Chapter 24 The Origin of Species

Speciation – The process whereby members of one species become another species – A species can evolve through time without splitting to create a second species – Speciation studies the factors responsible for causing one species to split into two species

Microevolution explains gradual change due to adaptation and natural selection – If only microevolution occurred then there would only be one highly adapted version of the first life form

Speciation is the focal point of evolutionary theory, because the appearance of new species is the source of biological diversity Evolutionary theory – Must explain how new species originate in addition to how populations evolve Macroevolution – Refers to evolutionary change above the species level

The Biological Species Concept The biological species concept – Defines a species as a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but are unable to produce viable fertile offspring with members of other populations – Members are connected through gene flow

Species - Concepts Biological – Hinges on Reproductive isolation Reproductive isolation (acting only on sexually reproducing organisms) – Is the existence of biological factors that impede members of two species from producing viable, fertile hybrids – Is a combination of various reproductive barriers

Reproductive Isolation (barriers between more closely related species may not be so obvious). Prezygotic barriers- block fertilization from occurring Impede mating between species prevent an attempted mating from being completed successfully Hinder fertilization if mating is completed successfully Postzygotic barriers – Often prevent the hybrid zygote from developing into a viable, fertile adult Stop gene flow

Prezygotic and postzygotic barriers Figure 24.4 Prezygotic barriers impede mating or hinder fertilization if mating does occur Individuals of different species Mating attempt Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation HABITAT ISOLATION TEMPORAL ISOLATIONBEHAVIORAL ISOLATION MECHANICAL ISOLATION (b) (a) (c) (d) (e) (f) (g) Breeding at diff. times, diff. seasons Live in diff. Habitats. Do Not meet Little or no sexual attraction Structural Diff in genitalia Or pollen

Viable fertile offspring Reduce hybrid viability Reduce hybrid fertility Hybrid breakdown Fertilization Gametic isolation GAMETIC ISOLATION REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN (h) (i) (j) (k) (l) (m) Gametes die before uniting or fail to unite Hybrids fail to develop or to reach sexual maturity Hybrids fail to produce functional gametes Offspring of hybrids are weak or infertile

Speciation The two main ways speciation can take place – Allopatric – Sympatric

(a) Allopatric speciation. A population forms a new species while geographically isolated from its parent population. (b) Sympatric speciation. A small population becomes a new species without geographic separation. Figure 24.5 A, B Species – may originate by Allopatric speciation “other country” population is divided into geographically isolated subpopulations Can be geologic change or colonization of a new area Reproductive barriers will probably arise due to genetic drift and selection.

Geologic events or Processes for Allopatric speciation Mountain range may emerge that fragments a population Large lake may subside into smaller lakes – splitting a population Populations remain separated based on how well they can move about. – Birds can fly – Wind blows pollen – Small rodents may not be able to cross rivers

Process of Allopatric speciation Once separated the gene pools may diverge due to – Mutations – Natural selection – Genetic drift Reproductive isolation may then arise as a by- product of selection or drift This type of speciation can happen very quickly (a few million yrs) to isolated populations (island) that has very little gene flow

Species – May originate by Sympatric Speciation “same country” – Reproductive barriers occur in populations that live in the same geographic area Less likely than allopatric Factors that reduce gene flow – Polyploidy – Habitat differentiation – Sexual selection

Sympatric speciation Polyploidy- extra sets of chromosomes – Usually due to an accident during cell division Two types – Autopolyploid- individual that has more than two chromosome sets that are all derived from a single species Failure of cell division – cell goes from 2n to 4n, thus producing offspring that must mate with other tetraploids 2n = 6 4n = 12

Sympatric speciation Allopolyploid- sterile hybrids can be changed into fertile polyploid Hybrid with 7 chromosomes is sterile. New combinations will have a diploid number and is a new species Rarely seen in animals. Seen in plants

Habitat Differentiation in sympatric speciation Genetic factors enable a subpopulation to exploit a habitat or resource not used by the parent population Sexual Selection in sympatric speciation Example females choosing males by coloration in fish produced a reproductive barrier that keeps gene pools separate

The Tempo of Speciation How long does it take new species to form? How many genes change when one species splits into two? Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe abrupt episodes of speciation punctuating long periods of little change, or equilibrium.

The punctuated equilibrium model – Contrasts with a model of gradual change (gradualism) throughout a species’ existence Figure Gradualism model. Species descended from a common ancestor gradually diverge more and more in their morphology as they acquire unique adaptations. Time (a) Punctuated equilibrium model. A new species changes most as it buds from a parent species and then changes little for the rest of its existence. (b)

Speciation Rates New species can develop rapidly once divergence begins The time between speciation events (the time that elapses before populations of a newly formed species start to diverge from one another plus the time it takes for speciation to be complete) can vary considerable. Millions of years may pass before a newly formed species will itself give rise to another new species. There is no “speciation clock” Speciation will begin only after gene flow between populations is interrupted Once gene flow is interrupted there must be a genetic change to an extent that causes reproductive isolation

Gene Changes Reproductive isolation can occurred due to a change in a single gene – Ex. Snail shell spiraling Or it can be influenced by larger numbers of genes and gene interactions

Speciation to Macroevolution as speciation occurs differences can accumulate and become more pronounced eventually leading to the formation of new groups of organisms that differ greatly from their ancestors One group may increase in size by producing many new species, another may shrink, losing species to extinction The cumulative effects help shape changes documented in the fossil record