Transcript Genetic Drift

Biología de la Conservación: Genética y Biología de Poblaciones Aplicada
Evolution – changes in population allele frequencies over time. The
population is the smallest unit which can evolve
Population – any group of organisms coexisting at the same time
and place that are capable of interbreeding with one another
Gene Pool – all of the alleles at all loci in the population
Natural Selection – differential survival and reproduction of individuals in a
population due to trait differences
Mutation – creation of new alleles
Genetic Drift – changes in the gene pool of a small population due to
chance. Random changes due to sampling errors in propagation of alleles
Gene flow – movement of genes between populations. Gain or loss of
alleles from a population due to migration of fertile individuals, or from the
transfer of gametes
Random Mating
Bottleneck Effect – population undergoes a drastic reduction in size as a
result of chance events. A cause of genetic drift
Founder Effect – a small group of individuals becomes separated from the
larger population. A cause of genetic drift
Selection
If individuals having certain genes are better able to produce mature
offspring than those without them, the frequency of those genes will
increase. This is simply expressing Darwin's natural selection in terms of
alterations in the gene pool. (Darwin knew nothing of genes.) Natural
selection results from
1) differential mortality and/or
2) differential fecundity
due to trait/phenotype differences.
Mutation
The frequency of “A” and “a” will not remain in Hardy-Weinberg
equilibrium if the “A” mutates into “a” (or vice versa) or into any
alternative alleles. By itself, this type of mutation probably plays only a
minor role in evolution; the rates are simply too low. However,
evolution depends on mutations because this is the only way that new
alleles are created. After being shuffled in various combinations with
the rest of the gene pool, these provide the raw material on which
natural selection can act.
Genetic drift
Drift is important for small populations, where chance events
may eliminate or change the frequency of alleles. Drift
produces evolutionary change, but there is no guarantee that
the new population will be more fit than the original one.
Evolution by drift is aimless, not adaptive, because it is chance
alone (not phenotype) which changes allele frequencies. Drift
is common in two population events: Genetic bottlenecks and
Founder events.
Gene flow
Gene flow occurs when alleles are exchanged between two
populations. Gene flow occurs when individuals migrate (immigrate or
emigrate) and breed in a new population (contributing their genes to that
population). Gene flow can also occur through hybridization: when
individuals from two separate populations (say, Pop A and Pop B) breed,
their offspring carry genes from one population (Dad’s Pop A genes) into
another (Pop B where the offspring lives). Gene flow increases the
variability of the gene pool by adding new alleles.
Nonrandom Mating
Nonrandom mating occurs when individuals have mating preferences rather than
randomly mating with any other individual in the population. There are several
ways nonrandom mating may occur:
1. Assortative mating – for example, when AA individuals preferentially mate with
other AA individuals. This increases the probability that “A” gametes will combine
with other “A” gametes, and decreases the probability of “A” combining with
“a”. In humans, people often mate assortatively according to height (tall with tall,
short with short).
2. Inbreeding – when close relatives mate.
3. Sexual selection. Female animals and plants frequently chose among many
possible fathers for their offspring, selecting the father that has the best genes or is
the best resource-provider. This increases the alleles contributing to the favored
phenotype and decreases all alternative alleles.
Minimum viable population size
A minimum viable population (MVP) size is an estimate of the
number of individuals required for a high probability of survival of
a population over a given period of time. A commonly used, but
somewhat arbitrary definition is > 95% probability of persistence
over 100 years
Population Viability Analyses (PVA) are used. These are
computer-based simulation models which project changes in
initial population abundance over a set time period and account
for processes such as inbreeding depression, density
dependence, catastrophes and environmental and demographic
stochasticity. Time periods for PVAs are typically 20, 50, 100,
200, or sometimes 1000 years.
Extinction and Small Populations – the fewer you are the more problems you
have.
Small populations are generally at a greater risk of extinction than large
populations. They are subject to rapid declines in numbers for three main
reasons:
loss of genetic variability and related problems of inbreeding and genetic drift
demographic fluctuations due to random variations in birth and death rate
environmental fluctuations due to variation in predation, competition, disease
and food supply; and natural catastrophes that occur at irregular intervals, such
as fires, floods, volcanic eruptions, storms and droughts.
The figures below illustrate the rapid increase in probability of extinction for bird
species as the number of breeding pairs decreases; and bighorn sheep as the
population size decreases
120 bighorn sheep (Ovis canadensis) seguidos durante 70 años: 100%
de las especies no manejadas que tenían 50 individuos o menos se
extinguieron, mientras que casi todas las que tenían 100 individuos
persistieron
Qué tan pequeña se puede mantener una población
para conservarla?
Empíricamente: 50 animales reproductivos para evitar
la depresión por consanguinidad, perdiendo 2-3%
heterocigosidad por generación
Para que haya nueva variación genética por mutación
(basados en trabajos con Drossophila) se necesitan
poblaciones de 500 individuos:
Regla 50/500
Lande (la tasa de mutaciones benéficas es menor)
Por lo tanto se necesitan problaciones de miles de
individuos para asegurar la conservación de su
variabilidad genética a largo plazo
Consecuencias de la reducción de la variabilidad genética:
Depresión por consaguinidad
Pérdida de flexibilidad evolutiva
Depresión por intercambio genético abierto (outbreeding)
Los cuales llevan a
Pérdida de variabilidad genética
Pérdida de adecuación
Mayor probabilidad de extinción
Biología Poblacional Aplicada
Volverse activistas
•Ambiente
•Distribución
•Interacciones bióticas
•Morfología
•Fisiología
•Demografía
•Conducta
•Genética
•Interacción humana
Métodos para le estudio de poblaciones:
•Literatura
•Información no publicada
•Trabajo de campo:
•Monitoreo de poblaciones
•Inventarios, “surveys”
•Estudios demográficos
Análisis Poblacional de Viabilidad
(PVA)
= estimación de riesgo, una
extensión del análisis
demográfico
Metapoblaciones
Monitoreo a largo plazo