Species and Speciation

Phylogenetic Species Concept

• A phylogenetic species is defined as an irreducible (basal) cluster of organisms that are diagnosably different from other such clusters.

• Within a phylogenetic species, there exists a parental pattern of ancestry and descent.

Evolutionary Species Concept

• An evolutionary species is a single lineage of ancestor-descendant populations.

• This lineage maintains its identity from other such lineages across both space and time.

• It possesses its own unique evolutionary tendencies and historical fate (Wiley, 1981).

• The phylogenetic species concept (PSC) and evolutionary species concept (ESC) define what species are, but do not specify how to identify them.

Species and Speciation

• Key questions include:

  • What defines a species?

  • How do new species arise (speciation)?

Reproductive Isolation

• Reproductive isolating barriers or mechanisms play a significant role in many cases of speciation.

• The inability to interbreed indicates that populations are likely separate species.

• However, the ability to interbreed does not necessarily confirm that populations belong to the same species.

• Reproductive isolation will eventually evolve when populations are isolated from one another.

Types of Reproductive Isolation

• Two basic types of reproductive isolation (Table 9.1):

  • Prezygotic barriers: No zygote is produced; these barriers can be pre-mating or post-mating.

  • Postzygotic barriers: A zygote is produced; these barriers can be extrinsic or intrinsic.

Prezygotic Barriers

• Figure 9.9 and 9.10 illustrate various prezygotic barriers (Ecological and Sexual).

Prezygotic Barriers - Box 9B

• Example:

  • Initial sample from the wild.

  • One group raised on starch.

  • Another group raised on maltose.

  • Illustrates a prezygotic isolation mechanism related to food source adaptation.

Sperm/Egg Incompatibility

• Sperm/egg incompatibility can act as a prezygotic barrier.

• This is particularly important in species with external fertilization.

Postzygotic Barriers

• Postzygotic barriers can be:

  • Extrinsic

  • Intrinsic

Extrinsic Postzygotic Barriers

• Figure 9.11 illustrates extrinsic postzygotic barriers.

• Example involves melpomene F_1 hybrid cydno, showing proportion attacked.

Intrinsic Postzygotic Barriers

• Hybrids either do not develop properly or are sterile.

• These barriers may evolve after speciation has begun.

Intrinsic Postzygotic Barriers

• Incompatible interactions of genes can lead to postzygotic isolation.

• Dobzhansky-Muller Incompatibilities

• Chromosomal structure (ploidy, fusions, etc.) differences can also cause intrinsic postzygotic isolation.

• These barriers may evolve after speciation has begun.

Dobzhansky-Muller Incompatibilities

• Figure 9.12 illustrates the Dobzhansky-Muller model.

• Ancestor population: A1A1B1B1

• Geographic separation leads to genetic divergence.

• Population 1: A1A1B1B1 -> A2A2B1B1

• Population 2: A1A1B1B1 -> A1A1B2B2

• F1 hybrids: A1A2B1B_2

Intrinsic: DMI Arabidopsis thaliana

• Gene duplication occurs.

• In one population, one copy accumulates mutations that destroy its function.

• In the other population, the other copy accumulates mutations.

• Hybrids are often homozygous for non-functional copies of both genes, leading to incompatibility.

Intrinsic Barriers: Chromosomal Structure

• Chromosomal structure differences (ploidy, fusions, etc.) can lead to postzygotic isolation.

• These barriers may evolve after speciation.

Chromosome Fusion

• Chromosome fusion in one race can produce meiosis problems in hybrids.

• Figure 9.14 shows an example with Novosibirsk hybrids and Tomsk.

• Frequency of Novosibirsk type varies with transect distance.

Case Study of Isolation Barriers: Monkeyflowers

• Examples:

  • M. lewisii (A)

  • M. cardinalis (B)

Reproductive Isolation in Monkeyflowers

• Different factors contribute to reproductive isolation:

  • Elevation

  • Pollinator

  • Pollen precedence

  • Hybrid germination

  • Hybrid fertility

• Figure 9.8 shows the contribution to isolation for M. cardinalis and M. lewisii.

Role of Reproductive Isolation Barriers (RIBs) in Speciation

• RIBs are not strictly necessary for speciation.

  • They could have evolved in isolation.

  • They could have evolved after other barriers.

• However, they may still play a significant role in maintaining species separation.

How Fast is Speciation?

• Polyploidy can lead to very rapid speciation.

• Otherwise, speciation rates are highly variable.

• Lake Victoria: ~1 speciation per 2000 years.

• Drosophila species pairs: 200,000 to 2.7 million years.

• Prezygotic mechanisms often evolve faster than postzygotic mechanisms.

• Many sister species are fully compatible post-

Causes of Speciation

• Paradox: Reproductive isolation reduces reproduction in individuals.

• Explanation: It doesn't take much gene flow to keep a population homogenous.

• Therefore, speciation often starts with geographic isolation.

• Something must interrupt gene flow before isolating mechanisms can evolve.

Reinforcement of Mechanisms

• Selection cannot usually cause or reinforce postzygotic mechanisms because selection works against them.

• Selection can reinforce prezygotic mechanisms if postzygotic mechanisms have already evolved.

When Will Selection Favor Prezygotic Mechanisms?

• When postzygotic mechanisms are already present.

• When populations are sympatric.

Example of Allopatry and Sympatry

• P. drummondii and P. cuspidata in Texas.

• (A) Map showing locations.

• (B) Allopatry vs. Sympatry.

• (C) Fruit set and relative hybridization based on flower color genotype.

Strength of Prezygotic Isolation

• Graph comparing allopatric and sympatric taxa.

• The strength of prezygotic isolation increases with genetic distance in sympatric taxa.

Polyploidy as a Cause of Speciation

• Differences in the number of sets of chromosomes can result in instant reproductive isolation.

Geography of Speciation

• Gene flow tends to counter divergence by selection or genetic drift.

• Speciation usually requires geographic isolation to begin.

Geography of Speciation Types

• Allopatric: Probably very common.

• Sympatric: Probably rare (with exceptions).

• Parapatric: Between allopatric and sympatric.