Evolution 17 & 18: Species Concepts & Mechanisms of Speciation

1. Evolution 17 & 18: Species Concepts&Mechanisms of Speciation

3. Species Diversity • The number of living species may be as great as 100 million! • Speciation may have occurred billions of times in the last 3.8 billion years • All cultures recognize different forms (species) and name them • The challenge to scientists is to name and classify species in a testable way

4. Systematics is the evolutionary biology subfield that deals with taxonomy, understanding speciation, describing species, inferring phylogenies, and similar endeavors

5. So what is a Species??? • Smallest independently evolving unit • Independent evolution occurs when mutation, selection, gene flow, and drift operate independently in different populations • Therefore, these species will follow independent evolutionary trajectories • The essence of speciation is lack of gene flow • All modern species “concepts” really fall under a lineage concept • The different species “concepts” are really criterion for determining when you have lineage formation • Defining species is extraordinarily contentious • Potentially 20+ different species concepts

6. What is a Species??? https://www.youtube.com/watch?v=9fOfFlMe6ek Another video if you need it: https://www.youtube.com/watch?v=dnfaiJJnzdE

7. Remember:A species is a hypothesis!!!!!!!!!Species will have a set of characteristics and certain kinds of characteristics are what are used to define what is a species under each concept---e.g., if reproductive isolation is the requirement and then we find out that these two species breed freely, we would reassess our hypothesisWhat is and isn’t a species is different under strict concepts…

8. Biological Species Concept (BSC) (as proposed by E. Mayr) • "Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups." • The Biological Species Concept is the first modern attempt to define species, but numerous problems with this concept, on both theoretical and practical grounds

9. Problems with Biological Species • Practical problems • asexual species? • allopatric species? (most borderline cases don't occur sympatrically) • geological time? • level of reproductive isolation? (hybrids occur naturally!) • Theoretical problems • reproductive compatibility is a primitive character; reproductive isolation is derived • many cases, sister lineages are reproductively isolated, but distantly related lineages are not • The BSC doesn't apply to allopatric species, parthenogenetic species, or fossil species; highly limited (long held by ornithologists)

10. Problems with Biological Species

11. Morphological Species Concept Based on morphological differences between individuals

12. Morphological Species Concept Problems (hint, each image shows a single species!) Morphological Polymorphism

13. Ecological Species Concept Based on ecological (habitat use, diets, overall niche) differences between individuals

14. Ecological Species Concept Based on ecological (habitat use, diets, overall niche) differences between individuals

15. Evolutionary Species Concept (ESC) • Closer to defining species as individuals and as lineages which have beginnings and ends • Evolutionary species concept (as proposed by Simpson, modified by Wiley)"A single lineage of ancestor-descendent populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate" • Lineages may separate, become species, and come back together in the future (at which time they are no longer different species). This a very mutable concept.

16. More ideas about the Evolutionary Species Concepts • All organisms, past and present, belong to some evolutionary species • Species must be isolated from each other to the extent that this is required to maintain their separate identities, tendencies, and fate. • Therefore, reproductive isolation in the BSC sense is not required • Moreover, Evolutionary species may or may not exhibit recognizable phenetic differences (# of species may be over or under-estimated, usually the latter) • No separate, single evolutionary lineage may be subdivided into a series of ancestral and descendent species

17. ESC: Benefits and Problems • Benefits: clear conceptually; applies to asexual species, through time, and allopatric species • Problems: application (difficult to know the future), asexual species (too many independent lineages), • The Evolutionary Species Concept separates the idea of what species are from how they are recognized. This leads us to: How are species recognized?

18. Phylogenetic Species Concept • The practical approach of dealing with the more philosophically grounded ESC • Phylogenetic species concept uses a two-step process to define species • 1. Grouping taxa (regardless of rank) using the criterion of monophyly, or at least potential monophyly…build a tree? • 2. Ranking groups by the criterion of finding the smallest diagnosable monophyletic groups • A problem is that almost any population can be diagnosed • Also, when species initially split, most characters will not show reciprocal monophyly. That is, the gene trees may not follow the species trees!!

19. Sp.2 Sp.1 Ind.2 Ind.1 Ind.3 Ind.4 Ind.5 Initial Barrier to gene flow Examining a gene of four individuals

20. Sp.2 Sp.1 Ind.2 Ind.1 Ind.3 Ind.4 Ind.5 If we looked at the genes after initial split. It would appear that members of sp. 1 are more closely related to sp. 2 (ind. 4 related to 3)

21. Therefore, only after a certain period of time through continued isolation from members of other species and continued genetic contact among members of the same species, will the species appear reciprocally monophyletic Sp.2 Sp.1 Ind.2 Ind.1 Ind.3 Ind.4 Ind.5 This is not a problem with the concept, but with our ability to detect lack of gene flow

25. Cryptic species in marine phytoplankton Historically two species id’d under microscope using morphology, but is this meaningful variation? Tested under a variety of concepts including genetic data (PSC) and reproduction between species (BSC) Confirms, plus additional species id’d! 8 total

26. Evolutionary significant units (ESU) = important populations for genetic diversity in a species, important to conservation efforts Almost extinct species of ray (skate); previously classified as one species, then two, then one again, new data (genetic) indicates two

27. The “CLASSIC” process of speciation

28. Mechanisms of Isolation While physical separation is typically the first step in speciation, anything that causes a reduction in gene flow between two potential species (populations) can be an isolating factor (e.g., a genetic event that changes number of chromosomes, different breeding times, etc.).

29. Physical Isolation Gene flow tends to homogenize population frequencies and reduce the differentiation of populations Recall the banded and unbanded watersnakes in Lake Erie-migration homogenizes the island populations What would happen if lake currents around islands made it extremely difficult for mainland snakes to swim to islands? Genetic processes of the island snakes (their evolutionary trajectory) is now independent of the mainland snakes This is the beginning of allopatric speciation-physical (geographic) isolation resulting in speciation

30. Allopatric speciation Allopatry can be due to dispersal (movement of individuals) or vicariance (an encroaching physical feature)

31. Geographic isolation via dispersal and colonization 3 Hawaiian Drosophila (diverse morphology) Hawaii=archipelago of islands that emerged over time Phylogeny shows the dispersal patterns of the species of Drosophila on the islands

32. Geographic isolation via vicariance Vicariance can be a fast or slow event (lava spewing through a habitat or the rise of mountain ranges) Snapping shrimp are marine and found on the Pacific and Atlantic side of Central America Did the rise of the isthmus of Panama cause speciation?

33. Geographic isolation via vicariance A DNA phylogeny (b) of the shrimp sister species shows that generally each pair has an Atlantic (Carribean) and a Pacific species The slow rise of the land bride is reflected in the estimated timing of the species pair divergences

34. Mutation can cause Isolation For some organisms (especially plants) changes in chromosome number can reproductively isolate species instantly Chromosome numbers need complement each other for reproduction to happen Changes in chromosome number due to mutation during meiosis can result in a new species without any physical isolation (sympatric speciation) These are differences ploidy (diploid organisms are what we typically think of, an extra set of chromosomes ups the ploidy to a tetraploid, which can only breed with other tetraploids) 2% to 4% of plant species are due to this! Also seen in some lizards, frogs, salamanders, etc.

35. Polyploidy

36. Temporal Isolation Timing isolation-can be different times for reproduction (flowering time in plants, pupation time in insects) Japanese winter moths are genetically isolated due to the timing of their reproductive cycle But only in northern regions, in south timing is the same

37. Other examples sympatric Japanese land snail synestration (the direction the shell curves in) Mutation that causes shell to curve in other direction---each direction can only mate with those that are also in that direction because genitals do not line up properly for reproduction Anatomical incompatibility

38. But…“True” Sympatric speciation is still considered rare Sympatric speciation and other “rare” speciation events https://www.youtube.com/watch?v=IdcEgJazOHc

39. Mechanisms of Divergence What causes the phenotypic differences we observe between species (not just the absence of gene flow, but what we “see”)? Even in similar species, a close exam can show some morphological differences between species Ecological factors (differing habitats) are important, acted on by natural selection Genetic drift may also contribute as well (but difficult to document)

40. Adaptation to different habitats Biotic factors (competition, predation, prey) and abiotic factors (climate) When populations of a single species occupy different habitats with different selection pressures, a frequent outcome is phenotypic and genetic divergence, which can result in speciation if gene flow is limited Example: Mimulus plants

41. Mimulus ecotypes Moving the coastal, perennial Mimulus to inland habitat and vice versa results in plants from the transplanted region to do not well (less fit to that environ) Genetic manipulation of a certain region related to life-history phenotypes making the plant more or less coastal/inland showed similar results (light colored lines for each)

42. Mimulus ecotypes (plus Heliconius) The inland, annual ecotype of Mimulus guttatus occurs in seasonally dry habitats and flowers early in the spring, whereas the sympatric coastal, perennial form is found in wetter areas and is dormant in the early spring and flowers later. Hybridization between these ecotypes would produce offspring that are less fit in either habitat. Traits that confer local adaptation to these distinct environments are located on an inversion (shown as a long rectangle) that preserves these phenotypic combinations81. e |  Heliconius butterflies are a classic example of Mullerian mimicry. Many species of the genus Heliconius (for example, Heliconius numata silvana and Heliconius numata aurora) mimic the wing patterns of Melinae spp. to avoid predators. Each of these wing patterns requires a distinct combination of alleles that influence colour and shape, and recombinants between these distinct types are maladapted. The different Heliconius mimics are closely related and occur sympatrically, yet hybrids are rarely found in nature. It has been shown that two phenotypically distinct mimics have an inversion that harbours at least two colour-pattern loci97.

43. Assortative Mating Traits the influence how species choose mates, where individuals choose partners with traits most similar to their own Strong assortative mating can lead to isolation and speciation Female crickets choose specific songs

44. Assortative Mating How does this lead to speciation? Created hybrid families of these crickets which had wide variation in songs and genetic makeup from each species Within each hybrid family, the pulse rate of the male song matches the female preference Could not separate male song and female preference despite how mixed up the genes had become! Genetic linkage

45. Multiple factors can act together Habitats influence traits that are naturally selected for, these traits can also be involved in mate choice and cause divergence in mating preferences as well Cichlid fish are a well studied group for understanding speciation

46. Multiple factors can act together Some places have very different looking species (red and blue), others have more intermediate phenotypes (low water clarity locales) The water depth and clarity affects how the fish see (LWS gene) The colors also reflect female preference to mates

47. Speciation via Sensory Drive In cichlids, water depth, female preference, the alleles for seeing red, and water clarity are all acting together Speciation by sensory drive Predictions -higher frequnct of red LWS allele and more red males found in deeper waters -female preference for red will be associated with having the red LWS alleles (females are NOT red though) -strongest associations between color, water depth, LWS allele frequencies, female preference will be high clarity water

48. Water clarity increases from left to right… Male color and depth distribution increases As does the LWS allele freq. Females have stronger pref.

49. Hybridization and gene flow Even separate species may hybridize Can be during secondary contact (when two sister species come back into geographic contact after previous divergence) Very common in plants What can keep species separate? Prezygotic isolation-zygotes never form because of no mating Postzygotic isolation-mating occurs but offspring less fit than either parental species

50. Possible outcomes of hybridization A shows speciation B, C, D show possible outcomes of hybridization Does not show something that can also happen---total breakdown of species barriers and merge into a single species