Chapter 24The Origin of Species

Macroevolution

• the origin of new taxonomic groups

Speciation: the origin of new species

• Anagenesis: accumulation of changes within a population, transforming that population into a new species

• Cladogenesis: branching evolution, whereby a new species arises from a parent species

Biological Species Concept

• Defines a species as a population or group of populations that have the potential to breed with each other in the wild and produce viable, fertile offspring

What are some of the limitations of the biological species concept?

Ecological Species Concept

• Defines a species in terms of its ecological niche

Morphological Species Concept

• Characterizes each species in terms of its unique set of structural features

Genealogical Species Concept

• Defines species in terms of its unique genetic history

Pluralistic Species Concept

• The factors that are most important for the cohesion of a species vary

Reproductive Isolation

• Factors that prevent interbreeding

Note: These are factors that intrinsic to the species itself and does not include geographic isolation

Reproductive barriers can either be prezygotic or postzygotic

• Zygote: fertilized egg

Prezygotic barriers

• Impede mating between species or hinder fertilization of ova if two members attempt to mate

Prezygotic Barriers

• Habitat isolation – if two species live within the same area but different habitats

• Behavioral Isolation – often depends upon courtship rituals

Prezygotic Barriers, cont’d

• Temporal Isolation: if two species mate during different times of the day, different seasons, or different years

• Mechanical isolation: two species are not anatomically compatible

• Gametic isolation: two gametes meet but fail to fertilize

Postzygotic Barriers

• Reduced hybrid viability: hybrids are not very healthy, don’t live to maturity

• Reduced hybrid fertility: hybrids are sterile• Hybrid breakdown: first-generation offspring

are fertile and viable but when these offspring mate with either each other or parent species, offspring are feeble or sterile

Modes of Speciation

• Based on how gene flow is interrupted

Allopatric Speciation

• Speciation takes place in populations with two geographically separate ranges

Sympatric Speciation

• Speciation takes place in geographically overlapping areas

• How might these barriers arise?

Punctuated Equilibrium Model

• Argues that species diverge in spurts of relatively rapid change instead of slowly and gradually

Exaptations

• Structures that evolve in one context but becomes co-opted for another function– Ex: feathers in birds

Evo-devo

• The link between evolutionary biology and the study of how organisms develop is called “evo-devo”

• Allows us to understand how small changes in the genome can lead to dramatic changes in an organism

Toolkit Genes

• Subset of genes used to pattern the body• Proteins encoded by these genes control the

formation, design, and patterning of most major features of animal design and diversity

Homeotic Genes

• Control placement and spatial organization of body parts

Hox genes

Ultrabithorax mutation

Eyeless

• First discovered in Drosophila. Humans have a homologous protein called Pax6.

• Homeotic gene. Responsible for turning on other genes (transcription factor)

• Loss of eyeless leads to a loss of eyes. (Flies without eyes)

If you turn on eyeless in another part of the fly -

Ectopic eyes!

Halder et al, 1995

Paedomorphosis

• When the adult species retains structures that were juvenile structures in an ancestral species

Ex: axolotl

Heterochrony

• Evolutionary change in the rate or timing of developmental events

Allometric Growth

• The relative growth rates of different parts during development

Allometric Growth Example – human and chimpanzee skulls