Chapter 24

24.1 Biological Species concept emphasizes reproductive isolation

  • Speciation: Process by which one species splits into two species.

    • Produced diversity and helped explain the unity of life.

    • Microevolution: Allele frequencies in a population change over time,

    • Macroevolution: Broad pattern of evolution above the species level.

  • Species is latin for kind or appearance.

Biological Species concept

  • Species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but not of other groups.

  • All human beings belong to the same species.. despite ethnicity or race however, they can reproduce

  • Limitations (# of species to which biological species concept can be applied is limited)

    • No way to evaluate reproductive isolation of fossils.

      • Asexual and fossils can’t have mating observed.

    • Species are designated by absence of gene flow.

    • Morphologically and ecologically distinct species

      • Grizzly bear and polar bears produce “grolar bears”


Reproductive Isolation

  • Existence of biological factors (barriers) that impede members of two species from interbreeding and producing viable, fertile offspring.

  • Hybrids for example blocked in the gene flow.

  • Prezygotic barriers (before zygote) block fertilization from occurring.

    • Impeding of members of different species to mate (different contact receptors)

    • Gametic isolation → Hindering fertilization by preventing the sperm from reaching the egg.

      • Surface proteins on sperm and eggs of different species bind poorly preventing fertilization

    • Prevention of a mating attempt.

  • Postzygotic barriers: After the zygote. Hybrid zygotes prevented from developing into viable, fertile adults.

    • Development errors or even complications after birth.

    • Reduced hybrid viability → Genes of different parental species interact in was which prevent development or survival.

      • Meiosis fails to produce normal gametes, resulting in sterility, if parent’s species have chromosomes of different number or structure.

        • Hybrid offspring of a male donkey and female horse, a mule, is a robust but sterile.

  • Reduced hybrid survivability → Neither beneficial or negative effects on fitness are observed, often leading to a stable population due to reproductive isolation between the species.

Other definitions of Species

  • Morphological species concept: Distinguishes a species by body shape and other structural features.

    • Can be applied asexual and sexual organisms. Useful because it can be used without information on next extent of gene flow.

    • Relies on subjective research criteria. Research can disagree on structural differences.

  • Ecological species concept: Defines species in terms of its ecological niche, sum of how members of species interact with nonliving and living parts of their environment.

    • Two species of oak trees might differ in their size of in their ability to tolerate dry conditions, yet still occasionally interbreed.

    • Due to different ecological backgrounds, they are different species despite being connected by gene flow.

    • States

      • State 1 (continuous adaptive variation)

      • State 2 (discontinuous adaptive variation with minor reproductive isolation)

      • State 3 (adaptative differences with reversible reproductive isolation)

      • State 4: Adaptive differences with irreversible reproductive isolation

  • Over 20 other specifies definitions have been proposed.

Concept 24.2 Speciation can take place with or without geographic separation

Allopatric Speciation

  • Allopatric (“other country”) speciation: gene flow disrupted when population divided into geographically isolated subpopulations.

    • Giant ass mountain blocking one population from another.

    • Canyon may create barrier for small rodents, but not for birds, coyotes, or pollen and seeds of plants.

  • Process of allopatric speciation involves species ability to move about. Once geographic isolation occurs, separated gene pools may diverge.

    • Different mutations arise, and natural selection and genetic drift may alter allele frequencies in different ways in separated populations.

    • Coyotes, birds and pollen/seeds not affected easily versus smaller rodents.

    • Gene pools are isolated, and isolated populations diverge through mutation

  • Evidence → Field studies show reproductive barriers develop between isolated laboratory populations subjected to different environmental conditions.

    • Fruit flies experiment → Fruit flies separated by food and they mutated to prefer different types of media, illustrating how environmental factors can lead to speciation.

      • Different pheromone production.

  • Geographic isolation prevents interbreeding between members of allopatric populations, physical separation is NOT a barrier.

Sympatric (“Same Country”) Speciation

  • Syn = “together”; not geographically separate. Less common than allopatric speciation

  • Sympatric speciation → Speciation occurs in populations that live in the same geographic area.

  • Sympatric Speciation can occur if gene flow is reduced by factors such as polyploidy sexual selection, and habitat differentiation.

    • Sexual selection and habitat differentiation can also promote allopatric speciation.

Polyploidy

  • Polyploidy Speciation: Phenomenon when a new species originates from an accident during cell division which results in extra sets of chromosomes (Polyploidy).

    • Common in plants around (80%)

  • Autopolyploid: Individual that has more than two chromosome sets that are derived from a single species.

    • Diploid cell error → Tetraploid cell → Meiosis → New species/tetraploid(4n)

    • Tetraploids can produce fertile offspring by self-pollination or mating with other tetraploids.

    • Triploids are possible by tetraploids and diploids breeding; reduced fertility.

  • Allopolyploid: Result of two different species interbreeding and producing hybrid offspring.

    • Can only breed with other allopolyploids or backcross with one of the parent species, leading to the establishment of a stable hybrid population.

Sexual Selection

  • Biggest hint at this was with the 600 species of cichlids in Lake Victoria in Africa. There is no reason that there should be 600 species.

  • Hypothesis states that subgroups formed as a result of adaption to difference food sources, which resulted in genetic divergence.

  • Sexual selection hypothesis suggests that due to females selecting males based on appearance, this led to the rapid diversification of traits that are attractive to them, further increasing the number of species within this enigmatic group.

Habitat Differentiation

  • Sympatric speciation can also occur when subpopulation exploits a habitat or resource not used by parent population.

    • North American apple maggot fly: As colonized apples trees introduced by European settlers.. they preferred these apple trees and adapted.

  • Natural selection helps support this. Certain trees grow fruits faster, resulting in more fed and growing subgroups.

Allopatric and Sympatric Speciation Review

Allopatric

  • Intrinsic barriers to reproduction with members of the parent population may arise a by-product of genetic changes that occur within an isolated population.

  • Different environmental conditions, genetic drift, and sexual selection can promote such speciation in allopatric speciation.

Sympatric

  • Requires emergence of a reproductive barrier that isolates a subset of a population from remainder of the population in the same area.

  • Sympatric speciation can occur when gene flow to and from the isolated subpopulation is blocked.

    • Typically, dude to polyploidy or sexual selection.

    • However, can happen when subset of population becomes reproductively isolated due to habitat or food source change from parent population.

Concept 24.3 Hybrid zones reveal factors that cause reproductive isolation.

  • A Hybrid zone is a region which members of difference species mate and produce hybrid offspring.

  • Hybrids are the result of mating between species with incomplete reproductive barriers.

  • Patterns within Hybrid Zones

    • Some hybrid zones form as narrow bands where habitats overlap, while others can be broader and more complex, influenced by various environmental factors and the movement of the species involved.

  • Hybrid Zones and Environmental Changes

    • Changing environmental conditions can result in relocation of existing hybrid zones.

      • Black-capped and Carolina chickadees hybrid zone has shifted northward due to climate change.

    • Changing environmental conditions can result in NEW hybrid zones.

      • Southern flying squirrel range expanded northward into Northern flying squirrel range.

  • Hybrid Zones over Time

    • If hybrids do not become reproductively isolated from their parent species, three alternate outcomes are possible:

      • Reinforcement

      • Fusion

      • Stability

    • Reinforcement: Strengthening of Reproductive Barriers

      • If hybrids are less fit than parent species, this can lead to the reinforcement of reproductive barriers, as natural selection favors traits that enhance the reproductive success of the parent species.

      • Sympatric males choose their own species over hybrids.

    • Fusion: Hybrids are often less fit due to a combination of genetic incompatibility and environmental challenges, which may eventually lead to the merging of parent species and a reduction in the differentiation between them.

    • Stability: In stable environments, hybrids might persist if they possess certain advantageous traits, but in fluctuating conditions, the selective pressures typically favor the parent species, sustaining reproductive barriers over time.

Concept 24.4: Speciation can occur rapidly or slowly and can result from changes in few or many genes.

  • Many questions remain concerning how long it takes for new species to form and how many genes change when one species splits into two.

  • Time course of Speciation

    • Rate of speciation can be studied by observing broad patterns in fossil record.

    • Morphological and molecular data can also be used to asses the time interval between speciation events in particular groups.

  • Patterns in Fossil Record

    • Punctured equilibria describes these periods of apparent stasis punctuated by sudden change.

    • Rather than a punctuated pattern, other species appear to have changed gradually over time.

  • Speciation Rates

    • Punctuated pattern in fossil record and evidence from lab studies suggest speciation can be rapid.

      • Sunflower Helianthus anomalous was formed by hybridization between two other sunflower species by rapid selection.

  • Study of Genetics of Speciation

    • Depending on the species, speciation might require change in a single gene or many genes.

      • Japanese Euhardra snails.

    • Monkey flowers, at least two loci affect flower color and influences pollinator preference

  • From Speciation to Macroevolution

    • Differences accumulate with successive speciation events; eventually new groups of organism’s form that differ greatly from their ancestors.

    • Other groups shrink in size as species are lost to extinction.

    • Macroevolution is the cumulative effect of many speciation and extinction events.