Evolutionary Synthesis

Main Points

  • Traits are passed down in discrete packages of DNA (genes), this is called inheritance.
  • Variation is natural within a population and arises from errors in DNA such as (but not limited to) mutation.
  • Natural selection allows for the sorting of the variation within a population.
  • Speciation (microevolution) can occur by many processes three main types include: allopatric, sympatric,
    and parapatric.
  • Macroevolution is long term change resulting from evolutionary processes

Evolutionary synthesis includes the principles of natural selection, genetics, DNA, and biogeography. Evolution and evolutionary synthesis do not speak to the origins of life, but rather scientists working in each field are trying to assess and understand the patterns of life.

Studies of evolution have been around since people started making observations about the natural world. So after the Greek philosophers, Shen Kuo, Robert Hooke, Nicolas Steno, Jean-Baptiste Lamarck, Georges Cuvier, Charles Lyell, and many more, Darwin then provided the idea of evolution occurring by natural selection. Around the same time that Darwin had written his famous novel, On the Origin of Species, Alfred Russell Wallace, an English naturalist, independently came up with the same idea of natural selection. This prompted several collaborations on evolutionary processes between Darwin and Wallace.

It is important to understand that Charles Darwin did not come up with evolution all on his own. He had a wonderful idea but did not understand the mechanics behind how evolution worked (genetics, see below). On his travels around the world he observed living and fossil animals, from these observations he was able to understand that biological change had been occurring through time. Darwin also provided geologists with an understanding to why fossil assemblages were changing through time. The idea of faunal succession was well understood but it was unclear why the fauna was changing. Darwin provided substantial evidence for evolution through the fossil record of extinct organisms (Boggs, 2012).

For a more in depth understanding of the key players in the start of evolutionary thinking click here or here.

Main Parts of Evolutionary Synthesis

Inheritance

This shows an example of how traits are passed down from Your Inner Fish by Shubin (2008).

The Theory of Inheritance states that traits are passed down in discrete packages of DNA (genes). This is the work that Gregor Mendel, a Czech scientist, accomplished through experiments in his garden. Although Mendel was conducting his research around the same time that Darwin and Wallace were pondering natural selection, the work did not collide. It was not until the early 1900s (some decades after his death) that Mendel’s work was necessary for understanding the results that new scientists were acquiring. This theory explains where morphology (the form, or body shape, of organisms) comes from. You acquire this genetic information from your parents, each providing you with a set of genomes. If you are interested in learning more about genomes and inheritance please check out YourGenome.

Let’s examine the figure. The couple at the top produces an offspring – notice the feet are the same as the parents but the nose is different. The next generation has floppy feat rather than normal feet. We can make an inference (=a conclusion based on evidence) about the other parent of this offspring. That parent likely had large feet so the offspring now has large feet. We can continue to make inferences about the morphological features of the other parent as we examine the tree.

If you are interested in learning more about the genetics behind inheritance (or really any of these parts of evolution) please head to Six Things Everyone Should Know about Genetics for an interactive journey through genetics!

Variation

A generalized example of variation within a population. All of the shapes represent a given population, and the shapes themselves as well as the different colors represent the variation among individuals within that population.

Variation is natural within populations. It can result from several different versions of any given gene allele. An allele is a variant or alternative form of a gene. These alternative forms of genes can arise from mutation, transcription error, or gene duplication. This type of variation is often referred to as genotypic variation (meaning it is at the gene level). But there is another type of variation called phenotypic variation. Phenotypes are the physical representation of your genotype. Think of it as one possible visualization of the underlying code. But phenotypes can be altered depending on the environmental conditions acting on these pre-coded genotypes. This can be misleading, especially in the fossil record when we rarely have access to genetic information.

Look to the figure on the right. Here is a very basic diagram of variation within a population. Call our population “Circle” and within population Circle we have circles of different sizes and colors. This is normal – maybe the small Circles are juveniles and there was interbreeding of different Circle colors producing a different color offspring. Or maybe there was a mutation that caused a Circle to become a Square. Most variation is harmless, but some cause issues such as disease, illness, or disabilities.

Sorting

Sorting is evolutionary change acting via natural selection on small isolated populations. This theory comes from much of Darwin’s work as well as an understanding of biogeographic processes (=how organisms have moved around on Earth throughout their existence). This process happens most often in small isolated populations because there are simply fewer organisms to breed, making the sorting of specific traits much faster than in a large population.

Survival of the fittest is simply defined as raising offspring to a breeding age. Fitness is not a measure of how smart or strong a species is, but how many viable offspring they produce to carry on their traits. If the traits are favorable in a given environment the organism will succeed. Natural selection allows for new species to arise as the composition of populations change with time. Those individuals that have these ‘favorable traits’ will have better odds of surviving and reproducing (Boggs, 2012).

We have our starting population as above, with a high degree of variation! Squares! Circles! Colors! In the middle image we have selection pressures acting on the population. We end up with less variation. The blue and orange circles create the final population from the most desirable traits.

Speciation

This is sometimes termed microevolution. A species (a population) is the unit upon which evolution acts. Individual organisms do not evolve, the whole population evolves and a split in the population’s gene pool causes speciation to occur! New species arise from a process called speciation. Speciation, in human/ecological time, is not usually a fast process – it takes some time. In the rock record, speciation is usually faster than the rock is deposited! Although speciation appears rapidly in the fossil record we must remember that the rock record of time is not linear and that rocks are not being continually deposited everywhere! Therefore, the time we are interpreting from the rock record is pulsed, so we are not always able to see gradual change.

There are three main types of speciation that are commonly recognized and discussed (there are more, of course):

Here are simplified examples of the three types of speciation discussed in the text to the left.
  • Allopatric speciation: Allo means different and patric means “fatherland” but here we will refer to it as space – so allopatric simply means different space! This means there is some break in the geographic area or range that a species lives. This break could be in the form of a mountain, a river, or an island, for example. The break in area causes isolation of a portion of that species population. This isolation is key in this mechanism for speciation. Once the population of a species is separated by a barrier of some sort (mountain, river, island, etc.), the separte populations may undergo speciation due to difference in the climate, habitat, or food availability.
  • Parapatric speciation: Para means next to or near and patric means space – so simply it means near-space. We can think of it as adjacent or next to. This is where a species population get separated with rare cases of gene flow, but it is essentially isolated near the original population. This type of isolation is very different from allopatric speciation as there are no physical barriers involved. The separation is commonly by some severe or extreme change in the environment the animals are living in.
  • Sympatric speciation: Sym means same and, again, patric means space – so same space! Sympatric speciation occurs within the original species population. There are many supported hypotheses suggested for how sympatric speciation can occur but at a basic level a generation of the group of animals becomes genetically separated from the main population. This can occur by polyploidy where the offspring have twice the number of chromosomes of the parents.

Macroevolution

This figure includes the relationships among horses, which horses lived where, diet, and body sizes of the horse family over the past 55 million years. Figure from MacFadden, 2005.

Macroevolution adds in another component of time. This is long term clade (=natural evolutionary grouping) level change resulting from evolutionary processes. We can track the shifts and changes in biodiversity through time – this includes both speciation events and extinction events of various groups. This aspect of time can be incredibly long, over millions to hundreds of millions of years, or it can happen much more rapidly. It must not only affect a single species but a high level grouping.

The study of macroevolution involves examining patterns within the history of life. Broad research aims include things like: examining the stability or dynamic nature of how a group evolves through time; when do new groups arise and why; what features in a group do poorly with crisis events that lead to extinction, and so many more! In the figure displaying horse macroevolutionary patterns, the study tried to address many questions. The figure includes where the animals lived and what they ate. There are ways to quantitatively (mathematically) address these patterns once you gather the data. To read more about Patterns in Macroevolution, click here!

Proceed to ‘Taxonomy’