Lecture 7: SPECIATION

Prezygotic Isolating Mechanisms

Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Incompatibility

A cline is a(n)

spatial gradient in the frequency of genotypes and phenotypes.

Sympatric Speciation (rare)

-Evolution of reproductive isolation within an initially randomly mating population, no spatial segregation of incipient species -differential selection must overwhelm gene flow

allopatric speciation (vicariance & peripatric)

-Populations are separated by a physical barrier -MOST COMMON mode of speciation in animals

Parapatric ("beside place") Speciation

-when geographically adjacent populations diverge into separate species WITHOUT an extrinsic barrier (gene flow is present) -selection, not isolation, initiates / dominates speciation process

Describe the primary difference between allopatric, parapatric, and sympatric speciation.

Allopatric speciation occurs when populations become geographically isolated from one another, preventing gene flow. Parapatric speciation occurs when two adjacent populations diverge in the absence of a geographic barrier. Sympatric speciation occurs between populations with no geographic barrier.

describe the potential effect of genetic drift in the peripheral isolate model of allopatric speciation?

Because the descendant populations can be founded by a relatively small number of individuals, genetic drift is more likely to have strong effects in the descendant populations.

How did the morphology of the Lake Apoyo cichlids help scientists understand the sympatric speciation process in this lake?

Body type morphology was indicative of fish that typically live at the bottom of the lake (benthic form) and near the upper surface (limnetic), separating the two species.

Scientists studying speciation often look at both nuclear DNA and mitochondrial DNA. Why is the genetic information obtained from mitochondrial DNA different from information obtained through nuclear DNA in sexually reproducing species?

In most organisms, mitochondrial DNA is inherited from the female, while nuclear DNA is inherited from both parents.

Parapatric speciation and sympatric speciation both describe how populations diverge into separate species without a geographic barrier to dispersal. What distinguishes these two modes of speciation?

In parapatric speciation, the populations are adjacent to one another and typically have a hybrid zone between the populations.

Which of the following statements about changes in ploidy is true?

It can result in instantaneous speciation.

Explain the difference(s) between prezygotic and postzygotic isolating mechanisms, and provide an example of each.

The key is to determine whether fertilization has taken place. In prezygotic isolating mechanisms, either mating is deterred or, if mating does occur, fertilization is not successful. In postzygotic isolating mechanisms, mating and fertilization have occurred but the offspring do not survive or they are sterile or they have dramatically reduced fitness. Examples given will vary but should be along the lines of what is listed in Table 14.2.

What is a fundamental problem with the application of the phenetic species concept?

The phenetic species concept uses phenotypic traits to create clusters, but phenotypic traits can be similar due to convergent evolution rather than common ancestry.

In which model of allopatric speciation is genetic drift a larger factor, and why?

Under the vicariance model, one large population is split into two or more populations of relatively equal size. Under the peripheral isolate model, one small population is split off from a larger population. Because genetic drift has a greater effect in small populations than in large populations, genetic drift will play a larger role in the peripheral isolate model.

The evolutionary species concept defines a species as

a group of populations that have a shared past and will have a shared future on a phylogenetic tree.

peripatric isolation

a small colony disperses from a larger population to a new area and, ultimately, evolves reproductive isolation

Modes of speciation

allopatric parapatric sympatric (hybridization)

The beginning of this chapter presents research conducted on mosquito populations. The researchers compared Culex pipiens and Culex pipiens molestus to confirm if they were separate species. How did they confirm they were separate species by applying the biological species concepts?

although all mating crosses between underground populations produced viable and fertile offspring, crosses between aboveground and underground individuals produced no offspring at all. The biological species concept is based on separation of species when there is not gene flow between the populations. Failure to reproduce indicates there is no gene flow between the mosquito populations; therefore, they can be considered separate species.

Host tree specificity of Rhagoletis pomonella reduces gene flow between the apple and hawthorn fly races by 4% to 6% in each generation, as individuals tend to mate with the same fruit preference. This is an example of

assortative mating

The ________ species concept defines a species as a cluster of individuals that occupy a particular niche.

ecological

In order to determine if individuals from separate populations are indeed distinct species, Drosophila biologists often test to see if individuals from different populations will mate with each other. This is an application of the ________ species concept.

biological

What is the most common reason for the disappearance of a hybrid zone, resulting in the completion of the speciation process?

when hybrids are at a selective disadvantage compared to offspring from within-population matings

Allopatric speciation requires

complete geographic isolation

Most sister species are rather similar in external appearance (morphology). But humans are clearly morphologically distinct from chimpanzee (although genetically quite similar) - which key evolutionary process explains this pattern?

extinction

A chromosomal rearrangement that involves the splitting of chromosomes is chromosomal

fission

Biological Species Concept (BSC)

groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups"

Postzygotic Isolating Mechanisms

hybrid inviability hybrid reproductive inviability

Ring Species

include a combination of parapatry & allopatry

Why are hybrid Ensatina salamanders at a selective disadvantage?

intermediate phenotype is not good for crypsis or camoflague

Changes in ploidy in plants are typically tolerated and have minor effects on survival or fertility, and it can often lead to rapid speciation. How are these changes preserved in the next generation?

the ability to self-fertilize helps the plants to reproduce itself, therefore preserving new ploidy

A small population of a species invades an island where the species is not found. The island population subsequently diverges from the ancestral population and becomes a new species. This is an example of

the peripheral isolate model of allopatric speciation.