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.