Island Evolution and Genetic Drift. The Role of Chance in Evolution

Transcription

1 Island Evolution and Genetic Drift The Role of Chance in Evolution

2 Biological Evolution on Islands Island Evolution Natural Selection Survival of the fittest Predictable Deterministic Genetic Drift Survival of the lucky Random Stochastic

3 Random/Stochastic Factors! Unpredictable in time and result! Uncommon components of an organism s environment! Big change in weather (drought, hurricane)! Big change in geology (volcano, earthquake, ice age)

4 Genetic Drift! Process that produces evolutionary change! Produces differential survival/reproduction for reasons other than fitness ( survival of the lucky )! Effects small populations more than large populations! Island populations tend to be small and are more likely to evolve by genetic drift

5 Genetic Drift! Reproduction produces combinations of alleles by chance! In small populations, greater chance of losing alleles by chance! Loss of one allele, cause the other allele to become fixed! The fixed allele is now the only option in that population

6 Genetic Drift and Natural Selection! Two processes can operate together

7 Genetic Drift and Natural Selection Genetic Drift Survival of the lucky Random process Doesn t occur due to environment challenges Non-directional Non-adaptive evolution Adaptive and/or neutral allele may disappear Loss of genetic variation Natural Selection Survival of the fittest Non-random process Occurs as result of environmental challenges Directional Results in adaptive traits Operates on any allele Increases genetic variation

8 Genetic Drift and Islands! Immigration and founder effect! Colonization is a chance event! Founding immigrants may not represent genetic diversity of source population! Island populations tend to start with low genetic diversity

9 Genetic Drift and Islands! Large reduction in a small population produces a bottleneck effect! May result in extinction! Remaining population not representing genetic diversity of original population

10 A Genetic Drift Story Think of two species that live on the same tiny island. One is a mouse. Total population, ten thousand. The other is an owl. Total population, eighty. The owl is a fierce and proficient mouse eater. The mouse is timorous, fragile, easily victimized. But the mouse population as a collective entity enjoys the security of numbers.

11 A Genetic Drift Story Say that a three year drought hits the island of owls and mice, followed by a lightning-set fire, accidental events that are hurtful to both species. The mouse population drops to five thousand, the owl population to forty. At the height of the next breeding season a typhoon strikes, raking the treetops and killing an entire generation of unfledged owls. Then a year passes peacefully, during which the owl and mouse populations both remain steady, with attritions from old age and individual mishaps roughly offset by new births. Next he mouse suffers an epidemic disease, cutting its population to a thousand, fewer than at any other time within decades. This extreme slump even affects the owl, which begins starving for lack of prey...

12 A Genetic Drift Story Weakened by hunger, the owl suffers its own epidemic, from a murderous virus. Only fourteen birds survive. Just six of those fourteen owls are female, and three of the six are too old to breed. Then a young female owl chokes to death on a mouse. That leaves two fertile females. One of them loses her next clutch of eggs to a snake. The other nests successfully and manages to fledge four young, all four of which happen to be male. The owl population is now depressed to a point of acute vulnerability.

13 A Genetic Drift Story Ten years pass, with the owl population becoming progressively less healthy because of inbreeding. A few further females are hatched but most turn out to be congenitally infertile. During the same stretch of time the mouse populations rebounds vigorously. Good weather, plenty of food, no epidemics, genetically it s fine. So the mouse population returns to its former abundance. Then another wildfire scorches the island, killing four adult owls and, oh, six thousand mice. The four dead owls were all breeding age females, crucial to the the beleaguered population. Among the owls there now remains only one female who is young and fertile. She develops ovarian cancer, a problem to which she s susceptible because of the history of inbreeding among her ancestors. She dies without issue. Very bad news for the owl species.

14 A Genetic Drift Story Let s give the mouse another plague of woe, just to be fair: a respiratory infection, contagious and lethal, causes eight hundred fatalities. None of this is implausible. These things happen. The owl population- reduced to a dozen mopey males; several dowagers; no fertile females - is doomed to extinction. The mouse population fluctuates upward in response to the extinction of owls, a rude signal that life is easier in the absence of predation. From Song of the Dodo, David Quamman