Evolution

Ecology and Evolution are inextricably connected

Ecology = the study of interactions between organisms and their environment (physical, chemical and biological conditions)

 

Evolution = changes in the genetic composition of a population with the passage of each generation (change through time)

= change in allelic (gene) frequency in populations over time (alleles are different versions of the same gene)

So what is evolution?

• Evolution is a change in the number of times specific genes that code for specific characteristics occur within an interbreeding population

• Individuals don’t evolve, populations do

• There is no implied “improvement” in evolution

Consider how the amount of genetic divergence (change) forms a

continuum:

Microevolution Macroevolution small changes large changes

Microevolution = adaptationMacroevolution = speciation

Evolution

There is no difference between macroevolution and microevolution. Macroevolution is merely a collection of microevolution events. They act together to form new species.

Four distinct mechanisms generate evolution (change in allelic frequency in populations over time):

1. mutation

2. gene flow

3. genetic drift

4. selection (natural and “artificial”)

1. Mutation = a heritable change in the nucleotide sequence of the genetic nucleic acid, resulting in an alteration in the products coded for by the gene

 

2. Gene flow = introduction or loss of new alleles into the population through immigration or emigration.

 

Wilson & Bossert, 1971

3. Genetic drift =stochastic shifts in allele frequencies in small populations

 

Wilson & Bossert, 1971

4. Selection = change in allele frequencies over generations due to differential survival and reproductive success of genotypes

 

Darwinian evolution is evolution by natural selection

A brief history of the theory of Evolution

Charles Darwin was born on February 12, 1809 in Shrewsbury, England.

From 1831 to 1836 Darwin served as naturalist aboard the H.M.S. Beagle on a British science expedition around the world.

He observed much variation in related or similar species of plants and animals that were geographically isolated from each other.

These observations were the basis for his ideas.

Darwin presumed that populations of individuals changed over time, and, in 1844, he developed the concept of the driving force for evolution. It wasn’t until many years later that he published his idea.

“I have called this principle, by whicheach slight variation, if useful, is preserved, by the term Natural Selection.”

—Charles Darwin from "The Origin of Species“, 1859

Darwin’s Theory of Natural Selection States

1. All living species arrived at their present biological structures through a historical process involving random inheritable changes

2. Those changes that enhanced an organism’s ability to survive and reproduce were passed along in increasing numbers from generation to generation

3. Those changes that hindered survival and reproduction were lost over generations

Natural Selection

Darwin knew nothing of genes, but what he did have were two observations and a little inference that provided the motive force for evolution.

Natural Selection

Observation 1: Organisms generally have more offspring than can survive to adulthood.

Observation 2: Offspring are not identical. There is variation in their appearance, size, and other characteristics.

Natural Selection

Inference: Those organisms that are better adapted to their environment have a greater likelihood of surviving to adulthood and passing these characteristics on to their offspring.

Survival of the “fittest.”

Darwin’s voyage aboard the H.M.S Beagle

Darwin focused on adaptation - How species adapt to succeed in changing or different environments? How do species fit into their ecological niche?  Descent with modification the phrase used by Darwin to describe what he had observed during his research. This concept was later worked into Darwin’s theory on evolution.

The Darwinian revolution

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The Darwinian revolution

It was a letter Darwin received on June 18, 1858, that precipitated the publishing of The Origin of Species.

Alfred Russell Wallace, exploring in Asia, had come to the same conclusion as Darwin.

The Darwinian revolution

Darwin, with a strong sense of honor, arranged for a simultaneous reading of his and Wallace’s papers before the Linnean Society.

The readings were met with silence, so Darwin published the full text of his ideas.

In Darwin’s lifetime he would be recognized as one of the great masters of science. By the 1870s almost all serious scientists in England had accepted evolution.

The Darwinian revolution

Jean Baptiste Lamarck placed fossils in an evolutionary context and published a theory of evolution in 1809.  His theory used two common ideas of his time:

a.  Use and disuse - parts of the body used on a regular basis become larger and stronger; those not used deteriorate.                                   b.  Inheritance of these acquired characteristics (I.e., the modifications) are passed onto the offspring.                         - Lamarck’s theory was based on observations of fossils, but gave no mechanism for evolution.  However, he set the stage for Darwin.

Evolution before Darwin

What is the mechanism of natural selection?

1. Genotypes within populations vary and this variability is heritable.

 2. Biotic and abiotic components of an organism’s

environment act as selection pressures. 3. Genotypes that are best adapted to these

selection pressures leave the most offspring. 

Natural selection occurs because of differential success in reproduction. 

It is driven by interactions of variable individuals with their environments.  Those individuals with the most favorable traits will succeed to produce more offspring.

The product of natural selection is adaptation (evolution) of organisms to their environments.

Closely examine these premises: 1. What introduces variability among

genotypes?

Mutations

Recombination

 Populations with diverse gene pools have a lot of variation in alleles.

How is this variability passed on (heritable)? Genotypes pass on this variability through reproduction

Variability within the gene pool In sexually

reproducing the genetic deck of cards gets shuffled every generation. That means that novel alleles that arise through mutations are immediately placed in a diversity of genetic environments.

Genotypes pass on this variability through reproduction

In contrast, recombination is not tied to reproduction in asexual organisms (e.g. bacteria, archaea, many species of algae, fungi ....). Recombination happens in asexual organisms, but it is not necessarily tied with reproduction.

Genotypes pass on this variability through reproduction

Recombination has major ramifications on how natural selection acts on variance in the populations.

2. What are selection pressures in an organism’s environment?

Examples of biotic factors:

predators competitors mutualists

Examples of abiotic factors:

resource availability physical conditions chemical conditions

Selection can be…

“natural” or anthropogenic…

3. Genotypes that are best adapted to these selection pressures leave the most offspring

Variation in genotypes lead to the concept of adaptation and fitness

1. anemia (a recessive trait) 2. blood cell sickling (co-

dominant) 3. resistance to malaria

(dominant)

Adaptation =

a genetically determined characteristic that improves an organism’s ability to survive and reproduce in a particular environment.

Adaptation = a genetically determined characteristic that improves an organism’s ability to survive and reproduce in a particular environment.

Adapt = the evolutionary process by which organisms become better suited to their environments

Fitness = the relative contribution by an individual’s descendants to future generations.

Some important properties of fitness:

• Fitness is specific to a particular environment.(Consider both the biotic and abiotic Consider both the biotic and abiotic environment)environment).

• As the environment changes, so do the fitness values of the genotypes

Notice the connection between ecology and evolution.

Some important properties of fitness:

• Fitness is a property of a genotype, not of an individual or a population.

• Individuals with the same genotype share the same fitness within the same environment.

• Fitness is measured over one generation or more.

New genotypes and alleles enter the population through mutation, immigration (horizontal gene transfer) etc. A new genotype that is fitter than the current one will gradually replace it. If the current genotype cannot be replaced by an invading one, it is said to represent the evolutionarily stable strategy or ESS (Maynard Smith and Price, 1973).

The concepts of fitness and adaptation are relevant ONLY in a particular ecological context. There is no such thing as fitness in an absolute sense.

Important facts

• Evolution is spurred by changes in the environment

• When conditions change, previously useful characteristics may become harmful or irrelevant

• Evolution of specific characteristics can happen slowly, quickly, or not at all depending on environment

Which of the 4 evolutionary mechanisms generates adaptation?

1. mutation2. gene flow3. genetic drift4. selection

Which of the 4 evolutionary mechanisms generates adaptation?1. mutation2. gene flow3. genetic drift4. selection

Only natural selection, the other mechanisms generate change, but the change has no linkage to improved survival in the environment

There can be multiple paths to higher fitness in response to many but not all types of natural selection

(e.g. Contrast the results of Lenski’s experiments of glucose starvation in E. coli with Bull’s experiments with high-temperature stress in a bacteriophage

                        Homologous structures:  anatomical signs of descent with modification.

Homology refers to a similarity in characteristics due to common ancestry.

Examples - skeletal components of forelimbs, and vestigial organs in some organisms.

Evidence of evolution

                                    Molecular evidence: DNA/RNA is a universal genetic code. Other evidence: homologies basically follow the tree of life.  More closely related organisms have more homologous molecules.

Similarity between the typical animal and plant cells

Genetic similarity between species

Observation and Inference

Let’s do a little exercise…

Dinosaur tracks are common occurrences in the southern and eastern U.S.

Here is a section of tracks that were recently uncovered. Can you answer the following questions?

•What is the size and nature of the organisms?

•Were the tracks made at the same time?

•How many animals were involved?

•Can you reconstruct the events that occurred?

•In what direction did the animals move?

•Did they change speed or direction?

•Was the soil moist or dry?

•In what type of rock were the prints made?

The following summer some more digging revealed more of the track. What additional information have you gained that allows you to refine your answers?

•Were the tracks made at the same time?

•How many animals were involved?

•Can you reconstruct the events that occurred?

•In what direction did the animals move?

•Did they change speed or direction?

In the final summer of the excavation one last part of the footprint trail was uncovered. Does this section provide additional information to refine your hypothesis?

So what happened?

What part of your hypothesis is observation? What part is inference? What part is conjecture?

This is how science is done.