2.6 - Adaptations

Learning Objectives

Foundations of Adaptation: Genetic Diversity

1. Random mutations: These occur when DNA is being copied. A change in the DNA, like replacing C with T, can result in an entirely new trait the organism previously lacked.

2. Crossing over: This occurs during meiosis, the creation of sex cells. Parts of chromosomes in the parent organism exchange genetic information, and that exchange can lead to new combinations of traits.

Adaptations and Fitness

• An adaptation increases the organism’s fitness.

• Fitness means the organism is more likely to survive and reproduce.

Natural Selection (Evolution at Work)

1. Organisms that are better adapted to their environment survive and reproduce more offspring.

2. They pass those beneficial traits onto their offspring.

3. Individuals without the adaptation are more likely to die off.

4. Over time, the prevalence or frequency of the trait spreads throughout the population. That shift is what we call evolution.

Selective Force / Pressure

• This is the environmental condition that kills off or “selects out” the individuals that don’t have the necessary adaptation.

• Example: In a mouse population, if gray fur allows blending into the environment (an adaptation), a hawk that preys on tan mice acts as the selective force. The tan mice have decreased fitness and get selected out, leaving the gray mice to survive and reproduce more, eventually producing a population of mostly gray mice.

The Role of the Environment

• A trait that’s highly beneficial in one environment could actually be harmful in a different environment.
• Analogy: Being tall is an advantage in basketball but a disadvantage in a marathon. The environment is key.

• As the environment changes, a previously advantageous trait may start to harm the organism, and a new trait may become the adaptation needed for the new conditions.

• Real world example (Daphne Major, Galapagos): During a severe drought in 1977, plants on the island made far fewer seeds. Medium ground finches ate up the small, soft seeds first, so the food that was left was mostly large, hard seeds. That made a large beak an adaptation: birds with bigger, deeper beaks could crack the hard seeds, while many birds with small beaks starved. Most of the population died, and the survivors and their offspring had larger beaks on average. Biologists Peter and Rosemary Grant measured this change on the island.

Factors Affecting the Pace of Evolution

1. Pace of environmental change: The more rapidly an environment changes, the lower the likelihood that species will be able to adapt.
• If change is too fast, like ocean temperature warming too quickly, species may be forced to migrate or may die off completely, such as fish suffocating due to low oxygen in warm water.
• If change is gradual, species have a better chance to stay and adapt.

2. Genetic diversity: Populations with higher genetic diversity have a higher likelihood of adapting, because there’s a greater chance some individuals already have beneficial mutations that are advantageous in the new environment.

3. Generation time / lifespan: The longer a species’ generation time or lifespan, the less likely it is to adapt quickly to rapid environmental change.
• Organisms with short lifespans, like bacteria and viruses, reproduce rapidly, which lets beneficial mutations spread quickly. They can evolve in a matter of days.
• Organisms with long lifespans, like humans, evolve very slowly, over thousands or millions of years, and are therefore more likely to go extinct if changes are rapid.

Responding to Change: Adjust, Move, or Die

1. Change behavior: Individuals adjust what they do so they stay inside their range of tolerance. Example: At unusually warm, low-elevation sites, American pikas (small mammals that overheat easily) spend the hottest hours inside the cool spaces of rock piles and are most active in the early morning, late afternoon, and at night.

2. Move: Individuals relocate to places where conditions still fit their tolerance range. Example: Biologists repeated an early 1900s survey of small mammals in Yosemite National Park about a century later. About half of the 28 species they tracked had shifted their elevation range uphill, by about 500 m on average, as the area warmed.

3. Perish: If individuals can't adjust or move, they die, and if that happens across the whole population, the species goes extinct. Example: The Bramble Cay melomys, a small rodent known only from one tiny, low island in the Torres Strait between Australia and Papua New Guinea, lost almost all its habitat as storm surges and higher seas repeatedly flooded the island and killed its food plants. It was last seen in 2009, declared extinct by Queensland in 2016 and by Australia's government in 2019, and it's considered the first mammal extinction linked to human-caused climate change.

Big picture: Natural selection is like an audition hosted by the environment. Genetic diversity provides the initial pool of different “actors” (traits). The environmental conditions, like extreme heat or a new predator, act as the unforgiving “director” (the selective force), who only chooses the actors (adaptations) best suited for the current performance. Only those successful traits continue into the next generation.