Lecture 6-Fossils and Evolutionary Processes

Focus Question-which four mechanisms drive the formation of new species?

In the previous lecture, we looked at many ecological and environmental processes that affect how an organism or populations can survive and thrive in its (their) habitat.  Today, we look at how populations of individuals that interbreed (that is, species) are affected through time. 

I.  Background History of the Ideas Related to Variation in Species

1.        Pre-20th century views of species-a continuum of thought ranging from thinking of species as distinct, unchanging forms to thinking of species as having a birth (species origination) a life (species duration) and a death (extinction), and that species can change.

John Ray, 17th c  species unchanging

Georges Cuvier, 18th c  species unchanging, but extinction occurs

Charles Darwin, 19th c species can change, new species can develop from older or existing species

Gregor Mendel, 19th c internal independent character traits exist that can be independently passed from one generation to the next (considered the Father of Genetics)

2.       Charles  Darwin’s 5 year voyage around the world as ship’s naturalist on board the HMS Beagle

               

A.       Galapagos Islands-Galapagos tortoises and Galapagos finches          

Both kinds of animals showed him how descendents from a common ancestor could show variation linked to their isolation on the different islands, and their food source specialization.

B.       On return to England, he read a book by Thomas Malthus “Essay on the Principle of Population”. The book described how human populations increase at a faster rate than food supplies, and so when a population exceeds its carrying capacity, a famine generally happened.   In that book, Darwin found the mechanism to explain what he saw in nature-that some individuals in a population would have characteristics that would favor survival, and others might not have those characteristics.   

C.      His ideas and hypotheses focused on the mechanism of Natural Selection, in which environmental and ecological pressures on populations could drive the selection of traits carried by individuals, and this could explain the rise of new species.  He published his book on this topic in 1859, On the Origin of Species by Means of Natural Selection.

 

3.        Gregor Mendel was a priest in the St. Thomas Monastery of the Augustinian Order in what is today Brunn (Brno) Czech Republic.  Through his work on the inheritance patterns in independent characters (factors) in pea plants, he discovered genes, and developed the concept of gene flow.  His work went largely unrecognized until the 20th century.

II. The four mechanisms driving the formation of new species

1.        Natural selection – nonrandom (and non-predictive)

2.       Gene Flow (Migration) - random

3.       Mutation-random

4.       Genetic Drift – random

 

III.  Genes-segments of DNA molecules carrying the information of heredity.  Most genes are structural genes, some are regulator genes.

1.        Homeobox “HOX” genes are regulator genes controlling body plan and switches for timing of development.  Mutations in HOX genes affect body plan and development timing.

 

2.        Mutations, spontaneous and random changes in sequences of base pairs within a gene, can happen in structural or regulator genes.  Mutations can be neutral (no change to the appearance of the individual (the phenotype) or non-neutral (a change occurs).

 

IV.  So, with these 4 mechanisms (natural selection, gene flow, genetic drift, mutation), how does speciation occur?  Through

1.        Geographic or behavioral isolation of a subpopulation of a larger group

2.       Reduction of gene flow (bottleneck or founder effect)

V.  Rates of speciation vary (some family lines of descent in plants and animals show very fast speciation, others slow).

                1.  Adaptive Radiation-sudden expansion of a lineage resulting in lots of species occupying numerous niches.  Example-the radiation of the mammals following the extinction of the dinosaurs.

                2.  Convergent Evolution-the descendents of very different lineages looking very similar, because the environment they occupy is the same.  Example-fish, marine mammals (e.g. whales), marine birds (e.g., penguins).  All are streamlined in body plan, an efficient swimming body plan. 

 

Next Lecture, Age and Formation of the Earth, Sun and Solar System