Warm-up- 5 minutes Explain the biological species concept.

Today’s structure Warm-up Lecture Solve it! With a partner Finish clover case study

Homework Read lab manual Due Friday Bring book on Friday for review Read pp 493-504 MasteringBiology HW, due Friday midnight

The Biological Species Concept The biological species concept: a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations Gene flow between populations holds the phenotype of a population together

(a) Similarity between different species Figure 24.2a Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity. (a) Similarity between different species

(b) Diversity within a species Figure 24.2b Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity. (b) Diversity within a species

Reproductive Isolation Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring Hybrids are the offspring of crosses between different species Reproductive isolation can be classified by whether factors act before or after fertilization

Prezygotic barriers (a) (c) (e) (f) (g) (d) (b) Figure 24.3_b Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Isolation Individuals of different species MATING ATTEMPT FERTILIZATION (a) (c) (e) (f) (g) (d) (b) Figure 24.3 Exploring: Reproductive Barriers

Postzygotic barriers Reduced Hybrid Viability Reduced Hybrid Fertility Figure 24.3_c Postzygotic barriers Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown VIABLE, FERTILE OFFSPRING FERTILIZATION (h) (i) (l) (j) Figure 24.3 Exploring: Reproductive Barriers (k)

Speciation Process by which one species splits into one or more species

Limitations of the Biological Species Concept The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) The biological species concept emphasizes absence of gene flow However, gene flow can occur between distinct species For example, grizzly bears and polar bears can mate to produce “grolar bears”

Grizzly bear (U. arctos) Figure 24.4 Grizzly bear (U. arctos) Polar bear (U. maritimus) Figure 24.4 Hybridization between two species of bears in the genus Ursus. Hybrid “grolar bear”

Other Definitions of Species Other species concepts emphasize the unity within a species rather than the separateness of different species The morphological species concept defines a species by structural features It applies to sexual and asexual species but relies on subjective criteria

The ecological species concept views a species in terms of its ecological niche It applies to sexual and asexual species and emphasizes the role of disruptive selection The phylogenetic species concept defines a species as the smallest group of individuals on a phylogenetic tree It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species

Allopatric (“Other Country”) Speciation In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations For example, the flightless cormorant of the Galápagos likely originated from a flying species on the mainland

The Process of Allopatric Speciation The definition of barrier depends on the ability of a population to disperse For example, a canyon may create a barrier for small rodents, but not birds, coyotes, or pollen

A. harrisii A. leucurus Antelope squirrels, Grand Canyon Figure 24.6 Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon. Antelope squirrels, Grand Canyon

Reproductive isolation may arise as a result of genetic divergence Separate populations may evolve independently through mutation, natural selection, and genetic drift Reproductive isolation may arise as a result of genetic divergence For example, mosquitofish in the Bahamas comprise several isolated populations in different ponds (a) Under high predation (b) Under low predation

Evidence of Allopatric Speciation Figure 24.8 Evidence of Allopatric Speciation 15 pairs of sibling species of snapping shrimp (Alpheus) are separated by the Isthmus of Panama These species originated 9 to 13 million years ago, when the Isthmus of Panama formed and separated the Atlantic and Pacific waters A. formosus A. nuttingi Atlantic Ocean Isthmus of Panama Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus). Pacific Ocean A. panamensis A. millsae

Regions with many geographic barriers typically have more species than do regions with fewer barriers Reproductive isolation between populations generally increases as the distance between them increases For example, reproductive isolation increases between dusky salamanders that live further apart

Degree of reproductive isolation Figure 24.9 2.0 1.5 Degree of reproductive isolation 1.0 0.5 Figure 24.9 Reproductive isolation increases with distance in populations of dusky salamanders. 0 50 100 150 200 250 300 Geographic distance (km)

EXPERIMENT Initial population of fruit flies (Drosophila Figure 24.10 EXPERIMENT Initial population of fruit flies (Drosophila pseudoobscura) Some flies raised on starch medium Some flies raised on maltose medium Mating experiments after 40 generations RESULTS Female Female Starch population 1 Starch population 2 Starch Maltose Figure 24.10 Inquiry: Can divergence of allopatric populations lead to reproductive isolation? Starch population 1 Starch 22 9 18 15 Male Male population 2 Starch Maltose 8 20 12 15 Number of matings in experimental group Number of matings in control group

Sympatric (“Same Country”) Speciation In sympatric speciation, speciation takes place in geographically overlapping populations

Solve it! 15 minutes You have two populations of birds from the same species. These two populations are geographically separated by a mountain range. One population is much smaller than the other. 1) How would you determine if these two populations’ genetic compositions were changing from one generation to the next? 2) What mechanisms would most likely lead to this evolutionary change in each of these populations? Explain these mechanisms. 3) How would you test whether or not these two populations have become two different species? 4) What type of speciation would this be categorized as?

Homework Read lab manual Due Friday Bring book on Friday for review Read pp 493-504 MasteringBiology HW, due Friday midnight

Finish Clover Case Study