World of Dinosaurs/Adaptation Exaptation
This page explores two frameworks for derivation in lineages of animals.Edit
Generally, a population of animals includes individuals with a wide range of trait expressions.
- When we learned in grade school about genes, we often compared eye color to our parents. Two parents with brown eyes can have a kid with blue eyes if each parent is carrying a recessive gene. They have a 1/4 chance of this occurring.
- Most traits in animals are not so simple. While our DNA includes the information to build our bodies cell-by-cell, it includes a ton of information that isn't necessarily put to use in our body at any given time.
- Gene expression is complex. Many different factors can influence how a body actually produces cells and structures day-to-day.
- Thus, the traits that we see expressed in individuals are not a one-to-one match with the information for potential traits that their offspring will inherit.
- Once offspring get their suite of DNA instructions, the actual execution of those instructions is subject to many factors influencing gene expression.
- If each individual animal lives for many years, populations can include multiple generations of offspring and parents.
Genetic drift refers to shifts in the typical DNA pattern found within a population or group.
- If one population splits into two, the typical gene coding and expression can shift in one of the groups.
- If the animals rejoin a mingled population, they might re-mix their DNA and the genetic drift wouldn't have much impact.
- If the animals do not rejoin into a mingled population, and they do not mix their DNA for some generations, then the typical genetic code in one population may become different enough from others that offspring are not possible, or are rare.
Many factors can lead to distinct populations.
- Geographic barriers can arise within a population, or present opportunities for populations to split.
- Habitat fragmentation can reduce mixing between individuals, creating sub-groups.
- Environmental conditions like seasonality (how early the flowers bloom; how late it stays dark) or climate can alter the challenges animals face. Individuals that handle these challenges differently may find themselves mingling with only a sub-set of their broader population.
- Animal behavior can also influence where, when, and how animals meet, court, and reproduce. This is a particularly powerful force in social animals that have complex cognitive behaviors associated with mating.
Traits that confer success can be reinforced in a population.
- If a trait helps an individual survive, that animal is kept in the population. The individual's reproductive or behavior actions may affect the population.
- If an individual expresses traits that aid reproduction, or that improves offspring survival, then ANY heritable trait the animal possesses may be passed to the next generation.
- Behavior traits can be spread without direct sexual reproduction.
- Gene expression can influence which traits are produced from a potential list within the DNA, and how traits interact with other physical and behavioral features.
- "Survival of the fittest" is a phrase often attributed to Charles Darwin, but not actually from his work. Darwin speculated about "decent with modification" a hundred years before scientists discovered DNA as a mechanism for the process. He was correct about some speculations, and incorrect about others.
- "Survival of the fittest" is an inadequate assessment of genetic and behavioral forces in evolution. It's also been used to justify cruel and inhumane slaughter of people and civilizations in recent history.
- A phrase we prefer in this class, taken from Dr. Frank Corsetti, is "survival of the minimally fit". This makes it easier to understand the existence of pandas!
When populations separate, new species can arise.
- For a new species to arise, a population needs to become distinct enough from its peers that they do not mingle their DNA.
- The kind of distinction varies, as explained above. Animals can cease to intermingle their genetics due to behavior, gene expression in anatomy, and environmental influences.
- The rate of this process differs with the kind of animal, behavioral complexity, habitat, geography, etc.
Natural Selection describes the processes by which specific traits are changed within populations.
- A trait that aids survival or reproduction may increase its expression within individuals.
- Hazards in an environment can remove traits from the population if the trait-holders die before reproducing, or cannot yield sufficient offspring.
- Parasites, diseases, and microbial growths can remove individuals from populations.
- An individual could have really awesome traits that are terrific for one environmental challenge, but could die or be removed from the reproductive cycle due to insufficient response to a different challenge.
We can observe natural selection influencing trait expression at a variety of scales over time and space. Natural selection is not always going to work in the same direction, on the same traits, or produce the same consequences.
Below we will learn about Adaptation and Exaptation.
- Basically, if a trait is exaggerated (enlarged, increased frequency, etc.) in the context of a particular function, we call this set of changes adaptation.
- A trait can also become exaggerated in the context of a totally new function, different from the context in which it first became valued. Think of this as the natural selective processes taking a sudden left-turn. This is called exaptation.
Some traits allow an animal to succeed in the context of a specific environmental challenge.
- Example: seeds are hard to open. A finch with a strong beak can crack strong seeds.
That environmental challenge can become a source of natural selection if it increases, or decreases, associated trait expressions in an animal population.
- Example: During a drought, birds that cannot open the tough seeds die. Birds that can open the seeds live. Strong beaks are more common in the hatchlings later that year.
Natural selection does not guarantee long-term change in a population.
- Example: The next three years the weather is mild and wet. Birds with keen eyesight that spot small but nutritious seeds do well, and produce a few generations of offspring with generalized beak shape.
IF a trait increases to be present in all of the members of a species, and if that trait increases in expression in the context of a continued environmental challenge for which it presents an advantage, then this is adaptation.
Sometimes a trait increases in the context of a challenge very different from the context in which the trait arose. Example:
- Maniraptorans (including Therazinosaurs, Oviraptors, and Utahraptor) have a half-moon shaped wrist bone that allows the hand to swivel differently than in other theropods.
- A humming bird has very large hands that flip back and forth to generate lift on the backswing and forward swing. This takes advantage of the flexible wrist. But the wrist bones did not originally appear in the context of flight.
- The wrist architecture was "hijacked" or "hacked" for a different function.