Big Idea 1

Notes – Ch. 24 – The Origin of Species

The biological species concept• Species is a Latin word

meaning “kind” or “appearance”

• Remember, a species is a group of organisms that can reproduce fertile offspring

• If we can’t get a new species by just breeding two different species together, how do they occur?

Similarity between different species.

Diversity within a species.

• Speciation, the origin of new species, is at the focal point of evolutionary theory

• Evolutionary theory must explain how new species originate and how populations evolve

• Microevolution consists of adaptations that evolve within a population, confined to one gene pool

• Macroevolution refers to evolutionary change above the species level

• Two basic patterns of evolutionary change:– Anagenesis (phyletic

evolution) transforms one species into another

– Cladogenesis (branching evolution) is the splitting of a gene pool, giving rise to one or more new species

Reproductive Isolation• Reproductive isolation is the existence of

biological factors (barriers) that impede two species from producing viable, fertile hybrids

• Two types of barriers: prezygotic and postzygotic

• Prezygotic barriers impede mating or hinder fertilization if mating does occur:– Habitat isolation– Temporal isolation– Behavioral isolation– Mechanical isolation– Gametic isolation

• Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers– Flycatchers (birds) are found in

different habitats in the same area. One species prefers open woods and farmland; one frequents beech trees; one is found in alders, one in conifer woods, and one in willowy thickets.

LE 24-4a

Prezygotic barriers impede mating or hinder fertilization if mating does occur

Postzygotic barriers prevent a hybrid zygote fromdeveloping into a viable, fertile adult

REDUCED HYBRIDVIABILITY

REDUCED HYBRIDFERTILITY

HYBRID BREAKDOWN

HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION GAMETIC ISOLATION

Reducedhybrid

viability

FertilizationViable,fertile

offspring

Reducedhybridfertility

Hybridbreakdown

Matingattempt

Gameticisolation

Fertilization

Mechanicalisolation

Behavioralisolation

Temporalisolation

Habitatisolation

Individualsof

differentspecies

• Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes

• Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers– Courtship rituals are only understood by species that share the same

communication system

• Mechanical isolation: Morphological differences can prevent successful mating– the genital organs of different

species are incompatible– The flowers of black sage and

white sage are structurally different and are pollinated by different species of insects. In this example, each insect species pollinates flowers of only one of the sage species. Therefore, interbreeding does not occur.

• Gametic isolation: the egg and sperm of different species are incompatible. – Gametic isolation is

particularly important in aquatic environments because many aquatic animals release their gametes into the water, where fertilization takes place.

LE 24-4aa

Prezygotic barriers impede mating or hinder fertilization if mating does occur

HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION GAMETIC ISOLATION

Matingattempt

Gameticisolation

Fertilization

Mechanicalisolation

Behavioralisolation

Temporalisolation

Habitatisolation

Individualsof

differentspecies

• Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:– Reduced hybrid viability– Reduced hybrid fertility– Hybrid breakdown

• Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development (lowered potential to survive compared to parents)

• Reduced hybrid fertility:when two different species mate with one another and produce NONFERTILE offspring. – The reason for this is that the 2

species have different chromosome numbers, so when the offspring go to perform meiosis, their chromosomes cannot segregate in a manner that allows them to be fertile.

– The classic example is when a horse is mated to a donkey, producing a mule. Both the horse and the donkey are obviously fertile, but the MULE is not, because it has a different chromosome number.

• Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile– an interspecies hybrid is viable

and fertile, but subsequent generations harbour detrimental genetic abnormalities.

LE 24-4ab

Postzygotic barriers prevent a hybrid zygote fromdeveloping into a viable, fertile adult

REDUCED HYBRIDVIABILITY

REDUCED HYBRIDFERTILITY HYBRID BREAKDOWN

Reducedhybrid

viability

FertilizationViable,fertile

offspring

Reducedhybridfertility

Hybridbreakdown

Limitations of the Biological Species Concept• The biological species concept does not apply to– Asexual organisms– Fossils– Organisms about which little is known regarding

their reproduction

Other Definitions of Species

• Morphological: defines a species by structural features

• Paleontological: focuses on morphologically discrete species known only from the fossil record

• Ecological: views a species in terms of its ecological niche

• Phylogenetic: defines a species as a set of organisms with a unique genetic history

Speciation can take place with or without geographic separation

• Speciation can occur in two ways:– Allopatric speciation– Sympatric speciation

LE 24-5

Allopatric speciation Sympatric speciation

Allopatric (“Other Country”) Speciation

• In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations

• One or both populations may undergo evolutionary change during the period of separation

• To determine if allopatric speciation has occurred, reproductive isolation must have been established

LE 24-6

A. harrisi A. leucurus

Sympatric (“Same Country”) Speciation

• In sympatric speciation, speciation takes place in geographically overlapping populations

• No physical barrier dividing the population – Usually caused by

mutations or a change in behavior

Polyploidy• Polyploidy is presence of extra

sets of chromosomes due to accidents during cell division– It has caused the evolution of

some plant species• An autopolyploid is an

individual with more than two chromosome sets, derived from one species (potato)

• An allopolyploid is a species with multiple sets of chromosomes derived from different species (wheat)

LE 24-8

Failure of cell division in a cell of a growing diploid plant after chromosome duplication gives rise to a tetraploid branch or other tissue.

Gametes produced by flowers on this tetraploid branch are diploid.

Offspring with tetraploid karyo-types may be viable and fertile—a new biological species.

2n = 64n = 12 4n

2n

LE 24-9

Species B2n = 6

Species A2n = 4

Normal gameten = 3

Normal gameten = 3

2n = 10

Unreduced gametewith 4 chromosomes

Unreduced gametewith 7 chromosomesHybrid with

7 chromosomesViable fertile hybrid

(allopolyploid)Meiotic error;chromosomenumber notreduced from2n to n

Habitat Differentiation and Sexual Selection

• Sympatric speciation can also result from the appearance of new ecological niches

• In cichlid fish, sympatric speciation has resulted from nonrandom mating due to sexual selection– the fish have genetic variation, and some of this

variation affects the fishes' ability to see different colors. Some fish have genes that enable them to see blue light better, while other fish have a red light advantage. Because of the differential penetration of light into the lake, fish with gene versions sensitizing them to blue light have an advantage in shallower waters because they can better find food and spot predators there, while fish tuned to red light have an advantage in deeper waters. So in different parts of the fishes' habitat, different color-sensitivity genes are favored by natural selection. Over many generations, if the fish don't move too much within their range, blue sensitivity will evolve to be more common among fish living near the surface and red sensitivity will become more common among fish living further down the slope.

Adaptive Radiation• Adaptive radiation is the evolution of diversely

adapted species from a common ancestor upon introduction to new environmental opportunities

Studying the Genetics of Speciation

• The explosion of genomics (a discipline in genetics that sequences, assembles, and analyzes the function and structure of genomes) is enabling researchers to identify specific genes involved in some cases of speciation

The Tempo of Speciation• The fossil record includes many

episodes in which new species appear suddenly in a geologic stratum, persist essentially unchanged through several strata, and then apparently disappear

• Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis punctuated by sudden change

• The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence

Macroevolutionary changes can accumulate through many speciation events

• Macroevolutionary change is cumulative change during thousands of small speciation episodes

Evolutionary Novelties• Most novel biological

structures evolve in many stages from previously existing structures– Some complex

structures, such as the eye, have had similar functions during all stages of their evolution

Changes in Rate and Timing• Heterochrony is an evolutionary change in the rate or timing of

developmental events and is used for comparison of different organisms

• For example, the timing of the development of fins in one fish species can be heterochronic to that of another resulting in different shapes or sizes of fins and other body parts.

• It can have a significant impact on body shape

• Allometric growth refers to different growth rates of parts of a particular organism.

• Allometric growth is the proportioning that helps give a body its specific form– This can refer to changes in growth rate

over time and explains why babies have a bigger head in proportion to limbs than an adult. • As a human grows, the growth rates of the

head and limbs are different.– Different allometric patterns contribute to

the contrasting shapes of human and chimpanzee skulls• in chimpanzee fetuses brain and head growth

starts at about the same developmental stage and present a growth rate similar to that of humans, but end soon after birth. Humans, on the contrary, continue their brain and head growth several years after birth

LE 24-15b

Chimpanzee fetus Chimpanzee adult

Human fetus Human adultComparison of chimpanzee and human skull growth

• Paedomorphosis – the retention by an organism of juvenile or even larval traits into later life.

• The rate of reproductive development accelerates compared with somatic development– The sexually mature species

may retain body features that were juvenile structures in an ancestral species

Ground-dwelling salamander

Tree-dwelling salamander

Changes in Spatial Pattern• Substantial evolutionary change

can also result from alterations in genes that control the placement and organization of body parts

• Homeotic genes cause the development of specific structures in plants and animals

• Homeotic genes determine such basic features as where wings and legs will develop on a bird or how a flower’s parts are arranged

• The products of one class of homeotic genes called Hox genes

• Hox genes provide positional information in the development of fins in fish and limbs in tetrapods

• Evolution of vertebrates from invertebrate animals was associated with alterations in Hox genes

•

LE 24-18

Chicken leg bud

Region ofHox gene

expression

Zebrafish fin bud

LE 24-19

Hypothetical vertebrateancestor (invertebrate)with a single Hox cluster

First Hox duplication

Hypothetical earlyvertebrates (jawless)with two Hox clusters

Second Hox duplication

Vertebrates (with jaws)with four Hox clusters

Evolution Is Not Goal Oriented• The fossil record often shows apparent trends in

evolution that may arise because of adaptation to a changing environment

LE 24-20

Pleistocene

Sinohippus

Recent

Pliocene

AnchitheriumMiocene

Paleotherium

Oligocene

Propalaeotherium

Eocene Pachynolophus

Hyracotherium

Mesohippus

Miohippus

Orohippus

Epihippus

Key

GrazersBrowsers

Hypohippus

Parahippus

Archaeohippus

Merychippus

CallippusMegahippus

Pliohippus

Nannippus

Hipparion Neohipparion

Hippidion and other generaEquus

• According to the species selection model, trends may result when species with certain characteristics endure longer and speciate more often than those with other characteristics

• The appearance of an evolutionary trend does not imply that there is some intrinsic drive toward a particular phenotype