Transcript Speciation
Working with the Biological Species Concept • Speciation is a two-part process – 1. Identical populations must diverge – 2. Reproductive isolation must evolve to maintain these differences • Speciation occurs much more readily in the absence of gene flow – This much more likely in geographically isolated populations • Populations can become geographically isolated for several reasons Consider how the amount of genetic divergence (change) forms a continuum: Microevolution small changes Macroevolution large changes Microevolution = adaptation Macroevolution = speciation The Evolution of Species • Significant changes in the gene pool could lead to the evolution of a new species over time. • The evolution of new species, a process called speciation occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment. The Evolution of Species • Geographic isolation occurs whenever a physical barrier divides a population. • A new species can evolve when a population has been geographically isolated. Reproductive isolation can result in speciation • Reproductive isolation occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring • A change in chromosome numbers- polyploidy can lead to speciation Isolating Mechanisms • Reproductive isolating mechanisms are the barriers that prevent genetic exchange between species – Prezygotic isolating mechanisms • Prevent the formation of zygotes – Postzygotic isolating mechanisms • Prevent the proper functioning of zygotes after they have formed Prezygotic barriers: Geographic isolation Ammospermophilus spp • Species occur in different areas – physical barrier – allopatric speciation • “other country” Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel Prezygotic barriers: Ecological isolation • Species occur in same region, but occupy different habitats so rarely encounter each other – reproductively isolated 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest Prezygotic barriers: Temporal isolation • Species that breed during different times of day, different seasons, or different years cannot mix gametes – reproductive isolation – sympatric speciation • “same country” Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer Prezygotic barriers: Behavioral isolation • Unique behavioral patterns & rituals isolate species – identifies members of species – attract mates of same species • • courtship rituals, mating calls • reproductive isolation Blue footed boobies mate only after a courtship display unique to their species Prezygotic barriers: Mechanical isolation • Morphological differences can prevent successful mating – reproductive isolation Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen. Plants Mechanical isolation Animals • For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer – lack of “fit” between sexual organs: hard to imagine for us… but a big issue for insects with different shaped genitals! Damsel fly penises Prezygotic barriers: Gametic isolation • Sperm of one species may not be able to fertilize eggs of another species – mechanisms • biochemical barrier so sperm cannot penetrate egg – receptor recognition: lock & key between egg & sperm • chemical incompatibility – sperm cannot survive in female reproductive tract Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse. Postzygotic barriers: • Prevent hybrid offspring from developing into a viable, fertile adult – reduced hybrid viability – reduced hybrid fertility – hybrid breakdown zebroid Forms of Selection • Selection is a statistical concept – One cannot predict the fate of any single individual – But it is possible to predict which kind of individual will tend to become more common in a population • Three types of natural selection have been identified – Stabilizing selection • Acts to eliminate both extreme phenotypes – Disruptive selection • Acts to eliminate intermediate phenotypes – Directional selection • Acts to eliminate a single extreme phenotype Three kinds of natural selection Natural selection acts on variations • Stabilizing selection is a natural selection that favors average individuals in a population. Selection for average size spiders Normal variation Natural selection acts on variations • Directional selection occurs when natural selection favors one of the extreme variations of a trait. Normal variation Natural selection acts on variations • In disruptive selection, individuals with either extreme of a trait’s variation are selected for. Selection for light limpets Normal variation Selection for dark limpets