

Individuals with traits that increase mating success are more likely to reproduce and pass these traits to offspring. Over generations these traits become more common, even if they reduce survival.

A scientific model is a simplified representation of a system used to explain, predict, or test scientific ideas.

Bright coloration increases mating success because females prefer brightly coloured males. However, bright coloration also increases predation risk because predators can see them more easily.

John Endler studied guppies in different environments with varying predator presence.

In habitats with many predators, male guppies evolved duller coloration because brightly coloured individuals were more likely to be eaten.
In habitats with few predators, males evolved brighter coloration because females preferred colourful mates.

These results demonstrate natural selection acting on variation in coloration depending on environmental selective pressures.

Scientific models are simplified representations of real systems or processes used to explain, visualize, test ideas about, and predict natural phenomena.