Origin of Species

Gen Bio 2, Unit 3: Macroevolution

Speciation

The focal point of evolutionary theory.

Evolutionary Theory

What explains how new species originate and how populations evolve?

Microevolution

It consists of changes in allele frequency in a population over time.

Macroevolution

It refers to broad patterns of evolutionary change above the species level.

Species

The Latin word meaning “kind” or “appearance”.

Biological Species Concept

* It states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations
* It cannot be applied to fossils or asexual organisms (including all prokaryotes)
* Emphasizes the absence of gene flow
* However, gene flow can occur between distinct species
* Example: Grizzly Bear + Polar Bear = Grolar Bear

Gene flow

_____ between populations holds the phenotype of a population together.

Reproductive Isolation

* It is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring


* It can be classified by whether factors act before or after fertilization

Hybrids

* They are the offsprings of crosses between different species
* The result of mating between species with incomplete reproductive barriers
* They often have reduced fitness compared with parent species

1. Prezygotic Barriers
2. Postzygotic Barriers

The 2 major types of reproductive barriers.

Prezygotic Barriers

They block fertilization from occurring by:

* Impeding different species from attempting to mate
* Preventing the successful completion of mating
* Hindering fertilization if mating is successful

Postzygotic Barriers

They prevent the hybrid zygote from developing into a viable, fertile adult by:

* Reduced hybrid viability
* Reduced hybrid fertility
* Hybrid breakdown

1. Habitat Isolation
2. Temporal Isolation
3. Behavioral Isolation
4. Mechanical Isolation
5. Gametic Isolation

The types of prezygotic barriers.

Habitat Isolation

* Prezygotic barrier
* Two species encounter each other rarely, or not at all, because they occupy different habitats, even though they are not isolated by physical barriers.

Temporal Isolation

* Prezygotic barrier
* Species that breed at different times of the day, different seasons, or different years cannot mix their gametes.

Behavioral Isolation

* Prezygotic barrier
* Courtship rituals and other behaviors unique to a species are effective barriers.

Mechanical Isolation

* Prezygotic barrier
* Morphological differences can prevent successful mating.

Gametic Isolation

* Prezygotic barrier
* The sperm of one species may not be able to fertilize the eggs of another species.

1. Reduced Hybrid Viability
2. Reduced Hybrid Fertility
3. Hybrid Breakdown

The types of postzygotic barriers.

Reduced Hybrid Viability

* Postzygotic barrier
* Genes of the different parent species may interact and impair the hybrid’s development.
* The offspring is born, but it will die shortly after

Reduced Hybrid Fertility

* Postzygotic barrier
* Even if hybrids are vigorous, they may be sterile.

Hybrid Breakdown

* Postzygotic barrier
* Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile.

1. Biological Species Concept
2. Morphological Species Concept
3. Ecological Species Concept
4. Phylogenetic Species Concept

The different species concepts.

Morphological Species Concept

* Species concept
* It defines a species by structural features.

Ecological Species Concept

* Species concept
* It views a species in terms of its ecological niche.

Ecological Niche

* It is the role and position of a species within an ecosystem
* It refers to the specific environmental conditions and resources that a species needs to survive, grow, and reproduce
* It includes its interactions with other organisms in the ecosystem, as well as the physical and chemical factors of the environment that it requires
* It is determined by its physical characteristics, behavior, and evolutionary adaptations

Phylogenetic Species Concept

* Species concept
* It defines a species as the smallest group of individuals on a phylogenetic tree.

1. Allopatric Speciation
2. Peripatric Speciation
3. Parapatric Speciation
4. Sympatric Speciation

The modes of speciation.

Allopatric Speciation

* Mode of speciation
* Gene flow is interrupted or reduced when a population is divided into **geographically isolated** subpopulations.

Peripatric Speciation

* Mode of speciation
* A small subset of a population is isolated (becomes peripheral), and genetic drift and natural selection drives the evolution of reproductive isolation between the source and founding populations
* New niche entered → in isolated niche
* Small group migrates to a new area or gets physically separated from the original group
* The isolated group may feel different environmental pressures or genetic drift leading to genetic differences

Parapatric Speciation

* Mode of speciation
* Speciation happens because of **divergence** in assumed **niches**
* New niche entered → in adjacent niche
* New environment but not physically separated from the original group
* The differences in their environments may lead to differences in their traits and behaviors, which can lead to reproductive isolation and the formation of new species

Sympatric Speciation

* Mode of speciation
* Speciation takes place in **geographically overlapping** populations.

Polyploidy

* It’s the presence of extra sets of chromosomes due to accidents during cell division
* It’s much more common in plants than in animals.

Autopolyploid

An individual with more than two chromosome sets, derived from one species.

Allopolyploid

A species with multiple sets of chromosomes derived from different species.

Hybrid Zone

* A region in which members of different species mate and produce hybrids
* It can occur in a single band where adjacent species meet