Lecture 4: Origins and Mechanisms of Evolution ctd                                              pdf _download pdf _download

Tutorial Sessions:

Review:   

 

So, having viewed some movies about shared traits among animals across time, and having looked at a few web links to observe how the retention of physical features and bone structures, -potentially cross referenced with radioactive attibutes of the bones suggest physical relationaships and "shared heritage" over time...

  Fossil Records in "DEEP TIME"

  Origins of HUMAN KIND?

  All in the FAMILY

Ideally all of these ideas of change(s) reflect back upon Darwin's appreciation of the inhabitants of the Galapagos islands, and the finding that there are forces at work which affect "if" and "how" different animals/plants in a given population are able to survive.

Of course all this evidence pre-supposes "honesty" among scientists......which is not always the case, either as a consequence of being unable to interpret the data correctly, or even as a consequence of outright fraud.

Some understanding of these characteristics will lead us (I hope) to a basic understanding of some of the terms used by biologists to understand these concepts, allowing us to appreciate that the forces which may be exerted upon organisms DO NOT exert themselves on individual organisms, but on populations of related individuals.

To put Darwin's ideas (Voyage of the BEAGLE published in 1832 - 1835) in to a 21st century context we will begin to coin a few vocabulary terms that might be pertinent to understanding how populations evolve in response to environmental constraints as the organisms that are most suited to certain environmental conditions have an advantage over others.... Darwin's idea of Natural Selection, which was based on his understanding of "artifical selection", a phenomeonon that was already being applied in England back in Darwin's day.

    

In modern terminology,

Natural Selection can be understood to be"the gradual process by which heritable biological traits become either more or less common in a population as a function of the effect of inherited traits on the differential reproductive success of organisms interacting with their environment".

Modern Genetics has since determined the mechanism(s) of heredity, which have provided a solid base that supports and substantiates -to a large extent- Darwin’s ideas.

- population: all individuals of the same species occupying the same area.

- gene: A unit of heritable information -usually associated (at the molecular level) with a specific region located on the chromosome.

- allele: - one of two or more slightly different forms, or "variants" of a given gene.

- genotype: a selection of the genes that make up an individual.

- phenotype: the consequence(s) of all the allelic interactions that give rise to a visibly determinable "type".

- gene pool: ALL the genotypes within a population.

Implicit within this and any understanding of evolution/genetics is that populations are "populations of individuals" not a group of identical "clones".

Populations are a group of individuals that exhibit an assorted array of similar phenotypic traits (the operative word being "similar"), and that each individual has a slightly different organismal "signature" that contributes to the collective "gene pool".

Stabilizing, Directional,and Disruptive selection change the "distribution" of the phenotypes that are potentially governed by genes at more than one locus.

In the last lecture we used the selective pressre of a a "cold snap" on the cliff swallows in Nebraska and the "black-bellied seed cracking finches of West Africa to discuss the potential selective influences that changes in the environment have upon the gene pool of the different organisms. The book uses the colouration of lizards against a changing background to distinguish among the different types of selective pressure.

What Darwin could not have known, however, because he had no concept of genes, alleles, genotypes etc. is that there are additional forces (inherent within the population) that can also have a MAJOR influence upon the makeup of any given gene pool.

Genetic drift: is the random change in allele frequency (in a population) over time and is a key mechanism of evolutionary change.

            

In the short term (i.e over a few generations), as the survivors pass on their respective set of alleles to future generations, one might expect allele frequencies to increase and decrease in a random, unpredictable way, as a result of genetic drift.

But at some stage the frequency of a particular set of alleles would become stable or fixed in a population for one reason or another. The change in the number of individuals that carry any particular allele for a given trait is more likely to be effected in a small population than it is in a large population.

Would you expect that the effects of genetic drift might be stronger in small populations (YES / NO?)

-the fewer individuals in the population, the fewer the variables, the fewer the number of individuals that carry different alleles, the more likely it is that random fluctuations will completely disrupt the allele frequency.

IIn the short term (i.e over a few generations), one might expect allele frequencies to increase and decrease in a random, unpredictable way, as a result of genetic drift.

In the longer term, however, the major consequences of genetic drift lead to a homogenization of phenotypes with in a population (everyone starts to look alike), especailly if there is some external selective pressure as certain alleles (read "variations") will be lost as others become "fixed". Thus genetic drift gives rise to a loss of variation in the gene pool (genetic variation).

Now couple this genetic drift with natural selection.... and you can begin to see how the effects of inherent populational changes along with environmental conditions can begin to alter the allele frequency within different populations over time....

     .

On the other hand, gene flow(?) would tend to increase genetic variation within a given population.

Gene flow is the intermingling of separate traits among similar, but distinct, populations. This increase occurs because individuals from other populations will bring in alleles that would otherwise be absent or rare (may be even lost) from the population that is being observed. In other words they would add variety to the gene pool.

 

Quick question. How might you think honey bees contribute to the gene pool among assorted pollenating flowers? Do they increase or decrease variety of alleles within the "gene pool" of the plants?

What would happen if Bees (like those humming birds in a previous lecture) prefer red flowers over blue flowers? Would this have

(A) a Stabilizing effect,

(B) a Destabilizing effect

(C) a Directional effect

(D) No effect at all, or

(E) Need more information on the gene pool of pollenating plants?

Might the effect also potentially be one which may also involve Gene Flow? (YES / NO...?)

In coining the idea of "natural selection" Darwin was able to provide a potential mechanism by which different phenotypic traits changed (and how they continue to change), an understanding of which, can be highly informative.

 

Again:

Natural Selection can be understood to be"the gradual process by which heritable biological traits become either more or less common in a population as a function of the effect of inherited traits on the differential reproductive success of organisms interacting with their environment".

 

Modern Genetics has since elucidated the mechanisms of heredity, which have provided a solid base that supports and substantiates -to a large extent- Darwin’s ideas.

 

There are also classic examples of other more severe "external" factors that can have significant effects upon the makeup of any gene pool

  

- bottleneck: severe reduction in population size due to intense selective pressure or a natural calamity or catastrophe, which alters the allele frequency... 

As I was talking about in the last lecture.... over the life time of the earth there have been a number of "significant" events, such as meteorite collisions and indirect interactions with comets , which have had a cataclismic affect / change on climatic conditions -changing the potential for life to exist on the earth. 

 

There is a considerable body of evidence to suggest that a meteorite may have been at least partially the cause of the somewhat abrupt mass extinction at the end of the Cretaceous period.. or alternatively...Volcanoes in the Deccan Traps

Are there Human bottlenecks??

- founder effect: whereby a couple or even a single organism becomes "dislodged" from a population, and all genetic variation is limited to the individual or isolated individuals.

Human founder effects ..... The Afrikaner population in South Africa and Huntington's disease traced back to 1652

Now, armed with all this "new" information (and vocabulary(?)), let's return to Darwin and some of his rather absurd notions that he wrote about in in his first book The Origin of Species, and let's ask a few questions .........

Given what we have determined, might the "forces of evolution" work equally well on populations that have a limited gene pool, as they do in those populations with a much larger -effectively infinite variety of alleles within a its gene pool? (YES / NO..... ?)

This would explain the potential changes in the frequency of phenotypic changes (due to genetic drift) in animals / finches / tortoises that Darwin noticed on or in the Galapagos islands vs. similar changes (or a lack thereof) in their main land relatives, Remember, Darwin sailed around the world, saw a great deal of flora and fauna, but it was the Galapagos islands that provide the important location for his idea of natural selection.

Really, all Darwin observed were phenotypic "variations of different shared phenotypes

Question: Are there any constraints to evolutionary changes?

Answer: Yes.

At this stage, however, the scientific method and our understanding of these "constraints" upon evolutionary change can get bogged down in details.......

-------

Remember, implicit within this understanding of evolution is that population works on populations of "individuals" -each contributing a slightly different genotypic "signature" to the population's gene pool.

How did we get such a diverse gene pool?

Unfortunately, that question we will have to leave for another day -once we have dealt with Genetics and Mendal and his findings.

For now, I would like to ask how do we now adapt Darwin's evolutionary principles and begin to determine that there are sufficient differences between populations with "similar, but different, genetic makeup", and assign to each a different name,.....

....In essence how do we determine and then justify the term SPECIES that Darwin uses in his book, The Origin of Species, ?

Darwin and his contempories used appearance(s) (phenotype[s]) of animals/plants to define differences......often to distinguish one species from another, or (at the very least) recognize members of the same species present in different geographical locations.

   

Always remembering, of course, that such observations could lead to the formation of associations and false lineage that could have been derived from insufficient knowledge... the Panda's "thumb", which evolved from a different source than our own 5th digit....and can be, therefore, considered as an example of a relatively new concept -convergent evolution.

 

Over almost 300 years ago, Carolus Linnaeus -following on from John Ray- created the system of naming organisms that we still use today. even so, neither of these men were the first to do so, officially (remember Aristotle).

Linnaeus classified over 4,000 organisms based on their physical characteristics and appearance, using a morphological concept of species, and his best-known accomplishment is creating a hierarchical system of organization for living organisms, using, what is called a binomial nomenclature. K P C O F G S

 

How many kingdoms are there....? (A) 3, (B) 4, (C) 5, (D) 6, (E) 8,

Monera/Archaea/Protista/Fungi/Plantae/Animalia/Insecta/Panera/Pinera?

Before this classification system was effectively put in to use, scientists had many names for the same organism, which -not too surprisingly- caused considerable confusion within the scientific community.

Even so, despite Linnaeus' revolutionary ideas (remember, Linnaeus' ideas preceded Darwin's reflections) scientists still clung to the belief that all species were present in their day, unchanged -as they always had been.

After Darwin, however, this classification scheme needed significant refinement on precisely what was meant by each of the terms, (especially the term "Species" ) and how they came about.More modern, updates of Linnaean classifications arose in 1940, when Ernst Mayr proposed the "biological" definition of species, or the Biological Species Concept [BSC]:

"Species are groups of interbreeding (or potentially interbreeding) natural populations which are reproductively isolated from other such groups."

But, the arguments and difficulties that we have in defining "Species" reflect very accurately the arguments that scientists have had over the centuries.........

Problems....

One cannot apply this definition to a considerable number of asexual populations. 

Application of the BSC toward self-pollenating plants is (by its own definition) somewhat limiting.

Moreover, it is often very difficult to apply (practically) anyway.  It requires breeding tests to verify the separation, but this is rarely used.  Indeed, were breeding tests to be used to define such criterea, natural environments would potentially play an overiding role. 

e.g. Do Homo erectus, Homo neaderthalensis and Homo sapiens represent the same or different species? This question is unresolvable using the BSC definition, especially when (as we now suspect) Neanderthals and Homo sapiens may have interbread.

Other Definition of Species include: The Phenetic (or Morphological) Species Concept -Cronquist (1988)- who proposed an alternative to the BSC that he called a "renewed practical species definition" , where He very clearly defines species as

"... the smallest groups that are consistently and persistently distinct and distinguishable by ordinary means."


As you can see, determining if two populations are actually different "species"a can be quite a difficult task, especially as the very process of speciation is often a gradual process.

Today genetic information is often used to help determine if morphologically similar organisms are actually different species, but even this approach is limited. If chimpanzees have a 99.7% similarity with humans -where is the cut-off that clearly designates the difference between different species?.

To avoid the obvious pit-falls in giving a hard and fast definition to the term "species", the concept of Genetic Integration has been coined, and used to differentiate between SOME types of species.


Genetic integration works on the premise that if individuals within a population mate with one another, but not with individuals of other populations, they are considered to be an "independent evolutionary unit", and can safely be called a Species.

But just how do/did New Species Arise?

Speciation is the process by which one "species" splits into two.

We have already observed some possible mechanism(s) of a potential speciation in action....have we not?...in the "black-bellied seed cracking finches of West Africa, or a differentially coloured background and the lizards.

But not all such evolutionary changes result in new species. The critical process in the formation of new species is the segregation within the gene pool of the ancestral species into two separate gene pools, that can no longer be considered to be of the same species. Consequently, such speciation is often facilitated by an interruption of gene flow between or among different populations.

Allopatric Speciation requires total genetic isolation.....or, when two or more parts of a single population become divided by a geographic barrier, alternatively known as geographic speciation.

Allopatric speciation is thought to be the dominant form of speciation among most groups of organisms.

New populations of fruit flies founded by individuals dispersing among the Hawaiian islands exemplify founder events, which appears to define a form of Allopatric Speciation, although the physical separation of the gene pool is not total... perhaps like "founder effects"?.

Even so, Darwin's seed-eating and bud-eating finches of the Galapagos archipelago demonstrate the importance of geographic isolation as a distinction that can enhance the process of speciation.

However, the effectiveness of any barrier -geographical or otherwise- in preventing gene flow depends on the size and mobility of the species in question.

eg. an impenetrable physical barrier for a terrestrial snail may not be as remarkable a hurdle for a butterfly.
Indirect evidence that most speciation among animals is allopatric is provided by patterns of species distribution.

These findings can lead to a tantalizngly plausible (but ultimately false) potential inference.............if geographic barriers give rise to speciation....all organisms that are separated by a geographical boundary become species......long-distance romances don't work?

Unfortunately, it is not true...they can work. 

Geographic isolation does not necessarily lead to reproductive incompatibility, as evidenced by the European and American sycamores. Apparently, 20 million years of separation isn't such a long time if neither partner has to change too much. Consequently, physical barriers cannot be the ONLY part of the "species" equation.  Anything that begins to separate the gene pool of the ancestral species will suffice. Also exemplified (to some extent) by red-winged blackbirds (Agelaius phoeniceus)

Speciation, in the absence of any geographical barrer -or the subdivision of a gene pool when members of the daughter species are not geographically separated, but the two species become distinct as a function of inherent genetic polymorphism.- is called Sympatric speciation. Any examples spring to mind??

Additionally, the book states that the most common means of sympatric speciation is polyploidy, an increase in the number of chromosomes.  As we haven't dealt with Mendel and his beans yet, we'll hold off on this notion as an example for now, and go with an example that we might be able to appreciate....

Fruit flies have speciated sympatrically in New York State for more than a hundred years.
These fruit flies originally courted, mated, and deposited eggs only on hawthorn fruits.
About 150 years ago, large commercial apple orchards were planted in New York.
Some fruit flies began to lay their eggs on the apple trees, perhaps by mistake.
Consequently, their offspring sought out apple trees as adults and, therefore, mated with other fruit flies of similar heritage.
Today there are two sympatric species of picture-winged fruit flies in New York.

Parapatric speciation separates adjacent population, that have separated as a consequence of reproductive isolation coupled with some degree of migration -again in the absence of a geographic barrier. Parapatric speciation is quite similar to sympatric speciation, but reflects some migration (in time and/or space).

eg. Metal-tolerant and metal-intolerant plants flower at different times, reducing gene flow between the two groups.
Metal-tolerant plants also self-pollinate more frequently and can also grow in polluted soils that require tolerance to different metals. So as populations of plants expand......
Consequently, reproductive isolation between metal-tolerant and metal-intolerant plants could, therefore, become complete.....potentially giving rise to two new species

eg. Temporal isolation of the three species of Rana Frogs..

Once a barrier to gene flow is established, the resulting daughter populations are then free to diverge genetically -as a consequence of the evolutionary "forces" that we discussed in the  last lecture. 

In Summary......Speciations....

With all its faults, the very foundation stone of the BSC line of thought - the ability to define "species" as a consequence of their reproductive isolation, is a powerful tool in defining species.

It also promotes a potential number of mechanisms that may initiate or at least promote reproductive isolation (i.e the mechanism that dictate the formation of traits that prevent interbreeding between populations). Collectuvely, these mechanisms can help us understand how species can form.

Reproductive Isolating Mechanisms

Prezygotic barriers -operate before mating

Five prezygotic reproductive barriers have been described:

Spatial isolation          -is the separation by location of inhabitance in the environment.

Temporal isolation      -is the separation by differing mating seasons, or times of day.

Mechanical isolation  -is the separation by differing shapes of reproductive organs.

Gametic isolation        -is the separation by the inability of sperm from one species                                           to fertilize the egg of another species.

Behavioural isolation  -is the separation by behaviour,  eg. lack of recognition of                                            potential mates as mating partners.

Postzygotic barriers -operates after mating.

If individuals of two different species still recognize one another and are able to mate, postzygotic reproductive barriers may prevent gene exchange.

There are three postzygotic reproductive barriers have been described, with the major two being:

Low Hybrid viability, the offspring just generally don't do so well.

Hybrid zygote abnormality which is the failure of a hybrid zygote to develop in to a reproductively viable stage of life.

Hybrid infertility is one example of a "postzygotic barrier" the inability of a hybrid to reproduce. Hybrid offspring may survive less well than offspring resulting from matings within each species.  All the offspring of one sex may be sterile, or all the offspring may be of only one sex. In nearly all cases of hybrid sterility and inviability, it is the sex that is heterozygous for the sex chromosomes that is absent or sterile.

Mules are one of the more readily known examples of this type of barrier, with a female mule being born every once in a while.

 



This information is given as a guide to the student attending the Bio2107 lectures as a means to review some of the information. It is not meant to replace the lecture. No emphasis as to what will be required of the student is given in this text, indeed information that is given in the these transcripts may make little sense if the student has not first attended the relevant lecture.

 

 


Copyright © Department of Biology, Georgia State UniversityView Legal Statement Contact Us

About

Quick Facts
Governance and Strategy
Administrative
University Policies
Contact Georgia State

Academics

Colleges & Schools
Degrees & Majors
Academic Guides

Admissions

Undergraduate
Graduate
College of Law
Financial Aid

Research

News
Programs
Commercial Development
URSA


Libraries

University Library
College of Law Library

Campus Life

Housing
Parking
Safety
Recreation
Counseling
Career Services


Athletics

Sports
Tickets
Recruits

Alumni

News
Giving
Events