- Slides: 23
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 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
Monitoreo a largo plazo