Macroevolution

Recap of last lecture

• Natural selection
• Populations produce more offspring than the environment can support
• Variation exists in population
• Natural selections acts upon this variation, selecting those with phenotypes best suited for that environment. The others die off.
• However, the surviving individuals must survive and reproduce, passing on their more adaptive genes to future generations
• Neodarwinism
• Synthesis of natural selection with modern genetics - brings alleles into the picture
• Selection can alter allele frequencies over time, but it does not act alone
• Genetic drift - random fluctuations in allele frequencies
• Founder effect - small island populations with different frequencies
• Speciation mechanisms
• Reproductive isolation - a barrier must form, separating gene flow between two populations
• Barriers: mountain ranges, island breaking off, biochemical, genetics, behavior

Patterns of speciation

• Darwin's theory of natural selection predicts slow, gradual changes in a population
• This is known as phyletic gradualism
• Is this seen in the fossil record? - YES
• However, if you look closely at the fossil record, there are some major jumps - is this due to the imperfection of the fossil record, or is this real?
• Close examination of complete records from the ocean reveal another pattern, punctuated equilibrium.

Does this mean that Darwin was wrong, or that other mechanisms of speciation must be in effect?

The Origins of Evolutionary Novelties

Let's look at one example - the birds

• Wings
• Feathers
• Hollow bones
• Endothermic
• Keeled Sternum

How did these structures evolve?

• One explanation for many of these structures is preadaptation
• Most biological structures have a phenotypic plasticity that makes alternative functions possible
• From a retroactive vantage point, evolutionary biologists term a structure preadapted if it evolved in one context and became co-opted for another function
• note that this concept does not imply that a structure somehow evolved in anticipation of a future use

Lets go back to our example of birds and look at these characteristics again

The above fossil is of Archaeopterix, one of the most important and well-known fossils known. This fossil clearly illustrates various transitional characteristics between birds and dinosaurs.

• Hollow bones - the ancestor of birds were small, bipedal dinosaurs. Having a lighter skeleton would make them far more agile and fast (the loss of 50% of mass from the bones does not mean that the bones are 50% more fragile)
• Feathers - the small, bipedal dinosaurs were presumably more active - their high surface area to volume ratio means they would lose a lot of heat. Feathers are far more effective insulators than scales
• Feathers - others has proposed that feathers evolved as a means to catch insects by "netting"
• Wings - the loss of the forelimb would facilitate the formation of a wing.
• Endothermic - more and more data indicate that many dinosaurs, especially smaller ones, were endothermic
• Keeled sternum - this is not evident in early birds, it evolved later (this is evidence that flight in birds evolved much later on in their history)

Based on the above information, we believe that primitive bird-like dinosaurs looked something like this:

Late Breaking Images! Click here for some cool reconstructions of more ancient birds

Now lets look at another example, humans

• Human evolution is one of the most controversial topics in biology today
• Studies have shown that the DNA of humans and chimps is over 99% similar
• Why do humans and chimps look so differently?
• These differences are due to the actions of regulator genes which affect rates of development
Baby Chimp
• If you look at a human baby and a chimp baby, the relative proportion of skull and limb dimensions are very similar
• However, as the chimp grows older, the limbs elongate and the lower jaw becomes larger - this is not seen in human adults
Adult Chimp - note elongate arms and large lower jaw
• These changes are due to a change in a few genes which regulate developmental rates - in short, adult humans are simply sexually mature while retaining juvenile features
• This is known as Paedomorphism

• Studies have shown that there is a link tying major shifts in evolution to mass extinctions
• What can cause mass extinctions?
• Major climatic shifts - the ice ages have reduced diversity
• Drying caused the reduction in forests which lead to the shift from arboreal primates to savannah-dwelling primates, ultimately leading to the evolution of humans
• Geologic causes
• Plate tectonics leading to large-scale geographic isolation
The position of the continents at four stages of geologic time
• Example - marsupials and placental mammals
• Astronomical factors - asteroids, etc