Notes
Speciation
Sections
Species
Definitions
- Morphological, wherein a species comprises individuals with similar characteristics that reflect relatedness.
- Biological, wherein a species comprises individuals that are capable of reproducing viable offspring.
- Evolutionary, wherein a species comprises individuals within a single lineage and shared ancestry.
- Ecological, wherein a species comprises individuals that are adapted to a particular environment.
Based on these characteristics and others, more than 20 species definitions have been proposed, but each has its shortcomings; for example, the morphological definition of species pre-dates molecular analysis, and the biological definition doesn't apply to asexually reproducing organisms.
Mechanisms
- Speciation occurs when a single population diverges into two or more reproductively isolated species.
- Allopatric and sympatric speciation are at extreme ends of a continuum; for example, peripatric and parapatric speciation are omitted here for simplicity.
Allopatric speciation:
- Occurs when a physical barrier, such as a river or mountain, creates a barrier to reproduction.
- Example: Single beetle species is divided by the formation of a new river; the two new populations continue to evolve independently until they form disparate species.
Sympatric speciation:
- Occurs when there are non-physical barriers to reproduction.
- Example: Non-physical disruptions in gene flow results in speciation.
- Disruptive selection, in which the phenotypes at the extreme ends of a continuum are selected for.
- Ecological isolation, in which portions of the population thrive in different ecological settings within the same geographic area (for example, some feed on apples and others on pears within the same orchard, and, thus, never cross paths).
- Genetic changes, such as polyploidy (the appearance of extra chromosomes).
Barriers to reproduction:
- Pre-zygotic, which includes barriers that block fertilization.
- For example, our first beetle species encountered a major pre-zygotic barrier to reproduction – the river separated them and lead to allopatric speciation.
- Post-zygotic, which includes barriers that reduce hybrid viability and/or fertility.
- For example, the sister species produced in our second beetle example may intermingle; however, they may encounter post-zygotic barriers, such as molecular incompatibilities, that prohibit development and/or fertility of hybrids.
Hybrid zones:
- Regions in which reproductive barriers are incomplete and the two species interbreed.
- Can result from process of sympatric speciation.
- The consequences of interbreeding vary, and include:
- Reinforcement of the reproductive barrier, which promotes further sympatric speciation; for example, if our sister beetle species interbreed, but post-zygotic barriers prevent the production of viable offspring, the species may continue to diverge.
- Fusion of species occurs when the reproductive barriers are relatively weak, and, eventually, speciation "reverses" – the two species fuse to form a single new species.
- Stabilization within the hybrid zone promotes the continued formation of hybrids.
Speciation Rates:
- Gradualism = evolutionary changes occur and accumulate at a slow, steady rate.
- Punctuated equilibrium = long periods of stability are "punctuated" by relatively rapid bursts of evolutionary changes and speciation.
These need not be mutually exclusive; it is likely that some species have evolved slowly and steadily, while others have passed through alternating periods of stability and rapid change.
Trait sorting based on evolutionary history:
Despite the vast number of species and the variation within and between them, several features, such as the eye and wing, show up in both closely related and unrelated species (be aware that this is relative; if we go back back far enough in history, all organisms share a common ancestor).
- Homologous traits, aka, homologies, are similar because they are derived from a common ancestor.
- Parallel traits are similar traits that evolve in separate species after divergence from a common ancestor.
- Parallel traits may appear in separate lineages due to similar ecological pressures.
- Analogous traits are those that arise independently long after divergence occurs.
- Products of convergent evolution.
- Similar features may arise in unrelated species due to similar ecological pressures.
Full-Length Text
- Here we will learn about the process of speciation and how to classify similar features by their evolutionary histories.
- To begin, start a table, and denote that species have been defined based on the following characteristics:
- Morphological, wherein a species comprises individuals with similar characteristics that reflect relatedness.
- Biological, wherein a species comprises individuals that are capable of reproducing viable offspring;
- Evolutionary, wherein a species comprises individuals within a single lineage and shared ancestry; and,
- Ecological, wherein a species comprises individuals that are adapted to a particular environment.
- Based on these characteristics and others, more than 20 species definitions have been proposed, but each has its shortcomings; for example, the morphological definition of species pre-dates molecular analysis, and the biological definition doesn't apply to asexually reproducing organisms.
- Next, denote that speciation occurs when a single population diverges into two or more species;
- Allopatric speciation occurs when a physical barrier, such as a river or mountain, creates a barrier to reproduction;
- Sympatric speciation occurs when there are non-physical barriers to reproduction.
Let's illustrate these concepts:
- First, show a single species of beetles;
- Then, show that a river forms and divides the beetle species into two subpopulations, which continue to evolve independently until they form disparate species.
- To illustrate sympatric speciation, re-draw our original beetle species;
- Then, indicate non-physical disruptions in gene flow results in speciation.
- Indicate that such barriers can include:
- Disruptive selection, in which the phenotypes at the extreme ends of a continuum are selected for;
- Ecological isolation, in which portions of the population thrive in different ecological settings within the same geographic area (for example, some feed on apples and others on pears within the same orchard, and, thus, never cross paths); or,
- Genetic changes, such as polyploidy (the appearance of extra chromosomes).
Be aware that allopatric and sympatric speciation are at extreme ends of a continuum; for example, peripatric and parapatric speciation are omitted here for simplicity.
- In both allopatric and sympatric speciation, the new species are reproductively isolated, which means that they can no longer mate and produce viable offspring.
- Indicate that barriers to reproduction can be classified as either:
- Pre-zygotic, which includes barriers that block fertilization, or,
- Post-zygotic, which includes barriers that reduce hybrid viability and/or fertility.
- For example, our first beetle species encountered a major pre-zygotic barrier to reproduction – the river separated them and lead to allopatric speciation.
- On the other hand, the sister species produced in our second beetle example may intermingle; however, they may encounter post-zygotic barriers, such as molecular incompatibilities, that prohibit development and/or fertility of hybrids.
- Denote in our table that the process of sympatric speciation can create hybrid zones, which are regions in which reproductive barriers are incomplete and the two species interbreed.
- The consequences of interbreeding vary, and include:
- Reinforcement of the reproductive barrier, which promotes further sympatric speciation; for example, if our sister beetle species interbreed, but post-zygotic barriers prevent the production of viable offspring, the species may continue to diverge.
- Fusion of species occurs when the reproductive barriers are relatively weak, and, eventually, speciation "reverses" – the two species fuse to form a single new species.
- Finally, stabilization within the hybrid zone promotes the continued formation of hybrids.
Biologists also study the rate at which speciation occurs;
- Indicate that, in gradualism, evolutionary changes occur and accumulate at a slow, steady rate;
- In punctuated equilibrium, long periods of stability are "punctuated" by relatively rapid bursts of evolutionary changes and speciation.
- These need not be mutually exclusive; it is likely that some species have evolved slowly and steadily, while others have passed through alternating periods of stability and rapid change.
Finally, let's learn how biologists sort similar traits based on their evolutionary history.
- Despite the vast number of species and the variation within and between them, several features, such as the eye and wing, show up in both closely related and unrelated species (be aware that this is relative; if we go back back far enough in history, all organisms share a common ancestor).
- So, first indicate that homologous traits, aka, homologies, are similar because they are derived from a common ancestor;
- To show this, indicate that an ancestral beetle has a purple shell phenotype;
- Then, indicate that its daughter species, A and B, also have purple shells.
- Next, write that parallel traits are similar traits that evolve in separate species after divergence from a common ancestor;
- To show this, indicate a new common ancestor (we'll give ours a teal shell), and show that it gives rise to daughter species with different-colored shells.
- Then, show that these give rise to their own daughter species, which both happen to have purple shells.
- Parallel traits may appear in separate lineages due to similar ecological pressures.
- Finally, write that analogous traits are those that arise independently long after divergence occurs; they are the products of convergent evolution.
- To illustrate convergent evolution, we need to show two common ancestor species; give one green shells, and the other, blue shells;
- Indicate that each species gives rise to daughter species, which then give rise their own daughter species.
- Indicate that both lineages eventually produce species with purple shells.
- As with parallel traits, similar features may arise in unrelated species due to similar ecological pressures.